A review of AC microgrid protection system design challenges from authors' experience

What is it about?

A microgrid is a type of resilient electrical power system. It's increasingly employed as part of the critical and non-critical infrastructure, such as hospitals, residential buildings, emergency systems, etc., to maintain or to increase the availability of services such infrastructure provides us and we've come to expect. Microgrids offer the benefits of operational flexibility, resiliency, reliability, power quality, accommodation, management, etc., of Distributed Energy Resources (DERs). However, they are complex, particularly when designing their protection systems. Microgrid protection systems must be designed to accommodate microgrids’ unique operational requirements, such as grid-tied and island modes, while ensuring the safety of the microgrid, microgrid-connected equipment, and personnel at all times and meeting the microgrid’s stability, reliability, and power quality requirements to the greatest extent possible. Although there are a few standards that provide guidance on DER capabilities and interconnection requirements, they are not always applicable or do not always allow for an extensive exploitation of microgrids for the benefits they can offer. The paper presents a review of microgrid protection system design challenges and discusses a few real-world experiences, based on the authors’ own engineering, design, and field experience, in using several approaches to address microgrid protection system design, engineering, and implementation challenges.

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Photo by Timo Müller on Unsplash

Photo by Timo Müller on Unsplash

Why is it important?

The quality of services we expect from critical and non-critical infrastructure are reliant on the energy systems—including, microgrids—that power that infrastructure. Most microgrids are deployed on existing distribution utility or utility customers’ systems, with existing and or with additional equipment and with protection design standards that do not always adequately support and or address the microgrid protection needs. Designing a microgrid’s protection system, therefore, requires a thorough understanding of all microgrid operational modes, configurations, transitional states, and how transitions between those modes are managed. As part of the microgrid protection design, speed and reliability of information flow between the microprocessor-based relays and the microgrid controller, including during microgrid failure modes, must be considered. These and other aspects contribute to the overall complexity and challenge of designing effective microgrid protection systems. In the paper, we presented a detailed review of the current challenges with protecting microgrids and an overview of several state-of-the-art protection strategies with their respective advantages and disadvantages in addressing those challenges based on the authors’ successful experiences in designing several effective microgrid protection systems globally. We also discussed the benefits and approaches to switching between protection setting groups with automatic correction as part of adaptive-protection schemes, as well as seamless planned and unplanned microgrid transitions between grid-tied and island modes. These approaches were successfully used in design and implementation of protection systems for several utility-grade, as well as behind the meter microgrids.

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