Current Source and Voltage Source Converters
Traditionally high power h.v.d.c. schemes have used thyristors with the d.c. current
always flowing in the same direction (current source converters). These are used
extensively for point-point transmission of bulk power and are available at d.c. voltage
ratings of up to ±800 kV and are able to transmit 6500 MW over a single overhead
Figure 9.2 Commonly used converters for h.v.d.c. schemes
h.v.d.c. link. These schemes use converters employing thyristors and a large inductor is connected on the d.c. side. As the inductor maintains the d.c. current more or less constant (other than a small ripple) these converters are called current source converters (CSCs) (Figure 9.2(a)). In these converters while the thyristors are triggered on by a gate pulse, they turn off when the current through them falls to zero. Thus current source converters are also called line (or naturally) commutated converters.
Current source converter h.v.d.c. has a number of advantages. It can be made up to very high power and d.c. voltage ratings and the thyristors are comparatively robust with a significant transient overload capability. As the thyristors switch off only when the current through them has dropped to zero, switching losses are low.
However, it suffers from a number of disadvantages. Both the rectifier and inverter always draw reactive power from the a.c. networks and a voltage source, usually synchronous generation, is required at each end of the d.c. link to ensure commutation of the valves. When the a.c. voltage drops, due to a fault on the a.c. network, the valves may stop operating and experience commutation failure. The a.c. current contains significant harmonics and, although these can be reduced by 12 or even 24 pulse connection of the converters, large harmonic filters are required (which also provide some of the reactive power). Conventionally, the filters use open terminal switchgear and air-insulated busbars and so both the valve hall and the filters occupy a large area. Although there have been several three terminal CSC h.v.d.c. schemes constructed, this technology is mainly applied for bulk transfer of power between two stable a.c. power systems.
Recently large transistors called Insulated Gate Bipolar Transistors (IGBTs) have been used in h.v.d.c. systems where the d.c. voltage is always the same polarity and the current reverses to change the direction of power flow. These schemes are used in underground and submarine cable links of up to 1000 MW. The converters used for these schemes have a large capacitor on the d.c. side thus maintaining the d.c. voltage more or less constant. They are referred to as voltage source converters (VSCs) (Figure 9.2(b)). As IGBTs can be turned on and off by their gate voltage, these converters are also called forced commutated converters.
- 3. have fast acting control;
- 4. can use voltage polarized cables;
- 5. produce good sine wave-shapes in the a.c. networks and thus use small filters; The main disadvantages of VSC h.v.d.c. schemes are that:
- 1. presently their rating is very much lower than CSC h.v.d.c. schemes.
- 2. their power losses are higher.