Protection of Converter-Interfaced Microgrids Using Modified Short-Time Correlation Transform

The limited and bidirectional fault current of inverter interfaced islanded microgrids make the fault detection and faulty feeder identification the major challenges for conventional overcurrent-based protection systems. To address the first challenge, this article proposes a fault detection strategy based on the modified short-time correlation transform (MSTCT) of feeder current. Using the superimposed component of MSTCT, a new directional element is defined to address the second challenge. The developed strategy relies on an intelligent electronic device in each feeder, making it a cost-effective scheme. The proposed scheme is effective for all types of fault and can differentiate normal switching events from the fault conditions. Also, it properly operates in the presence of nonlinear loads and measurement noises. The reliable performance of the proposed protection scheme is assessed on the MATLAB/Simulink simulation model of the CIGRE benchmark low-voltage microgrid network.

I. Introduction

Global climate change and ever increased energy demand have resulted in increasing integration of distributed energy resource (DER) units and emergence of the microgrid concept [1]. This integration is changing the conventional passive distribution network to the modern active ones. Conventionally, the overcurrent relays are employed to protect the distribution network. The operating principle of these relays is that the magnitude of fault current supplied by the main grid is several orders higher than that of normal line current. In the case of grid-connected microgrids, there is no problem for the detection of faults due to the high short-circuit capacity of the main grid. In the case of standalone microgrids, where the grid does not contribute to the fault current, there are two conditions, depending on the DER interface type. If DER units are interfaced to the microgrid through rotating machines, overcurrent relays can properly operate due to large fault current supplied by these machines. However, fault detection in inverter-interfaced standalone microgrids is a serious challenge [2]. It is due to this fact that the low thermal inertia of voltage-sourced converters (VSCs) makes it necessary to protect the semiconductor switches against the large currents. To this end, VSC current is limited to two times the rated current, commonly using a current limiting strategy embedded in the VSC control system. On the other hand, another protection challenge arisen in both grid-connected and standalone microgrids is bidirectional power flow in the presence of DER units. Conventional overcurrent relays are set based on unidirectional power flow from the main grid to the loads and proper selectivity is achieved using the inverse time-current characteristic curve. Thus, low fault current and bidirectional power flow make the conventional overcurrent-based protection ineffective for active distribution networks.