Practical Challenges of Inverse and Definite-Time Overcurrent Protection Coordination in Modern Industrial and Commercial Power Distribution System

The characteristics of an overcurrent relay (OCR) in protection coordination are determined by adjusting the current and time delay function. This characteristic is adjusted once, fixed, and expected to overcome any disturbances in the system, which raises the risk of incorrect operation of the protection system, such as an overlapping trip or the incapability to limit the current exposure in the protected equipment. This article summarizes different efforts to overcome the modern challenges of overcurrent protection coordination (OCPC) in low or medium voltage industrial and commercial distribution systems. A brief background is introduced from a general perspective and then specifically describes in detail the problem of equipment coordination with a thorough link among mathematical concepts and fundamentals in practice. The operation challenges and ideas to avoid incorrect performance, under normal operation, are discussed. Finally, the article proposes a new optimization method based on the combination of the firefly algorithm and target remedy to overcome such challenges. The optimization considers the combination of inverse-time (ANSI 51) and definite-time (ANSI 50) function in OCR. The method is described and validated in the IEEE 242.2001 to meet the existing regulation of protection coordination. By assessing the critical cases, this article is expected to enhance the OCPC under different fault scenarios with complex coordination of time-current curves.