A voltage-sag compensation scheme based on the concept of power quality control center

The role of the power quality control center (PQCC) in mitigating the impacts of voltage sags on sensitive loads is examined. The center consists of back-to-back power converters, a dc bus, and a fuel cell distributed generator (DG) connected to the dc bus. It is shown that the range of voltage sags the system can ride through is determined by the current rating of the PQCC input-side inverter. Therefore, a three-stage operation scheme is proposed to improve on the system voltage sag ride-through capability. Stage 1 functions by maintaining the dc-link voltage at the same level as that before the sag. The PQCC input-side inverter would be protected from overloading during sag mitigation. The fuel cell remains undisturbed under this stage. Stage 2 is designed for compensating more severe sags. The purpose of introducing this stage is to decrease the input-side current through reducing the active power supplied by the upstream system. This is achieved by reducing the dc-link voltage and forcing the fuel cell to supply the balance of the active power. It is shown that this strategy can compensate for a voltage deviation of up to approximately double that under Stage 1. For even more severe sags that cannot be compensated under Stage 2, the PQCC would reduce the dc-link voltage even further, such that the DG supplies the entire protected loads. Simulation results are used to illustrate the validity of the proposed control scheme.