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The electric power system maintains a dynamic and delicate balance between generation and load in normal operatio condition. A disturbance, such as a sudden change of load, a power system fault, or a trip of a large generation unit, may break the balance, cause the oscillations among the generator rotor angles and force the generators to adjust to a new operating condition. The adjustment will not happen instantaneously due to the inertia of the generator prime movers. The oscillation will cause either stable and/or unstable power swings. During a power swing, the impedance trajectory seen by a distance relay may enter the fault detection zones and cause unwanted relay operation. During the Blackout on August 14th, 2003 in North America, stable and unstable power swings occurred and caused the operation of several distance relays. In 2007, Northeast Power Coordinating Council (NPCC) formed a joint team to start an investigation on the issue with the goal to improve the performance of distance protection in such events. This paper reports on distance relay testing that was performed by using simulated data of the blackout from NPCC's SS-38 working group. The simulations include both stable swings and unstable swings. The goal of these tests was to evaluate solutions which could be implemented to avoid unwanted distance relay operations for such events. The paper discusses the feasibilities and limitations of using load blinders to prevent relay operations during stable power swings. Several power swing blocking methods are discussed, and their advantages and disadvantages are compared. This paper describes the test procedures, and presents and discusses the test results. Relays were programmed for both mho and quadrilateral characteristics for the tests. Some recommendations and comments are made on the protection for the conditions of stable and unstable power swings. |
