Introduction to HVDC

High voltage dc transmission (HVDC) Basics components, comparison between ac and dc transmission,

1. UNIT-1 HVDC System Configuration and Components

2. Transmission System An efficient transmission system has to meet the following requirements: Bulk power transmission over long distances, Low transmission losses. Less voltage fluctuations. Possibility of power transfer through submarine cables. System of interconnection.

3. Transmission System Up to the 1980s, ultra high voltage ac (UHV-AC) transmission lines above 765 kV were used for bulk power transmission, and due to the development of accurate control in thyristor , the HVDC (high voltage direct current) transmission lines are using which are having a distinct superiority over UHV-AC transmission lines.

4. What is High Voltage DC (HVDC) Transmission System? The High Voltage Direct Current (HVDC) transmission system uses direct current for the transmission of power over long distances . The HVDC transmission system provides efficient and economic transmission of power even to very long distances that meet the requirements of growing load demands . Due to its simple constructional feature and less complexity, research and development authority discovered its usage in modern power transmission.

5. Principle of HVDC Transmission The HVDC transmission system mainly consists of converter stations where conversions from ac to dc (rectifier station) are performed at sending end and at the receiving end the dc power is inverted into ac power using an inverter station. Hence, the converter stations are the major component of the HVDC transmission system . Also, by changing the role of the rectifier to inverter and inverter to rectifier the power transfer can be reversed which can be achieved by suitable converter control. The below shows the schematic diagram of the HVDC transmission system.

6. Principle of HVDC Transmission The ac substations at both ends of the HVDC line consist of ac switchgear, bus bars, current transformers, voltage transformers, etc. The converter transformers are connected between converter values and ac bus valves which transfers power from ac to dc or vice-versa. Smoothing reactors are necessary for converter operation, and for smoothing the dc current by reducing ripples obtained on the dc line . The electrode line connects the midpoint of converters with a distant earth electrode.

7. Comparison Between HVDC and HVAC System HVDC Transmission System It is economical for transmission of power above break-even point i.e., for long distances . The number of conductors required for transmitting power is less . Does not require any intermediate substations for reactive power compensation. HVAC Transmission System It is economical for transmission of power below break-even point i.e., for small distances . The number of conductors required for transmitting power is more . Requires intermediate substations for compensation.

8. Comparison Between HVDC and HVAC System HVDC Transmission System Very fast and accurate power flow control is possible . Skin effect is absent resulting in uniform distribution of current density across the cross-section of the conductor . Corona loss and radio interferences are absent resulting in less insulation level required for the transmission line. HVAC Transmission System Power flow control is slow and I Skin effect is present due to which current density is non-uniformly distributed across the cross-section f the conductor. s very difficult . Corona loss and radio interferences are more due to which high insulation level is required for the transmission line.

9. Comparison Between HVDC and HVAC System HVDC Transmission System Voltage in the line does not fluctuate with the load. Does not require a double circuit, in this systems earth return is used . Transmission through underground or marine is also possible . Transmission losses are less due to the absence of flow of reactive power. HVAC Transmission System Voltage in the line fluctuates with the load . Always requires a double circuit . Limit is imposed on the length of the cable . Transmission losses are more due to the flow of reactive power .

10. Comparison Between HVDC and HVAC System HVDC Transmission System The fault levels of the two networks are unaffected and remain unchanged when interconnected . The cost of right of way is less and the cost of supporting towers is less, as this system requires narrow towers . DC breakers used in this system are of high cost, as it is difficult to break dc currents. HVAC Transmission System Fault levels of two networks get added up and are increased after the interconnection . The cost of right of way is more and the cost of supporting tower is more as this system requires lattice-shaped towers . The circuit breakers used in this system are of low cost when compared to dc breakers.

11. Components of an HVDC Transmission System The essential components in a HVDC transmission system are 6/12/24 pulse converters, converter transformer with suitable ratio and tap changing, filters at both DC and AC side, smoothening reactor in DC side, shunt capacitors and DC transmission lines.

