Generator Protection

Generator Protection. 1. Loss of Prime Mover : Generator Motoring 2. Generator Reverse Power Protection 3. Generator Over/Under Frequence Protection. 4. Over Excitation : Volt per hertz Protection. 5. Under Excitation Protection. 6. Generator Minimum Impedance Protection 7. Generator Negative Phase Sequence Current Protection 8. Generator Inter turn Fault Protection 9. Thermal Over Load. 10.Over Voltage. 11.Loss of Synchronism. 12.Diff Protection. 13.Diff Over All Differential Protection. 14.Rotor Earth Fault Protection. 15.Stator Earth Fault Protection

Excitation System Protection and Limiter. 1) 2) 3) 4)

Field Grounds. Field Over Excitation. Field Under Excitation. Practical consideration.

Distributed Generator Intertie Protection. 1) Power Quality Protection. 2) Power System Fault Protection. 3) System Protection for faults on distributed. 1)Loss of Prime Mover : Generator Motoring(32) Loss of prime mover is due to the loss of Steam supply to turbine.In case of loss of prime mover , ie loss of mechanical inputs, The generator continue to remains synchronized with the grid. The generator now running as synchronous motor. The generator now draws a small amount of active power from the grid in order to drive the turbine. At the same time the generator supply reactive power to grid since its excitation is intact (AVR is supplying field voltage & field current). Running in this mode is not harmful to generator but is definitely harmful to prime mover (Steam Turbine). Loss of Steam supply to the Prime Mover (Steam Turbine) causes a churning of trapped steam in the turbine, which causes the temperature rise and may damage the turbine blades.

Therefore, it imperative to detect the Loss of Prime Mover quickly and followed by the tripping of Generator.

Preventive action:Directional relay. During the normal condition generator stator supply full load current to the grid. During the loss of Prime Mover the magnitude of stator current is smaller than when it was generating and the stator current goes 180 deg. Phase shift. The magnitude of phase current is very small compared to forward current, hence the degree sensitivity of Directional relay used for Loss of Prime Mover should be very hi compared to Direction relay used for over current application. Therefore, these can be protected by using Directional Relay. The Directional relay input are:- Phase Voltage & Phase Current , Output:- Tripping to Generator CB. 2)GENERATOR REVERSE POWER PROTECTION (32) Reverse power can be occur when there is loss of Prime Mover as already discussed or when two or more generator are running in parallel. if one generator does not accept load usually because its unloaded running speed is less than the other generator (called speed droop) then the other generator will motorized the drooping generator. The generator are classified by their Prime Mover which determine the amount of Reverse power they can motor. Sr.No Prime Mover Motorizing Power in % of Unit Rating 1 Gas Turbine (single shaft) 100% 2 Gas Turbine (Double Shaft) 10-15% 3 4 Cycle Diesel 15% 4 2 Cycle Diesel 25% 5 Hydraulic Turbine 2-100 6 Steam Turbine 1-4% (Conventional) 7 Steam Turbine (Cond 0.5 to 1.0% Cooled) Say for example Generator Capacity-37500KVA Generator Terminal Voltage:- 11KV CT-2400/1, PT -11KV/110V, 50Hz. Let us assume Unit motoring power rated as 1% of ratedKVA=375KVA

Generator full load at Unity PF=>, 37500/11*1.732=1968 Amp Primary During Motoring Condition Im(Primary)=375/11*1.732=19.68 Im (Sec)= 19.68*1/2400=0.0082 Amp. Therefore, during motoring condition, when 0.0082 amp flows, Relay gives trip command with time delay setting. AT 30MW ACBPL Setting for Reverse Power Protection in REM 545 Function Code=UPOW6ST_ (Three Phase Under Power or Reverse Power Protection) 1) Relay can be operative either in Operation Mode=Reverse Power or Operation Mode=Under Power) 2) This function is applicable for following types of Prime Mover a. Steam Turbines b. Francis & Kaplan Hydro Units c. Gas Turbine d. Diesel Turbine. 3) Parameters Setting a. For Reverse Power = 3%. b. Angle set=-90°. c. Wait time=

