An HVDC converter converts electric power from high voltage alternating current (AC) to high-voltage direct current (HVDC), or vice versa. HVDC is used as an alternative to AC for transmitting electrical energy over long distances or between AC power systems of different frequencies.Arrillaga, Jos; High Voltage Direct Current Transmission, second edition, Institution of Electrical Engineers, , 1998, Chapter 1, pp 1-9. HVDC converters capable of converting up to two gigawatts (GW)Davidson, C.C., Preedy, R.M., Cao, J., Zhou, C., Fu, J., Ultra-High-Power Thyristor Valves for HVDC in Developing Countries, IET 9th International Conference on AC/DC Power Transmission, London, October 2010. and with voltage ratings of up to 900 kilovolts (kV)Skog, J.E., van Asten, H., Worzyk, T., Andersrød, T., Norned – World's longest power cable, CIGRÉ session, Paris, 2010, paper reference B1-106 . have been built, and even higher ratings are technically feasible. A complete converter station may contain several such converters in series and/or parallel to achieve total system DC voltage ratings of up to 1,100 kV.
Almost all HVDC converters are inherently bi-directional; they can convert either from AC to DC (rectification) or from DC to AC (inversion). A complete HVDC system always includes at least one converter operating as a rectifier (converting AC to DC) and at least one operating as an inverter (converting DC to AC). Some HVDC systems take full advantage of this bi-directional property (for example, those designed for cross-border power trading, such as the Cross-Channel link between England and France).Rowe, B.A., Goodrich, F.G., Herbert, I.R., Commissioning the Cross Channel h.v.d.c. link, GEC Review, Vol. 3, No. 2, 1987. Others, for example those designed to export power from a remote power station such as the Itaipu scheme in Brazil,Praça, A., Arakari, H., Alves, S.R., Eriksson, K., Graham, J., Biledt, G., Itaipu HVDC Transmission System - 10 years operational experience, V SEPOPE, Recife, May 1996. may be optimised for power flow in only one preferred direction. In such schemes, power flow in the non-preferred direction may have a reduced capacity or poorer efficiency.
Types of HVDC converters HVDC converters can take several different forms. Early HVDC systems, built until the 1930s, were effectively rotary converters and used electromechanical conversion with motor-generator sets connected in series on the DC side and in parallel on the AC side. However, all HVDC systems built since the 1940s have used electronic (static) converters.
Electronic converters for HVDC are divided into two main categories. Line-commutated converter (HVDC classic) are made with electronic switches that can only be turned on. Voltage-sourced converters are made with switching devices that can be turned both on and off. Line-commutated converters (LCC) used mercury-arc valves until the 1970s,Peake, O., The History of High Voltage Direct Current Transmission , 3rd Australasian Engineering Heritage Conference 2009 or thyristors from the 1970s to the present day. Voltage-source converters (VSC), which first appeared in HVDC in 1997,Asplund, G., Svensson, K., Jiang, H., Lindberg, J., Pålsson, R., DC transmission based on voltage source converters, CIGRÉ session, Paris, 1998, paper reference 14-302. use transistors, usually the Insulated-gate bipolar transistor (IGBT).
As of 2012, both the line-commutated and voltage-source technologies are important, with line-commutated converters used mainly where very high capacity and efficiency are needed, and voltage-source converters used mainly for interconnecting weak AC systems, for connecting large-scale wind power to the grid or for HVDC interconnections that are likely to be expanded to become Multi-terminal HVDC systems in future. The market for voltage-source converter HVDC is growing fast, driven partly by the surge in investment in offshore wind power, with one particular type of converter, the Modular Multi-Level Converter (MMC)Lesnicar, A., Marquardt, R., An innovative modular multi-level converter topology for a wide power range, IEEE Power Tech Conference, Bologna, Italy, June 2003. emerging as a front-runner.
Electromechanical converters As early as the 1880s, the advantages of DC long-distance transmission were starting to become evident and several commercial power transmission systems were put into operation. The most successful of these used the system invented by René Thury and were based on the principle of connecting several motor-generator sets in series on the DC side. The best-known example was the 200 km, Lyon–Moutiers DC transmission scheme in France, which operated commercially from 1906 to 1936 transmitting power from the Moutiers hydroelectric plant to the city of Lyon.Black, R.M.,The History of Electric Wires and Cable, Peter Peregrinus, London, 1983, , p 95 KimbarkKimbark, E.W., Direct current transmission, volume 1, Wiley Interscience, 1971, pp3–4. reports that this system operated quite reliably; however, the total end to end efficiency (at around 70%) was poor by today's standards. From the 1930s onwards, extensive research started to take place into static alternatives using gas-filled tubes – principally mercury-arc valves but also thyratrons – which held the promise of significantly higher efficiency. Very small mechanical rotary convertors remained in use for niche applications in adverse environments, such as in aircraft and vehicles, as a power conversion method from batteries to the high voltages required for radio and RADAR, until the 1960s and the transistor era.