12. Converter Unit HVDC transmission requires a converter at each end of the line. The sending end converter acts as a rectifier which converts AC power to DC power and the receiving end converter acts as an inverter which converts DC power to AC power. This unit usually consists of two three phase converter which are connected in series to form a 12 pulse converter. The converter consists of 12 thyristor valves and these valves can be packaged as single valve or double valve or quadri valve arrangements. Due to the evaluation of power electronic devices, the thyristor valves have been replaced by high power handling devices such as gate turn-off thyristors (GTOs), IGBTs and light triggered thyristors .

13. Converter Transformers The transformers used before the rectification of AC in HVDC system are called as converter transformers. The different configurations of the converter transformer include three phase- two winding, single phase- three winding and single phase- two winding transformers. The valve side windings of transformers are connected in star and delta with ungrounded neutral and the AC supply side windings are connected in parallel with grounded neutral. The design of the control transformer is somewhat different from the one used in AC systems . These are designed to withstand DC voltage stresses and increased eddy current losses due to harmonic currents. The content of harmonics in a converter transformer is much higher compared to conventional transformer which causes additional leakage flux and it results to the formation of local hotspots in windings. To avoid these hotspots, suitable magnetic shunts and effective cooling arrangements are required.

14. Filters Due to the repetitive firing of thyristors , harmonics are generated in the HVDC system. These harmonics are transmitted to the AC network and led to the overheating of the equipment and also interference with the communication system. In order to reduce the harmonics, filters and filtering techniques are used. Types of filters include: AC filter, DC filter and High frequency filter

15. AC filters These are made with passive components and they provide low impedance and shunt paths for AC harmonic currents. Tuned as well as damped filter arrangements are generally used in HVDC system. DC filters Similar to AC filters, these are also used for filtering the harmonics. Filters used at DC end, usually smaller and less expensive than filters used in AC side. The modern DC filters are of active type in which passive part is reduced to a minimum. Specially designed DC filters are used in HVDC transmission lines in order to reduce the disturbances caused in telecommunication systems due to harmonics. High frequency filters These are provided to suppress the high frequency currents and are connected between converter transformer and the station AC bus. Sometimes these are connected between DC filter and DC line and also on the neutral side.

16. Reactive Power Supplies (Shunt capacitors) Due to the delay in the firing angle of the converter station, reactive volt-amperes are generated in the process of conversion. Since the DC system does not require or generate any reactive power, this must be suitably compensated by using shunt capacitors connecting at both ends of the system.

17. Smoothening reactor These are large reactors having high inductance as high as 1 H connected in series with each pole of converter station. It can be connected on the line side, neutral side or at an intermediate location They serve the following purpose: Decrease harmonic voltages and currents in DC line. Prevent commutation failure in inverters. Prevent current from being discontinuous at light load. Limit the crest current in the rectifier during the short circuit in DC line.

18. Transmission medium or lines or cables Overhead lines act as a most frequent transmission medium for bulk power transmission over land. Two conductors with different polarity are used in HVDC systems to transfer the power from sending end to receiving end. The size of the conductors required in DC transmission is small for the same power handling capacity to that of AC transmission. Due to the absence of frequency, there is no skin effect in the conductors. High voltage DC cables are used in case of submarine transmission. Most of such cables are of an oil filled type. Its insulation consists of paper tapes impregnated with high viscosity oil.

19. DC and AC switchgear The switchgear equipment provides the protection to the entire HVDC system from various electrical faults and also gives the metering indication. The switchgear equipments include isolator switches, lightening arrestors, DC breakers, AC breakers, etc.

