Thermal Stress Evaluation of a Multifunctional Modular Multilevel Converter – STATCOM Operating as Active Filter

The Modular Multilevel Converter (MMC) has proved to be an attractive technology in medium and high voltage Static Synchronous Compensator (STATCOM) applications. Nevertheless, once reactive power compensation profile is not constant over time, there are periods that a MMC-STATCOM works below its nominal capacity. In these periods, the converter can be used to perform ancillary services, such as Active Power Filtering. This paper analyzes the thermal stresses in the capacitors and semiconductor devices (i.e. IGBTs and diodes). The analyses take into account reactive compensation and harmonic current compensation of 5th and 7th harmonic components of a MMC based STATCOM. In addition, the limits of harmonic compensation for 5th and 7th harmonic orders are analytically derived. The results demonstrated lower thermal stresses during Active Power Filtering than the traditional STATCOM operation. Moreover, the 5th harmonic compensation is the most stressed one among the cases studied in this work.