Chapter 7 - STATCOM and DSTATCOM with modular multilevel converters

Abstract

The static synchronous compensator (STATCOM) is a power electronic converter designed to be shunt-connected with the grid to compensate for reactive power. Although they were originally proposed to increase the stability margin and transmission capability of electrical power systems, there are many papers where these compensators are connected to distribution networks for voltage control and power factor compensation. In these applications, they are commonly called distribution static synchronous compensator (DSTATCOM). In contrast to transmission systems, electric distribution networks generally operate with voltage imbalances caused by the connection of unbalanced loads, short circuits between the coils of transformers and motors, asymmetries of feeders’ impedances, among other problems. Although balancing single- and two-phase electric loads between the distribution feeders can contribute to minimize this undesired effect, the penetration of grid-connected single-phase distributed generation systems, especially those based on alternative energy sources, such as rooftop photovoltaic systems, and the increasing number of battery chargers for electric vehicles, may compromise the voltage unbalance indexes of modern electric networks. Multilevel converters have the advantage of not requiring complex transformers, high-order harmonic filters, or sophisticated magnetic interface structures to be connected to the mains. Among them, the modular multilevel converter (MMC) is one of the promising topologies in multilevel converter family, especially for medium- and high-voltage applications. Thus, the MMC-DSTATCOM can be connected to medium-voltage distribution networks to regulate the positive-sequence voltage and to compensate the negative- and zero-sequence voltage imbalances at the point of common coupling. In addition to providing details regarding the operation and control of modular multilevel converters, this chapter presents the steps to derive mathematical models for designing the positive-, negative- and zero-sequence voltage and current control loops for DSTATCOM based on an MMC capable to synthesize output voltages with 29 levels.