Voltage Source Converter
Figure 9.26 shows a one-leg voltage source converter. When switch T is on, the voltage at A becomes Vd with respect to N and Vd/2 with respect to O. When switch T2 is on, the voltage at A becomes zero with respect to N and —Vd/2 with respect to O. The two switches use IGBTs that can be switched off as well as on rather than thyristors which turn off only when the current flowing through it drops to zero. If each IGBT is turned on and off for 10 ms, the resulting output waveform is a square wave at 50 Hz.
For three-phase applications, three one-leg converters are placed in each phase as shown in Figure 9.27. This is the same bridge topology of a CSC (shown in Figure 9.11) but with important differences. The d.c. link voltage is held constant with a capacitance rather than the d.c. link current being maintained with an inductance. The connection to the a.c. circuit is through a coupling reactor and so the fundamental operation of the VSC is of a voltage source behind a reactor in a manner similar to that of the synchronous generator described in Chapter 3.
The VSC is normally connected to a transformer to bring the output voltage to be equal to the a.c. system voltage. If the transformer is connected in star, then with
Figure9.26 One-leg VSC
respect to the neutral point the voltage of the a-phase looks the same as VAq of Figure 9.26. Other phase voltages will be phase shifted by 120°.
The simple bridge VSC of Figure 9.27 with each device switched once per 50 Hz cycle is not normally used due to the high harmonic content at the output. A square wave contains all the odd harmonics (excluding the triplen harmonics). As IGBTs can be switched many times during one a.c. cycle, the two switches of each phase are switched using a sine-triangular pulse width modulation (PWM) technique. The switching instances are determined by comparing a sinusoidal modulating signal with a triangular carrier signal (see Figure 9.28). A sinusoidal modulating signal representing the desired output voltage is compared with a high frequency triangular carrier signal. When the magnitude of the carrier is higher than the modulating signal, the upper switch is turned on. On the other hand, when the magnitude of the carrier is lower than the modulating signal, the lower switch is turned on.
Figure 9.27 Bridge VSC
Figure 9.28 PWM waveform (only half a cycle is shown)
In high power applications, such as h.v.d.c., each switch in the bridge is made up of a large number of IGBTs connected in series to form a valve. It is important to operate every IGBT in a valve at the same time. This requires complex circuitry. Another disadvantage of this arrangement is that high frequency switching (around 2 kHz) of high current and voltage results in much higher losses than the equivalent CSC arrangement where each thyristor valve is switched on and turns off only once in each cycle.
An alternative VSC topology is to connect a large number of one-leg converters in series to form a multi-level converter. One leg of a three-level converter is shown in Figure 9.29. The ground shown in the figure is the solidly earthed neutral of the three phase converter transformer. Point A provides 0, Vd/2 and — Vd/2 depending on the switches that are conducting. For example, the right hand side figure of Figure 9.29 shows the switching sequence when the current is at unity power factor. When T23 and T24 are on (which connects the positive rail to point A), the voltage at A with respect to ground becomes the voltage of the positive rail, that is Vd/2. On the other hand when T21 and T22 are on, the voltage at A with respect to ground becomes the same as the voltage of the negative rail, that is —Vd/2. During the positive half cycle of
Figure 9.29 A three-level modular converter with the switching sequence for unity power factor operation the current, if T23 and T14 are on, then the voltage at A with respect to ground becomes zero. During the negative half cycle of the current the voltage at A with respect to ground becomes zero, if T21 and T12 are on. However, when the power factor of the current is lagging or leading, the switching sequence becomes more complicated.
A five-level modified modular converter could be obtained with the same number of one-leg converters as Figure 9.29, but with two series switches as shown in Figure 9.30. Switch T is on during the positive half cycle and upper two one-leg converters constructing the positive half cycle of the output. On the other hand, switch T2 is on during the negative half cycle and lower two one-leg converters constructing the negative half cycle of the output.
Other multi-level configurations such as diode-clamped topology and the capacitor-clamped topology exist. However industrial practice is presently converging on modular topologies as this approach makes the implementation easier. A
Figure 9.30 A five-level modular converter
multi-level converter used for h.v.d.c. application may have as many as 300 levels to make a waveform close to a sinusoid.
Compared to the bridge shown in Figure 9.27, the modular converter has lower losses as the valves in a bridge VSC operating with PWM turn on and off many times per cycle. In the modular VSC each valve switches only once per cycle. However modular converters require complex balancing circuits to balance the voltage in each d.c. capacitor.
-  For ease of explanation it was assumed that the windings of the converter transformer connected to theVSC is star connected and the neutral is earthed.