Average modelling of medium frequency dc-dc converters in dynamic studies

Summary form only given. This paper studies the average value modelling (AVM) of medium frequency (200Hz-2000Hz) dc-dc converters, which are becoming increasingly important in high power dc grids and distributing micro grids. The most convenient modelling is based on transfer function and power components which are available in common simulation platforms like PSCAD/EMTDC. However these models are found to require very small simulation step and the achieved time domain accuracy is not satisfactory for medium frequency converters. The core issue of accuracy loss is identified and corresponding improvement is proposed. User written state-space models in the abc frame and the dq frame are analyzed in depth. Different solvers including the Dommel´s method, the Heun´s method and the Runge-Kutta method were adopted and compared in solving the state-space models. A high power 2kHz LCL dc-dc converter is taken as the test system. It is found that the AVM in dq rotating frame based on Runge-Kutta solver is able to generate very accurate time domain responses compared to the detailed switching model with almost 100 times improvement in simulation speed. It is demonstrated that a dc-dc converter with 2kHz inner AC circuit frequency can be accurately simulated at 50μs time step.