3)GENERATOR OVER/UNDER FREQUENCY PROTECTION. (14A/14T/81) The Generator are designed to give rated output voltage & frequency of 50Hz. The operation within the safe limit of +-5% (47.5 to 52.5)of rated frequency (50Hz)is recommended to protect various apparatus in a network , Generator , Turbine & Transformer. Over frequency condition occurs when excess generation or when load is thrown off. This situation can be corrected quickly by a reduction in power output via governor system. The Over Speed of a generator would result in Over Frequency operation of generator. Steam turbines being the key elements of such power plants, are running closer and closer to their mechanical limits and must be protected

efficiently against vital risks. Steam turbines normally run with constant rotor speed of 7059 rotations per minute (rpm) in a 50 Hz grid . Due to the large masses of rotors and blades over-speeding the turbine is very critical and lead to blade cracks, blade losses and heavy rotor and bearings defects. Under frequency caused by load in excess of generation. And mostly used for load shedding purpose. Protection is required for the following reasons. 1) With under frequency, Generator Stator & Rotor cooling effect reduces thus load carrying capability reduces. 2)

Steam Turbine blades are designed and tuned for operation at rated frequency rotation. Under frequency protection is used to prevent blade resonance & fatigue damage in the turbine.

3) The Electromagnetic flux ф is proportional to Terminal Voltage Vt & Inversely Proportional to Frequency. Ф α ….. Vt……….. Freq. Therefore, if the frequency decreases then Flux increases as a result field current would be on the higher side than at the rated freq. This will causes a Over Fluxing. The over fluxing will causes the over- heating of Stator & rotor circuit.

4)OVER EXCITATION PROTECTION V/Hz (24)

The over excitation protection system are used to protect Generator against the excessive Flux density and saturation of the magnetic core. The saturation leads to the stray flux which may cause eddy current and severe overheating in non-laminated parts of a generator. v/f ratio are commonly used to denote flux density. 1) This condition arises during abnormal operating conditioni.e heavy voltage fluctuation at lower frequency 2) Due to AVR malfunctioning.

The Electromagnetic flux (ф)is proportional to Terminal Voltage Vt & Inversely Proportional to Frequency. Ф (webers) α ….. Vt……….. Freq. The magnetic flux density B (Tesla)=ф (weber)/Area (m2). Since the area of Magnetic core is fixed quantity. hence, Increase in Flux ф will increase in Flux density (B). The Flux density is depends up on the type of material used for the constructions of Core in the Generator.

The properties of different materials are as follows:,

1.Standard sheet steel, annealed. 2) Silicon sheet steel, annealed, Si 2.5%. 3.Soft steel casting. 4. Tungsten steel . 5) Magnet Steel. 6) Cast Iron. 7. Nickel 99%. 8) Cast Cobalt. 9) Magnetite, Fe2O3.

The relationship between magnetic field strength (H) and magnetic flux density (B) is not linear in such materials. If the relationship between the two is plotted for increasing levels of field strength, it will follow a curve up to a point where further increases in magnetic

field strength will result in no further change in flux density. This condition is called magnetic saturation. If the magnetic field is now reduced linearly, the plotted relationship will follow a different curve back towards zero field strength at which point it will be offset from the original curve by an amount called the remanent flux density or remanence. If this relationship is plotted for all strengths of applied magnetic field the result is a sort of S- shaped loop. The 'thickness' of the middle bit of the S describes the amount of hysteresis, related to the coercivity of the material. Its practical effects might be, for example, to cause a relay to be slow to release due to the remaining magnetic field continuing to attract the armature when the applied electric current to the operating coil is removed.

Protection System 1) Normally Over Excitation protection inbuilt in the AVR system. 2) Back up Protection is provided by Generator Protection Relay.ie REM 545. The core of 30MW generator is made up of Silicon Sheet Steel, whose B value is 1.4 Approx.

Measuring mode:

1) Voltage And its frequency from Potential Transformer. 2) Phase to Phase or Phase to Line Voltage.

Calculation methods:Generator rated Voltage: 11000V Rated Frequency: 50Hz V/F ratio: 11000/50= 220 220 is Base value, PU=1.

Running Parameters: V: 11470V, F: 50.11 V/F ratio: 11470/50.11=228.89 PU: 228.89/220=1.04 Sampling Time of relay:- 10msec at 50Hz.