20. Types of HVDC Transmission Systems The HVDC transmission systems are mainly classified into the following types on the basis of arrangement of the pole (line) and earth return. They are: Mono-polar HVDC System - An HVDC system having only one pole and earth return. Bipolar HVDC System - An HVDC system with two poles of opposite polarity. Homo-polar HVDC System - It has two poles of the same polarity and earth return. Back to Back HVDC Coupling System - It has no dc transmission line. The rectification and inversion are taken place at the same substation by a back-to-back converter. Multi-Terminal HVDC Systems - It has three or more terminal substations.

21. Mono-polar HVDC System An HVDC link that uses only a single conductor is known as a mono-polar link . Usually, in this type of link, only a single conductor with negative polarity is used, in order to reduce corona and interference . Earth or water is used as the return path. However, a metallic conductor is used as a return path when earth resistivity is very high. The power and current flows only in one direction . For mono-polar transmission systems, the rated current is from 200A to 1000A. The below figure represents the mono-polar HVDC link.

22. Advantages and Disadvantages of Mono-polar Link Advantages of Mono-polar Link : It uses only a single conductor. Hence, the design is very simple. It requires less maintenance. Because of high charging currents, these links are technically feasible than HVAC systems. It is economical. Disadvantages of Mono-polar Link : When a fault occurs on the conductor the entire transmission system is shut down. These are used only for low-power rating links, like cable transmission. It affects the magnetic compasses of ships when it passes over underwater cables.

23. Bipolar HVDC Transmission System An HVDC link that uses two conductors for transmitting the power and current is known as bipolar links. Generally, these type of systems uses two conductors. One with positive polarity and the other with negative polarity. 

24. Bipolar HVDC Transmission System Under normal conditions, the current in the two poles is the same. Hence, the ground current is absent. Whenever a fault occurs on these systems then they automatically switch to the mono-polar system by using earth as a return path conductor i.e., when one pole undergoes fault condition, the other will continue to supply the load . A single bipolar high voltage direct current line is equal to two ac transmission lines. When compared to the mono-polar link the voltage is twice between the poles in this system. The mid-point of the converters are grounded.

25. Advantages of Bipolar HVDC Link The transmission of power between two stations or on the mainline is continuous. The fault on one link does not affect the operation of another link. During fault conditions, this link can also be used as the monopolar link. The direction of power flow can be changed by changing the polarities of the two poles. The voltage in the bipolar link is twice between the poles when compared to the voltage between the pole and the earth of a monopolar link.

26. Disadvantages of Bipolar HVDC Link Corona and radio interference is more when compared with a homo-polar link. The connection of a converter to a pole is complicated. It is quite costly when compared to mono-polar links.

27. Homo-polar HVDC Transmission System These links also use two conductors but of the same polarity, usually of negative polarity . When a fault occurs on the conductor the converters of the healthy pole are quite enough to feed the remaining conductors, Which are able to supply more than 50% of the power. In this type of link, the earth is used as a return conductor. It also acts as a mono-polar link during faulty conditions.

28. Advantages of Homo-polar HVDC Link It is comparatively cheaper than a three-phase ac line of the same ratings. Corona and radio interference are greatly reduced with the use of negative polarity conductors. These links can be operated independently under faulty conditions. The connection of the converter to the pole is not so complicated as the bipolar link.

29. Disadvantages of Homo-polar HVDC Link The presence of ground current may have an adverse effect on the pipelines passing through the nearby areas. It has limited applications due to the presence of ground currents. The cost of the line increases for higher voltages.

30. Back to Back HVDC Coupling System It has no dc transmission line. Rectification and Inversion are done in the same substation by a back-to-back converter. The figure below shows the back-to-back HVDC coupling. For example, the Vindhyanchal back-to-back system in India, which has a capacity of 250MW is capable of transmitting and receiving power in between Uttar Pradesh and Madhya Pradesh power grids i.e., from the northern region to the western region.

31. Back to Back HVDC Coupling System The back-to-back HVDC coupling is mainly used to interconnect two ac networks operating at different frequencies. It also provides features like improving system stability, rapid variations in the power exchange, and control over the magnitude of voltage and frequency independently in two networks.

