Dynamic voltage restorer based on multilevel inverter with adjustable dc-link voltage

Dynamic voltage restorer (DVR) is a power electronic converter-based custom power device used to compensate for voltage variations. The inverter used in the DVR structure can have different topologies. Multilevel inverters are a good candidate to be used in the DVR structure especially in higher voltage level. This study proposes a new scheme for DVR based on a multilevel inverter. In the proposed DVR, a dc-dc converter is attached to the multilevel inverter. The dc-link voltage is regulated by the dc-dc converter considering the voltage sag magnitude. In this way, the output voltage of the multilevel inverter has always its maximum possible number of levels. This results in better quality of output voltage in both deep and shallow voltage sags. It is important to emphasise that the existing inverter-based DVR topologies do not have this capability. Also, in the proposed DVR, there is no need for an output filter. Moreover, the fundamental frequency control method for the multilevel inverter can be used instead of pulse width modulation-based methods. This can result in lower switching losses and lower switching stresses. The mathematical analysis of the operation range of the proposed DVR is given in detail. The simulation results using Power System Computer Aided Design/Electromagnetic Transient including DC (PSCAD/EMTDC) as well as the experimental results from a laboratory prototype verify the proposed DVR scheme.