Grounding the AC Microgrid

Grounding strategy of an ac microgrid affects its line-to-ground fault response, personnel/equipment safety, service continuity, insulation requirements, and protection criteria. Therefore, a comprehensive knowledge of the available grounding strategies and their effects is essential for design and operation of the microgrid components and especially its protection. In this paper, characteristics of different ac distribution system grounding devices and grounding configurations are investigated. Subsequently, ac microgrid grounding requirements and issues are identified based on the unique characteristics and constraints of microgrids. The results of the aforementioned investigations are used to identify the best grounding strategies, i.e., grounding configurations, transformer connection types, and grounding devices, for the ac microgrid. The study results indicate that the four-wire multigrounded configuration is the most suitable choice for the majority of microgrids, especially in North America. The Yg/Δ and Yg/Yg transformers are identified as the most appropriate choices for distributed energy resources (DER) interface to the four-wire multigrounded microgrid. Besides, low-reactance and low-resistance grounding devices are recommended for the DER interface transformers.

I. Introduction

Alternating current (AC) microgrids offer significant advantages, such as improving power quality, reliability, and efficiency, while facilitating grid integration of distributed energy resources (DER) including distributed generation units and energy storage systems [1], [2]. Similar to the traditional ac distribution systems, the grounding strategy of an ac microgrid drastically affects (i) its fault-induced overcurrent (OC) and overvoltage stress, and thereby its protection system design and coordination., (ii) safety of personnel and equipment, (iii) reliability of energy supply, (iv) harmonics, and (v) maintenance costs [4]–[6].