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 capacitorsas energy sources. The inverter is a 13-level cascaded H-bridgeconverter and can be connected directly to a 20-KV network without any series injection transformer. With capacitorsas energy sources, some new challenges are introduced. First, the capacitorsare not able to provide active power during voltage sagcompensation. Second, the charging of capacitorsto 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 capacitorsand allows voltage compensationwith the minimum energy approach. The proposed structure can develop a nearly sinusoidal waveformat 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.