Control of a Three-Phase Four-Wire Modular Multilevel Converter as a Grid Emulator in Fault Scenarios

Grid emulators enable the test of grid-connected distributed power generators in an efficient manner. Two-level and three-level voltage source inverters are the most commonly used topologies for grid emulators. An issue of these inverters is the rather complex expansion to higher dc voltages. Their transient performance is also inadequate. This paper presents a technique for using a three-phase four-wire configuration to control a modular multilevel converter as a grid emulator in fault scenarios. These scenarios can either be balanced scenarios such as triple line-to-ground voltage sags or be unbalanced scenarios such as single line-to-ground voltage sags. In comparison with other conventional grid emulator topologies, the modular multilevel converter benefits from a smaller ac filter and a faster response due to its higher voltage level. However, energy control is always a crucial topic for modular multilevel converters, especially under unbalanced conditions.This paper investigates the strategy to control a modular multilevel converter as a grid emulator. It also exams the influence of energy balancing on the steady-state performance and the transient performance of the grid emulator. The control strategy is verified using simulation results of a 400V dc-link modular multilevel converter comprised of 32 sub-modules, where the steady-state performance and the transient performance are analyzed. It is demonstrated that the transient performance of the modular multilevel converter is faster than the one of a conventional inverter. In addition, higher-order harmonics distortion scenarios can be implemented with the modular multilevel converter than a two-level voltage source inverter.