EXP MN SE060 en R0 Electrical Power Generation
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ELECTRICAL MAINTENANCE ELECTRICAL POWER GENERATION
TRAINING MANUAL Course EXP-MN-SE060-EN Revision 0
Field Operations Training Electrical Maintenance Electrical Power Generation
ELECTRICAL MAINTENANCE ELECTRICAL POWER GENERATION SUMMARY 1. OBJECTIVES ..................................................................................................................6 2. INTRODUCTION - ELECTRICAL GENERATION ...........................................................7 2.1. ALTERNATING CURRENT GENERATORS, THE ALTERNATOR...........................7 2.2. DIRECT CURRENT GENERATORS, ROTATING MACHINES ................................9 2.3. DIRECT CURRENT GENERATORS, BATTERIES...................................................9 3. GENERATION OF ELECTRICAL CURRENT................................................................10 3.1. GENERATION OF DIRECT CURRENT..................................................................10 3.1.1. Batteries ..........................................................................................................10 3.1.2. Photovoltaic cells.............................................................................................11 3.1.3. T.E.G. Thermo Electric Generator ...................................................................12 3.1.4. Rotating generators .........................................................................................13 3.1.4.1. Energy conversion .....................................................................................13 3.1.4.2. Symbol .......................................................................................................13 3.1.4.3. Construction ...............................................................................................13 3.1.4.4. Principle of a DC generator ........................................................................14 3.1.4.5. Different DC machine types: ......................................................................17 3.2. GENERATION OF ALTERNATING CURRENT ......................................................19 3.2.1. Principle of an AC generator ...........................................................................19 3.2.2. Permanent magnet generator..........................................................................20 3.2.3. Principle of a basic alternator ..........................................................................22 3.2.3.1. Alternator with 2 pairs of poles ...................................................................22 3.2.3.2. Alternator with 'x' pairs of poles..................................................................23 3.2.4. Battery Chargers / Inverters ............................................................................24 4. SYNCHRONOUS MACHINES –ALTERNATORS .........................................................25 4.1. PRINCIPLE AND FUNCTION OF POWER ALTERNATORS .................................25 4.2. MAIN COMPONENTS.............................................................................................27 4.2.1. Stator...............................................................................................................27 4.2.2. Rotor................................................................................................................28 4.2.3. Exciter .............................................................................................................28 4.2.4. Bearings ..........................................................................................................29 4.2.5. Resistance Temperature Detectors .................................................................30 4.2.6. Space Heater ..................................................................................................30 4.2.7. Supporting Frame............................................................................................30 4.3. ALTERNATOR CONSTRUCTION ..........................................................................31 4.3.1. The single phase synchronous generator........................................................31 4.3.2. The three phase’s synchronous generator ......................................................32 4.3.3. AC generator in general ..................................................................................33 4.3.4. Rotor construction ...........................................................................................35 4.3.5. Insulation .........................................................................................................36 4.3.6. Cooling ............................................................................................................37 4.3.7. Neutral Earthing Resistor ................................................................................38 Training manual EXP-MN-SE060-EN Last Revision: 29/09/2008
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4.3.8. Insulated Bearings...........................................................................................38 4.4. GENERATOR EXCITATION ...................................................................................40 4.4.1. Conventional excitation ...................................................................................40 4.4.2. Static excitation ...............................................................................................40 4.4.3. Brushless excitation (general case).................................................................41 4.4.4. Brushless excitation (without pilot exciter).......................................................42 4.4.5. Brushless excitation (with pilot exciter)............................................................42 4.4.6. Diode bridge ....................................................................................................43 4.4.7. Alternator parts................................................................................................45 5. ALTERNATOR CONNECTIONS AND PROTECTIONS ................................................47 5.1. GENERATOR CONNECTIONS ..............................................................................47 5.1.1. The Delta system ............................................................................................47 5.1.2. Delta connected generator ..............................................................................48 5.1.3. The wye (star) system .....................................................................................50 5.1.4. Wye (star) connected generator ......................................................................51 5.2. GENERATOR PROTECTIONS...............................................................................52 5.2.1. ANSI codes for Protections .............................................................................53 5.2.2. Typical one line diagram generator protection.................................................54 5.2.3. Details for generator protections .....................................................................56 5.2.3.1. Protection functions connected to generator neutral current transformers.56 5.2.3.2. Protection functions connected to voltage transformers.............................56 5.2.3.3. Protection functions connected to line-side current transformers (for parallel operation only) ........................................................................................................56 5.2.3.4. Generator mechanical protection functions connected to sensors .............57 5.2.4. Practical checks by operators and maintenance technicians ..........................57 5.2.4.1. Review .......................................................................................................57 5.2.4.2. Active reverse power protection .................................................................58 5.2.4.3. Reactive reverse power protection (Loss of excitation) ..............................59 6. ALTERNATOR OPERATION AND CONTROL..............................................................61 6.1. LOAD ADJUSTMENT OF A GENERATOR (OR ALTERNATOR)...........................61 6.2. AUTOMATIC VOLTAGE REGULATORS (AVR) .....................................................63 6.2.1. AVR set-point ..................................................................................................63 6.2.2. AC Generator voltage regulation .....................................................................63 7. GENERATORS PARALLELING AND SYNCHRONISING.............................................65 7.1. CONDITIONS FOR PARALLELING........................................................................65 7.1.1. Condition 1: same phase operation .................................................................66 7.1.2. Condition 2: same frequency...........................................................................68 7.1.3. Condition 3: same voltage ...............................................................................70 7.1.4. Condition 4: Synchronising or phasing ............................................................70 7.2. SYNCHRONISATION / PARALLELING ..................................................................74 7.2.1. Ready for coupling ..........................................................................................74 7.2.2. Coupling operations of a one phase alternator with lamps ..............................74 7.2.3. Coupling operations with a three-phase alternator lamp .................................76 7.2.4. Coupling operations with a synchronoscope ...................................................77 7.2.5. Tolerances for coupling / synchronising ..........................................................78 7.3. PARALLEL CONTROL OPERATION......................................................................79 7.3.1. Taking the load................................................................................................79 7.3.2. Load sharing....................................................................................................79 Training manual EXP-MN-SE060-EN Last Revision: 29/09/2008
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7.3.3. Droop and Isochronous mode of control..........................................................81 7.3.4. Load Shedding ................................................................................................83 8. MAINTENANCE OF ALTERNATORS ...........................................................................85 8.1. DIODE REPLACEMENT .........................................................................................85 8.2. VARISTOR REPLACEMENT ..................................................................................89 8.3. DRYING WINDINGS ...............................................................................................90 8.3.1. Space Heaters.................................................................................................90 8.3.2. Forced Air........................................................................................................90 8.4. OPERATIONAL DIFFICULTIES..............................................................................91 8.4.1. Troubleshooting table ......................................................................................91 8.4.2. Insulation Resistance ......................................................................................91 9. ELECTRICAL GENERATION ON SITE.........................................................................93 9.1. TYPES OF ENGINE GENERATOR SETS..............................................................93 9.2. RATED POWER FOR GENERATOR SET APPLICATIONS ..................................94 9.3. TYPICAL APPLICATIONS ......................................................................................96 9.3.1. Stand-by generator sets ..................................................................................96 9.3.2. Production generator sets ...............................................................................99 9.4. OPERATION OF GENERATOR SETS .................................................................101 9.4.1. Starting and stopping of generator sets.........................................................101 9.4.2. Stand alone operation ...................................................................................102 9.4.3. Parallel operation with utility supply...............................................................103 9.4.4. Parallel operation with other generator sets ..................................................103 9.5. TRANSFER SCHEMES AND SYNCHRONISATION ............................................105 9.5.1. Automatic transfer on loss of supply..............................................................105 9.5.1.1. Residual voltage transfer..........................................................................105 9.5.1.2. Fast transfer .............................................................................................105 9.5.2. Maintenance transfer – back to normal supply ..............................................106 9.5.3. Synchronization of generator circuit-breaker .................................................106 9.5.4. Synchronization of bus-tie, bus-coupler, or utility incoming circuit-breakers..107 9.6. GENERATOR SET PROTECTION .......................................................................108 9.6.1. General protection philosophy.......................................................................108 9.6.2. Electrical protection .......................................................................................109 9.6.2.1. Particularities of generator short-circuit currents ......................................111 9.6.2.2. Possible delaying of circuit-breakers........................................................111 9.6.3. Machine protection ........................................................................................112 9.7. CONNECTION OF GENERATORS TO ELECTRICAL NETWORK ......................113 9.7.1. Connection to generator circuit-breaker ........................................................113 9.7.2. Connection of generator neutral point ...........................................................113 9.7.2.1. Stand-alone generator set........................................................................113 9.7.2.2. Operation in parallel with utility or other sets............................................113 9.8. LOAD SHEDDING.................................................................................................114 9.8.1. Gradual increase in load................................................................................115 9.8.2. Loss of a generator .......................................................................................115 9.8.3. Electrical faults ..............................................................................................115 9.9. INTERFACING GENERATOR WITH ELECTRICAL DISTRIBUTION SYSTEM ...116 9.9.1. Typical split of supply between generator set manufacturer and switchgear manufacturer ...........................................................................................................116 9.9.2. Information to be exchanged .........................................................................117 Training manual EXP-MN-SE060-EN Last Revision: 29/09/2008
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9.9.3. Integration of generator set into electrical distribution supervisory system....118 9.10. INSTALLATION AND MAINTENANCE OF GENERATORS SETS .....................118 9.10.1. Location.......................................................................................................118 9.10.2. Air intake and exhaust .................................................................................119 9.10.3. Compliance with local regulations ...............................................................119 9.10.4. Special tools and spare parts ......................................................................120 9.11. CONCLUSION ....................................................................................................120 10. GLOSSARY ...............................................................................................................121 11. FIGURES...................................................................................................................122 12. TABLES .....................................................................................................................125
Training manual EXP-MN-SE060-EN Last Revision: 29/09/2008
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1. OBJECTIVES At the end of this course, the participant should be able to: Define the generation principle for alternating current Define the generation principle for direct current List the different types of generators and/or alternators Explain the use and operation of alternators Determine the regulation factors for an alternator Couple an alternator to a network Define the principles and use of electric protection for an alternator Be familiar with the basic maintenance of a power alternator on a site Differentiate Loading, Load Sharing, Load Shedding,…
Training manual EXP-MN-SE060-EN Last Revision: 29/09/2008
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2. INTRODUCTION - ELECTRICAL GENERATION This can be: an external source, a self-contained electricity plant, turbo-generators, generators with gas/diesel engines, generator sets, wind turbines, solar panels, etc. but not forgetting direct current generation with the battery sets and rectified current supplies (UPS). On-site, you will mainly encounter turbo-generators or generators driven by gas or diesel engines. Whatever the size of the generator, it is always represented in the same manner in the diagrams. Figure 1: Electricity generation However, for the alternator, the number of wires is not systematically represented (threephase, single-phase).
2.1. ALTERNATING CURRENT GENERATORS, THE ALTERNATOR The three-phase generator is the "indispensable" source on site. The one-line diagram representation can be as shown here and by specifying the power and the voltage, e.g. 600kVA, 3x400V, windings in star (or Y) configuration. The power diagram specifies 3 distributed phases, neutral not distributed with a voltage of 400V between the phases. "Three-pole" protection by circuit breaker at the LV switchboard. Figure 2: One-line and power diagram representation for the alternator' –neutral to ground
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This is the same representation but for a 10MVA three-phase generator with 3x5.5kV and neutral earthed through an impedance The voltage between phases is 5.5 kV. The protection (and/or the disconnect components) is of course in the HV switchboard. Figure 3: One-line and power diagram of alternator – neutral to ground with impedance Here, the generator operates at a low voltage of 3x400V but with distributed neutral. The protection (or the disconnect components) at the LV switchboard must be on the 4 poles of a circuit breaker. In this distribution with 400V between phases, what is the voltage between phase and neutral? Figure 4: Generator with neutral distributed Three-phase distribution, voltage between phases and between phase and neutral: V=1 U/2 = 0.866 U
30° U/2 = 0.866
V
Figure 5: Vector representation of a star shape three-phase distribution system Training manual EXP-MN-SE060-EN Last Revision: 29/09/2008
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It depends on the relationships in a right-angled triangle and on the vector diagram above. "Imagine" the 3 phases in star (or Y) configuration, ‘V’ is the voltage across the terminals of a winding and ‘U’ is the voltage between phases. Form 2 right-angled triangles on one of the 3 segments. U/2 = V x sine 30° = V x
3 /2
and
U = 2 U/2 = 2 V x
3 /2 = V x
3 =U
Thus when U = 400 V between phases, between phase and neutral V = 230 V (rounded). For U = 380V, V= 220V For the delta configuration, there is no distributed neutral and there are just the voltages between phases.
Figure 6: The Delta shape 3 phase distribution system
2.2. DIRECT CURRENT GENERATORS, ROTATING MACHINES See in other paragraph and here in the following chapter(s). Direct current generators are very rare on a production site. However, direct current motors (machines strictly identical to the generators) are associated with the turbogenerators' auxiliaries (oil pump, cooling fan,…etc powered by batteries when no other sources are available).
2.3. DIRECT CURRENT GENERATORS, BATTERIES See the specific "batteries" course. You will systematically find battery packs with inverters supplying alternating current from batteries and also associated with the rectifier cubicles to supply the instrumentation circuits, fire safety circuits, electrical safety circuits, etc.
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3. GENERATION OF ELECTRICAL CURRENT 3.1. GENERATION OF DIRECT CURRENT Of course, an « alternator » is not producing « Direct Current », but without this D.C. there would be no « Alternative Current » produced by the A.C generator !......(think and debate about this…..) Several sources of direct current are present on the site - batteries, photovoltaic cells, rectifiers, rotating generators. This is the subject of the following paragraphs.
