1. HVDC TRANSMISSION Subject code:10EE751
Course by
A.Velu
Assistant Professor
Department of Electrical and Electronics Engineering

2. Text Books:
Direct current Transmission by EDWARD WILSON KIMBARK(Wiley interscience, New york,1971).
High Voltage D.C.Power Transmission system by K.R.PADIYAR IISc Bangalore, New Age International Publishers Ltd.

3. Overview:
General Aspects of DC transmission and comparison of it with AC transmission.
Converter Circuits
Analysis of the Bridge converter
Control of HVDC Converters and Systems.
Protection.

4. PART A (Unit 1 & 2)
General Aspects of DC Transmission and comparison of it with AC Transmission
Historical Sketch
Constitution of EHV AC and DC links.
Limitations and Advantages of AC and DC transmission.

5. Historical Sketch:
Evolution of Power Systems: Late 1870s- Commercial use of electricity. In First Electric power system which includes Generator, Cable,fuse,Load designed by Thomas Edison at Pearl Street station in New york. It was DC System (Low Voltage 110V),underground cable is used to distribute the power to consumers. Only 59 consumers are benefited by this low voltage DC system. Incandescent lamps are used as a load. In 1884-Motors were developed by Frank Sprague. After the invention of motors electricity is used more effectively or it was appreciated. In Limitation of DC High losses and Voltage Drop Transformation of Voltage required.

6. Continues…
Transformer and AC distribution (150 lamps) developed by William Stanley of Westing house.
In First AC transmission system in USA between Willamette falls and Portland, Oregon. It was 1-Phase,4KV,Over 21 Km.
Before that in the year of 1888-N.Tesla developed Poly Phase system and had patents of Generator,Motor,Transformer, transmission lines. Later Westing House bought it.
In 1890-Controversy on whether industry should standardize AC or DC.
Edison-DC System Westing House-AC System
Later because of features of AC System, its dominated
Voltage increase is possible
Simpler and cheaper generators and motors.

7. Continues… In 1893-First 3-Phase line ,2.3KV,12 Km in California .
Improvement in voltages year by year, KV KV KV KV KV KV KV KV

8. Continues…
Standard voltages are 115,138,161,230KV preferred for High Voltage (HV)lines.
Remaining 345,500,765KV are Extra High Voltage(EHV) lines.
For interconnection of AC systems, We need fixed frequency.
60Hz-US and Canadian countries
50Hz-Europe and Asian countries

9. Entry of HVDC system:
HVDC transmission was designed by a French Engineer, RENE THURY. Simultaneously AC system was also developed slowly.
In between ,atleast 11 Thury system were installed in Europe. The prominent was Mouteirs to Lyons(France) in It comprises 180Km(4.5 km underground cable),4.3MW,57.6KV,75A.
Features :
DC series generators were used.
Constant control current mode.

10. Continues….
In1920-Transverter(Mechanicalconverter-polyphase transformer)were developed. Again AC system dominated.
In 1938-All the Thury system were dismantled. Because in DC system, we need frequent maintenance , cost also is not effective.
Again AC revolution back till In the year of 1950, Mercury arc valves (Bulky converter) it was possible to convert AC to DC.
In 1954, first HVDC System between Sweden and Gotland island was commissioned by cable. Conversion carried out by Mercury arc rectifier. Again people think about DC transmission because of the limitation in AC system.

11. Limitations of HVAC Reactive power loss Stability
Current carrying capacity
Skin and Ferranti effect
Power flow control is not possible.

12. 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

13. Continues… 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

14. 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.

15. Comparison of AC and DC Transmission
The relative merits of the two modes of transmission(AC and DC) which need to considered by a system planner are based on the following factors:
Economics of Transmission
Technical performance
Reliability
A major feature of power systems is the continuous expansion necessitated by increasing power demand .
This implies that the establishment of a particular line must be consider as a part of an overall long term system planning.

16. Economics of power transmission:
The cost of transmission line includes the investment and operational costs.
Investment cost includes,
Right of way
Transmission towers
Conductors
Insulators
Terminal equipment
Operational costs includes
It mainly due to cost of losses

17. Right of Way(RoW):
An electric transmission line right-of-way (ROW) is a strip of land used by Electrical utilities to construct, operate, maintain and repair the transmission line facilities.
Rights of way may also include the purchase of rights to remove danger trees. A danger tree is a tree outside the right of way but with the potential to do damage to equipment within the right of way. If the danger tree falls or is cut down, it could strike poles, towers, wires, lines, appliances or other equipment and disrupt the flow of electricity to our customers.

18. Images for (RoW)

19. Continues…

21. Continues…
This Implies that for a given power level, DC lines requires less RoW, Simpler , and cheaper towers and reduced conductors and insulator costs.
The power losses are also reduced with DC as there are 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.
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.

22. Continues…
The other factors that influence the line cost are the cost of compensation and terminal equipment.
In dc lines do not require compensation but the terminal equipment costs are increased due to the presence of converters and filters.

23. Variation of cost with line length:

24. Description:
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.

25. 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.
Advantages:
Full control over power transmitted.
The ability to enhance transient and dynamic stability in associated AC networks.
Fast control to limit fault currents in DC lines. This makes it feasible to avoid DC breakers in two terminal DC links.

26. Continues… 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 as a function of distance.
But in DC lines which is unaffected by the distance of transmission.

27. Power transfer capability Vs. Distance

28. Continues… VOLTAGE CONTROL
The voltage control in AC lines is complicated by line charging and inductive voltage drops.
The voltage profile in a AC line 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 mid point voltage is reduced for line loading higher than SIL and increased for loadings less than SIL.

29. Variation of Voltage along the line:

30. Continues… 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.

31. A whole picture of FACTS devices family:

32. Continues… 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

33. Continues…
The controllability of power flow in DC lines eliminates all the above problem. In addition, for asynchronous DC ties, there is no need of coordinated control.
It is obvious that two systems which have different nominal frequencies cannot be interconnected directly with AC ties and require the use of DC links.

34. Continues…
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( Monopolar 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.

35. Reliability:
The reliability of DC transmission is quite good and comparable to that 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.

36. Continues… There are two measures of overall system reliability
Energy availability
Transient reliability

37. Energy availability:
Equivalent outage time is the product of the actual outage time and the fraction of system capacity lost due to outage.

38. Transient reliability:
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.

39. HVDC outage statistics:
The average failure rate of thyristors in a valve is less than 0.6% per operating year. The maintenance of thyristor valves is also much simpler than the earlier mercury arc valves.

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

41. 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.

42. 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

43. 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.

44. 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

45. 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

46. 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. .

47. Principle parts of HVDC Transmission:

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

49. 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) .

50. Various Thyristor Ratings:

51. 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.

52. 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.

53. 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

54. 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

55. 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.

56. 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.

57. 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.

58. 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.

59. 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.

60. 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.

61. 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.

62. 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)

63. 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.

64. 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

65. 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.

66. 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.

67. 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.