EHVAC & DC Unit- 1 (1)

EHV AC AND DC TRANSMISSION SYSTEM

Unit-I Constitution of EHV a.c. and d.c. links, Kind of d.c. links, Limitations and Advantages of a.c. and d.c. transmission, Principal application of a.c. and d.c. transmission, Trends in EHV a.c. and d.c. transmission, Power handling capacity. Converter analysis garetz circuit, Firing angle control, Overlapping

ROLE OF EHV AC TRANSMISSION  Industry requires vast amount of energy such as hydro, thermal, oil for transportation and industry, natural gas for domestic of which electrical energy forms a major fraction.  It is only 120 years since the installation of the first central station by Edison using dc. But the world has already consumed major portion of its natural resources in this short period and is looking for sources of energy.  Hydro-electric and coal or oil-fired stations are located very far from load centers for various reasons which requires the transmission of the generated electric power over very long distances. This requires very high voltages for transmission. The very rapid development of dc transmission since 1950 is playing a major role in extra-long-distance transmission, complementing or supplementing EHV ac transmission.

BRIEF DESCRIPTION OF ENERGY SOURCES AND THEIR DEVELOPMENT

Two broad categories: (1) Transportable (2) Locally Usable. 1. Transportable type is obviously hydro-electric and conventional thermal power. But locally generated and usable power is by far more numerous and exotic. These are also called 'Alternative Sources of Power'. Twelve such sources of electric power are listed here. 2. Locally Usable Power  Conventional thermal power in urban load centers  Micro- hydel power stations  Nuclear Thermal: Fission and Fusion

BRIEF DESCRIPTION OF ENERGY SOURCES AND THEIR DEVELOPMENT

 Wind Energy  Ocean Energy: (a) Tidal Power, (b) Wave Power, and (c) Ocean thermal gradient power  Solar thermal  Solar cells or photo-voltaic power  Geo-thermal  Magneto hydro-dynamic or fluid dynamic  Coal gasification and liquefaction  Hydrogen power  Biomass Energy: (a) Forests (b) Vegetation (c) Animal refuse.

PROBLEMS/LIMITATIONS POSED IN USING HIGH VOLTAGE  Increased Current Density because of increase in line loading by using series capacitors.  Use of bundled conductors.  High surface voltage gradient on conductors.  Corona problems: Audible Noise, Radio Interference, Corona Energy Loss, Carrier Interference, and TV Interference.  High electrostatic field under the line.  Switching Surge Over voltages which cause more havoc to air-gap insulation than lightning or power frequency voltages.  Increased Short-Circuit currents and possibility of Ferro resonance conditions.

PROBLEMS/LIMITATIONS POSED IN USING HIGH VOLTAGE

 Use of gapless metal-oxide arresters replacing the conventional gap-type Silicon Carbide arresters, for both lightning and switching-surge duty.  Shunt reactor compensation and use of series capacitors, resulting in possible sub-synchronous resonance conditions and high short circuit currents.  Insulation coordination based upon switching impulse levels.  Single-pole reclosing to improve stability, but causing problems with arcing

1.2 ADVANTAGES OF HVDC No reactive power loss No Stability Problem No Charging Current No Skin & Ferranti Effect Power control is possible Requires less space compared to ac for same voltage rating and size Ground can be used as return conductor Less corona loss and Radio interference

ADVANTAGES OF HVDC Cheaper for long distance transmission Asynchronous operation possible No switching transient No transmission of short circuit power No compensation problem Low short circuit current Fast fault clearing time

1.3 DISADVANTAGES OF HVDC Cost of terminal equipment is high Introduction of harmonics Blocking of reactive power Point to point transmission Limited overload capacity Huge reactive power requirement at the converter terminals Cooling of HVDC sub-station HVDC system control

1.4 COMPARISION OF AC & DC TRANSMISSION

• The relative merits of the two modes of transmission (AC and DC) which need to be considered by a system planner are based on the following factors:

• Economics of Transmission • Technical performance • Reliability

1.4.1 Economics of power transmission Cost of a transmission line includes • Investment includes:  Right of Way (ROW)  Transmission towers  Conductors  Insulators  Terminal equipment • Operational costs includes:  Cost of losses. Cont..

• The characteristics of insulators vary with the type of voltage applied. For simplicity, if its assumed that the insulator characteristics are similar for AC and DC and depend on the peak level of voltage applied with respect to ground, then it can be shown that for lines designed with the same insulation level, a DC line can carry as much power with two conductors (with positive and negative polarities with respect to ground) as an AC line with 3 conductors of the same size.