3.1.1. Batteries See the "Batteries" course. The term "battery" in languages other than French also refers to all cells used in everyday ‘items’ (radios, mobile telephones, torches, etc.). Figure 7: Examples of batteries These cells are also used in instrument cabinets and computers as "back up" devices. The alternative French term ‘pile’, or cells is open to confusion as they are referred to as rechargeable and non-rechargeable. In theory, a cell cannot be recharged. The term "accumulator" should be used for rechargeable cells (note: this paragraph refers specifically to the French term "pile"). The term ‘accumulator’ refers to car batteries, which is correct. The word accumulator should be used for any source of direct current in static form which may be discharged and recharged.
Figure 8: Examples of accumulators Training manual EXP-MN-SE060-EN Last Revision: 29/09/2008
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The conventional lead acid battery (accumulator) is present on site in vehicles (onshore, obviously….), to start combustion engines for fire pumps and emergency generators. The set of batteries is present in ‘UPS’, incorporated in cabinets or in a battery room.
Figure 9: Examples of UPS And if the EDG battery is flat, if the UPS batteries do not have the capacity to last for the duration of the shut-down, it will not be easy to restart the main alternator.
3.1.2. Photovoltaic cells These are "sun panels" for platforms supplying indicator lights and/or remote transmission, instruments, etc. Photovoltaic cells take their energy from any light source and "light" in general. Solar energy is an inappropriate term, "light energy" would be more accurate. A photovoltaic cell is an electronic component which, when exposed to light (photons), generates electric voltage (this effect is known as the photovoltaic effect). Direct current is obtained at approximately 0.5V. Figure 10: Photovoltaic cell Photovoltaic cells consist of semi-conductors with a silicon (Si), cadmium sulphide (CdS) or cadmium tellurium (CdTe) base. They are also known as "photo-galvanic They exist in the form of two thin plates in close contact. They are also known as “photo-galvanic” Figure 11: Structure d’une cellule photovoltaïque Training manual EXP-MN-SE060-EN Last Revision: 29/09/2008
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This semi-conductor is sandwiched between two metal electrodes and the entire unit is protected by glass. Photovoltaic cells are mounted on the panels in series and in parallel. The set of cells is connected to a set of batteries (with regulator).
3.1.3. T.E.G. Thermo Electric Generator
Figure 12: ‘TEG set’s installed on a Total platform (Peciko)
Figure 13: TEG principle Training manual EXP-MN-SE060-EN Last Revision: 29/09/2008
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A TEG unit consists of cells which have the particular characteristic of producing voltage and current when subject to a certain temperature. On a platform, natural gas is burned in each TEG, the heat emitted in combustion is transformed into electric energy in each cell. As is the case for photovoltaic cells, the TEG cells are mounted in series and in parallel and are connected to a set of batteries and a voltage regulator.
3.1.4. Rotating generators 3.1.4.1. Energy conversion The term direct current (DC) machine would be more accurate, as a DC generator (or a dynamo, exciter or rotary convertor) is the same machine as the DC motor. Let's take a look at DC generators and then we can forget about motors….. Electric energy supplied = Motor = Useful mechanical energy Mechanical energy supplied = Generator = Useful electric energy
3.1.4.2. Symbol
Figure 14: Rotating D.C. generator symbols
3.1.4.3. Construction The machine includes: A magnetic circuit including a stationary part, the stator, a rotating part, the rotor, and the air gap, the space between the two components. A magnetic field source known as the field system (mounted on the stator) is created using coils or permanent magnets. Field coils for both motors and generators are always supplied with a secondary and direct source of energy. Training manual EXP-MN-SE060-EN Last Revision: 29/09/2008
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An induced electric circuit (the rotor), is subject to the effects of this magnetic field; the collector and the brushes allow for access to the electric circuit of the rotor.
Figure 15: Magnetic circuit of a two-pole machine
3.1.4.4. Principle of a DC generator Suppose a one turn coil rotating in a magnetic field (field systems). The current generated in the turn is "collected" via the 2 sections of a slip ring with brushes (in graphite) on the stationary part (stator) consisting of 2 commutator (or switch) segments (in copper, attached to the rotor) which "switch" with each rotation. The switching function is essential. Each side of a turn rotating on its axis passes near to a north pole, and subsequently a south pole, and so on. Therefore, the magnetic induction intercepted by the turn regularly changes direction due to the angular position of the rotor. To avoid the torque produced by the electromagnetic force (emf) reversing at the same rate, the current in the turn must be regularly inverted. This is the task of the collector. Training manual EXP-MN-SE060-EN Last Revision: 29/09/2008
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Figure 16: Simple DC generator
Figure 17: EMF (‘e’) pulses produced by a DC generator The torque produced by the passage of the current in a turn would be approximately sinusoidal without the presence of the collector. This resembles a rectified sinusoid thanks to the collector. Pulses are not however desirable. Therefore DC machines are equipped with several turns, each connected to a pair of segments on the collector. The figure opposite shows the torque smoothing effect obtained by using 2 turns rather than 1 turn at the rotor, and therefore 4 rather than 2 segments at the collector. Figure 18: Smoothing effect Training manual EXP-MN-SE060-EN Last Revision: 29/09/2008
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This assembly shows the magnetic field of the stator created by an electromagnet, with 6 turns (or coils) on the rotor.
+
+
+
Consequently, the current and the voltage generated are 6 times more regular with the same quantity of collector segment pairs (split-ring commutator or switch) as coils
+ + +
Figure 19: Six turn assembly
e
For a smoother electromagnetic torque, DC machines are created with a large number of turns and segments. t
Figure 20: The smoothing effect
Current and voltage are thus "smoothed" out. Only ‘sinusoid’ peaks are switched.
Figure 21: DC machine Training manual EXP-MN-SE060-EN Last Revision: 29/09/2008
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3.1.4.5. Different DC machine types: Question: why is the term "machine" used in the above paragraph instead of "generator"? Permanent magnet machine: DC generators with permanent magnets exist (coil-free field system) as a low power threephase alternator exciter. Other machines: 3 types exist corresponding to the type of wiring between the rotor and stator The general advantage of the DC machine is its flexibility to speed. A DC generator can provide the same voltage over a range of speeds, simply by modulating the current in the "field systems or inductors" of the stator. A DC motor can achieve a wide range of speeds by varying the current either in the “armature” or in the inductors (or the field systems) or in both. "Shunt" machine: The stator and rotor are connected in parallel.
N
S S
The shunt motor produces a constant torque, independently to speed.
Figure 22: Shunt machine Figure 23: Shunt motor
Training manual EXP-MN-SE060-EN Last Revision: 29/09/2008
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"Series" machine:
Armature
The stator and rotor are connected in series. The series motor produces high torque, particularly at low speed
S
N
Field windings Figure 24: Series machine Figure 25: Series motor "Compound" machine: Combination of Shunt and Series, the field systems are partially connected in series and partially in parallel with the armature.. This leads to combined advantages for the motor, which is the most used of the 3. It drives the oil pumps, and the cooling water for a generator. Figure 26: :"Compound" machine
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3.2. GENERATION OF ALTERNATING CURRENT 3.2.1. Principle of an AC generator
Figure 27: Principle of the generation of alternating current If a turn is rotated in a magnetic field, voltage is induced at the terminals. This varies the angle α between the turn plane and the magnetic induction. The faster the rotation, the less time required for cos α to pass from 1 to -1 and vice versa. The amplitude of the induced voltage created is proportional to magnetic induction and the rotation speed of the turn. Figure 28: Turn in a magnetic field
This is the principle behind an alternator. The following diagrams demonstrate the sinusoidal form.
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Figure 29: Production of AC The coil is moving (rotating) anti-clockwise. From (a) to (b) at 90° From (b) to (c) at 90° From (c) to (d) at 90° From (d) to (a) at 90°
3.2.2. Permanent magnet generator This is an AC generator, with a sinusoidal form, with a (permanent) magnet creating an "emf" in a "peripheral" coil. Training manual EXP-MN-SE060-EN Last Revision: 29/09/2008
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This generator can be used for a bike. In this case the term "bike dynamo" is used, however the term "dynamo" does, in principle, apply to a DC generator and this term is therefore incorrect when referring to an alternator installed on a "bike". In the figure below, 2 coils are shown (in series), however one single coil would be enough. V
Axis Sin V
N α
Cos α
S V = e = emf = Figure 30: bike dynamo The voltage produced can be called ‘e’ or equally ‘U’, ‘V’, ‘v’, ‘u’, ‘E’, etc… Standards exist in this domain (regarding the representation abbreviation) but they are not consistent at an international scale and are often criticised.
Figure 31: Voltage induced by a magnet rotating in a coil By rotating the magnet, the value of ‘U’ at time ‘t’ represents exactly the value of the sinus of the positioning angle (α) of the magnet. A sinusoid curve is created for each complete rotation of 360 deg. This is known as an "alternation". U (at time ‘t’) = U sin α Training manual EXP-MN-SE060-EN Last Revision: 29/09/2008
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Questions/Exercises: and without the use of formulas, please! With "bike dynamos", 1 full 360° rotation for 1 second produces an induced voltage at a frequency of 1 hertz. 1) How many revolutions per second are required to achieve 50Hz? 2) The speed of rotating machines is expressed in RPM (Revolution Per Minute). At what speed must the machine rotate in rpm to reach 50Hz? 3) At what speed must the machine rotate in rpm to reach 60Hz? 4) Is the central rotating magnet known as the "rotor" or the "stator"? 5) Are the coils capturing the induced energy (peripheral) in the "stator" or the "rotor"? As you have answered these questions easily, you are aware that, with a bit of logic (without using formulae), you can understand the relation between speed and frequency. This is the basis of "synchronism".
3.2.3. Principle of a basic alternator The "bike dynamo" mentioned in the above paragraph is in fact a single-phase alternator. The permanent magnet rotating at 1 revolution per second produces a voltage (and a current) induced at the frequency of 1Hz at the terminals of the alternator. By rotating the magnet at 50 revolutions per second, a frequency of 50Hz is produced, corresponding to a synchronism speed of 50 revolutions per second, i.e. 3000 rpm and this for a "magnet" with one pair of poles (one ‘North’ and one ‘South’ pole).
3.2.3.1. Alternator with 2 pairs of poles +V Sin α
V
S
V
N
α
N -V
1 Hertz
S
V = e = emf 1 tour
Figure 32: Alternator with 2 pairs of poles Training manual EXP-MN-SE060-EN Last Revision: 29/09/2008
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This is the "same" alternator as in the previous paragraph with a second permanent magnet added to the rotor. There are now 2 pairs of poles. With one complete revolution, the passage of the stator coil next to the poles will be increased, it will pass twice by a maximum and minimum. One revolution produces 2 Hz. For a frequency of 50 Hz, a speed of 25 revolutions per second will be required, i.e. 1500 rpm which is the synchronism speed (the speed producing 50hz) for an alternator with 2 pairs of poles. Question: In view of the relation between frequency and speed (still no formulae), at what speed (rpm) must an alternator with 2 pairs of 'North American' poles rotate to produce a frequency of 60Hz?
3.2.3.2. Alternator with 'x' pairs of poles Questions: This is obvious, and you will easily work out the different configurations of frequency/speed/pairs of poles for all types of alternators (note: the same logic applies for single and three-phase alternators). What is the synchronism speed (rpm) for: An alternator with 1 pair of poles and a frequency of 60Hz? An alternator with 3 pairs of poles and a frequency of 50Hz? An alternator with 3 pairs of poles and a frequency of 60Hz? An alternator with 4 pairs of poles and a frequency of 50Hz? An alternator with 4 pairs of poles and a frequency of 60Hz? All these types of alternators exist. This is simply the basic principle behind the alternator. The following chapter will consider "true" alternators, those producing electrical power on sites. Simply replace the permanent magnet on the rotor with coiled field systems supplied with secondary direct current and make the unit "a bit bigger" to create the 'synchronous machine'
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3.2.4. Battery Chargers / Inverters This refers to UPS, battery chargers which produce "rectified" current from AC (generated by an alternator). This is not true direct current, which can be generated only by a battery. +V
t
Figure 33: One phase rectified voltage/current with smoothed signal (rectifier or dynamo The power alternator or the 'synchronous machine' requires rectified current within the rotor to produce electromagnetic induction, generally in the form of one phase rectified current. The alternator generally uses an exciter (at the end of the shaft) to produce the energy required for the magnetic field. If this exciter is a DC machine, the armature (the rotor) is directly powered (with a set of rings/brushes). The exciters are now (small) alternators producing rectified AC. The exciter/bridge rectifier unit is mounted on the main shaft. The technology will be considered in more detail later in the course.
Figure 34: One phase bridge rectifier and three-phase Graetz bridge Rotor "field systems" do not require "smoothed" rectified current. The average value of a one-phase rectified current is adequate (get your instructor to explain this to you if you do not “grasp” it).
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4. SYNCHRONOUS MACHINES –ALTERNATORS This machine is both the industrial alternator for our sites and the synchronous motor which is rarely found on "our" sites. A Synchronous machine, known as an ALTERNATOR; it supplies AC when it acts as a generator. When operating as a MOTOR, its rotation speed is imposed by the frequency of the alternating current which supplies the stator windings.
4.1. PRINCIPLE AND FUNCTION OF POWER ALTERNATORS A synchronous generator transforms electric energy (T, Ώ) into electric energy (V, I at frequency f).
Introduction/reminder The alternator is the key to energy for a facility, a site. The AC generator converts the mechanical energy produced by the turbine (or heat engine or any type of prime mover) into electric energy via electromagnetic induction. Two types of "core" and "field" windings are required to achieve this. The "main" current or operating current comes from the core (generally the stator). DC (or rectified current) is injected in the field windings in order to create a magnetic field of fixed direction (or polarity).
Figure 35: Alternator windings
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Relative movement between the core and field windings is required to produce voltage. ‘Relative movement' means movement of the magnetic field through the conductors of the core or movement of the conductors through the magnetic field. The stator and the rotor are used to produce this relative movement. The latter will rotate in the stator windings, thus creating induced voltage. According to standards and the country, the relation between frequency and voltage is as follows for standard alternators:
Frequency
Voltage
60 Hz
50 Hz
480
380 / 400
600
440
2400
3300
4160
5500 / 6000
13800
11000
Table 1: Relation between frequency and voltage
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4.2. MAIN COMPONENTS Alternators have six main components: stator, rotor, bearings, space heater, temperature detection and the supporting frame. We will now consider each of these components in detail.
