A control approach for UPQC connected to weak supply point

The shunt component of a UPQC (unified power quality conditioner) uses a VSI (voltage source inverter), which is typically connected to the grid through a series interface inductor. This inductor reduces the switching harmonics injected by the shunt VSI into the distribution network. However, if the UPQC is connected to a weak supply point, the load side voltage can become unacceptably distorted due to the switching frequencies in the supply current. In order to prevent the switching frequencies generated by the shunt inverter entering into the grid, LC or/and LCL filters are used for interfacing the shunt VSI with the distribution network. However, the incorporation of a second or higher order filter involves more complex control techniques to maintain system stability. In the case when an LCL filter is used for interfacing the shunt VSI with the grid, a hysteresis current controller would fail to track the reference if the current injected into the grid is tracked. But, the hysteresis based control strategy is very attractive from the practical implementation point of view, because of its simplicity. In this paper a control approach for avoiding the stability problem is proposed with application to the shunt component of the UPQC, incorporating an LCL filter, while using the hysteresis control technique. It is shown that if the inverter output current is tracked, rather than the current into the grid after the filter capacitor, stability is maintained using the hysteresis controller.