Mitigating Zero Sequence Effects in Dynamic Voltage Restorers

This paper presents an in-depth analysis of the effects of zero sequence components on series custom power devices in particularly on the dynamic voltage restorer (DVR). It is shown that the conventional three-wire DVR could not properly compensate for voltage sags containing a zero sequence component due to the lack of zero sequence current circulation path within the restorer. It is proposed that a four-wire DVR can mitigate unbalanced voltage sags with zero sequence component effectively as it provides a circulation path within the DVR for zero sequence current. However, the interaction of DC-side with AC-side in this topology leads to an unbalance in the DC-link capacitor voltages and it causes zero sequence resonance especially when the DVR is under open loop control. The closed- loop control can provide additional damping to mitigate possible harmonic resonance. However, the DC-link ripple voltage due to the fundamental zero sequence current cannot be avoided. This may need larger capacitors in the DC-link than that is normally required. The DC-link voltage ripple can be eliminated by introducing a fourth leg in the DVR and by controlling the DVR under closed-loop control. Simulation results are presented to verify the theoretical analysis.