4.2.1. Stator The stator is built with high-grade silicon steel laminations, precision punched, and individually insulated. Low voltage windings are random-wound coils in lined, semi-closed slots. High-voltage windings are form-wound in lined slots. Wound cores are repeatedly impregnated with thermosetting synthetic varnish, and baked for maximum moisture resistance, high dielectric strength, and high bonding qualities. Windings are braced to withstand shock loads such as motor starting and short circuits. Space heaters are available to minimise condensation during long shutdowns A space heater in the interior of the generator prevents the formation of condensate in the generator windings after shutdown. The space heater is automatically switched on and off by the turbine control system during the start-up and shutdown sequence unless the systems select switch is in the off position. The only operator action necessary is the verification of the heater’s operation upon generator shutdown. Optional RTD (Resistance Temperature Detector) sensors may be installed to monitor generator winding temperatures.
Figure 36: Stator assembly
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4.2.2. Rotor At the centre of the rotor assembly is a high grade steel shaft that has four metal pieces extending outward, two sets of cooling fan blades and a brushless exciter armature mounted an the forward end. The flour metal "poles" are used to form the magnetic poles. Coils of wire are wrapped around each metal pole to form a magnetic field opposite from the one next to it (Figure under). All four metal poles of the rotor form the second type of winding, the field winding. During generator operation, dc current is passed through each pole winding to form alternate north and south poles, which makes up the rotating magnetic field.
Figure 37: Rotor assembly When a load is connected to the generator, current flows in the system. As this current passes through the armature windings, heat is created that must be dissipated to prevent damage. On some generators air is drawn in from both ends by the fan blades on the rotor and circulated around the rotor and stator windings to remove the heat. The cooling air usually exits through the top of the generator.
4.2.3. Exciter A second, smaller generator is mounted inside the main generator. This smaller generator is mounted on the forward end of the rotor shaft and is referred to as the brushless exciter armature. In this exciter the armature rotates inside the stationary field windings mounted on the frame. The brushless exciter produces three-phase ac voltage for use in the four magnetic poles of the rotor. To use this ac voltage, it must be rectified to dc by the six diodes located at Training manual EXP-MN-SE060-EN Last Revision: 29/09/2008
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the forward end of the rotor shaft. The six diodes furnish three-phase, full-wave rectification for a very smooth pulsating dc current, which is fed directly to the main field windings on the projecting poles. The brushless exciter eliminates commutator, collector rings, brushes, and brush holders making the generator a low maintenance machine. The exciter consists of a 3-phase rotating armature type ac generator and a 3- phase full wave rectifier. Excitation is available when the generator is carrying 150 percent rated current for one minute. The rotating armature and the rotating rectifier assembly are mounted on the generator rotor shaft and are electrically interconnected with each other and with the generator field windings. The stator for the exciter consists of a wound-laminated core installed in a flange ring, which forms an integral part of the generator front bearing bracket. The complete exciter is enclosed and protected by a removable cover.
Figure 38: Assembly of main parts stator/rotor/exciter
4.2.4. Bearings The bearings in the typical mid-range generator are either the self-lubricated anti-friction type or the sleeve bearing type. If the sleeve bearing type is used, the generator cooling air effectively cools the oil passing through the bearings and oil reservoirs. A sight gage located below the bearings is used to check the oil level in the reservoir.
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4.2.5. Resistance Temperature Detectors Resistance temperature detectors (RTD) monitor the temperature of the generator windings and bearings. The RTD’s are connected to remote sensors that have warning and shutdown set points to protect the components against high temperatures. The RTD’s have a positive coefficient of resistivity. That is, the element’s resistance increases as the temperature applied to the detector increases. Six detectors, two per phase, are installed in the stator slots between the top and bottom stator coils. An RTD is also located on each of two bearings in the generator A signal is sent to remote sensing devices that monitor the temperature of the stator and bearings. The winding and/or bearing temperature monitors have warning and shutdown set points, which are, initiated it the temperature problem persists.
4.2.6. Space Heater A space heater in the interior of the generator prevents the formation of condensate in the generator windings after shutdown. The space heater is automatically switched on and off by the turbine control system during the start-up and shutdown sequence unless the systems select switch is in the off position. The only operator action necessary is the verification of the heater’s operation upon generator shutdown.
4.2.7. Supporting Frame The generator frame supports the rotating and stationary components and serves as an enclosure to protect the internal components. The generator nameplate contains a host of valuable information including rpm, amperage, insulation rating, power factor, voltage, KVA, and maintenance information. The operator and the maintenance technician should know the location of the generator nameplate and become familiar with the information on it.
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4.3. ALTERNATOR CONSTRUCTION 4.3.1. The single phase synchronous generator (Compared with a 3-phase). Faraday’s induction law states that a conductor which rotates in a magnetic field will induce an electromotive force (emf). In a generator the magnetic field is created by the electromagnets; in other words the poles of the generator. An exciter (generator) is used to obtain this direct-exciter current. The exciter mounted onto the alternating-current generator will be of the internal-pole type. The stationary part, in which a single-phase winding is fitted, is located on the outside. The moving part used to generate the magnetic field, is located on the inside (see fig. under). The main advantage of the internal-pole type is that the alternating current from the stator can be fed through stationary connections.
Figure 39: Construction principle of the internal pole of a single phase alternator The emf induced in the stator winding will have a sine wave form relative to the time. The direct current is fed to the rotor by means of carbon-brushes and two slip rings. The relationship between the speed (n), the frequency (f) and the pole pair (p): Training manual EXP-MN-SE060-EN Last Revision: 29/09/2008
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f =
60 xf nxp or n = 60 p
n = rpm
It is time now to have a minimum of formulae even if you do not need it. You know already the relation between frequency, speed, and pair of poles, seen in the previous chapter.
4.3.2. The three phase’s synchronous generator In a three-phase or alternating-current machine the stator has three windings instead of the one winding as on a single-phase machine. These windings are located and axed at 120° between them.
Figure 40: Generator with three stator windings: U-X, V-Y and W-Z. The rotor excited by direct current has a north and a south pole. The rotor has therefore one pair of poles; p = 1. The rotating (changing) main field generates, or induces, voltage in the three-stator windings. The three emf's have the same frequency and are 120° “out of phase relative to each other. Training manual EXP-MN-SE060-EN Last Revision: 29/09/2008
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By one revolution of the rotor (R= 1) in “t” seconds the stator coils embrace the maximum rotational field at intervals of 1/3 x t seconds relative to each other. In ‘t’ seconds the field rotates 3000 times (for 50 Hz and one pair of poles), which means that each stator emf's reach its maximum value 120° in rotation or 1/3 T = 6.66 milliseconds in time after the next one.
Figure 41: Electromotive force for each stator winding Question: Rewrite the phrase in italic above for a frequency of 60 Hz And why not for 2 pairs of poles and 50 Hz? No problem, you are able to find it, “logically” without formulae
4.3.3. AC generator in general Figure under shows, in cutaway form, a typical A.C. generator in the 15-megawatt (20 000 hp) size range. The generator itself is enclosed in a box or “hood”; this is both to exclude noise and to contain the closed ventilation system. It also assists purging before starting if gas has been present. The rotating parts are coloured yellow and the stator blue. The armature (normally the stator) windings carry the load current, which varies with the loading. These windings have resistance and generate heat at a rate proportional to the square of the current (W = I² R). The field’s exciting winding (normally on the rotor) also carries current. It too has resistance and generates I² R heat.
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Figure 42: Cutaway of a typical A.C. generator – prime mover (turbine, motor) not represented These two sources of heat, together with iron loss heating, combine to raise the temperature of the machine. All the heat must be taken away by the cooling system if the temperature rise is to be held below the designed limit. The generator is cooled by a shaft-driven fan which circulates air in a closed air circuit through all the windings. The air, in circulating, passes through an air/water heat exchanger. The stator (armature) carries a 3-phase winding consisting of insulated conductors in slots round the inside face. These conductors must be insulated up to the full working voltage of the system. Serious or sustained excess temperature of the winding will cause this insulation to deteriorate or even to break down completely, resulting in an internal flashover. The rotor windings, which provide the field, operate at a much lower voltage of the order of 70 or 120 V D.C. (as a basic general range), so insulation is less of a problem. Note: there is a “main exciter” and a “pilot exciter”. To be seen in next chapter excitation and voltage control. Training manual EXP-MN-SE060-EN Last Revision: 29/09/2008
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4.3.4. Rotor construction A.C. generators with rotating fields have rotors which fall into two-types: salient and cylindrical Pole’s. They are both shown in the figure under
Figure 43: A.C. Generator rotor types The salient-pole type is by far the most common with offshore generators and also with the smaller sizes onshore. The salient-pole rotor is commonly used with 4-pole generators. Where there are six or more poles, this is the only type which is practical. The cylindrical rotor (sometimes also called “turbo type”) is, as the name implies, completely cylindrical and has no projections. The field windings are embedded and wedged into slots in the rotor surface in a similar way to the stator slots. The rotor slots cover only part of the surface and are disposed either side of the poles, the whole field winding forming a spiral around each pole centre. Cylindrical rotors are very sound mechanically and are favoured for large, high speed generators (3 000 or 3 600 rev/min), where centrifugal forces on a salient-pole rotor would Training manual EXP-MN-SE060-EN Last Revision: 29/09/2008
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present severe problems. Consequently cylindrical rotors are common with 2-pole generators and are sometimes used with 4-pole types. They are never used with six poles or more, where the rotor construction would become far too difficult. Question: For the rotor picture above, how many pair of poles for the “salient” type? How many pair of poles for the “cylindrical” type?
4.3.5. Insulation Generator windings are insulated against the highest voltages to which they may be subjected, and the insulation must withstand a certain specified maximum temperature without deteriorating. There are many insulating materials: The classification is as follows (as example of standard BS2757) Class
Typical Insulating Material
Ultimate Temperature
Y
Cotton, silk, paper, etc. ,unimpregnated
90 °C
A
Impregnated cotton, silk, etc., paper, enamel
105° C
E
Paper laminates, epoxies
120° C
B
Glass fibre, asbestos (unimpregnated), mica
130° C
F
Glass fibre, asbestos, epoxy impregnated
155° C
H
Glass fibre, asbestos, silicone impregnated
180° C
C
Mica, ceramics, glass, with inorganic bonding
> 180° C
Table 2: Insulation materials It should be noted that the classification letters do not follow an alphabetical sequence. This is because there were originally only three classes - ‘A ‘, ‘B ‘and ‘C ‘. Most platform and shore-installed generators are Class ‘B’ or ‘F’. It does not depend on temperature rise alone; if, for instance, the ambient temperature is 40°C, a Class ‘B’ material may be used if the designed temperature rise will not exceed 90°C, so making the ultimate maximum temperature 130°C. Training manual EXP-MN-SE060-EN Last Revision: 29/09/2008
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4.3.6. Cooling Generators used on platforms and in shore installations are either air-cooled. (However, you will find many water-cooled equipment’s) The air is circulated past the stator and rotor windings by a fan on the generator shaft. The warmed air itself may be discharged to atmosphere and not used again (‘Circulating Air ‘ or ‘CA”); or it may be water cooled in a separate cooler with a forced water circulation ( ‘ Circulating Air, Forced Water’ or ‘CAFW’ ); or in a radiator-type cooler ( ‘Circulating Air, Natural Water’ or ‘CANW’). A new international coding system for cooling methods has been introduced for all rotating machines (BS 4999, Part 21). First Digit
Second Digit
0 Free circulation
0 Free convection
1 Inlet duct ventilated
1 Self-circulation
2 Outlet duct ventilated
2 Integral component mounted on separate shaft
3 Inlet and outlet duct ventilated
3 Dependent component mounted on the machine
4 Frame surface cooled 5 Integral heat exchanger (using surrounding mediums)
5 Integral independent component
6 Machine-mounted heat exchanger (using surrounding medium )
6 Independent component mounted on the machine
7 Integral heat exchanger (not using surrounding medium)
7 Independent and separate device or coolant system pressure
8 Machine-mounted heat exchanger (not using surrounding medium )
8 Relative displacement
9 Separately mounted heat exchanger Table 3: Coding system for cooling methods Training manual EXP-MN-SE060-EN Last Revision: 29/09/2008
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Where it is desired to specify the nature of a coolant, the following letter code is used in conjunction with the cooling code: Gases:
Liquids:
Air
Water
A H N C L
Hydrogen Nitrogen Carbon dioxide Helium
W U Oil
4.3.7. Neutral Earthing Resistor The star-points of all high-voltage generators on platforms are earthed through a current limiting ‘neutral earthing resistor’ (NER). Its purpose is to limit the fault current flowing through the generator if an earth fault develops anywhere on the system. Neutral earthing resistors are therefore given a maximum current rating for a maximum time - for example, '200A for 30 s’. (in High Voltage) The NER unit sometimes contains also a current transformer to measure the presence of any earth-fault current in order to initiate the protection. See course “Ground and neutral systems” SE070 for neutral system management.
4.3.8. Insulated Bearings Bearings of a large machine are often insulated to prevent stray currents (Eddy currents) from circulating through them. Such currents can arise from emf's being generated in the rotor shaft due to stray magnetic fields. Under fault conditions these stray fields can be very large. The figure shows how such currents may flow through the bearings. These currents, if allowed to flow, would arc across the bearing surface and cause small craters, acting like corrosion, destroying quickly the same bearing. (Corrosion is caused by natural electric current between 2 metallic parts being in contacts through an electrolyte – same as a battery).
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Figure 44: Insulation of bearings For reasons of safety the shaft must be at earth potential. The insulation of the pedestal is carried out by a shim of insulating material between the base of the pedestal and its stool.
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4.4. GENERATOR EXCITATION Or the different ways to supply the Direct Current inducing magnetic field in the rotor.
