Analytical circuit oriented modelling and performance assessment of modular multilevel converter

This study presents a detailed time domain model of a modular multilevel converter (MMC) to provide an alternative form of evaluation of the converter dynamic performance, for example, during AC or DC fault conditions. The purpose of the model is to accurately reflect the converter behaviour to be used in control systems design. It can also be beneficial to predict the steady state or transient state of the converter with fixed or variable inputs. In an MMC, the capacitor voltages are modulated using pulse-width modulation (PWM) to fabricate a sinusoidal output voltage. With time, and because of the PWM action, the capacitors get charged or discharged. Therefore the circuit behaves as a time-variant circuit. The analysis is based on the equivalent circuit derived from the converter differential expressions at any time instant in a piecewise fashion. The usefulness and accuracy of the theoretical study is validated for the three-phase MMC topology by simulation results with respect to normal operation, start-up process, cell short-circuit fault, line-to-ground and DC-side fault conditions. Experimental tests are carried out through a scaled-down laboratory prototype.