32. Back to Back HVDC Coupling System The dc voltage between two converters can be freely selected because of the short length of the conductor . A back-to-back system gives or provides more stability for the system. The power can be transmitted very fast and accurately. The power flow can be controlled in a system by controlling the magnitude and direction of power in a network. By using these types of systems the power can be transmitted from one station to another station or it can be received from the other terminals i.e., these systems possess the ability to receive or transmit power from the same station itself .

33. Advantages of Back to Back HVDC System The voltage and frequency can be controlled independently in two networks. The power flow is fast, accurate, and fully controllable. We can determine the power flow in the link. Short circuit levels can be limited. Coupling of two networks at different frequencies. Daily and seasonal costs can be determined.

34. Disadvantages of Back to Back HVDC System Harmonics are generated. These systems are very expensive because of complicated converters and dc switchgear. When the system is nearer to the sea coast, water gets contaminated with insulators.

35. Multi-Terminal HVDC System A multi-terminal HVDC system consists of three or more converter substations in which some of the converter stations act as the rectifiers and some of them as the inverters. The substations are either connected in series or parallel according to the requirements. The below shows the bipolar multi-terminal HVDC system.

36. Multi-Terminal HVDC System The multi-terminal HVDC system configuration consists transmission line and more than two converters connected in parallel or in sequential. In this multi-terminal HVDC configuration, the power is transmitting between two or more AC substations and the frequency conversion is possible in this configuration.

37. Advantages of HVDC Transmission System The HVDC transmission requires narrow towers, whereas ac systems require lattice shape towers, this makes the construction simple and reduces cost. The ground can be used as the return conductor. No charging current, since dc operates at unity power factor. Due to less corona and radio interference, it results in an economic choice of the conductor. Since there is no skin effect in dc transmission the power losses are reduced considerably. Large or bulk power can be transmitted over long distances. Synchronous operation is not required. Low short-circuit current on dc line. Tie-line power can be easily controlled. Power transmission can be also possible between unsynchronised ac distribution systems (interconnection of ac systems of different frequencies). Cables can be worked at a high voltage gradient, which makes them more suitable for undersea cables. Power flow through the HVDC line can be quickly controlled.

38. Disadvantages of HVDC Transmission System It is very difficult to break the dc currents hence it requires a high cost of dc circuit breakers. It is not possible to use transformers to change the voltage levels. Due to the generation of harmonics in converters, it requires ac & dc filters, hence the cost of converting station is increased. It requires continuous firing or triggering thyristor valves hence is it is complex. Converters have little overload capability. HVDC substations have an additional loss at converter transformers and valves. These losses are continuous.

39. Applications of HVDC Transmission System Long-distance bulk power HVDC transmission by overhead lines. Underground or underwater cables. Interconnection of ac systems operating at different frequencies.Back -to-back HVDC coupling stations. MTDC asynchronous interconnection between 3 or more ac networks. Control and stabilization of power flow in ac interconnection of large interconnected systems.

40. Application1: Interconnection of two AC systems DC link is an economical option than the AC link to interconnect two AC systems. This system is more effective, efficient and technically superior compared to the AC link. The biggest advantage is the there is no effect of frequency in the DC link. And the frequency disturbance of one system does not transfer to other systems. It does not affect the transient stability and there is no change in the short circuit levels of both the systems. The direction of power flow maintains properly through the DC link.

41. Application2: Long-distance power transmission line This is the main purpose to use the HVDC system. Because in the HVAC system, the length of a line is the biggest constraint. The length of the line cannot more than a certain length to keep control of the thermal effect of the conductor. And it needs an intermediate substation every 300 km of line. But this problem solved by the use of the HVDC line. In the HVDC line, the generated AC power is stepped up by the transformer. The high voltage AC converted in High voltage DC with the help of a rectifier at sending end of the line. Power transmitted to long-distance with the help of the HVDC line. To transmit more power bipolar HVDC system used. At the receiving end, high voltage DC power converted into high voltage AC with the help of an inverter. The HVDC line is economical only for long-distance. The breakeven point is at 800 km. hence, at 800 km of line, the HVDC line is more economical than the HVAC line. And there is no need to build an intermediate substation in between of lines irrespective the length of a line. The cost of the tower and the conductor is less in the HVDC line.

