Power Quality Control Strategy of MMC Rectifier as Solid State Transformer in MVAC Network

The development of electricity technology in distribution networks such as Electric Vehicles and Distributed Generation (Rooftop PV, etc.) can lead to increased instability due to the characteristics of generation that tends to fluctuate and the characteristics of power electronic devices. Conventional transformers behave passively to the problems faced by the distribution network. One solution to this problem is the use of solid state transformers (SST). This study will describe the consideration of topology selection and rectifier control system as one of the main converters of SST. The rectifier topology used is Modular Multilevel Converter (MMC). MMC is chosen because it has the flexibility of feature development and low THD compared to other converter topology options. The Higher Level Control algorithm used is Vector Current Control and the Lower Level Control uses Nearest Level control. Simulation results show that the MMC rectifier in the designed SST is able to support reactive power by sending and absorbing reactive power according to load demand or dispatch commands on the MVAC network. Moreover, the designed MMC rectifier is able to respond to the emergence of power quality problems in the form of voltage sag and swell.

I. Introduction

The development of electricity technology increases the challenges of power distribution. This is due to several factors such as the application of energy storage (ES), electric vehicles, and distributed generation (DG) which are mostly based on power electronic devices [1]. Furthermore, these challenges can lead to increased instability of the power grid due to the unstable generation characteristics and power electronic device characteristics. The increased use of such power technologies results in power quality problems, such as intermittency, harmonics, power flow direction, and other incidents [2]. Conventional transformers, which are components for stepping up and down voltage, behave passively to the problems faced by the power grid caused by the integration of renewable energy and electric vehicles. An example of the passive behavior of conventional transformers is that they cannot correct the power factor because the power factor depends on the load. One solution to this problem is the use of solid state transformers (SST).