4.4.1. Conventional excitation
Figure 45: Conventional excitation Typical schematic of a ‘conventional’ method where a driven D.C. exciter (in this case beltdriven) feeds its D.C. output through slip-rings to the main generator field. The Field current for the exciter itself (the D.C. generator) is supplied by a rectifier bridge itself piloted by the voltage regulator (AVR) of the main generator. Note: at start of this unit, there is no voltage, no power from the main 3 phase’s distribution in which the AVR is taking its “energy”. The D.C. exciter needs current in its (stator) field winding to provide in turn the current in the main generator (rotor) field winding to build power output… The exciter is using at start the “remanent magnetic field” of its own iron frame (it is like a small permanent magnet) which can provide at least a small current in its output, enough to have voltage output of the main generator, and the AVR can start to have current to “help” the remanent field. And the loop is going on, increasing up to the regulation values.
4.4.2. Static excitation The rotating D.C. exciter is replaced by a static electronic exciter. Note: at start, same as for the conventional excitation there is no power output of the generator, no D.C. current to give to the rotor windings. Two solutions: for small generators, this system is also using the remanent field magnetism of the rotor, building gradually the voltage up at generator output.
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In practice, (second solution), the rectifier bridge is supplied by a common 3 phase’s source and not directly from the concerned generator itself. This applies in ‘important’ distribution where several generators are in parallel.
Figure 46: Static excitation
4.4.3. Brushless excitation (general case)
Figure 47: Brushless excitation – general case A further significant development is shown in above schematic. Here the shaft-driven rotating exciter has been restored, but it now takes the form of an A.C. generator of the fixed-field type mounted on the main shaft itself. Its A.C. output is taken through connections inside the shaft, through a diode bridge which rotates with the shaft, to the main rotating field of the generator. The field is thus excited by D.C. without the need for brushes and slip-rings. It will be seen that this exciter cannot be belt-driven; it must be integral part of the main shaft. The principal advantage of brushless excitation over the other two first types is that the absence of brush-gear and slip-rings greatly eases the maintenance problem.
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Under short-circuit conditions or sudden ‘big’ load request (on main 3 phase’s distribution) the generator’s output voltage will drop heavily – it might even vanish. To overcome this, a method, improvement of the present one here, is employed which makes use of the short-circuit currents themselves to provide the missing excitation, this is the next paragraph
4.4.4. Brushless excitation (without pilot exciter) Three heavy duty current transformers are arranged in the generator output lines as shown in Figure under Under short-circuit conditions when the generator output voltage is very low, the shortcircuit CT's pick up the heavy short-circuit conditions - a necessary requirement in network operation so that protection may operate reliably.
Figure 48: Brushless excitation without pilot exciter
4.4.5. Brushless excitation (with pilot exciter) With large brushless generators this different method is used. Instead of drawing excitation power from the generator output, the AVR has only a voltage-sensing connection. As in the conventional case, the excitation of the generator is now independent of the generator’s output voltage and so is maintained even under short-circuit conditions and without the use of short-circuit CT's. This is the arrangement on almost all platforms main generators. Training manual EXP-MN-SE060-EN Last Revision: 29/09/2008
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Figure 49: Brushless excitation with pilot exciter
4.4.6. Diode bridge On the Figure, the diodes are shown for clarity as inside the shaft between the exciter and the main generator. The exciter output is 3-phase, and the diodes are in fact a 3-phase full-wave bridge, requiring six diode elements. Clearly they cannot be buried in the middle of the shaft, and in practice they are mounted on a rotating plate on the extreme end of the shaft at the exciter end, (as shown in Figure, in green). This makes them easily accessible for inspection, testing or replacement. A point on the use of diodes should be noted: If one of the six should fail, either by open or short-circuiting, harmonic currents flow in the main field circuit. These harmonics are reflected into the field circuit of the main exciter and are detected by a ‘ diode failure’ relay tuned to respond to the principal harmonic frequency; the alarm (or trip) signal from this relay is time-delayed by 10 or 15 seconds to prevent false operation. Caution: (for maintenance operation) When megger testing a generator field system (exciter + main field winding) all diodes must be first disconnected or short-circuited to prevent the megger voltage (500V) being applied across them and breaking them down.
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Figure 50: Diode Bridge of an A.C. generator
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4.4.7. Alternator parts
Figure 51: Brushless generator - exercise
Exercise: Name the different part of this machine, even those not requested and even those not yet seen… And then only you can go to next page…
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Stator armature + windings 3 phase
Power output connection terminals
Shaft mounted fan for air cooling
Exciter ( small A.C. generator)
Diode Bridge Rotor and 4 poles windings
Figure 52: Brushless generator – exercise solution Simple recommendation: Suppose that you have the responsibility to assemble such a generator (French, Leroy-Somer, 50 Hz 1500 rpm) with a diesel engine (American, Caterpillar set for 60 Hz at 1800 pm). When you make running test, specially over speed test (set at 2100 rpm on engine for 60 Hz), do not be surprised to have the generator rotor “loosing” its winding parts…….. (Over speed of generator – for 50 Hz - being 1750 rpm). And if it happens, bad luck that day, the over speed of engine was set even higher than 2100 rpm and the generator had already run hours (at 1800 rpm) for load test, please do not blame the generator manufacturer….
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5. ALTERNATOR CONNECTIONS AND PROTECTIONS 5.1. GENERATOR CONNECTIONS AC generators are usually constructed so that they have two types of output connections, wye (star) or delta. (Y or Δ) The output wires are called leads. There may be 6 leads or 12 (and even more). They are connected to the three-phase winding in the armature and then brought out to be connected externally to switchgear. It is the way that they are connected externally that determines whether a system is delta or wye. Each coil group in an armature is wound with a designed number of wires in each coil of the group. Each coil has a start wire and a finish wire, and to make a coil group, the coils are simply connected together with a start end and a finish end. One coil group is installed in the stator to form one phase.
5.1.1. The Delta system For the delta system the phases are arranged in a triangle shape. The important thing to remember is that to connect a winding for a delta output, you must connect the start of one coil group to the finish of another, and this is done for all starts and finishes, T1 to T6, T2 to T4, T3 to T5 T1 T5 T6
T1 2’ 1
3’ 3 T3
2’
3’
3
T3
1’
1’
2
T6
1
2
T4 T4
T5
T2
T2
Figure 53: Delta system In a delta system, line voltage is equal to phase voltage.... Training manual EXP-MN-SE060-EN Last Revision: 29/09/2008
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E line = E phase ...while line current is equal to phase current times the square root of 3. I line = I phase x
3
I (amp) line
I (amp) phase
Balanced Load E phase
E line
Figure 54: Current and voltage in a Delta system This relationship is an important factor in making the delta system a desirable one to use. If you are asked why a delta system should be used, you can explain that delta phase current is less than line current. So for 17 amps line for example, the generator phase has only 10 amps in it. This allows cooler generator operation under load conditions.
5.1.2. Delta connected generator The next illustration is an example of a delta system. It is a three-wire delta connected generator. Note the wires leaving the generator from T1, T2, and T3. Metering of these wires is taken through potential and current transformers. For delta connection, the start and finish wires are connected to form the triangle. T1 to T6, T2 to T4, and T3 to T5. T1, T2, and T3 go to the system as line leads, as seen in the previous paragraph
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Figure 55: Typical delta connections, 3 wire generator
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5.1.3. The wye (star) system Wye connected systems can be either three-wire or four-wire depending on the needs of the plant. In a wye-connected generator, the coil groups are connected in a way that forms the letter Y. To connect wye, all three finish wires are connected to external load circuitry, while all start wires are connected in a common junction T1, T2, and T3 are line leads going to the distribution system, while T4, T5, and T6 are junctioned together for a common or neutral connection. The neutral may or may not be grounded and the system may be operated as either a three-wire or four-wire system. T1
T1
T5
1 1’
T4 T6
2’ T5
2’
2 T2
3’
3
T3
1’
3’
T6
1
2
3 T3
T4
T2
Figure 56: Connection diagram and windings arrangement for a wye connection In the wye (or "star") system, the relationship of line current to phase current is that they are equal... I line = I phase ... and line voltage is equal to phase voltage times the square root of 3. E line = E phase x
3
Let’s look at an example to illustrate the advantage of this system. If the voltage line to line were 400 volts, then the phase voltage would be 400 volts divided by the square root of 3.
E phase =
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Since the square root of 3 is 1,732 then in our example we have 440 / 1.732 = 230 volts Many power distribution systems use 220/230 volts for lighting and portable power. Some typical lines to line voltages produced at 50 cycles per second are: 380 - 415 - 3000 - 3300 - 5500 - 6000 - 6300 - 6700 - 11000 Phase voltage for any of the above cases would be that voltage divided by
3
NOTE: Since some generators produce high voltages (wye or delta connected), any energised readings must be taken with the proper equipment and all high voltage safety rules must be observed. A precautionary procedure is to take voltage readings at the secondary of step down transformers, rather than at the primary voltage terminals in the generator connection box located on top of the generator or elsewhere. The nameplate of any generator will show the number of phases, usually three-phase, and line to line voltages as well as other pertinent data.
5.1.4. Wye (star) connected generator This illustration shows a generator in an installation, which is connected as a typical Wye four-wire system. On the wye drawing the wires leaving the generators are N, T1, T2, and T3. These also go through potential and current transformers for the purposes of metering. For the wye system the connections should be T1, T2, and T3 to load, and T4, T5, and T6 together as a common or neutral. So as you can see below all the start wires are connected together and all the finish wires are connected to the load.
Figure 57: wye (star) connection
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Figure 58: Typical wye connected, 4 wire, generator
5.2. GENERATOR PROTECTIONS The protection system of a generator aims to protect the machine against the internal defects and protect the network against dysfunction, which can disturb it The principal defects, which can affect a generator, are: The overload The external short-circuits between phases (on the network) Training manual EXP-MN-SE060-EN Last Revision: 29/09/2008
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The short-circuit interns between phases The intern defect between phase of the stator and mass The defect between the rotor and the mass The cut of a phase or the inversion of two phases The loss of excitation Generator running as a motor A frequency too weak or too high A voltage too weak or too high
5.2.1. ANSI codes for Protections The required protections are using specific ANSI coded relays Relay function
Code ANSI
Differential protection
87 G
Stator ground-fault protection
51 NG
Under impedance protection
21 G
Over-voltage protection
59-1 and 59-2
Rotor ground-fault protection
64 F
Field failure protection
40
Under-voltage protection
27
Reverse power protection
32-1
Current unbalance protection
45 P and 46 G
Overfluxing (frequency) protection
59 / 81 G
Overload protection
51-1 G
Overload protection
49 G
Table 4: ANSI codes for protections
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5.2.2. Typical one line diagram generator protection
Figure 59: Typical one-line diagram generator protection
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Generator Protection
500 KV A
2000/1
2000/1 B 500/1 500/1 400 MVA 500/16 KV
mcb
3Ø VT
mcb Power supply
87 T
Transf. Diff.
87 T
Transf. Diff.
51 G
E/F back up
59
Over voltage
59 N
Earth fault
V / HZ Over excitation 51 AVR
15000/5
400 MVA 16 KV 60 HZ
87 G
Gen. Diff.
32
Rev. Power
40
Loss of field
64 F
Field E/F
21
Imp. Prot.
46
Negative sequence
SCR G
15000/5
15 KV 3
10 A
Time log O/C
27 N3 59 N1
100 % E/F
Figure 60: Example for generator synchronised on network
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5.2.3. Details for generator protections 5.2.3.1. Protection functions connected to generator neutral current transformers 32P: reverse active power 32Q: reverse reactive power serving as loss of field (for generators above 1 MVA) 46: negative sequence (for generators above 1 MVA) 49: thermal image 51: overcurrent 51G: earth fault 51V: voltage restrained overcurrent 87G: generator differential protection (for generators above 2 MVA) (Note: 46, 49, 32P and 32Q can also be connected to the line-side current transformers)
5.2.3.2. Protection functions connected to voltage transformers 25: synchronism-check (for parallel operation only) 27: undervoltage 59: overvoltage 81: overfrequency and underfrequency
5.2.3.3. Protection functions connected to line-side current transformers (for parallel operation only) 67: directional overcurrent (not required if 87G is used) 67N: directional earth fault (on core balance CT for better sensitivity)
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5.2.3.4. Generator mechanical protection functions connected to sensors 49T: stator temperature (recommended for generators above 2 MVA) 49T: bearing temperature (recommended for generators above 8 MVA) 64F: rotor earth fault protection
5.2.4. Practical checks by operators and maintenance technicians All the numbers, type of electrical protections enumerated here above are (usually) not within the burdens of a production operator or a maintenance technician. The main purpose of listing them is for you, when wandering inside the electrical switchgear room to be able to understand a “minimum” about these equipments. However, the maintenance technician will need, in due time to know how to check the calibration… Nevertheless, the “normal” and/or “universal” operator is supposed to check the parameters of his ‘power plant’, principally here the power indication active reactive powers and the power factor (cos φ) as well as the running parameters of equipment’s (oil pressure, oil level, temperature of windings, bearings, etc). And you, maintenance (electrical) technician, ensure yourself that the operator is not cheating you by leaving to you the entire responsibility of “his” power plant., forgetting even the simple survey during his shift period.
