Design Trade-Offs of Modular Multilevel Converter-Based Arbitrary Wave Shape Generator for Conventional and Unconventional High Voltage Testing

Abstract

This paper comprehensivelyinvestigates the design trade-offs of a Modular Multilevel Converter (MMC) operations as an Arbitrary Wave shapesGenerator (AWG) to perform High Voltage (HV) dielectric testing of different grid assets. HV AWG applications pose unique operatingconditions to the MMC, which influences the selection of the various system parameters. This influence of the MMC system parameters is studied analytically, with MATLAB-Simulink simulations and a down-scaled MMC prototype. It is found that the Phase-Shift Carrier (PSC) modulation technique proves to be a superior modulation technique over Nearest Level Control (NLC). The correct choice of arm inductance and series damping resistance improves the harmonic performance of the output voltage waveform. The fast switching SiC MOSFETs are well suited to generate complex waveforms with high bandwidth. The adapted control system with the proportional controller can accurately generate the different waveforms with Total Harmonic Distortion (THD) less than 5%. The circulating current in the MMC is negligible for the HV AWG application, which explains why the submodule capacitor voltages are balanced even when asymmetric complex wave shapes are generated from the MMC. Additionally, the submodule capacitor ripple expression is derived for this unique application, and it matches well with the simulation and experimental results. For this application, submodule capacitance in the $\mu$F range is sufficient to keep the ripple within 1% of its average value. Moreover, the challenges of realizing the full-scale MMC setup are discussed. The discussed design guidelines are applied to simulate the full-scale prototype with 67 submodules per arm.

Keywords

• MMC
• AWG
• conventional and unconventional dielectric testing
• complex waveforms
• modulation techniques
• down-scaled prototype