Important Considerations in Testing and Commissioning Digital Protective Relays

Abstract

The proven advantages of digital technology for power system protective relays are now commonplace in the power producing and delivery industry. Digital relays provide unsurpassed reliability and extended capabilities at a very economical cost. Keeping pace with testing and commissioning requirements of these devices has proven to be a challenge for both protective relay engineers and technicians. While testing procedures have been well-defined for single function electromechanical protection devices, modern relay test procedures have been left to the utility to develop, creating possible shortcuts that may compromise the protection system operation. Extended options and settings, complex trip logic equations, and advanced communication options can lead to overwhelming difficulties in ensuring a multifunction IED is properly tested. Observations within the industry indicate that a common reason for potential errors, are shortcuts being implemented, primarily to simplify the process and meet regulatory recordkeeping requirements. Some of these testing shortcut practices include setting and logic changes to accommodate easy testing, creating test values based on single element settings rather than the actual applications, and failure to test the entire enabled capabilities of the protection system. This paper presents examples of some common mistakes typically observed during testing and commissioning, and ways to avoid them with simple to understand guidelines. The paper demonstrates the importance of testing protection systems, rather than single protective elements, to avoid protective relay misoperations once in service.