5.2.4.1. Review In “electricity” and in “measures” courses, you can see details about active, reactive power and cos φ. Let’s see here the principle of having a reverse power Neutral point
rotation
P: active power (+)
G
V
φ
3 Ph
Q: reactive power (+)
S: apparent power
I amp Ph I
Synchronized on a network
Figure 61: Principle of reverse power
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An alternator synchronised on a network must provide active power (P) and reactive power (Q), with a power factor (cos φ) in the range 0.85 to 0.93 (as per the load)
5.2.4.2. Active reverse power protection (Code ANSI 32 P)(P for active) This protection is used to detect an inversion of the sign of the active power in the absence of electric fault. This protection is used in particular to: protect a motor against an operation in generator when there is a supply shutdown and is continued to run by its load; protect a generator against an operation in motor which can deteriorate the driving engine. Neutral point rotation P: active power (-) Q: reactive power (+) I
V φ
S: apparent power (-)
G 3 Ph
I amp Ph
Synchronized on a network
Figure 62: Active reverse power protection A generator set connected to a power network continues to turn synchronously even if the prime mover (diesel or turbine) is no longer energy supplied, the main breaker being kept closed. The alternator then functions as a synchronous motor. Operating in such a way may be detrimental to the prime mover. Figure 63: Active power protection relay schematic representation Training manual EXP-MN-SE060-EN Last Revision: 29/09/2008
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5.2.4.3. Reactive reverse power protection (Loss of excitation) (Code ANSI 32 Q)(Q for reactive) This protection is used to detect the field loss of the synchronous drives Neutral point
rotation I S: apparent power (-) -φ
G
Q: reactive power (-) V
P: active power (+)
3 Ph I amp Ph
Synchronized on a network
Figure 64: Reactive reverse power protection The break or the short-circuiting of the excitation coil of an alternator is a serious fault. It either causes the alternator to function as an asynchronous generator, or it stops the conversion of energy and causes an increase in speed. The consequences are an overheating of the stator because the reactive current can be raised and an overheating of the rotor because it is not dimensioned for the induced currents. An important induced current circulates in the rotor and causes an overheating. D.C. current crossing the rotor (called inductor) carries out the energisation of the synchronous drives The field loss can be due to a fault in the DC feeder or to a fault of the rotor (breakdown, short-circuit, etc). When a field loss appears, the drive compensates the drop of the magnetising power of the rotor by absorbing reactive power on the network. The reactive power of the machine is then negative.
Figure 65: Protection against excitation losses by a reactive reverse power relay. Training manual EXP-MN-SE060-EN Last Revision: 29/09/2008
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In practice the relay can be set to check the cos φ permanently, threshold being between 0.91 and 0.93. It means that the relay “sees” the reactive power going dangerously towards ‘0’ with the possibility of becoming negative; On your power plant, please check value of cos φ. A power factor of 0.93 is very good, power consumption speaking but it is going to a limit for the safety of the generator.
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6. ALTERNATOR OPERATION AND CONTROL 6.1. LOAD ADJUSTMENT OF A GENERATOR (OR ALTERNATOR) Function of an alternator is to deliver active power. The fact that there is also reactive power is not the “responsibility” of the generator; it is the consequence of the load characteristics. Reactive power management is done on power distribution side (with a synchronous machine running as synchronous motor, or with capacitances, for example…….) The load control is therefore done on active power. For example on an EDG (Emergency Diesel Generator) working in manual control there are only 2 potentiometers, one for motor speed, the second for voltage adjustment. Speed adjustment is seen in turbine and/or engine courses, from the generator it is simply a digital or analogical instrument request: “please, increase or decrease” WHAT ARE THE CONSEQUENCES OF AN INCREASE OF THE ACTIVE POWER DELIVERED BY A GENERATOR
ON THE FREQUENCY?
The frequency decreases When the load of an alternator increases, its speed decreases
ON THE VOLTAGE?
The voltage decreases Three causes are at the origin of this reduction
• The speed • The voltage drop by load increase
If speed decreases, the frequency decreases in the same proportions
• The armature reaction which decreases the inductive flux
HOW TO MAINTAIN CONSTANT THE VOLTAGE AND THE FREQUENCY ?
Figure 66: Active power management Training manual EXP-MN-SE060-EN Last Revision: 29/09/2008
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A motor (hydraulic or thermoelectric turbine) provides the mechanical energy to the alternator Speed adjustment
A generator with D.C. current, exciter (or other system) is installed at the shaft end to provide the D.C. current necessary to the inductor of the generator. Field excitation current adjustment
To maintain the frequency and the voltage constant these two simultaneous operations are necessary:
• Increase speed by action on the turbine (or engine) regulation • Increase the excitation current by action on the shunt field rheostat of the exciter (as per the drawing above) or through any static of rotating adjustment device Practically these 2 operations can be carried out by a speed regulator and a voltage regulator
Figure 67: Example of Load Management on Emergency Diesel Generator To maintain the frequency and the voltage constant, these two simultaneous operations are necessary: Control speed by action on the turbine (or engine) regulation Control the excitation current by action on the shunt field rheostat of the exciter (see drawing above) or through any static of rotating adjusting device Practically, these two operations can be carried out by a speed regulator and a voltage regulator
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6.2. AUTOMATIC VOLTAGE REGULATORS (AVR) The name AVR for Automatic Voltage Regulation could be interpreted an other way. As in fact, it is he load (the power) which is regulated, an increase in current decrease the voltage and inversely, it could be said Ampere Volt Regulator. (It is a personal interpretation which is not an official one…) The AVR are nowadays entirely electronic; they take their operating power from either the main output or the shaft-driven high-frequency sub-exciter (typically at 400Hz) or from the network or UPS’s.
6.2.1. AVR set-point Like any closed-loop servo, an automatic voltage regulating system holds the voltage constant within stated errors at whatever level it has been set. This level is referred as being the ‘set-point’.
6.2.2. AC Generator voltage regulation When a load is applied to the terminals of a generator previously running at no load and without AVR control, the terminal voltage will drop by an amount which depends on the nature of the load.
G 3 Ph Receiving current for 1, 2 or 3 phases
Field winding
Receiving voltage values
AVR
Generator breaker
Power supply
Voltage adjustment
Modulating field current
Figure 68: Voltage regulation AC generator
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This drop of voltage is called the ‘regulation’ of the generator at that load. It is usually quoted at full rated load, that is, at the full-load rated current and rated power factor and is expressed as a percentage of the no-load or system voltage. Thus, if V0 is the no-load voltage and V the generator terminal voltage at full rated load and power factor and with the excitation unaltered, then: V0 − V × 100% is the percentage full-load regulation. V0
The AVR is a “box” receiving the “image” essentially of voltage, either between 2 or 3 phases and sending in output the required current in field winding to compensate the voltage variation. The AVR is like an “ETC” as per the instrumentation standard of “Voltage Transmitter Controller” … Many AVR’s includes the “image” of line current which acts as a derivative action anticipating the load demand. The AVR then becomes like a “JTC” for Power Transmitter Controller…
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7. GENERATORS PARALLELING AND SYNCHRONISING 7.1. CONDITIONS FOR PARALLELING At some time in the life of an industrial plant, the existing load carrying capabilities may need to be increased. It may be necessary to accommodate a peak load during a portion of a day, or a permanent expansion of the load may be planned in the installation. One solution would be to replace the existing generator set with a larger unit. A more cost effective and efficient solution would be to place another generator set into the system to assist in carrying the load, that is, parallel the first unit with the second. On line Generators G 3Ph
Stand-by Generators
G 3Ph
G 3Ph
G 3Ph
G 3Ph
? When to close? Bus bars Load distribution
Figure 69: Distribution with several generators in parallel When two sources of power are placed in parallel, the system voltage will be that of the individual sources, but the amperage capacity of the system will be the sum of the amperages of the units in parallel. This means that by operating two or more units in parallel, system voltage can be maintained at the desired value, and the load carrying ability of the system is increased. Units of different kW ratings can be paralleled as long as individual voltages are the same. Several units can be paralleled as long as they can divide the load proportional to their individual ratings. Successful paralleling depends on similar response of engines and sensitivity of the speed control governors, similar response of voltage regulators and presence or absence of cross current compensation devices. There are four conditions that must be met by the on-coming generator and the bus (network) before paralleling can take place. Training manual EXP-MN-SE060-EN Last Revision: 29/09/2008
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They must have the same phase rotation. They must operate at the same frequency. They must operate at the same voltage. They must be synchronised, or in-phase. And then, only when those 4 conditions are simultaneously present, the stand-by generator is running at synchronous speed, the voltage is adjusted, in phase, it means that it is synchronised, closing of the main loading breaker can be done
On line Generators G 3Ph
Stand-by Generators
G 3Ph
G 3Ph
G 3Ph Ready? – Go !
G 3Ph
Order to close
Bus bars Load distribution Figure 70: Generator synchronising Closing of the main breaker is the finalisation of the synchronising phases. Let’s see the 4 “pre” conditions in detail.
7.1.1. Condition 1: same phase operation Phase rotation is determined by the connections to the bus. That is, Phase A of one source must be met by Phase A of the second source. Phase B of one meets Phase B of the other, and C phase meets C phase
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Figure 71: Same phase operation: wrong conditions It means not only rotating the same direction, but each pair of phases matching
Figure 72: Phases are matching – OK!
Figure 73: Phase rotation OK, corresponding phase to be connected together
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This requirement is critical, and if not met, the powerful magnetic forces in the stators would cause the rotors to turn in reverse direction. The angle position of the phases are not important, B C A to A B C or C A B to A B C are equally correct. A phase rotation meter can be used to check phase sequence of the bus and of the oncoming generator. If potential or instrumentation transformers are used to step down the generator and bus (network) voltages for the phase rotation meter of synchronising lamps, extreme care must be taken to insure that proper primary to secondary polarities are maintained, so as to give the correct signal to the phase rotation meter. (Leave it to the commissioning electrician!) Swapping or interchanging any two of the generator lines can change the phase sequence. We could also change the phase sequence by changing the direction of generator rotation.
7.1.2. Condition 2: same frequency The second condition for paralleling is that both sets operate at the same frequency. In the figure you can see that turbine generator set 1 and turbine generator set 2 have different frequencies. TG2 is running faster than TG1, Figure 74: Frequency differential 1
The difference in speed is called "slip frequency". To match the oncoming generator frequency to bus frequency adjust the speed control switch/potentiometer on the generator panel until both frequency meters indicate the desired frequency. The "slip rate" is the time rate of change or the speed with which the generator frequency is approaching the bus frequency. Figure under shows both generators running at the same frequency.
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TG 1
1 Hertz TG 2
Figure 75: Frequency differential 2
TG 1 TG 2
1 Hertz 1 Hertz Figure 76: Both generators running at same frequency 1 Both curves can be superposed
TG 1 TG 2
Figure 77: Both generators running at same frequency 2 Training manual EXP-MN-SE060-EN Last Revision: 29/09/2008
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7.1.3. Condition 3: same voltage A third condition, voltage matching is accomplished by providing a raise (or lower) voltage adjusting signal to the voltage regulator via the voltage adjust rheostat located on the control panel. Check the voltmeters on each generator panel. Figure under shows a difference in voltage amplitude and not in frequency. V1
TG 1
1 Hertz V2 TG 2
Figure 78: Voltage differential (frequency OK)
7.1.4. Condition 4: Synchronising or phasing Generators that are synchronised will have their rotors north poles facing in the same position (Figure under). We consider the North pole giving the maximum induction (so maximum voltage) to one phase coil at an instant time‘t’. At the same instant South pole is giving half the induction (sin 30°) to the2 other phases (in reverse voltage). The north poles can be at any simultaneous clock position, not just the condition shown in figures. (A two-pole rotor has been used for simplicity.) The act of paralleling in the example shown would entail closing the output circuit breaker of the oncoming generator (assuming one unit is connected to the bus/network).
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Figure 79: Two rotors with two north pole / phase facing the same direction at ‘t’ CAUTION The circuit breaker is never to be CLOSED when the rotors of two generators are not in the same position, because the powerful magnetic forces generated in the rotors will cause the rotors to spin rapidly in an attempt to achieve identical positions. The rule of "likes repel, unlike attracts" is true, and when violated, as in figures under, the rotors will stop instantly and reverse direction or spin rapidly in the same direction in an attempt to achieve the same directional positions.
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When this occurs, damage to the turbine generating system will occur. Damage may consist of broken shear couplings, shafts, gearboxes, line voltage/current surges, and loss of power. TG 1
TG 2
TG 1
TG 2
Figure 80: Phases ‘A’ in opposition at 0 & 180° and in the same opposition at 90 & 270° It is possible to have voltages and frequencies matched, and still be "out of phase". Figure under shows bus voltage (TG-1) and oncoming generator voltage (TG-2) to have the same voltage amplitude. Speed is also equal because the two rotors have completed one revolution in the same amount of time. Ph A 30° out of phase
TG 1
TG 2 Ph C
Ph B
Generator on line or network
Generator to synchronise
Figure 81: Generators to synchronise 30° out of phase The phase angles, however, are not the same. Phase angle is the relative electrical degrees between the oncoming unit voltage wave and the bus voltage wave. Detection of the relative phase angle and closure of the circuit breaker at or as nearly possible to zero phase angle is the most important and critical factor in paralleling. The figures under illustrates voltage phase angles in electrical degrees.
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TG 1 TG 2
Ph A
Ph A
TG 1
TG 2
Ph C
Ph B
Ph C
Generator on line or on network
Ph B
Generator to synchronise
Figure 82: In phase 0° phase angle between the two generators TG 1
TG 2
Ph C
Ph A TG 1 TG 2 Ph C
Ph A
Ph B Ph B
Generator to synchronise
Generator on line or network
Figure 83: 90° out of phase or 90° phase’s angle TG 1
TG 2
Ph A
Ph C
TG 1
TG 2
Ph C
Ph B
Ph B
Generator on line or on network
Ph A
Generator to synchronise
Figure 84: 120° out of phase or 120° phase’s angle TG 1
TG 2
Generator to synchronise
Ph A TG 1
Ph B Ph C
Ph C
Ph B TG 2
Generator on line or on network
Ph A
Figure 85: 180° out of phase or 180° phase’s angle Achieving zero phase angle will be described under the heading "methods of paralleling".
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7.2. SYNCHRONISATION / PARALLELING 7.2.1. Ready for coupling The conditions in the previous paragraph have (almost) been satisfied. The phases are correctly positioned, "commissioning" have done their job or these machines have already been paralleled. The unit (turbine + generator) has been rotating for a (short) while at synchronism speed (approximately) i.e. 3000 or 1500 rpm. Voltage has been adjusted. You simply need to bring the 2 "rotating fields" in phase and press on the close button of the coupling circuit breaker by turning or modifying the turbine or engine speed (4th condition). The synchronism system is clearly manual, hassle-free under automatic version, it runs by itself.
