Three phase phase angle converter & Commutation in PAC
Three phase – phase angle converter:
Fig: A three-phase Phase Angle Converter; bottom: The input three-phase voltage waveforms
- These converters may be either AC-DC, DC-DC or DC-AC converters.
- The AC-DC Phase Angle Converter, (PAC) continues to be used in much high power and very high power converters where the application is non-critical or the non-state-of-the-art is preferred for operational advantages.
Commutation in PAC:
Fig: Significant voltage and current waveforms of a single phase converter highlighting the overlap instants and the corresponding converter terminal and output voltages
- In a three phase PAC, nominally balanced three phase voltages VR, VY and VB are connected to the three legs of the converter via three inductances LS, which can be considered to represent the leakage reactance of the supply transformer.
- At any instant, two devices are conducting, phase voltage VR is most positive and VB most negative. Subsequently, at the crossover point, VY becomes most negative and SCR2 is more forward biased with respect to SCR6.
- The incoming SCR does not take the full load current IL, nor does the outgoing SCR turn-off immediately. There ensues an ‘overlap’ period when three SCRs conduct for a transient period.
- It is evident that with the simultaneous conduction of SCR2 and SCR6 there is a short circuit at the converter terminals with the short circuit current ISC being limited by the per-phase series inductances LS. Line voltage VYB drives this current.
- With no delay in triggering (as if the SCRs are all replaced by diodes) the SCRs, they would be triggered 600 after the zero crossing of the corresponding line voltage. The triggering on this line voltage is delayed by the trigger angle α from this 600 point.
- There are a few significant effects of the commutation process when three devices conduct.
- The voltage waveforms at the output and at the converter input terminals reflect the commutation process. All-SCR (fully-controlled) converters, which are capable of operating with trigger angles α between 00 to 1800 ideally, are restricted in the inverter mode to operate within the ‘margin-angle’. This angle is of the order of 1600 and the output voltage is limited.