A novel constraint and non‐standard characteristics for optimal over‐current relays coordination to enhance microgrid protection scheme

Over the last few decades, the inverse over‐current protection scheme has been one of the most common schemes utilised in protecting the distribution networks (DNs). Although over‐current relays (OCRs) give an excellent protection in extreme failure situations, but continuous integration of distributed generation (DG) and the multi‐looped structure of modern DN has made it difficult for the existing industrial OCR's to secure suitable coordination between the different OCR's. The problem of coordinating over‐current schemes in the presence of DG needs a complementary between the optimisation techniques employed and the limitation of the manufactured relays existed in the DN for proper discrimination between OCRs. This study investigates the influence of excessive fault current due to DG penetration on conventional IEC characteristics. A novel constraint had been proposed to be added to the formula of the coordination problem – taking into consideration – the limitation of conventional IEC tripping characteristics utilised in nowadays numerical relays. Furthermore, a new non‐standard tripping characteristic is suggested to increase the applicability of the optimisation techniques with the existing OCR's. A benchmark IEC microgrid is implemented in ETAP Package. The genetic algorithm is employed to investigate the effectiveness of the two proposed approaches to the conventional optimisation techniques in different operational modes.