7.2.2. Coupling operations of a one phase alternator with lamps I.e. a GT2 alternator (one phase) which we suggest couple in parallel with several others connected to the PN bars, or coupling bars. The network (or GT1) has an emf E1. If we organize the unit as shown in the schematic in which L1 and L2 refer to the lamps connected across each pole of the coupling switch terminals Figure 86: One phase alternator, lamp coupling a) Alternator GT2 rotates at a speed near to the synchronism speed, N = 60 f / p (f = frequency of the voltage. E1 between the bars). b) GT2 has been excited in order to have E2 (between its output terminals) at the same value as E1, the indications being on 2 different voltmeters connected to points E1 and E2.
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The first condition for coupling has been satisfied. We then observe that lamps L1 and L2 show beats. The closed circuit (P – GT2 –N) is subject to an emf equal to e1 + e2 in instantaneous value (algebraic sum) and to: Voltage at lamp terminal ‘E’
E1
E = E1 + E2
E2
Figure 87: Algebraic value of ‘E’ (vector quantity) However, as the frequency of E2 is not equal to that of E1 (the speed needs to be ‘gently” adjusted), vector E2, as compared with vector E1 which is considered as fixed, rotates with an angular speed equal to the slip between the pulses for the two emf. Therefore E varies between ‘0’ and (E1 + E2) limits. E = 0
E = E1 + E2 = 2E1 = 2 E2
With E1 supposedly equal to E2 Figure 88: Variations of E In the first case, lamps L1 and L2 are extinguished. in the second case, lamps L1 and L2 are lit at maximum. Caution: the lamps must be able to accept twice the voltage of E1 or E2 as the 2 voltages act cumulatively (in instantaneous values). c) Let us act on the speed of alternator GT2, in order to slow the beats of the lamps. The coupling switch can be closed when lamps stay extinguished for approximately three seconds. Vector E is zero at extinction of a lamp, i.e. emf E1 and E2 are in phase in the shared circuit (P – GT2 –N) to the outside network.
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7.2.3. Coupling operations with a three-phase alternator lamp Exactly the same process is used, the phase order having been defined earlier.
Figure 89: Coupling with a three-phase alternator lamp The first three conditions are satisfied; therefore the three lamps come on and go out simultaneously at a speed which is inversely proportional to the slip in speed between the two sources E1 and E2. The speed of GT2 must be adjusted (carefully) until the lamp beating speed is low (lit for at least 5 seconds) and when the lamps are out, close the coupling switch. Should the 3 lamps start operating randomly, this means that the first condition of the "phase order" is not satisfied. Configuration could be similar to the following figure.
Figure 90: Example with "unsatisfied phase order" Training manual EXP-MN-SE060-EN Last Revision: 29/09/2008
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With this situation, do not attempt to couple….. start again from the beginning Tip: with three-phase circuits, in commissioning (and initial testing), at least 2 lamps must be connected to the terminals of 2 phases of the coupling circuit breaker/switch, even if all "other devices" demonstrate that the circuit is working fine. An error is always possible. With at least 2 lamps, you are 100% certain to couple the right phases…
7.2.4. Coupling operations with a synchronoscope Coupling may be manual or automatic, lamps are replaced with a "rotating field slip indicator", i.e. the synchronoscope, included in the coupling unit.
Figure 91: Example with the ABB system, complete and compact SYNCHROTACT CSS, ready for assembly
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U1 : Network voltage U2 : Alternator voltage DP : Main breaker G : Alternator AVR : Voltage regulator TR : Turbine regulateur ORDRE : Closing sequence U+, U- : Voltage adjustment F+, f- : Frequency adjustment
Figure 92: Diagram of the device operating principle In MAN mode, the functions are manually adjusted using push buttons from the front section. Measurement values will be indicated on the instruments. The shut down order will be released in the conditions corresponding exactly to the phases if the release and close push buttons are pressed simultaneously. In AUTO mode, the voltage and frequency of the alternator will be automatically adjusted to the tolerance value. The shut down order for the circuit breaker considers its closing time with the exact corresponding phase’s conditions (at that time of closing).
7.2.5. Tolerances for coupling / synchronising The permissible limits or differences for Voltages, frequencies, and synchronising are generally : : Voltages : + or - 5% Frequencies: 0,1 to 0,5 Hz – Network Frequency is then monitored by a specific relay . Synchronising or phasing : 5° max
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7.3. PARALLEL CONTROL OPERATION 7.3.1. Taking the load Considering one generator on line on a network, in parallel with others generators, coupling (synchronising) has been successful, we want it to take some load. Only 2 controls are available on the couple turbine/generator or engine/generator, the Voltage adjustment of the generator (AVR) and the speed control of the prime mover. To take load for the “new arrived” generator, the speed control will increase/decrease the active power (“real” power on the shaft). When the voltage increases, the active power increases. In fact, the voltage on the network does not move. Instead the current output increases and the (reactive) power taken by the generator increases. In manual control, the operator increase up to the desired value. In automatic control, the “share” of the power will go (gradually) up to the pre-set value which can be in equal percentage for all generators or depending a ratio function of power capacity of the concerned generator.
7.3.2. Load sharing Or the balancing of power between generators and this depending the configuration of the power plant Identical generators On line Generators G 3Ph 3 MW
G 3Ph 3 MW
G 3Ph 3 MW
Stand-by Generators G 3Ph 3 MW
G 3Ph 3 MW
G 3Ph 3 MW
Bus bars Load distribution
Figure 93: Identical generators load distribution In this power plant, all generators identical, generally, power is shared in equal percentage
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Unbalanced power capacities On line Generators G 3Ph 10 MW
G 3Ph 10 MW
G 3Ph 3 MW
Stand-by Generators G 3Ph 3 MW
G 3Ph 1 MW
G 3Ph 0.6 MW
Bus bars Load distribution
Figure 94: Non-identical generators It is impossible to ask the last generator (0.6KW) to share same power with the 10MW ones. There will be a load sharing system which can be: By ratio of individual power capacity, or by permanent manual adjustment By centralised power sharing system, given independent orders to each generator control system, it is the “Load sharing” in which an additional control box is added to each unit in complement of its AVR and speed control And if you are familiar with site electrical installation, Woodward control material is installed on numerous plants, here after some pictures of Load control, Load & speed control devices. The 2301A Load Sharing and Speed Control is available in forward- or reverse-acting systems and in several speed ranges for applications requiring either droop or isochronous speed control. Models are available with either accelerating or decelerating ramps. Figure 95: Load sharing and speed control The Automatic Generator Loading Control (AGLC) can be used with any Woodward load sharing and speed control system with either built-in or external load sensors. It is designed to provide soft loading or unloading of a generator set to an isochronous load sharing system or to base load setting at controlled rates. The electronic ramps are easily adjusted from five seconds to five minutes for 100% load change. The load and unload ramp rates adjust separately Figure 96: Automatic Generator Loading Control Training manual EXP-MN-SE060-EN Last Revision: 29/09/2008
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And so on, we cannot edit here the complete Woodward operating and instruction manuals and anyway the load sharing + control of alternators is a complete course by itself. Hereafter a schematic configuration with load sharing example. We shall see nevertheless in next paragraph the meanings of “isochronous” and “droop”. Prime movers Gas Turbines T1
T2 SC1
G1
T3 SC2
G2 AVR1 LS1
T4 SC3
G3 AVR2
SC4 G4
AVR3
LS2
T5
LS3
T6 SC5
G5 AVR4 LS4
SC6 G6
AVR5 LS5
AVR6 LS6
Communication bus or wires between LS’s LS0
Load distribution bus-bars Total load and V volt inputs for LS0
Figure 97: Load sharing principle example schematic SCx is for Speed Controller receiving the rpm indication and sending back signal to prime mover governor AVRx is the Automatic Voltage Regulator receiving voltage and current of its generator and sending back field current LSx is the Load Sharing “box” dialoguing with each generator / prime mover SC & AVR and interconnected with the master LS0 which checks the total load
7.3.3. Droop and Isochronous mode of control Isochronous operation provides constant turbine speed for single unit operation and for parallel units provides proportional division of load between units while maintaining fixed frequency on an isolated bus. Speed control, in isochronous mode, for each prime mover is (nearly) independent, the turbine (or engine) governor acts as a single regulator, watching and adjusting “its” speed to the fixed synchronism value.
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52 Hz 51Hz Time
50 Hz 49 Hz 48 Hz
Isochronous response to increase in load
47 Hz Figure 98: Isochronous response form for frequency on a bus bar In Speed Droop the prime mover governor operates to decrease speed with increasing load. This is the mode that is commonly used to operate generators in parallel, as it allows them to share load in proportion to rated load.
52 Hz 51Hz Time
50 Hz 49 Hz 48 Hz
Droop response to increase in load
47 Hz Figure 99: Droop response But, what about the frequency control? I want the network to be at 50 Hz permanently! Other generators have to be in ‘isochronous’ on the network to keep the frequency at desired value. The one in “droop”, generally a smaller one, running “under speed” is just (by this system) maintaining its load at the same value leaving the other generator the task of taking the increase. The AVR, not concerned by the droop is still controlling at the set voltage. Droop mode is pres-set as project/commissioning values; they are in the range of 3% or 5%, for 50%, 100% of load,
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52 Hz
5% droop for rated Hz at 100%
51Hz
3% droop for rated Hz at 100%
50 Hz 3% droop for rated Hz at 50%
49 Hz
5% droop for rated Hz at 50%
48 Hz 47 Hz 50%
Load
100%
Figure 100: Example of droop mode settings Only one setting is applied to one machine, if all generators in parallel have the same droop curve, the frequency varies accordingly, the user have to accept the change in frequencies function of the load Problems begin to occur when machines in parallel have different droop settings. Leave it to the specialist….
7.3.4. Load Shedding It is common to see people mixing the terms and functions of “Load Shedding” and “Load Sharing” and not only because they are written nearly identically. Principle : Objective of a “Load Shedding’ is to keep a network on line not going to a general shutdown, when one generator trips revealing suddenly the insufficiency of available power. The only alternative, at time of the trip, is to cut immediately part of the distributed power, the one non essential. This «Load Shedding » function is dedicated to a specific PLC (Programmable Logic Computer). When a generator trips, the PLC, watching permanently the ratio between available and used power, can decide the opening of a certain number of breakers, this depending the analysed power to be “sacrificed’. PLC having acknowledged the trip of a turbine / generator, analyse at that time, the value of frequency (generally) and if that one goes under a threshold, shedding is decided. The more frequency drop, the moreshedding) Training manual EXP-MN-SE060-EN Last Revision: 29/09/2008
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Circuits to be opened are pre programmed in the PLC, numbers being function of the power still available. Of course, all this shedding operation should be completed in the shortest time to avoid the overloading of generators kept on line. A lasting overload would conduct inevitably towards a general shutdown. Generator trip detection + Frequency drop detection + Conditions analysis by PLC + Opening orders towards circuits + Response time of breakers = 40 to 50 milliseconds maxi. The figure / diagram is self-explanatory for the understanding of the Load Shedding principle.
Instant ‘T’: 4 generators on line 12 kW available 11 kW used Generators on line G 3Ph 3 MW
G 3Ph 3 MW
Generators in stand-by
G 3Ph 3 MW
G 3Ph
G 3Ph
3 MW
3 MW
G 3Ph 3 MW
busbars Load Distribution Trip of 1 generator
Instant ‘T+t1’: 3 generators on line 9 kW available 11 kW used Generators on line Trip
Analyse P.avail. / P.used
Generators in stand-by
G 3Ph 3 MW
G 3Ph 3 MW
G 3Ph 3 MW
G 3Ph 3 MW
G 3Ph 3 MW
G 3Ph 3 MW
busbars Load Distribution
Load Shedding action by PLC x
x
x
Instant ‘T+t2: shedding = open non essential circuits to have total used power by consumers under available power from generators
Figure 101: Load Shedding principle
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8. MAINTENANCE OF ALTERNATORS WARNING: before initiating maintenance procedures, disconnect all power sources to the machines and accessories; replace all grounding connections prior to operating. Failure to observe these precautions may result in injury to personnel. GENERAL: The instructions related to general maintenance, cleanliness, inspection, and cleaning insulation as covered in the generator Instructions in any manufacturer maintenance instruction manual apply to al type all alternators (including the brushless excitation systems). Cleaning the exciter windings and rectifier assembly at regular intervals is recommended. CAUTION: if it becomes necessary to take out and dry a rotor (in oven), remove the rectifier assembly prior to dry the rotor.
8.1. DIODE REPLACEMENT This work has to be done by a qualified technician, or at least permanently supervised by one; it is an operation/intervention relatively frequent on an alternator, let’s say the first cause of trouble and if it becomes necessary to replace any of the silicon diodes, the following Instructions should be observed. The listing of directives given hereafter is for information taken out from an instruction manual.
Figure 102: Diode bridge assembly on a brushless generator
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It is recommended that identical diodes, as originally furnished, be used as replacements. The replacement diodes should be ordered by the manufacturer’s renewal part number. Always tighten or loosen a diode by turning the nut and holding the hex on the diode body stationary. Clean the heat sink thoroughly around the diode-mounting hole. Be sure there are no raised areas that would prevent the diode from seating tightly against the heat sink. This mounting surface and the diode-mounting surface must be flat, smooth and clean to permit maximum heat transfer from the diode to the heat sink. Diode pigtails should be positioned such that they are not in tension and do not exert a strain on the diode. Some diodes may have leads soldered to them. If it becomes necessary to solder a new lead to the diode, it must be removed from the heat sink, or the lead must be installed before the diode is mounted. Since diodes can be damaged by excessive heat during soldering, use a low-melting solder such as 60 percent tin, 40 percent lead, and apply heat just long enough to make the solder connection. Use only rosin core solder, and clean surfaces before soldering. During the soldering operation the diode can be held by installing the nut on the diode stud and lightly clamping the nut in a vice. Care must be exercised in holding the diode during soldering to avoid providing a good heat sink to the diode which may result in overheating of the diode and/or a poor solder joint. Check the threads on the diode stud to see that they are clean and free of burrs. The nut should turn freely by hand the full length of the thread. It the diode is one which was removed from a 530 type rectifier assembly, and is to be reinstalled, remove all trace of glue ( “Loctite”) from the stud and nut threads prior to Installation. Before mounting the diode, apply a coating of Burndy "Penetrox A" or equivalent to the diode heat sink mounting surface. When installing a diode, use one of the following procedures appropriate for the type of rectifier assembly furnished:
•
TYPE 1. Install the diode in its proper position, Install the locking plate, and install the diode nut. Torque the nut to its proper value (see Table under), and bend up the tab on the locking plate to lock the nut in position.
