Modelling of VSC converters in the rotating dq Frame for DC fault study

As the future DC grids will involve numerous converter systems, the accuracy and speed of their modeling becomes of high importance. The modelling in a rotating dq frame is able to transform the quantities from ac variables to dc variables resulting in a significantly improved simulation speed. dq modelling is normally used for VSC converter in dynamic studies, while dc fault study is typically studied separately in static abc frame. This creates problems if converters are studied for transient DC faults. Commonly, the diode bridge is assumed to be a current source converter which gives inaccurate results for a diode bridge which is a part of VSC converter. To solve these challenges, a new modelling method for modelling VSC converter in the rotating frame for dc fault study is proposed. A unified modelling of the VSC converter for both dynamic and dc fault study is then further developed. Simulation results verified that the proposed model is accurate compared with detailed switching model. The proposed model is able to replace the detailed switching model and conventional dq frame model for both dc fault and dynamic study. A new modelling platform in the dq frame could be further developed employing the method proposed in this paper.