42. Application3: Multi-terminal HVDC interconnection The frequency does not affect in DC system. Therefore, if the frequency is not the same, then also these systems can connect with the HVDC link. Three or more AC systems can be interconnected asynchronous using a multi-terminal HVDC system. Due to this, bulk power can be transferred.

43. Application4: Parallel AC and DC link DC link operates with the parallel to the existing AC line. In this way, more amount of power can transmit. Due to this, there is a decrease in the fault level and an increase in the stability of the system.

44. Application5: Underground or submarine cable transmission In the AC system, it is difficult to transmit power through underground cable or submarine cable because of the temperature rise due to the charging current. This will limit the length of the line. But this problem solved in the HVDC line as an absence of the charging current. Therefore, it is easy to implement the underground and submarine cable with the HVDC transmission line.

45. Application6: Back to back asynchronous tie station If two tie lines have different frequencies than it tends to not possible to interconnect. Therefore, back to back asynchronous tie station becomes very useful for the interconnection of two AC systems which has different frequencies. For example. One tie line has 50 Hz and the other has 60 Hz frequency. (Generally, is much of frequency difference will not create in the same country. But tie lines which connect the different countries which use different frequencies.) The interconnection of these tie lines can be done by the HVDC system. This cannot be done by the AC system. The converter substation used to connect two asynchronous AC systems. There is no DC transmission line used. The two AC lines connected through back to back converters. Power flow can easily control from one system to other systems. Smoothing reactor, filters and converter transformer used in this station.

46. Analysis of HVDC Converters Introduction: HVDC converters converts AC to DC and transfer the DC power, then DC is again converted to AC by using inverter station. HVDC system mainly consists of two stations, one in rectifier station which transfers from AC to DC network and other is inverter station which transfers from DC to AC network. For all HVDC converters twelve pulse bridge converters are used. Same converter can act as both rectifier as well as an inverter depending on the firing angle ‘α’. If firing angle α is less than 90 degrees the converter acts in rectifier mode and if the firing angle α is greater than 90 degrees the converter acts in inverter mode.

47. Choice of Converter configuration

48. Choice of Converter configuration For a given pulse number select the configuration such a way that both the valve and transformer utilization are minimized. In general converter configuration can be selected by the basic commutation group and the no. of such groups connected in series and parallel. Commutation group means set of valves in which only one valve conducts at a time. Let ‘q’ be the no of valves in a commutation group, ‘r’ be the no of parallel connections, ‘s’ be the no of series connections, then the total no of valves will be = qrs

49. Choice of Converter configuration Valve Voltage Rating: Valve voltage rating is specified in terms of peak inverse voltage (PIV) it can withstand. The valve utilization is the ratio of PIV to average dc voltage. Converter average DC voltage is

50. Choice of Converter configuration i ) Peak inverse voltage(PIV): If q is even: then the maximum inverse voltage occurs when the valve with a phase displacement of π radian in conducting and this is given by PIV = 2Vm If q is odd: the maximum inverse voltage occurs when the valve with a phase shift of π+π /q in conducting and this is given by PIV = 2Vm Cosπ /2q

51. ii) Utilization factor:

52. Analysis of Graetz circuit (6-pulse converter bridge )

53. This Graetz circuit utilizes the transformer and the converter unit to at most level and it maintains low voltage across the valve when not in conduction. Due to this quality present in Graetz circuit, it dominates all other alternative circuits from being implemented in HVDC converter. The low voltage across the valves is nothing but the peak inverse voltage which the valve should withstand . The six-pulse Graetz circuit consists of 6 valves arranged in bridge type and the converter transformer having tapings on the AC side for voltage control. AC supply is given for the three winding of the converter transformer connected in star with grounded neutral. The windings on the valve side are either connected in star or delta with ungrounded neutral. The six valves of the circuit are fired in a definite and fixed order and the DC output obtained contains six DC pulses per one cycle of AC voltage wave.