•
TYPE 2. Install the diode In Its proper position and Install the spring washer such that the surface at the outside diameter is In contact with the heat sink. Coat the threads of the diode stud with ‘Loctite’ "Screwlock", or equivalent, put the nut on the stud, and torque the nut to its proper value Immediately (see Table under).
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Stud size
Hex size across flats (inches)
Torque (inch/pounds) Maximum
Minimum
1/1” – 28
11/16
30
25
3/18” – 24
1 1/16
100
95
¾” - 16
1 1/4
300
285
Table 5: Semiconductor mounting torque CAUTION: after the Loctite has been applied to the diode threads and the nut installed, the nut must be torqued to its proper value as quickly as possible and before the Loctite begins to set. Failure to do so may result in false torque readings, improper diode mounting, and diode failure. When installing diodes, a torque wrench must be used. The nut on the diode should be torqued to its specified value. For diodes furnished, the torque limits of the table must be observed. CAUTION: Both forward and reverse polarity diodes are used in the exciter an arrow on the diode case indicates rectifier assembly diode polarity. When replacing diodes be certain that replacement rectifiers on each heat sink are of the proper polarity.
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Figure 103: Typical type ‘1’ rectifier assembly
Figure 104: Typical Type ‘2’ rectifier assembly
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8.2. VARISTOR REPLACEMENT
Figure 105: Leroy Somer alternator showing varistor use A varistor is a device whose resistance decreases as the voltage across it increases. Its use is to eliminate the voltage spikes and have resistance load. If it should become necessary to replace one of the varistors, the following instructions should be observed. Since the varistors have special characteristics, they should be replaced only with the same type as originally furnished by the generator manufacturer. A varistor can easily be replaced by following the procedure outlined for the type rectifier assembly furnished. When removing a varistor or varistor assembly, observe how the parts are assembled so that they can be installed In the identical manner.
•
Type 1. Remove the stud and insulating bushing on which the varistor is mounted, and unbolt the varistor leads from the heat sinks
•
Type 2. Remove the connection jumper between the varistor heat sinks, and remove the bolt, which secures the varistor assembly to the heat sink.
Prior to mounting a new varistor, check all mounting surfaces, such as heat sinks, shims, and the varistor faces, to see that they are flat and smooth. Tighten the nut and bolt which secure the varistor assembly to the heat sink only sufficiently to make a good electrical connection. Excessive tightening may crack or damage the varistors.
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8.3. DRYING WINDINGS Generators in service may inadvertently have their windings exposed to splashing or sprayed water. Units that have been in transit or storage for long periods of time may be subjected to extreme temperature and moisture changes causing excessive condensation. Regardless of the source of moisture, wet windings should be thoroughly dried out before operating the unit. If this precaution is not taken, serious damage to the generator can result. The following procedures may be utilized in drying the generator’s windings. The method selected will be influenced by winding wetness and situation limitations.
8.3.1. Space Heaters An electric heater may have been supplied with the generator. When energized from a power source other than the generator, the heater will gradually dry the generator. This process can be accelerated by enclosing the unit with a covering and inserting additional heating units. A hole should be left at the top of the covering to permit the escape of moisture. Care should be taken not to overheat various accessory equipment mounted with the generator. When intervention is done on a generator (same for a motor equipped with space heater) never forget to insulate (switch off and lock off) the space heater supplied generally in 220 or 380V………..
8.3.2. Forced Air Another method to dry the generator is to run the set with no excitation. The natural flow of ambient air through the generator will tend to dry the windings. This method can be accelerated by adding a source of heat at the air intake to the generator. Heat at point of entry should not exceed 80°C.
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8.4. OPERATIONAL DIFFICULTIES Occasional careful inspection of machines during operation is essential to detect any improper operation, which may, in time, result in a serious failure. Some operating difficulties of the brushless excitation system, which may occur, and their causes, are given in Table under and should be corrected as soon as discovered.
8.4.1. Troubleshooting table Affected part
Exciter
Difficulty
What to check
Excessive exciter Field current
Defective diode or varistor Shorted field turns in exciter or generator Short in system wiring Overloads
Generator output voltage will not build up
Reversed field leads Exciter residual lost Open circuit in excitation system Defective regulator
No control of generator output voltage
Defective regulator Open or short circuit in exciter system
Generator
Table 6: Generator troubleshooting
8.4.2. Insulation Resistance If a generator has become damp in shipment or in storage or after ‘inactivity’, it is advisable to measure the Insulation resistance of the stator and rotor winding with a megger (voltage adapted to the alternator voltage on stator and on rotor) CAUTION: When using a megger to check insulation resistance of the stator, be certain to disconnect all control equipment and/or radio-suppression capacitors at the generator and exciter terminals.
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To megger the rotor of a generator furnished with an ac brushless exciter, short across the heat sinks before applying power. Failure to observe these precautions may result in damage to the rectifiers or other solid state devices connected in these circuits In accordance with established standards, the recommended minimum insulation resistance for the stator winding is as follows: Rm = kV + 1
where
Rm = recommended minimum insulation resistance of the entire stator winding In megohms at 40°C (obtained by applying direct potential to the entire winding for one minute), and kV = rated machine voltage in kilovolts. The above formula should also be used to establish the recommended minimum Insulation resistance of the field winding by using field voltage in kilovolts in the above formula. INSULATION RESISTANCE VALUES
Extract From Total Spec: SP-COM-511 PRECOMMISSIONING ACTIVITIES Values at commissioning time for a GENERATOR The connections used for the insulation resistance tests shall be similar to the ones used on the high voltage test. A 5000 V Megger shall be used for testing the 5.5 kV windings and a 1000 V Megger shall be used on the 440 V windings, and exciter windings. A 500 V Megger shall be used for the anti-condensation heater and bearing pedestal tests. The minimum acceptable insulation value shall be: 5.5 kV generator windings - 150 Megohms 400 V generator windings - 100 Megohms Exciter windings - 100 Megohms Anti-condensation heaters - 10 Megohms Bearing Insulation - 1 Megohm If the insulation resistance of a generator winding is below the minimum acceptable value the polarization index (see course SE070 “Ground and Neutral” and course SE050 “Measurements in electricity”)) should also be measured. A motorized Megger, or similar equipment, is required for the test.
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9. ELECTRICAL GENERATION ON SITE For this specific part of course SE060, let’s do a résumé of what has been seen here and in other courses in form of Merlin – Gérin “Cahier Technique” N° 196
9.1. TYPES OF ENGINE GENERATOR SETS The main types of prime movers used in engine driven generator sets for industrial sites and commercial buildings are Diesel engines, gas turbines, and steam turbines. Turbines are used mainly for production sets whereas Diesel engines can be used for both production and standby sets.
Figure 106: turbo-generators 2 x 10 MW (gas turbine) onsite Peciko Most of the topics covered in this chapter are not dependant on the type of term generator set will be used. The choice of the prime mover is determined by such considerations as the availability and type of fuel (gas and or diesel for turbine and as well for motors). Since Diesel engines are very often used some specific information about Diesel generator sets will be given. Training manual EXP-MN-SE060-EN Last Revision: 29/09/2008
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Figure 107: diesel generator set 1 MW for Myanmar site
9.2. RATED POWER FOR GENERATOR SET APPLICATIONS The power output requirement for the generator set is probably the most important criterion to be defined. The output of a generator set is typically defined on the active/reactive power graph as represented in figure.
Figure 108: Active/reactive power graph showing operating limits The active power output depends on the type of fuel used, and on site conditions including ambient temperature, cooling medium temperature, altitude, and relative humidity. It also Training manual EXP-MN-SE060-EN Last Revision: 29/09/2008
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depends on load characteristics such as possible overloading and load variations over time. The ISO 3046-1 standard for Diesel engines defines three different types of power ratings, and a standard definition of overload capability. The different power ratings are: Continuous power rating: The engine can supply 100% rated power for an unlimited time. This rating is normally used for production sets. Prime power rating: The engine can supply a base load for an unlimited time, and 100% rated power for a limited time. The base load and acceptable time for 100% rated power are different for each manufacturer. Typical values are a base load of 70% of the rated power, and 100% rated power during 500 hours per year. Standby power rating: This is the maximum power that the engine can deliver and is limited in time, typically less than 500 hours per year. This rating should only be applied to generator sets which are used exclusively for emergency power. Since the engine is incapable of supplying more power, a security factor of at least 10% should be used when defining the standby power rating. The standard overload capacity is defined as 10% more power during 1 hour for every 12 hours of operation. There is no overload capacity with a standby power rating. Most manufacturers allow the standard overload capacity with the continuous power rating and the prime power rating, but since there are exceptions, the overload capacity should always be specified together with the type of power rating used. A typical example is a Diesel engine having a continuous power rating of 1550 kW, a prime power rating of 1760 kW, and a standby power rating of 1880 kW. When generator sets are used as a prime source of electrical energy the following points should be considered: provide for parallel operation with other sets and/or with utility, allow for long maintenance periods (overhaul), ensure black-start capabilities, use low speed equipment for long life (maximum 750 rpm for Diesel engines). When used as a standby source: ensure quick and reliable start-up and loading, implement reliable load shedding to avoid overloading or stalling, Training manual EXP-MN-SE060-EN Last Revision: 29/09/2008
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allow for periodic testing under load, provide for parallel operation with utility if set is used during peak loads, supply magnetizing current for distribution transformers. One common application for standby generators is to supply UPS (uninterrupted power supply) equipment during power outages. Since the generator has a relatively high impedance as compared to a utility supply, voltage waveform distortion can occur due to harmonic currents generated by the UPS. Generator manufacturers normally derate their machines by up to 60% to ensure correct voltage waveforms when loads are UPS equipment without harmonic current filtering. The engine must also be able to supply the power absorbed by the UPS which is determined by
For preliminary generator set sizing where detailed UPS information is unavailable, the battery charger kW can be estimated to be 25% of the UPS output kW, and the UPS efficiency can be estimated to be 90%. Final determination of the generator set should be based on specified values of acceptable voltage distortion, and the actual UPS data such as efficiency, and harmonic currents.
9.3. TYPICAL APPLICATIONS 9.3.1. Stand-by generator sets The typical supply of essential loads for commercial buildings, small industrial sites or for emergency power to unit substations in a larger site, is shown in figure. Under normal operating conditions the essential load is supplied from the utility supply. Upon loss of this supply the bus-tie circuit-breaker Q3 is tripped, the generator set is started, and then load is supplied by the standby generator set by closing the generator circuit- breaker Q2. Critical loads which cannot accept any power outage are supplied by the UPS. The UPS is equipped with a static switch which will immediately bypass the rectifier/inverter module in case of an internal fault and thus ensure a continuous supply of electrical power. Typical generator set sizes for this scheme are 250 kVA to 800 kVA.
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Figure 109: Typical emergency supply for small industrial sites The advantage of this scheme is its simplicity and clarity. All essential loads are connected to the same busbar as the generator set and therefore no load shedding is required. UPS backup time can normally be limited to 10 minutes since the UPS will be supplied by the emergency supply. Both the normal and the backup supply to the UPS should be taken from the essential busbar.
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Figure 110: Typical emergency supply for large industrial sites For large industrial sites a centralized emergency power supply system as shown in figure is often used. The main emergency switchboard is normally supplied from the utility, although in some sites one of the generator sets may be in constant operation. The emergency switchboard is designed to allow generator sets to operate in parallel and also to be connected to the utility supply.
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The automatic transfer from the utility to the emergency supply is performed in each unit substation. Since the emergency switchboard is normally energized, fast transfers (described in paragraph 9.5.1) without loss of plant load can be used. The use of a centralized emergency supply has the following advantages: fewer generator sets for the site (normally maximum of 2), permanently energized emergency supply allowing fast transfer schemes to be used, no loss of emergency supply due to maintenance of one generator set. Generator sets for such systems are normally in the 1-4 MW range.
9.3.2. Production generator sets For remote sites having no utility supply, several generator sets are used. A typical distribution system is shown in figure (of this paragraph). The number of sets “N” will depend on the power required, but since generator sets require periodic maintenance, plant power should be able to be supplied by (N – 1) sets without any load shedding. The generator set size should be such that they are loaded at least 50%. A poor load factor can be detrimental to the sets. For example Diesel engines loaded at less than 30% will not achieve a good operating temperature resulting in poor combustion and degrading of lubrication oil. Plant operation at (N – 2) sets should also be considered, this case occurring when one set is being maintained and there is a loss of an additional set. The highest initial load factor F that can be used with N installed generators such that load N −2 shedding is not required for (N – 2) operation can be determined from: F = N −1 For example the highest load factor for N = 6 will be 80%.
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Figure 111: Industrial site without utility supply Bus-tie circuit-breakers are often used for maintenance purposes. During normal plant operation all bus-tie circuit-breakers are normally closed. Short-circuit calculations should always take operation with N generators into account since it is normal to connect standby sets prior to switching off sets for maintenance. A power supply using local generation is generally much weaker than a utility supply and therefore it is probable that load shedding will be required to maintain system stability during fault conditions. Determination of how much load must be shed requires dynamic simulation of the network for different fault conditions such as a loss of a generator or a short-circuit. Prior to the study it is necessary to determine which operating configurations are to be considered. Operating conditions with the bus-tie circuit-breaker both in the open and the closed positions will greatly increase the complexity of the load shedding system since each busbar can be operated independently and will require specific load shedding criteria. For most plants it is recommended that only the standard operating configuration be used for the dynamic simulations and definition of the load shedding strategy. Training manual EXP-MN-SE060-EN Last Revision: 29/09/2008
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Figure of this paragraph shows each generator having its own transformer. The use of generator transformers has several advantages: provides flexibility in the choice of generator voltage, reduces peak short-circuit current at main board, allows use of high impedance generator grounding (reduces possible damage to generator).
