A dynamic voltage restorer using multilevel cascaded inverter and capacitors as energy sources

In this paper a dynamic voltage restorer is presented based on a cascaded multilevel inverter with capacitors as energy sources. The inverter is a 13-level cascaded H-bridge converter and can be connected directly to a 20-KV network without any series injection transformer. With capacitors as energy sources, some new challenges are introduced. First, the capacitors are not able to provide active power during voltage sag compensation. Second, the charging of capacitors to a reference voltage and keeping voltage balance among them (during sag compensation) is not straightforward. The adopted control strategy in this paper provides voltage balance among the dc link capacitors and allows voltage compensation with the minimum energy approach. The proposed structure can develop a nearly sinusoidal waveform at the ac side with almost a 90 degree phase shift with respect to the line current. Several simulations have been carried out in PSCAD/EMTDC environment in order to verify the validity of the proposed control scheme.