Development of a Protection Scheme for Microgrids with High Photovoltaic Penetration

In light of the rapid development of distributed energy resources (DERs), especially photovoltaic (PV) systems, a new kind of grid has been formulated called MGs. These grids can be operated in both grid-connected and islanded modes. Their faults currents can be bi-directional and vary in a wide range between two operation modes. Moreover, for achieving a successful islanding of MGs, the fault ride through (FRT) and dynamic voltage support requirements of DERs and PVs should be fulfilled. Therefore, the protection concept for such new grids is totally different from the traditional distribution networks. The protection scheme of MGs should not only protect them against all kind of faults but also be coordinated with the interface protection of the connected DERs. Although a number of studies have undertaken the MG protection into consideration, there is still room for further development. In this paper, the authors were determined to propose a new protection scheme that can protect MGs against all faults and ensure their smooth transition between two operation modes. The proposed protection scheme was validated on a MG simulated in the software environment DIgSILENT|PowerFactory with real data provided by a local utility company.

I. Introduction

Recently, with the high integration of Distributed Energy Resources (DERs) a new kind of grid at low-voltage (LV) level has been arisen called MG [1]. The protection philosophy applied for LV microgrids (MGs) can be different from that for traditional networks. Actually, these grids may be required to operate in both operation modes, including grid-connected and islanded modes. In the islanded mode, the low level of the fault currents supplied by PV systems, and possibly Battery Energy Storage System (battery) is not sufficient for the responsible protective devices to operate within an acceptable time frame. In addition, fast and accurate operation of the MG interface protection must be ensured to guarantee successful islanding of the MG. Currently, MGs are connected to external networks via transformers protected by a fuses or a Circuit Breakers (CBs) with integrated Time Over-current Protection (PTOC) elements. These protective devices cannot detect external faults to facilitate the MG islanding.