9.4. OPERATION OF GENERATOR SETS 9.4.1. Starting and stopping of generator sets Since Diesel generator sets are often used for emergency power, it is necessary that steps be taken to ensure that the set will start correctly and quickly when required. An example of measures to be taken is lubrication and heating of the cooling water when the set is not operating. The Diesel generator set manufacturer should list all such measures and the design should take into account the availability of all auxiliary supplies necessary during times when set is not operating. A starting time of 15 seconds from the start order to the closing of the generator circuitbreaker can be guaranteed by manufacturers. Specifying shorter starting times should be avoided since the decrease in starting time will be small and could increase the cost of the set. Critical equipment must be supplied by an UPS in any case. Two techniques are commonly used for starting. These are compressed air and battery, compressed air generally being used for larger sets. The starting equipment should be designed for a minimum of 3 consecutive starts. It should be carefully monitored in order to enable preventive maintenance to be carried out prior to a failure during an attempted start. Failure to start is most often due to a problem with the starting battery. Where reliable starting is essential, consideration should be given to using compressed air. When a generator is operating in parallel with another source, it will be synchronized as described in paragraph 9.5.3 hereafter, and gradually loaded. When a generator set is operating alone, the load will be applied in one or more steps. The variation in frequency and voltage will depend upon the size of the step loads. As an example, step loads of 90% can be applied to a Diesel generator set without the frequency varying more than 10% and the voltage more than 15%. Should specific limits on frequency and voltage variations be required, they should be specified together with the type of load which is to be connected. This information should Training manual EXP-MN-SE060-EN Last Revision: 29/09/2008
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include motor starting characteristics such as the starting current, and the type of starting (direct-on-line, wye-delta). Several steps may be required should the frequency and voltage tolerance be small. When stopping a generator set, the power output should be reduced to zero by transferring the load to other sources, and the circuit-breaker then tripped. The generator set should be run for several minutes to allow it to cool down prior to shutdown. In some cases the cooling system should continue to operate after shutdown in order to remove latent heat from the machine. Manufacturer’s recommendations for shutdown should be followed. Generator set start and stop sequences should be handled by the generator set control equipment. Generator sets should be operated periodically. For installations where short power outages are not critical, opening the normal incoming circuit-breaker will cause the set to start and automatically pick up the emergency load. After the required minimum operating time, the generator circuit-breaker can be tripped and the normal source circuit-breaker closed. For plants where power outages mean unacceptable production losses, it must be possible to test generator sets without first switching off the supply. This is normally done by using a maintenance transfer. The generator set is started, and after it is ready to take load, it is synchronized to the incoming supply (see paragraph 9.5.3 below). The generator circuit-breaker (or bus-tie circuit-breaker depending on the scheme) will then be closed and the generator will thus be paralleled with the incoming supply. The closing of the circuit-breaker will cause tripping of the incoming supply and the loads will be supplied by the generator. The transfer to the normal incoming supply is done in the same manner without power interruption. Since the supplies are paralleled only for a few hundred milliseconds, it is not necessary to dimension the switchboard for the combined short-circuit power of both the normal incoming supply and the generator. Where equipment has been designed to operate in parallel on a permanent basis, it is not necessary to trip the incoming supply after connection the generator to the load. For this case, however, the switchboard must be designed for the combined short-circuit power of the incoming supply and the generator.
9.4.2. Stand alone operation Generator sets are often designed to operate independently (isochronous mode). In such cases the system frequency will be controlled by the engine governor. Overloads exceeding the maximum power output (standby power rating for Diesel engines as described in paragraph 9.2) of the set will cause the system frequency to decrease and this can be used for initiating load shedding. Training manual EXP-MN-SE060-EN Last Revision: 29/09/2008
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The generator voltage regulator will determine the system voltage. Generators can normally operate at a power factor of 0.8 and therefore supply most industrial loads without additional power factor compensation equipment.
9.4.3. Parallel operation with utility supply In some cases permanent operation of the generator set in parallel with the utility supply is required. Since the utility supply is much stronger it will determine the system frequency and the system voltage. The governor will therefore be used to control the active power output of the engine, and the voltage regulator will control the reactive power output of the generator. The generator set must know in which configuration it is operating in order to be able to switch the governor and voltage regulator operation from frequency and voltage control (isochronous operation) to active and reactive power control (parallel operation). Auxiliary contacts from the switchboard are normally used to provide the necessary information to the generator sets.
9.4.4. Parallel operation with other generator sets In this case generator sets are operated in parallel with other generator sets of approximately the same size. There are three basic schemes used. a) All generator sets but one have fixed active and reactive power output settings. One generator set is in the isochronous mode and will provide the active and reactive power necessary to keep the system frequency and voltage within the allowable limits. Any synchronizing instructions for frequency or voltage changes will be sent to the generator set in the isochronous mode. Since all power fluctuations will be absorbed only by this generator set, this scheme cannot be easily used where there are large variations in load. b) All generator sets operate in the droop mode. The active and reactive power is then shared equally among the sets or in proportion to their rated power if sets with different ratings are used. Variations in load will cause voltage and speed fluctuations due to the droop characteristic which is normally 4% from zero to 100% load. Since synchronizing of the sets with another source can only be done by adjusting the droop setting, this scheme is normally not used when parallel operation with another source is required.
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c) All generator sets are interfaced in order to share the active and reactive power. An example of how this is done is shown In figure. Each engine governor receives the active power set point from the active load dispatcher which also provides frequency regulation. Similarly each excitation regulator receives the reactive power set point from the reactive power dispatcher which also provides voltage regulation. This scheme allows for large load variation without changes in frequency or voltage.
Figure 112: Parallel operation using a load dispatcher
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9.5. TRANSFER SCHEMES AND SYNCHRONISATION 9.5.1. Automatic transfer on loss of supply An automatic transfer normally occurs when there is a loss of the normal supply and the load is to be supplied from the back-up supply with a minimum outage time. The transfer is blocked should the reason for the loss of supply be a fault on the busbar. Closing the emergency supply circuit-breaker onto a busbar fault will result in loss of the emergency supply and could result in damage to the equipment. Two techniques for transferring are generally used, their choice being based on whether or not the plant can accept a brief loss of supply.
9.5.1.1. Residual voltage transfer This is the most commonly used automatic transfer scheme and has the following basic steps: trip the incoming breaker to isolate the load from the supply start the generator set shed any loads which cannot be supplied from the generator set close the generator circuit-breaker after the generator set is able to be loaded, and the residual voltage on the busbar is less than 30%.
9.5.1.2. Fast transfer A fast transfer scheme is used when the process cannot accept any power outages. Such a system requires that the backup supply be permanently available and that the load is transferred to the backup supply before drives have had time to slow down. The time window for such switching is about 150 ms. In order to avoid the mechanical stresses and large currents due to out-of-phase switching, it is necessary to give the closing order to the emergency supply circuit-breaker such that the voltage generated by the decelerating motors is close to being in phase with the emergency system voltage when the circuit-breaker closes. Control gear for such transfer systems take into account the closing time of the circuitbreaker in order to anticipate the correct switching moment. If switching does not occur Training manual EXP-MN-SE060-EN Last Revision: 29/09/2008
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during the 150 ms time gap, the fast transfer is blocked and a residual voltage transfer is made including any required load shedding.
9.5.2. Maintenance transfer – back to normal supply After the normal supply has returned, the load should be transferred from the emergency supply back to the normal supply. This is normally initiated manually as described at the end of paragraph 9.4.1 above.
9.5.3. Synchronization of generator circuit-breaker Any time parallel operation of a generator set is required, it is necessary to be able to synchronize it to the system. Synchronization basically consists in adjusting the generator frequency and voltage to values close to the system values. Since the system frequency and voltage can vary within a few percent, it is necessary that both the engine speed and the generator voltage be able to be adjusted for synchronization purposes. The engine speed and generator voltage are controlled by the governor and voltage regulator. Adjustments in the frequency and voltage are normally achieved by momentarily closing contacts connected to the governor and voltage regulator. When the generator voltage is almost in phase with the system voltage a closing order is given to the generator circuit-breaker. Synchronization is normally done automatically by means of relays which measure generator and line voltages, frequencies, and phase angles. The relay automatically adjusts the speed and voltage of the generator set and closes the circuit-breaker when the phase angle between the generator and line voltages is sufficiently small. One set of automatic synchronization equipment can be used for several generators by selecting the appropriate voltage transformers and sending the ± voltage, ± speed as well as the closing order to the selected circuit-breaker. Manual synchronizing should be provided in all cases, either as a back up to the automatic synchronizing system, or for use in applications where synchronization would only rarely occur. For manual synchronization the operator uses push buttons to provide the voltage and speed adjustment signals. A synchroscope will let the operator know when the line and generator voltages are sufficiently in phase to close the circuit-breaker. For manual synchronization use of a synchronism check protection relay is recommended which will inhibit closing of the circuitbreaker unless all conditions of frequency, voltage, and phase angle have been satisfied. Synchronization across the generator circuit-breaker is often included as a standard feature in generator set control equipment. Training manual EXP-MN-SE060-EN Last Revision: 29/09/2008
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9.5.4. Synchronization of bus-tie, bus-coupler, or utility incoming circuitbreakers When several generator sets are used, they are often connected to different busbars in order to facilitate maintenance. It is therefore possible at times to have generator sets supplying loads on busbars which are not connected together. In order to have all busbars connected it will be necessary to synchronize groups of generator sets across bus-tie or bus-coupler circuit-breakers. Specific synchronization equipment is normally required for such applications since the generator set normally allows synchronizing across the generator circuit-breaker only. A similar situation can occur when plant load is being supplied by generator sets and it is necessary to connect the loads to the utility. Synchronization across the utility circuitbreaker will be necessary. Synchronization requires voltage and speed adjustments. As described in paragraph 9.4.4 above, synchronization of a group of generator sets is possible when one set is in the isochronous mode, or when a load dispatcher is used which will change the power output (and therefore speed) of all sets. When a set is in the isochronous mode, the voltage and speed adjustment signals will be sent to that set and the others will follow according to their droop characteristic. When a load dispatcher is used, the ± frequency signal will be sent to the load dispatcher which then sends appropriate signals to the individual governors. The voltage regulators used in such cases are sometimes connected to the voltage transformer of the busbar to which they are to be synchronized and can therefore adjust their excitation accordingly without receiving a separate ± voltage signal. For both schemes, once the required frequency, voltage, and phase angle have been achieved, the circuit-breaker can be closed. Some manufacturers of load dispatching systems offer adjustment of the voltage in addition to adjustment of the speed. Specifications for synchronization equipment should therefore clearly specify all the functional requirements thereby allowing suppliers to choose the best solution.
Training manual EXP-MN-SE060-EN Last Revision: 29/09/2008
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9.6. GENERATOR SET PROTECTION “Already seen” above in this document but nevertheless re-included here as with the “original” comments.
9.6.1. General protection philosophy
Figure 113: Recommended generator protection Training manual EXP-MN-SE060-EN Last Revision: 29/09/2008
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Since generators are a source of electrical power, the overcurrent protection relays should be connected to current transformers on the neutral side of the stator windings in order to cover faults occurring in the windings. Additional protection relays are required at the generator circuit-breaker only for applications where generator sets will be operating in parallel with other generator sets or with the utility, and will pick up faults on the line side of the generator. The current transformers for these protection relays are installed at the generator circuitbreaker in order to cover the whole connection to the generator. Reverse active and reverse reactive power relays are normally connected to current transformers on the neutral side of the generator as shown in the picture of this paragraph. They can also be connected to the current transformers associated with the circuitbreaker. The location will depend on the split of works as described in paragraph 9.9.1.
9.6.2. Electrical protection The recommended protection functions are shown in figure of previous paragraph. Function reference numbers are the following: Protection functions connected to generator neutral current transformers: 32P : reverse active power 32Q : reverse reactive power serving as loss of field (for generators above 1 MVA) 46 : negative sequence (for generators above 1 MVA) 49 : thermal image 51 : overcurrent 51G : earth fault 51V : voltage restrained overcurrent 87G : generator differential protection (for generators above 2 MVA) (Note: 46,49, 32P and 32Q can also be connected to the line-side current transformers)
Training manual EXP-MN-SE060-EN Last Revision: 29/09/2008
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Protection functions connected to voltage transformers: 25 : synchronism-check (for parallel operation only) 27 : undervoltage 59 : overvoltage 81 : overfrequency and underfrequency Protection functions connected to line-side current transformers (for parallel operation only): 67 : directional overcurrent (not required if 87G is used) 67N : directional earth fault (on core balance CT for better sensitivity) Generator mechanical protection functions connected to sensors 49T : stator temperature (recommended for generators above 2 MVA) 49T : bearing temperature (recommended for generators above 8 MVA) 64F : rotor earth fault protection The following table gives typical settings for each protection function, and what action should be taken. This information should be verified with the generator set manufacturer for each application. A general shutdown means tripping and locking out the generator circuit-breaker, switching off the excitation, and closing the fuel supply to the engine. Function
Typical setting
Action
27
0.75 Un, T » 3 s T > longest time of 51, 51V, 67
General shut-down
32P
1-5 % for turbine, 5-20 % for Diesel, T = 2 s
General shut-down
32Q
0.3 Sn, T = 2 s
General shut-down
46
0.15 In, inverse time curve
General shut-down
49
80% thermal capacity = alarm 120% thermal capacity = trip time constant 20 min operating time constant 40 min standstill
Trip breaker only, overload may be temporary
51
1.5 In, 2 s
General shut-down
51G
10 A, 1 s
General shut-down
Training manual EXP-MN-SE060-EN Last Revision: 29/09/2008
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51V
1.5 In, T= 2.5 s
General shut-down
59
1.1 Un, 2 s
General shut-down
81
Overfrequency: 1.05 Fn, 2 s Underfrequency: 0.95 Fn, 2 s
General shut-down
87G
5 % In
General shut-down
67
In, 0.5 s
General shut-down
67N
Is0 » 10 % of earth-fault current, 0.5 s
General shut-down
25
Frequency < ±1 Hz, Voltage < ±5 %, Phase angle
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