Mode II operation of Single-phase Current Source Inverter
Mode II operation of Single-phase Current Source Inverter:
Fig: MODE II (1phase CSI) Fig: Equivalent circuit of MODE 2
- Diodes, D2 & D4, are already conducting, but at t = t1, diodes, D1 & D3, get forward biased, and start conducting. Thus, at the end of time t1, all four diodes, D1–D4 conduct.
- As a result, the commutating capacitors now get connected in parallel with the load (L). For simplicity in analysis, the circuit is redrawn as shown in Fig. , where the equivalent capacitor is C/2 , as C1 = C2 =C .
- The equation for the current at the node is, I i0 = ic (= ic1 ic2) where, ic1 = ic2 = ic/2. The voltage balance equation is
- The solution of the equation is, where, A, B & K are constants, natural frequency,
- The initial conditions at t = 0 are, i0 = I and di0/dt. It should be noted that the time, t is measured from the instant, the diodes, D1 & D3, start coducting, i.e., from the instant, mode I is over. Using the initial conditions stated earlier, the current is, .
- The capacitor current is
- The voltage across capacitor is,
- This expression can also be obtained as,
- As the current,ic1 tends to reverse, diode, D3 prevents its reversal. Similarly, the diode, D4 prevents the reversal of the current,ic2 . From the initiation of mode II, a time, t2 must elapse for the current,ic1 to become zero (0). The time,t2 is
- The capacitor voltage at time,t2 is, Note that this is also the maximum value. Now, the load current is, i0 = I.(2 -1) = I. This shows that the load current has reversed from I to –I during mode II, after time, t2. It is also seen that the capacitor voltage changes by 2.Vco (from –Vco to Vco) during each commutation interval. The time t1, after substituting Vco , comes out as,
- At the end of the process, constant current flows in the path, Th1, D1, load (L), D3, Th3, and source, I. This continues till the next commutation process is initiated by the triggering of the thyristor pair, Th2 & Th4.