Commutation phases of Three Phase CSI
MODE – I operation of a three phase CSI:
Fig: MODE I ( 3 Phase CSI)
- The commutation process starts, when the thyristor, Th3 in the top half, is triggered, i.e. pulse is fed at its gate. Immediately after this, the conducting thyristor, Th1 turns off by the application of reverse voltage of the equivalent capacitor.
- Mode I now starts. 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 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 & C5 (C` =C/2).
- It may be shown that 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 other two capacitors, C3 & C5 is I/3. When the voltage across the capacitor, C1 (and also the other two) decreases to zero, the mode I ends.
MODE – II operation of a three phase CSI:
Fig: MODE –II (3 phase CSI) Fig: Three-phase CSI with two thyristors, Th3 & Th2 conducting
- After the end of mode I, the voltage across the diode, D3 goes positive, as the voltage across the equivalent capacitor goes negative, assuming that initially (start of mode I) the voltage was positive. It may be noted that the current through the equivalent capacitor continues to flow in the same direction. Mode II 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 II ends. The diode, D1 turns off, as the current goes to zero.
- So, at the end of mode II, the thyristor, Th3 & the diode, D3 conduct. The polarity of the voltage across the equivalent capacitor (at the end of mode II) 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.