MMC Applied to Pumped Hydro Storage using a Differentiable Approximation of a Square Wave as Common-Mode Voltage during Low-Frequency Operation

The application of Modular Multilevel Converters (MMC) to Variable-Speed Pumped Hydro Storage (VS-PHS) is one of the currently under research options to implement large-scale energy storage units with fast dynamics and ancillary services. It is well known, though, that the MMC has a strong tendency to present high levels of submodule voltage ripple when operating under low-frequency, a problem usually counterbalanced with common-mode voltage injection. In this work, the application of a differentiable approximation of a square wave as common-mode voltage is proposed in order to reduce the high-frequency components of this injection and possibly decrease problems such as bearing current circulation. The performance of the proposed waveform is compared with the commonly studied trapezoidal-wave injection, taking into account limitations imposed by main circuit design. The analysis is based on a detailed PSCAD/EMTDC VS-PHS model with a full-size MMC directly feeding a synchronous machine. A pumping mode start-up procedure which attempts to reproduce the one applied in the currently operational Grimsel 2 VS-PHS is applied.