Mode I operation of Single-phase Current Source Inverter
Mode I operation of single phase current source inverter:
Fig: Voltage and current waveforms Fig: Circuit diagram
- The assumptions taken are: Starting from the instant,t = 0- , the thyristor pair, Th2 & Th4, is conducting (ON), and the current (I) flows through the path, Th2, D2, load (L), D4, Th4, and source, I.
- The commutating capacitors are initially charged equally with the polarity as given, i.e.,vc1 = vc2 = -Vc0 . This means that both capacitors have right hand plate positive and left hand plate negative. If two capacitors are not charged initially, they have to pre-charged.
- At time, t = 0, thyristor pair, Th1 & Th3, is triggered by pulses at the gates. The conducting thyristor pair, Th2 & Th4, is turned OFF by application of reverse capacitor voltages. Now, thyristor pair, Th1 & Th3, conducts current (I). The current path is through Th1, C1, D2, L, D4, C2, Th3, and source, I. Both capacitors will now begin charging linearly from (-Vco ) by the constant current, I. The diodes, D0CV−2 & D4, remain reverse biased initially
- The voltage, vD1 across D1, when it is forward biased, is obtained by going through the closed path, abcda as . It may be noted the voltage across load inductance, L is zero (0), as the current, I is constant. So
- As the capacitor gets charged, the voltage across vD1, increases linearly. At some time, say t1, the reverse bias across D1 becomes zero (0), the diode, D1.starts conducting. An identical equation can be formed for diode, D3 also. Actually, both diodes, D1 & D3, start conducting at the same instant, t1. The time t1 for which the diodes, D1 & D3, remain reverse biased is obtained by equating,. The time is given by,.
- The capacitor voltages vc1 = vc2 = -vc , appear as reverse voltage across the thyristors, Th2 & Th4, when the thyristors, Th1 & Th3, are triggered. The value of Vc is , which, if computed at t = t1, comes out as, , using the value of t1 obtained earlier. This means that the voltages across C1 & C2, varies linearly from -Vco to zero in time, t1. Mode I ends, when t1= 0, and vc = 0. Note that t1 is the circuit turn-off time for the thyristors.