54. Operation without overlap The six pulse converter without over lapping valve construction sequence are 1-2, 2-3, 3-4, 4-5, 5-6 , 6-1 . At any instant two valves are conducting in the bridge. One from the upper group and other from the lower group. Each valve arm conducts for a period of one third of half cycle i.e., 60 degrees. In one full cycle of AC supply there are six pulses in the DC waveform. Hence the bridge is called as six pulse converter.

55. Operation without overlap For simple analysis following assumptions are much: i ) AC voltage at the converter input is sinusoidal and constant ii) DC current is constant iii) Valves are assumed as ideal switches with zero impedance when on(conducting) and with infinite impedance when off(not conducting) In one full cycle of AC supply we will get 6-pulses in the output. Each pair of the devices will conduct 60 degrees. The dc output voltage waveform repeats every 60 degrees interval . Therefore for calculation of average output voltages only consider one pulse and multiply with six for one full cycle. In this case each device will fire for 120 deg.

56. Firing angle delay Delay angle is the time required for firing the pulses in a converter for its conduction. It is generally expressed in electrical degrees. In other words, it is the time between zero crossing of commutation voltage and starting point of forward current conduction. The mean value of DC voltage can be reduced by decreasing the conduction duration, which can be achieved by delaying the pulses ie ., by increasing the delay angle we can reduce the DC voltage and also the power transmission form one valve to another valve can also be reduced. when α = 0, the commutation takes place naturally and the converter acts as a rectifier. when α > 60 deg, the voltage with negative spikes appears and the direction of power flow is from AC to DC system without change in magnitude of current. when α = 90 deg, the negative and positive portions of the voltage are equal and because of above fact , the DC voltage per cycle is zero. Hence the energy transferred is zero. when α > 90 deg, the converter acts as an inverter and the flow of power is from DC system to AC system .

57. Let valve 3 is fired at an angle of α. The DC output voltage is given by From above equation we can say that if firing angle varies, the DC output voltage varies

58. DC Voltage waveform The dc voltage waveform contains a ripple whose frequency is six times the supply frequency. This can be analyzed in Fourier series and contains harmonics of the order h= np , Where p is the pulse number and n is an integer. The r.m.s value of the hth order harmonic in dc voltage is given by

59. Although α can vary from 0 to 180 degrees, the full range cannot be utilized. In order to ensure the firing of all the series connected thyristors , it is necessary to provide a minimum limit of α greater than zero , say 5 deg. Also in order to allow for the turn off time of a valve, it is necessary to provide an upper limit less than 180 deg. The delay angle α is not allowed to go beyond 180-γ where γ is called the extinction angle ( sometimes also called the marginal angle). The minimum value of the extinction angle is typically 10 deg, although in normal operation as an inverter , it is not allowed to go below 15deg or 18deg.

60. AC current waveform: It is assumed that the direct current has no ripple (or harmonics) because of the smoothing reactor provided in series with the bridge circuit. The AC currents flowing through the valve (secondary) and primary windings of the converter transformer contain harmonics . The waveform of the current in a valve winding is shown in fig.

61. By Fourier analysis, the peak value of a line current of fundamental frequency component is given by,

62. Now the RMS value of line current of fundamental frequency component is given by where I = Fundamental current n = nth order harmonic. The harmonics contained in the current waveform are of the order given by h = np + 1 where n is an integer, p is the pulse number. For a 6 pulse bridge converter, the order of AC harmonics are 5, 7, 11, 13 and higher order. They are filtered out by using tuned filters for each one of the first four harmonics and a high pass filter for the rest.

63. The Power Factor