The DC link energy control method in dynamic voltage restorer system

Dynamic voltage restorer (DVR), which is installed between the supply and a critical load, can restore the load voltage to the pre-fault voltage during voltage sag. To restore load voltage, DVR should inject the equivalent of dropped voltage, which represents the voltage difference between pre-fault and fault voltage through series connected transformer. Voltage restoration of DVR needs to inject active power and energy from DVR to distribution system. However, the capability of energy storage that usually consists of capacitors in DVR is limited. Therefore, it must be considered how the injection energy can be minimized and the load voltage can be made close to the pre-fault voltage. This paper describes conventional restoration techniques like pre-sag compensation, in-phase compensation, and phase advance method. In addition, this paper proposes a new concept of restoration technique to inject minimum energy during DVR compensation. The proposed method is based on the definition of voltage tolerance of the load. In the proposed method, some particular disturbances can be corrected with less amount of energy discharge than those of conventional methods.