1. HVDC Transmission part 2

2. Types of HVDC links: Monopolar link Bipolar link Homopolar link

3. Monopolar link: Having one conductor (-Ve Polarity) and ground is used as return path. We can operated either in +Ve or –Ve polarity,but usually preferred -Ve polarity in order to reduce the Corona effect. 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.

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

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

6. Application of HVDC: 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. Based on the interconnection, three types of HVDC is possible.  Bulk Power transmission  Back to back connection  Modulation of AC system

7. Purpose of HVDC based on interconnection: Bulk power transmission (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. 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

8. Continues… To provide stability to 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. HVDC is the better option for above cited purposes while compare with its AC system.

9. Principle parts of HVDC Transmission:

10. Various Parts of HVDC transmission: Converters Converter transformers Smoothing reactors Harmonic filters Overhead lines Reactive power source Earth electrodes

11. CONVERTERS Converters are the main part of HVDC system. Each HVDC lines has atleast 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. Note*- Valves (Combinations of several thyristors).

12. Various Thyristor Ratings:

13. Continues… How to achieve required voltage and current ratings? The current rating of converter stations can be increased by putting  Valves in parallel  Thyristors in parallel  Bridges in parallel  Some combinations of above. The voltage ratings of converter stations can be increased by putting  Valves in series  Bridges in series  Combination of above. Bridge converters are normally used in HVDC systems.

14. Main requirement of the Valves are: 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. Smooth control of conducting and non conducting phases.

15. Continues… Two versions of switching converters are feasible depending on whether DC storage device utilized is. An inductor-Current source converter A Capacitor-Voltage source converter. CSC is preferable for HVDC system VSC is preferable for FACTS like STATCOM,SVC,etc

16. Comparison of CSC and VSC: CSCVSC Inductor is used in DC sideCapacitor is used in DC side Constant currentConstant voltage Higher lossesMore efficient Fast accurate controlSlow control Larger and more expensiveSmaller and less expensive More fault tolerant and more reliableLess fault tolerant and less reliable Simpler controlComplex control Easily expandable for in seriesEasily expanded in parallel for increased rating

17. 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. Here entire winding are completely insulated.

18. Continues… 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 star-star 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.

19. SMOOTHING REACTORS: As its name, 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.

20. 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. Filter 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. 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.

21. 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. The basic principle of design of DC overhead lines is almost same as AC lines design such as configurations,towers,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.

22. Reactive power source As such 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 and transient, special designed machines is used.

23. 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 electrode 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. 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 upto 500 meters into the earth.

24. Constitution of EHV AC and DC links: EHV transmission links, superposed on a lower voltage AC networks, or interconnecting two such networks, or connecting distant generating plants to an ac system, are compared as to their principle components and arrangements thereof, according to whether the line operates on AC or DC. Below single line diagram, is single circuit three phase AC line. In such system requires transformer at both ends-step up transformers at the sending end and step down transformer at the receiving end. Most long overhead AC lines require series compensation of part of the inductive reactance.(one bank of series capacitor)

25. Continues… The three phase AC lines cannot be operated, except for a very short time(less than 1 sec) with one or more conductors are open, because such operation causes unbalanced voltages in the AC system and interference in phone telephone lines. Therefore three-pole switching is always used to clear the permanent faults, although such fault may involve in any one conductor. This being so, two parallel three phase circuits required for reliable transmission.

26. Continues… The line itself usually has two conductors, although some lines have only one, the return path being in the earth or sea water or both. At both end of the lines are converters, the components of which are transformers and group of mercury arc valves. The converter at the sending end- Rectifier. The converter at the receiving end-Inverter. Either converter can function as rectifier or inverter, permitting power to be transmitted in either direction. Of course it is preferred for AC line, also has this reversibility. The circuit breaker are installed only on the AC side of the converters. These breakers are not used for clearing faults on the dc line or misoperations of the valves, for these faults

27. Continues… Can be cleared more rapidly by grid control of the valves. However breaker is also required for clearing the faults in transformers or taking the whole DC link out of service. Harmonic filters and shunt capacitors for supplying reactive power to the converters are connected to AC sides of the converter. Large inductance called dc smoothing reactors are connected in series with each pole of the DC line.

28. Continues… If higher reliability is required of a DC line than that provided by two conductors, three or four conductors may be provided. Here one pole of four conductor line is shown with two converters per terminal. The bus-tie switches 1 are normally open. If a permanent fault occurred on the lower conductor, the converters connected to it would be controlled so as to bring the voltage and current on it to zero. Then switches 3 would be opened, isolating the faulted line.

29. Continues… Next the converter voltages would be raised to equality with those of the respective adjacent converters, after which switch 1 would be closed. The capability of all converter would be usable, and the power normally carried by two conductors would then be carried by one.