The corona effects on DC conductors tend to be less significant than for AC and this also leads to the choice of economic size of conductors with DC transmission. The other factors that influence the line costs are the costs of compensation and terminal equipment

Comparison of ROW • This Implies that for a given power level, DC lines requires less ROW with Simpler, cheaper towers reduced conductors and insulator costs. • The power losses are also reduced with DC as only two conductors are used. • No skin effect with DC is also beneficial in reducing power loss marginally. • The dielectric losses in case of power cables is also very less for DC transmission.

Comparison of ROW • The corona effects tends to less significant on DC conductors than for AC and this leads to choice of economic size of conductors with DC transmission. • The other factors that influence the line cost are the cost of compensation and terminal equipment. • DC lines do not require compensation but the terminal equipment costs are increased due to the presence of converters and filters.

Variation of cost with line length  AC tends to be more economical than

DC for distances less than Break even distance and costlier for longer distances. The breakeven distances can vary from 500Km to 800Km in overhead lines.

1.4.2 Technical performance  The DC transmission has some positive features which are lacking in AC transmission. These are mainly due to the fast controllability of power in DC lines through converter control. The advantages are: 1. Full control over power transmitted 2. The ability to enhance transient and small signal stability in associated AC networks 3. Fast control to limit fault currents in DC lines. This makes it feasible to avoid DC breakers in two terminal DC links.

STABILITY LIMITS The power transfer in AC lines is dependent on

the angle difference between voltage phasors at the two ends. For a given power level, this angle increases with distance. The maximum power transfer is limited by the considerations of steady state and transient stability. The power carrying capability of an AC line is a function of distance but in DC lines it is unaffected by the distance of transmission.

VOLTAGE CONTROL The voltage control in AC lines is complicated by line charging and inductive voltage drops. The voltage profile in a AC line is relatively flat only for fixed level of power transfer corresponding to surge impedance loading (SIL) or normal loading. The Voltage profile varies with the line loading. For constant voltage at the line terminal, the midpoint voltage is reduced for line loading higher than SIL and increased for loadings less than SIL.

LINE COMPENSATION AC lines require shunt and series compensation in long distance transmission, mainly to overcome of the line charging and stability limitations. Series capacitors and shunt inductors are used for this purpose. The increase in power transfer and voltage control is possible through the Static Var Systems (SVS). In AC cable transmission, it is necessary to provide shunt compensation at regular intervals.

PROBLEMS OF AC INTERCONNECTION • When two power systems are connected through AC ties (Synchronous interconnection),the automatic generation control of both systems have to be coordinated using tie line power and frequency signals. • Even with coordinated control of interconnected systems, the operation of AC ties can be problematic due to  The presence of large power oscillations which can lead to frequent tripping.  Increase in fault level  Transmission of disturbances from one system to the other

GROUND IMPEDANCE • In AC transmission, the existence of ground (Zero sequence) current cannot be permitted in steady-state due to high magnitudes of ground impedance which will not only affect efficient power transfer, but also result in telephone interference. • But ground impedance negligible for DC currents and a DC link can operate one conductor with ground return (Mono polar operation). The ground return is objectionable only when buried metallic structures (such as pipes) are present and are subject to corrosion with DC current flow.

1.4.3 RELIABILITY • The reliability of DC transmission is quite good and comparable to that of AC systems. • An exhaustive record of existing HVDC links in the world is available from which the reliability statistics cab be computed. • It must be remembered that the performance of Thyristor valves is much more reliable than mercury arc valves and further developments in devices, control, protection is likely to improve the reliability level.

Cont..

There are two measures of overall system reliability • Energy availability • Transient reliability Energy availability=100 (1–{Equivalent outage time})% Total Time Transient reliability = 100*No. Of times HVDC systems performed as designed No. Of recordable AC faults

Cont..

This is the factor specifying the performance of HVDC systems during recordable faults on the associated AC systems. Recordable AC system faults are those faults which cause one or more AC bus phase voltages to drop below 90% of the voltage prior to the fault. It is assumed that the short circuit level after the fault is not below the minimum specified for satisfactory converter operation. Both energy availability and transient reliability of existing DC systems with thyristors valves is 95% or more.

