Three-phase Current Source Inverter
Three phase current source inverter:
Fig: Three phase CSI Fig: Phase current waveform of 3 phase CSI
- The circuit of a Three-phase Current Source Inverter (CSI) is shown in figure. The type of operation is Auto-Sequential Commutated Inverter (ASCI).
- As in the circuit of a single-phase CSI, the input is also a constant current source. The output current (phase) waveforms are also shown.
- 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. It may be noted that six capacitors are equal, i.e.C1 = C2 = ……= C6 = C.
- 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 same, as used in a three-phase VSI, with the thyristors being triggered in sequence as per number assigned.
- The circuit, when two thyristors, Th1 & Th2, and the respective diodes, are conducting. The current is flowing in two phases, A & C. The three capacitors in the top half, are charged previously, or have to pre-charged. But the capacitors in the bottom half are not shown.