HVDC converters
The simplest type of converter bridge is the 6-pulse bridge, also known as the Graetz bridge. Converter bridges are the building blocks of HVDC systems (Figure1).
The diode turns on when the anode voltage is more positive than the cathode and turns off when the cathode voltage is more positive than the anode. Each diode conducts for 120∘ in every 360∘ power system frequency cycle. The amplitudes of the three phases change with time. When the voltage B–C becomes larger than the voltage A–C, valve 3 takes over the current that had been flowing in valve 1. This is called commutation. The successive conducting pairs of valves are valves 1 and 2, valves 2 and 3, valves 3 and 4, valves 5 and 6, and valves 6 and 1. The DC voltage resulting from the rectification with the 6-pulse diode converter bridge is shown in Figure 2
The thyristor can conduct the current only in one way, but it can be controlled when it goes in conduction. The thyristor has an extra terminal, the gate, and in order to enter conduction, this gate terminal must also be energized. The thyristor is turned on by triggering the gate, but can only be turned off by the circuit connected between the anode and the cathode.
Figure 3 shows a thyristor circuit. When a pulseIg is applied and a forward voltage Vth is present between the anode and the cathode, the thyristor will start to conduct current (Ia). The conduction continues as long as current flows in forward direction. When the current tries to reverse, the thyristor turns off. Hence a thyristor converter requires an alternating voltage supply in order to operate as an inverter, and that is why a thyristor-based converter is known as a line-commutated converter (LCC).
When we replace the diodes in the Graetz bridge by thyristors, we get the scheme of Figure 4
The thyristor bridge has the possibility of controlling the DC output volt- age by changing the firing angle of the thyristors. Note that with a zero firing angle, the thyristors behave as diodes, and the controlled rectifier bridge (Figure 4) turns into the uncontrolled one (Figure 1). There is always a pair of thyristors within the 6-pulse bridge turned on, so there is a continuous current path through the converter. Most LCC HVDC converters use the more complex 12-pulse bridge, which consists of two separate phase-displaced 6-pulse bridges. In this way the converter’s harmonic performance is improved.
An LCC depends on the AC system voltage for its proper operation and operates at a lagging power factor, because the firing of the thyristors has to be delayed relative to the voltage crossing to control the DC voltage. IGBTs and GTOs made it possible to build the voltage source converter (VSC). IGBTs and GTOs are devices that can force commutation, and they enable the VSC to operate in all four quadrants of the P–Q plane. Commutation is independent of the AC system voltage.[1][2]