Line-commutated converters Most of the HVDC systems in operation today are based on line-commutated converters (LCC). The term line-commutated indicates that the conversion process relies on the line voltage of the AC system to which the converter is connected in order to effect the commutation from one switching device to its neighbour.Arrillaga, Jos; High Voltage Direct Current Transmission, second edition, Institution of Electrical Engineers, , 1998, Chapter 2, pp 10-55. Line-commutated converters use switching devices that are either uncontrolled (such as diodes) or that can only be turned on (not off) by control action, such as thyristors. Although HVDC converters can, in principle, be constructed from diodes, such converters can only be used in rectification mode and the lack of controllability of the DC voltage is a serious disadvantage. Consequently, in practice all LCC HVDC systems use either grid-controlled mercury-arc valves (until the 1970s) or thyristors (to the present day).
In a line-commutated converter, the DC current does not change direction; it flows through a large inductance and can be considered almost constant. On the AC side, the converter behaves approximately as a current source, injecting both grid-frequency and harmonic currents into the AC network. For this reason, a line-commutated converter for HVDC is also considered as a current-source converter. Because the direction of current cannot be varied, reversal of the direction of power flow (where required) is achieved by reversing the polarity of DC voltage at both stations.
Line-commutated six-pulse bridge The basic LCC configuration for HVDC uses a three-phase Graetz bridge rectifier or six-pulse bridge, containing six electronic switches, each connecting one of the three phases to one of the two DC terminals.Kimbark, E.W., Direct current transmission, volume 1, Wiley Interscience, 1971, pp 71–128. A complete switching element is usually referred to as a valve, irrespective of its construction. Normally, two valves in the bridge are conducting at any time: one to a phase on the top row and one (from a different phase) on the bottom row. The two conducting valves connect two of the three AC phase voltages, in series, to the DC terminals. Thus, the DC output voltage at any given instant is given by the series combination of two AC phase voltages. For example, if valves V1 and V2 are conducting, the DC output voltage is given by the voltage of phase 1 minus the voltage of phase 3.
Because of the unavoidable (but beneficial) inductance in the AC supply, the transition from one pair of conducting valves to the next does not happen instantly. Rather, there is a short overlap period when two valves on the same row of the bridge are conducting simultaneously. For example, if valves V1 and V2 are initially conducting and then valve V3 is turned on, conduction passes from V1 to V3 but for a short period both of these valves conduct simultaneously. During this period, the DC output voltage is given by the average of the voltages of phases 1 and 2, minus the voltage of phase 3. The overlap angle μ (or u) in an HVDC converter increases with the load current, but is typically around 20° at full load.
During the overlap period, the output DC voltage is lower than it would otherwise be and the overlap period produces a visible notch in the DC voltage. An important effect of this is that the mean DC output voltage decreases as the overlap period increases; hence the mean DC voltage falls with increasing DC current.
The mean DC output voltage of a six-pulse converter is given by:Williams, B.W., Power Electronics - devices, drivers and applications, Macmillan Press, , 1992, pp 287–291.
Where: VLLpeak - the peak value of the line to line input voltage (on the converter side of the converter transformer), α - the firing angle of the thyristor Lc - the commutating inductance per phase Id - the direct current
The firing angle α represents the time delay from the point at which the voltage across a valve becomes positive (at which point a diode would start to conduct) and the thyristors being turned on.Kimbark, E.W., Direct current transmission, volume 1, Wiley Interscience, 1971, p 75. From the foregoing equation, it is clear that as the firing angle increases, the mean DC output voltage decreases. In fact, with a line-commutated converter, the firing angle represents the only fast way of controlling the converter. Firing angle control is used to regulate the DC voltages of both ends of the HVDC system continuously in order to obtain the desired level of power transfer.
The DC output voltage of the converter steadily becomes less positive as the firing angle is increased: firing angles of up to 90° correspond to rectification and result in positive DC voltages, while firing angles above 90° correspond to inversion and result in negative DC voltages.Mohan, N., Undeland, T.M., Robbins, W.P., Power Electronics - converters, applications and design, John Wiley & Sons, , 1995, pp 148-150. However, the firing angle cannot be extended all the way to 180°, for two reasons. Firstly, allowance must be made for the overlap angle μ, and secondly for an additional extinction angle γ which is needed for the valves to recover their ability to withstand positive voltage after conducting current. The extinction angle γ is related to the turn-off time tq of the thyristors. A typical value of γ is 15°. α, γ and μ are inter-related thus:
(in degrees)
Line Commutated Twelve-pulse bridge With a phase change only every 60°, considerable harmonic distortion is produced at both the DC and AC terminals when the six-pulse arrangement is used. Large filtering components are needed to restore the waveforms to sinusoidal. An enhancement of the six-pulse bridge arrangement uses 12 valves in a twelve-pulse bridge. A twelve-pulse bridge is effectively two six-pulse bridges connected in series on the DC side and arranged with a phase displacement between their respective AC supplies so that some of the harmonic voltages and currents are cancelled.