Internal Energy Balancing Control of a Modular Multilevel Converter as Part of a Hybrid Transformer

Hybrid transformers (HTs) can provide voltage and current compensation in the utility grid. HTs include a line-frequency transformer and a converter rated for partial transformer power. This paper focuses on the internal energy balancing process of a modular multilevel converter (MMC) as part of an HT. The output voltages of the MMC have equal frequency and equal phase angles, but different amplitudes in this application. In this paper it is shown that these two features result in steady-state control errors and variable control loop gains when a conventional MMC internal energy balancing method is applied. In this article, a customized control scheme for the MMC is developed that solves these problems. The theoretical analysis is validated through tests on a scaled-down prototype.