Investigation of Harmonic Content in Chain-Link STATCOM With a Non-Integer Frequency Modulation Index

Abstract

The technology around multilevel converters has emerged in the past decades and many new topologies have been introduced. Many multilevel converter topologies are equipped with floating capacitors which, if not controlled properly, face serious voltage imbalance issues. These issues are a result of the harmonic distortion in the output current and voltage. By proper choice of modulation technique, the harmonic performance of a multilevel converter can be significantly improved. One of the modulation techniques that improves the harmonic performance is the PS-PWM. However, this method does not eliminate the voltage imbalance issue that cause the capacitors to deviate, which leads to a need for additional individual balancing controllers. It has been concluded that a non-integer modulation frequency index significantly improves the imbalance problem by slowing down the deviation process during non-ideal conditions and almost completely preventing it during ideal conditions. In this thesis, the affect of a non-integer modulation frequency index voltage balancing in a modular multilevel converter with PS-PWM has been investigated. The harmonic behavior has been derived analytically and verified through PSCAD simulations.