Demonstration of Scalable Series-Connected Submodule of Modular Multilevel Converter-based Arbitrary Wave shape Generator used for High Voltage Testing from Off-the-Shelf Component

Abstract

To test high-voltage (HV) equipment with increasingly complex transients obtained from various power system studies, this article demonstrates a hardware implementation of a medium-voltage (MV) submodule (SM) to be used in a modular multilevel converter (MMC)-based HV arbitrary wave shape generator (AWG). The MV SM is scalable with its own onboard auxiliary power supply (APS), and it is constructed by connecting three full-bridge SMs in series from the commercially available component. The designed MV SM can be operated for a wide voltage range of 0.8-2.7 kV to incorporate different test objects ranging from HV insulation material to MV equipment and generate a wide output range of 0.12-1.2 kV. Considering the hardware nonidealities in the APS, gate driver, and switches, the series operation of three SMs is ensured using an arm energy controller. Based on the current-based model of APS, SM capacitance design criteria are updated for variable-frequency output waveform, and the minimum dc-link voltage is calculated for the proper start-up of this scalable MMC module. Apart from the variable voltage per SM, the HV AWG application poses different conditions, such as a low value of SM capacitance value and the HV dc sources with a current rating of a few tens of milliamperes. Hence, this article proposes exclusive design guidelines for the proper start-up, steady-state, and shutdown operation of the MMC-based AWG. In addition, this article dives deeper analytically into the soft start-up algorithm to understand its working principle and to design the average charging current within the limit for any number of SMs of the arm. In the end, their performance is showcased with a single MV SM per arm, operating at a different voltage (0.8-2.7 kV) and frequency levels (1-600 Hz) and generating different wave shapes, such as triangular, sinusoidal with different harmonics, and pulse waveforms. In addition, the fault ride-through capability is verified for the MMC-based HV AWG.