Analog Simulator Tests Qualify Distance Relay Designs to Today’s Stringent Protection Requirements

Deregulation, increased loads, and limited resources/rights-of-way available for the building of new transmission lines are routinely pushing modern power transmission networks to operating limits. New generations of microprocessor relays provide design opportunities that help stressed transmission networks meet protection requirements. To verify that increasingly complex microprocessor relays fulfill design specifications and utility protection requirements, China EPRI has designed an analog simulator test system to test many new designs of name-brand microprocessor relays. This paper draws upon many years of test experience to provide a summary of the common areas presenting the greatest challenges to distance relay designs. These challenges include evolving faults, faults with high fault resistance, power swing blocking, and reliable fault clearance during system power swings. The paper introduces ways that relay manufacturers have adapted and improved performance of their relays in these problematic areas. The paper compares the differences between analog and digital simulator systems and points out unique characteristics of each system. This paper focuses mainly on distance relay qualifications and applications for high-voltage transmission systems. I. MODERN REQUIREMENT FOR DISTANCE PROTECTION In the past several years, power transmission systems in China have expanded enormously. Increased power system complexity calls for corresponding protection device improvement. As the Chinese economy has expanded, transmission lines have needed to transmit more power to satisfy increased load. Following newly established policies of transporting power from western to eastern China and interconnecting regional power networks, transmission lines have become increasingly longer. To improve transmission line capacity, the Chinese have added series-capacitor compensation including Fixed Static Var Compensator (FSC) and Thyristor-Controlled Series Capacitor (TCSC) to the power system. Distance protection relays, although widely used for transmission line protection, must meet the following stringent requirements of power system development: • Relay operating time must be less than 20 ms for metallic faults along the entire length of the protected line, which means a sub-cycle tripping time for 50 Hz transmissions systems. • Transient overreach error of distance elements must be less than 5 percent. • Relays must correctly identify the faulted phase for single-line-to-ground (SLG) faults. • Distance relays must not misoperate during power swings. These power swings include those occurring during normal system operations when all three breaker poles are closed or those occurring during a single-phase open period following a successful trip of an SLG fault (phase-open or unsymmetrical power swings). • Distance relays normally have a power swing blocking (PSB) function to detect power swings and block distance elements. Any PSB must be removed to allow distance protection operations after the relay detects additional faults during a power swing. • Distance relays must operate correctly for seriescapacitor-compensated (SC) systems. • Relays must correctly identify the faulted phase when there is no overlap between external and internal faults in an evolving fault situation. Although designing a distance relay to meet all of these requirements is challenging, safe power system operation depends upon such a design. In China, a newly developed protection relay must undergo a comprehensive validation test to verify that it will meet all requirements before the relay can be installed on Chinese high-voltage transmission systems. During the test, a relay is subjected to several thousand faults or abnormal operating conditions. Several different system models are used to generate the different fault scenarios and operating conditions. These simulator tests often reveal design flaws and allow relay manufacturers to correct or improve firmware and algorithms before the relay is placed in service. The simulator test dramatically reduces the possibility of relay misoperations in the actual power system. II. ANALOG SIMULATOR TEST FOR DISTANCE PROTECTION In China, qualification testing of protection relays is done mainly through use of an analog simulation system. The dynamic simulation test is essential to determining the overall performance of a protection relay. The test center at the China Electric Power Research Institute (CEPRI) has tested various kinds of distance protection relays. Models that accurately represent the actual power systems and well-designed test