Dynamic DQ Model of DC-Modular Multilevel Converter for DC grid applications

This paper presents an accurate dynamic model of DC-modular multilevel converter (DC-MMC) which can be used for the design and analysis of the converter and DC grid studies. The main challenge of dynamic analytical modelling of DC-MMC is the nonlinear multiplication terms in ABC frame equations for modulating oscillating signals. This paper uses the rotating DQ frame for mitigation of this challenge. This requires simultaneous modelling in zero sequence, fundamental frequency DQ and second harmonic D2Q2 frames. The proposed dynamic model is implemented in state-space within MATLAB environment. A reduced-order model is also developed by equating the second harmonic components of the proposed model to zero. The validity and accuracy of the two models are verified against a detailed DCMMC model in PSCAD/EMTDC in time domain. The results show that the proposed full-order dynamic model is in excellent agreement with the detailed PSCAD model across all variables, both in steady-state and in transient operating phases. The results of the reduced-order model show that the accuracy is decreased if the second harmonics are not considered in the model. A simple eigenvalue analysis is also presented as an application of the model.