3-phase current source inverter (CSI) operation in this case is also Auto-Sequential Commutated Inverter (ASCI). As in the circuit of a single-phase CSI, the input is also a constant current source. In this circuit, six thyristors, two in each of three arms, are used, as in a three-phase VSI. Also, six diodes, each one in series with the respective thyristor, are needed here, as used for single-phase CSI. Six capacitors, three each in two (top and bottom) halves, are used for commutation.
In 3-Phase Current Source Inverter it may be noted that six capacitors are equal, i.e. The diodes are needed in CSI, so as to prevent the capacitors from discharging into the load. The numbering scheme for the thyristors and diodes are the same, as used in a three-phase VSI, with the thyristors being triggered in sequence as per number assigned.
The output current (phase) waveforms are shown is
Commutation Process in 3-Phase Current Source Inverter (CSI)
The commutation process starts, when the thyristor, Th3 in the top half, is triggered, i.e. pulse is fed at its gate. After this, the conducting thyristor, Th1 turns off by the application of a reverse voltage of the equivalent capacitor. As the diode D1 is still conducting, the current path is via Th3, the equivalent capacitor, D1, and the load in phase A (only in the top half). The other part, i.e. the bottom half and the source, is not considered here, as the path there remains the same.
The current, I from the source now flows in the reverse direction, thus the voltage in the capacitor, C1 (and also the other two) decreases.It may be noted the equivalent capacitor is the parallel combination of the capacitor, C1, and the other part, being the series combination of the capacitors, C3 and C5 (C’ = C/2). It may be shown the its value is Ceq= C/3, parallel combination of C & C/2 as C1 = C3 = C5 = C .
Also, the current in the capacitor, C1 is (2/3)I, and the current in the other two capacitors, C3 and C5 is I/3. When the voltage across the capacitor, C1 (and also the other two) decreases to zero, the mode1 ends.
After the end of mode1, the voltage across the diode, D3 goes positive, as the voltage across the equivalent capacitor goes negative, assuming that initially (start of mode 1) the voltage was positive. It may be noted that the current through the equivalent capacitor continues to flow in the same direction. Mode 2 starts. Earlier, the diode, D1 was conducting. The diode, D3 now starts conducting, with the voltage across it being positive as given earlier.
A circulating current path now exists between the equivalent capacitor, two conducting diodes, D1& D3 and the load (assumed to be inductive − R & L, per phase) of the two phases, A & B, the two loads and also the two diodes being now connected in series across the equivalent capacitor. The current in this path is oscillatory, and goes to zero after some time, when the mode 2 ends. The diode, D1 turns off, as the current goes to zero. So, at the end of mode 2, the thyristor, Th3 & the diode, D3 conduct.
This process has been described in detail in the earlier section on single-phase CSI (see mode 2). It may be noted that the polarity of the voltage across the equivalent capacitor (at the end of mode 2) has reversed from the initial voltage (at the beginning of mode I). This is needed to turn off the outgoing (conducting) thyristor, Th3, when the incoming thyristor, Th5 is triggered. The complete commutation process as described will be repeated. The diodes in the circuit prevent the voltage across the capacitors discharging through the load
The circuit shown in the above fig with two thyristors, Th3 & Th2, and the respective diodes conducting. The current now flows in two phases, B & C, at the end of the commutation process, instead of phase A at the beginning. It may be noted the current in the bottom half (phase C) continues to flow, and also the thyristor, Th2 & the diode, D2 remains in conduction mode. This, in brief, is the commutation process, when the thyristor Th3, is triggered and the current is transferred to the thyristor, Th3 & the diode D3 (phase B), from the thyristor, Th1 & the diode, D1(phase A).