Frequency Behavior of an MMC Test Bench System

Interactions of modular multi-level converters (MMCs) compromising the system stability can be assessed by investigating the frequency behavior of the converter. The MMC Test Bench system replicates full-scale converters on a low-voltage level. It can emulate high-voltage direct current systems representing meshed offshore grids as well as the influence of wind farms connected to MMCs. Measuring the MMC Test Bench's frequency behavior allows for assessing the frequency behavior of the applied control systems by avoiding simplifications of the electrical part of the MMC. This paper presents a setup for measuring the frequency-dependent impedance of the laboratory MMCs. The impedance is compared to small-scale as well as full-scale converter models. It is shown that the measured impedance can be scaled up for full-scale HVDC studies by means of derived correction factors. The results further demonstrate that simplified time-domain models can represent the MMC Test Bench but fail to represent the exact frequency behavior at low and high frequencies.