Kind of D.C. Links The DC links are classified into three types:

Monopolar 2) Bipolar 3) Homopolar • Monopolar link:

Monopolar link cont.. Having one conductor (-ive Polarity preferred in order to reduce the Corona effect) and ground is used as return path. The major drawback in this system is power flow is interrupted due to either converter failure or DC link. The ground return is objectionable only when buried metallic structures (Such as pipes) are present and are subject to corrosion with DC current flow.

Bipolar link There are two conductors, one is operated at positive and other is negative. During fault in one pole it will operate as Monopolar link. This is very popular link in HVDC.

Homopolar link • In this link, two or more conductors have same polarity. Normally negative polarity is used (less corona loss and radio interference). Ground is always used as return path. During fault in one pole it works as Monopolar.

1.6 Application of EHV- AC transmission • Voltage can be stepped up or stepped down in transformer substation to have economical transmission voltage. • Line can be tapped easily, extended easily. • Parallel line can be added. • Control of power flow in the network is simple and natural. • Power flow in a particular line cannot be controlled easily and quickly. • System incorporation. • Back born transmission network.

1.7 Application of EHV- DC transmission • The main areas of application based on the economics and technical performances, are Long distance bulk power transmission.  The underground of submarine cables. Asynchronous connection of AC system with different frequencies. Control and stabilize the power system with power flow control.

1.8 Based on the interconnection, three types of HVDC is possible.

Bulk Power transmission Back to back connection Modulation of AC system 1.8.1 Bulk power transmission: The transfer the power from one end to another end without tapping power in between. For this DC system is the best option. (or) HVDC transmission where bulk power is transmitted from one point to another point over long distance.

1.8.2 Power flow control (Back to Back HVDC)

• If two regions are very nearby, we can monitor the power flow from one region to another to control, emergency support as per our requirement.(Or) Back to Back link where rectification and inversion is carried out in the same converter station with very small or no DC lines

1.8.3 Modulation of AC system This is basically used to control the power and stabilize the system. It is also used to connect two different frequencies system. (Modulation of AC) AC system is connected parallel with DC system.(or)Parallel connection of AC and DC links. Where both AC and DC run parallel. It is mainly used to modulate the power of AC lines.

1.9 Principle parts of HVDC Transmission

Schematic diagram of a typical HVDC converter station

2.0 Various Parts of HVDC transmission • • • • • • •

Converters Converter transformers Smoothing reactors Harmonic filters Overhead lines Reactive power source Earth electrode

2.0.1 CONVERTERS Converters are the main part of HVDC system. • Each HVDC lines have at least two converters, one at each end. • Sending end converter works as Rectifier (It converts AC power to DC power). However converter as receiving end works as Inverter (It converts DC power to AC power). • In case for reversal of operation, Rectifier can be used as inverter or vice versa. So generally it is call it as CONVERTERS. • Several thyristors are connector in series and/or in parallel to form a valve to achieve higher voltage / current ratings.

How to achieve required current & voltage rating

Required current rating • Valves in parallel • Thyristors in parallel • Bridges in parallel • Combination of above. Required voltage rating • Valves in series • Bridges in series • Combination of above.

Main requirements of the Valves Bridge converters are normally used in HVDC systems

• To allow current flow with low voltage drop across it during the conduction phase and to offer high resistance for non conducting phase. • To withstand high peak inverse voltage during non conducting phase. • To allow reasonably short commutation angle during inverter operation.

Cont.. • Smooth control of conducting and non conducting phases. • Two versions of switching converters are feasible depending on whether DC storage device utilized is an inductor-Current source converter or Capacitor-Voltage source converter. • CSC is preferable for HVDC system • VSC is preferable for FACTS like STATCOM, SVC, etc.

Comparison of CSC and VSC Inductor is used in DC side

Capacitor is used in DC side

Constant current

Constant voltage

Higher losses

More efficient

Fast accurate control

Slow control

Larger and more expensive

Smaller and less expensive

More fault tolerant and more reliable

Less fault tolerant and less reliable

Simpler control

Complex control

Not easily expandable for in series

Easily expanded in parallel for increased rating

2.0.2 CONVERTER TRANSFORMERS • For six pulse converter, a conventional three phase or three single phase transformer is used. • However for 12 pulse configuration, following transformer are used. • Six single -phase two windings • Three single- phase three windings • Two three- phase two windings

• In converter transformer it is not possible to use winding close to yoke since potential of its winding connection is determined by conducting valves hence entire winding are completely insulated. • As leakage flux of a converter transformer contains very high harmonic contents, it produces greater eddy current loss and hot spot in the transformer tank.

