Modular Multilevel Converter (MMC) based High Voltage Test Source

Abstract

Conventional high voltage test sources, i.e., transformers (cascaded and resonant), impulse generators, and rectifier circuits generate 50 Hz/60 Hz sinusoidal, lightning and switching impulses, and DC respectively. For arbitrary wave shapes, function generator and HV amplifier setup are available up to 50 kV and few kHz signal bandwidths. Superimposed signals (impulses or transients on a sinusoidal or DC) are generated from a careful design of superposition of two voltage sources. The demand of these unconventional test sources is increasing as new electric stresses are introduced in the current electrical power network due to the rise of power electronics. This motivates the search for a novel high voltage test source which can generate arbitrary wave shapes with larger signal bandwidth at higher voltage level. Modular multilevel converter (MMC) topology is a promising solution for high power applications because of its high efficiency, modular structure, and reduced filter requirement. Hence, the idea of a MMC-based high voltage test source is proposed for arbitrary wave shape generation. Such a converter-based test source can also be adapted as a mobile test system for site acceptance and diagnostic testing and opens an entire new high voltage research area.

This master’s thesis demonstrates the proof of concept of a MMC-based high voltage test source using a mathematical and simulation model. The first step is to identify critical differences between existing MMC applications versus high voltage testing applications. Based on this analysis, control methodology and converter parameter design are adapted for high voltage testing application. Since a power electronic converter generates an output signal with harmonics, a filter is required to obtain a smooth signal that can be applied to the test objects. The quality of the generated signal is measured in terms of the peak value and the slope of the output voltage. Also, the protection system and DC source requirements are investigated briefly for a comprehensive picture of the MMC-based high voltage test system. The thesis demonstrates the feasibility of the MMC-based high voltage test source with a proof of principle on paper and with simulations. Recommendations are given for the next steps to realize a high voltage test source for arbitrary wave shape generation.