A Switched Capacitor Series Voltage Controller with Fault Current Limiting Capability for DC Microgrid Application

A multiple bus dc distribution network can be made more reliable by providing voltage regulation of buses, protection and enhanced system stability. In this paper, Switched Capacitor Series Voltage Controller (SCSVC) is proposed to address the voltage regulation and protection issues in a dc microgrid. The SCSVC limits short-circuit current rapidly by utilizing the passive components of the converter during fault condition. In normal situation, the SCSVC regulates the bus voltage by injecting a dynamic voltage in series with the line which is equal to the line drop. Moreover, the fault current limiting capability eliminates the costly solid-state Circuit Breaker (CB) at the SCSVC-bus, and hence the voltage of source bus does not experience much sag during fault. The dynamic performance of the proposed converter under various operating conditions is checked through OPAL-RT based real-time simulation. The results show the effectiveness of SCSVC as a fault current limitter and series voltage regulator in a dc microgrid.

I. Introduction

The clean and sustainable energy sources based on Distributed Generation (DG) is becoming popular in power sector. By connecting DGs (e.g. solar, wind, fuel cell etc.) with local loads and energy storage, a microgrid is formed which can enhance the reliability of utility and consumers [1]. The dc microgrids have more advantages compared to its ac counterparts in terms of efficiency, flexibility, power density, absence of reactive power and frequency related issues [2]. Fig. 1 shows a simplified multi-bus dc distribution system [3]. The DGs (e.g. PV and battery) are connected through a dc-dc converters at bus-A and the critical loads (e.g. Constant Power Loads (CPLs) and Constant Voltage Loads (CVLs)) and non-critical loads (e.g. resistive loads) are connected at the remaining buses. The power flow is taking place from its source-bus to the end-bus in one direction. Hence, the voltage levels at different points of the grid are dependent on load power due to voltage drop along the line resistances [4] - [5]. Moreover, CPLs require tightly-regulated voltage at its bus, otherwise these loads exhibit negative incremental impedance and tend to destabilize the system [6]. Therefore, voltage regulation of critical load buses (e.g. bus-C, bus-D and bus-E in Fig. 1) which are far away from the generation and connected though line impedances is essential to meet system specifications. Fig. 1.

The typical structure of dc microgrid