Although most practical ac ASDs employ PWM voltage source inverters, current source inverters are still used in high-power vector controlled drives. Typically, they are based on the relatively slow GTOs, for which the square-wave operation mode is very appropriate. Current source inverters are fed from controlled rectifiers allowing bidirectional power flow. The current source for the inverter (the rectifier with a current feedback and the dc-link reactor) automatically limits the short-circuit currents, increasing reliability of the system.
Current control in current source inverters differs from that in voltage source inverters.
A given reference vector, i* = /fZ®*, of stator current is realized by separately adjusting the dc-link current, /;, in the supplying rectifier and selecting such a state of the inverter that results in a position, s, of the current vector closest to ®*. A simplified block diagram of control scheme of the current source inverter is shown in Figure 7.11. The reference stator current signals, i$s and /^s, from the field-orientation
FIGURE 7.11 Control scheme of the current source inverter.