Electromagnetic compatibility of digital protective relays installed in medium voltage switchgear

Digital microprocessor based protective relays, installed in low voltage compartments of medium voltage switchgear and/or in control panels, become more and more exposed to EMC disturbances due to interferences that may affect the power quality of electrical networks. End-users sometimes report unexpected circuit breaker tripping and significant financial consequences due to process discontinuity. After analysis, it is quite easy to demonstrate that these incidents are caused either by a relay failure that went to a fall back position, incorrect operation or improper installation/cabling rules. Due to higher market requirements and more demanding applications, mainly in the Oil & Gas sector, digital protective relays have to integrate more powerful protection, control, monitoring and communication functionalities. Although these relays comply with the latest IEC 60255 and IEC 61000 standards and associated subsets, they have to be installed by switchboard manufacturers, whoever they are, in air insulated or Gas Insulated switchgear. Improper installation rules can sometimes result in unexpected tripping.