• In case of 12-Pulse configuration, if two three phase transformers are used, one will have starstar connection, and another will have star delta connection to give phase shift of 30 . • Since fault current due to fault across valve is predominantly controlled by transformer impedance, the leakage impedance of converter transformer is higher than the conventional transformer. • On-line tap changing is used to control the voltage and reactive power demand.

2.0.3 SMOOTHING REACTORS • These reactors are used for smoothing the direct current output in the DC line. • It also limits the rate of rise of the fault current in the case of DC line short circuit. • Normally Partial or total air cored magnetically shielded reactor are used. • Disc coil type windings are used and braced to withstand the short circuit current. • The saturation inductance should not be too low.

2.0.4 HARMONIC FILTERS: • Harmonics generated by converters are of the order of np 1in AC side and np is the DC side. Where p is number of pulses and n is integer. • Filters are used to provide low impedance path to the ground for the harmonics current. • They are connected to the converter terminals so that harmonics should not enter to AC system.

Cont.. • However, it is not possible to protect all harmonics from entering into AC system. • Magnitudes of some harmonics are high and filters are used for them only. • These filters are used to provide some reactive power compensation at the terminals.

2.0.5 OVERHEAD LINES • As Monopolar transmission scheme is most economical and the first consideration is to use ground as return path for DC current. • But use of ground as conductor is not permitted for longer use and a bipolar arrangement is used with equal and opposite current in both poles. • In case of failure in any poles, ground is used as a return path temporarily.

Cont.. • The basic principle of design of DC overhead lines is almost same as AC lines design such as configurations, towers and insulators etc. • The number of insulators and clearances are determined based on DC voltage. • The choice of conductors depends mainly on corona and field effect considerations.

2.0.6 REACTIVE POWER SOURCE • Converter does not consume reactive power but due to phase displacement of current drawn by converter and the voltage in AC system, reactive power requirement at the converter station is about 50-60% of real power transfer, which is supplied by filters, capacitors and synchronous condensers. • Synchronous condensers are not only supplying reactive power but also provide AC voltages for natural commutation of the inverter. • Due to harmonics transient special designed machines are used.

2.0.7 EARTH ELECTRODES • The earth resistivity of at upper layer is higher (~4000 ohm-m) and electrodes cannot be kept directly on the earth surface. • The electrodes are buried into the earth where the resistivity is around (3-10 ohm-m) to reduce transient over voltages during line faults and gives low DC electric potential and potential gradient at the surface of the earth.

Cont…. • The location of earth electrode is also important due to Possible interference of DC current ripple to power lines, communication systems of telephone and railway signals etc. Metallic corrosion of pipes, cable sheaths etc. Public safety. The electrode must have low resistance (Less than 0.1 ohm) and buried up to 500 meters into the earth.

Power Handling Capacity and Line loss • The Power Handling Capacity of a single circuit is P = E2 sin δ / LX ………………………… (1) • At unity p.f., at the load P, the current flowing is P = E sinδ / √3 LX …………………….. (2) • The total power loss in the three phase will amount to P= 3 I2 r ……………………… (3) • Therefore the percentage power loss is % P = 100 r sin δ / X ………………….. (4)

Cont… • The following important and useful conclusion can be drawn for preliminary understanding of trends relating to power handling capacity of A.C. transmission lines and line losses. 1.One 750kv line can normally carry as much power as four 400kv circuits for equal distance of transmission. 2. The power handling capacity of line at a given voltage level decreases with line length, being inversely proportional to length L.

Firing Angle Control Two basic requirements for the firing pulse generation of HVDC valves The firing instant for all the valves are determined at ground potential and the firing signals sent to individual thyristors by light signals through fiber optics cables. The required get power is made available at the potential individual thyristor for electrically triggered thyristor valves. Howe ever, for light triggered thyristor valves, the light signal can be used to directly fire for individual capacitor.

Cont.. While the signal pulse is adequate to turn on a thyristor, the gate pulse generator must send a pulse whenever required. There are two basic firing schemes • Individual phase control (IPC). • Equidistant pulse control (EPC).

Garetz Circuit:

Assumptions made for analysis : Without overlap

With overlap

Analysis of Mode-1

Analysis of mode-2

RGPV QUESTIONS

RGPV QUESTIONS