5)GENERATOR UNDER EXCITATION OR LOSS OF EXCITATION Under excitation means that the excitation of synchronous generator is less than required for stable operation at that particular power Out Put. The excitation limit determines the steady state stability characteristic of generator. If the excitation is not sufficient to provide power demand then this stability limits exceeds. As a result generator speed will increase above the synchronous speed and operate as an induction Generator taking its reactive power needs from the System. Loss of excitation results in 1)

Heavy overloading of armature winding.

2) Induce eddy current in the rotor surface and rotor windings at a slip frequency resulting in a heavy thermal heating in rotor body. 3) 4)

Large voltage drop in transmission line, loss of system stability. Loss of magnetic coupling between stator & Rotor.

Loss of excitation may occur as a result of 1) Under excitation or loss of excitation can be result of short circuit or open circuit in the excitation system. 2) Mal operation of AVR.

Generator Reactive Power OutPut. The reactive power output of a generator can be expressed QT=3[Eq.Vt cosδ –vt2]=QF+

Xd

QN

xd

Eq & Vt are generator Internal & Terminal Voltage respectively. Power angle δ is the angle between Eq & Vt. This equation shows that reactive power output consist of 2 components. Qf:- Internally generated reactive power, which is determined by the internal voltage (field current) and the load angle δ. Qn:- supplied to the generator from the external network. The equation “QT=3[Eq.Vt cosδ –

vt2] can be rearrange as QT=3 Vt [Eq. cosδ –

vt] Xd When

(Eq.

cosδ –

vt)>0

,

xd

Xd

reactive power is flowing from generator to system.

Operating under lagging power factor. In this mode generator is said to be over exited. When

(Eq. cosδ –vt)=0,

total reactive power measured at the machine erminal

will be zero. Operating at Unity PF & delivering only active power to the system. Operation is this mode is limited by allowed armature current. When

(Eq.

cosδ –

vt), x= (1-Is/I R). If Is/I R =10 With a resistor design to pass the rated current of the machine on a fault at machine terminal. Differential protection setting of 10% will protect 90% of the winding. And 10% of the winding is being unprotected. Biased / Percentage differential Protection. Biased differential protection has bias feature. The effect of biased feature is to reduce the impedance of relay operating coil for the through fault stability. The bias feature is obtained by circulating the through (external) fault current through an additional winding (restraining coil). Restraining coil exerts a

restraining forces on the relay. Normally no current flows in operating coil under through fault conditions. but there are some splii current due following reason: 1) Imperfect matching of CT. 2) Current imbalance due Tap changer. 3) Zero sequence current. 4) Inrush magnetizing current. This spill current will flow through the relay operating coil but will not cause a operation still relay bias setting is increase. If

Generator Winding

I

I

R

x I2

I1

Y

1.0 B If

R

I1-I2

Restraining /Stabilizing Coil Operating Coil

Fig.- Biased Differential protection Relay operating force Id (differential Current) = K(I1-I2)No. Relay Restraining force Ir (Stabilizing Current)= K(I1+I2)Nr. 2 Where No & Nr are no of turns in operating coil & restraining coil. Under normal condition operating forces = restraining forces. =>, (I1-I2) No= K(I1+I2)Nr. 2 =>, (I1-I2) =Nr/No (I1+I2) /2 Under normal condition differential current must be Id=0, in practical Id slightly deviate from zero due deviation in CT’s accuracies. The bias characteristic is determined by the ratio of Nr/No. the required value of stabilizing resistance is very low, thus giving a lower relay voltage than Unbiased relay.

The setting parameters are 1) Basic Setting ie. Minimum Id (differential current) required for tripping. a. Id= 5% of In 2) The Starting Ratio or the Slope. a. The Starting ratio is define as turning point T1 & T2. 3) Differential Over Current Protection Id>>.

Id /In Id>>

Trip zone T2 Restraining/ Safe zone

T1 Id Ib/In=1

Ib/In=3

The differential protection relay characteristic.

Ib/In (Stabilizing current)

Relay Operates Case 1) If Ib/In < T1 Id= Basic Setting. Case 2) if T1