Operation of a Modular Multilevel Converter Controlled as a Virtual Synchronous Machine

Abstract

This paper presents an implementation of a Virtual Synchronous Machine (VSM) based on a Modular Multilevel Converter (MMC). The control system relies on the internal simulation of an electromechanical swing equation, which emulates the inertia and damping effect of a synchronous machine and provides the frequency and phase angle needed for controlling the MMC to operate as a VSM. The ac-side control is based on cascaded voltage and current controllers in a synchronous reference frame defined by the virtual swing equation. The reference signals for the ac-side voltage control are generated from a reactive power control loop and a quasi-stationary virtual impedance which emulates the equivalent voltage drop over the stator windings of a synchronous machine. The control system also includes internal control loops for regulating the double frequency circulating currents of the MMC. The presented VSM implementation is tested in a laboratory environment using an MMC with 18 half-bridge sub-modules per arm. Experimental results demonstrate the operation of the VSM-controlled MMC when connected to an external ac grid and in islanded mode feeding a resistive load.