AC load conditioner and DC bus conditioner for a DC distribution power system

A PEBB (power electronics building block) based power converter system is described to provide a secondary utility bus in a DC distributed power system on a ship. The power converter system consists of a main inverter and a multi-functional load conditioner to achieve low voltage distortion under unbalanced and/or nonlinear loads. A four-legged three-phase power converter topology is used for both the main inverter and the load conditioner. Modeling and control design aspects of the power converter system are addressed with emphasis on the impacts of different load conditions. Experimental results show that the utility sub-system has a high performance under different load conditions. However, when an unbalanced load or nonlinear load is connected to the secondary utility bus, the whole utility sub-system behaves like a nonlinear load to the DC distribution bus by drawing a low frequency ripple current. The nonlinear loading effect has been identified as one of the system level control issues which impacts greatly on the stability of the DC distribution bus in a large signal sense. Two approaches are suggested to eliminate the nonlinear loading effect and to improve the stability of the DC distribution bus. One is to use the load conditioner with a modified control reference, another is to use a DC bus conditioner, which is an extension of the AC load conditioner concept by performing active filter and damping functions. Simulation results prove that both approaches are effective. Tradeoffs and system design considerations are discussed.