Combinational voltage booster technique for fault ride-through capability enhancement of squirrel-cage induction generator

This paper proposes a combinational voltage booster technique for fault ride-through (FRT) capability enhancement of squirrel-cage induction generators (SCIGs) by a multifunctional series compensator (MFSC). This technique uses a couple of antiparallel thyristors across the ac terminals of a traditional dynamic voltage restorer (DVR). Two control schemes are applied: a DVR mode and a multifunctional mode. The MFSC can operate in the DVR mode to compensate the slight voltage fluctuations/imbalances at the point of common coupling (PCC). Also, when serious voltage sags occur in the line, the MFSC controller deactivates the faulty phase inverter and activates its corresponding thyristors to insert the equivalent impedance to maintain the voltage level during the fault. Then, it switches to DVR mode to improve the recovery time after the fault. An operational algorithm is applied to deal with different voltage sag depths caused by various fault conditions (both symmetrical and asymmetrical faults). Comprehensive simulations were carried out by using PSCAD/EMTDC software, and results show that the proposed technology is a very effective device to improve the FRT capability of the SCIG. A scaled-down experimental verification was carried out. Experimental results confirm the effectiveness of the combinational voltage booster technique of the MFSC for both slight and serious voltage sags.