The major differences between electromechanical and microprocessor based technologies in relay setting rules for transformer current differential protection

The current rules and practices for setting transformer current differential protection using electromechanical relays are discussed. These rules are compared with the ones, which are currently available on microprocessor-based technology. Major differences in relay setting approaches are outlined. With the electromechanical technology, selection of proper current transformer (CT) ratios and settings for a small size transformer connected to a high fault duty bus has been and continues to be a challenge as the system grows. A sample of such case is analyzed through examples using both electromechanical and microprocessor based technologies and some guidelines are suggested for each case.