Zero-setting Power-swing Blocking Protection

Modern distance relays have integrated numerous protection functions including power-swing blocking and out-of-step or pole-slip tripping functions. The main purpose of the power-swing blocking function is to differentiate faults from power swings and block distance or other relay elements from operating during stable or unstable power swings. Most power-swing blocking elements are based on traditional methods that monitor the rate of change of the positivesequence impedance. The required settings for the power-swing blocking elements could be difficult to calculate in many applications, particularly those where fast swings can be expected. For these cases, extensive stability studies are necessary to determine the fastest rate of possible power swings. This paper describes a zero-setting power-swing blocking protection method that is independent of network parameters and is based on monitoring the line swing-center voltage rate of change. The new method does not require any stability studies or user settings for the proper blocking of relay elements that are prone to operate during stable or unstable power swings. The method is applicable to long, heavily loaded transmission lines that pose great problems in the application of power-swing blocking elements based on traditional methods. This new method offers a great advantage in its ability to detect three-phase faults that may occur during power swings and allows the protective relays to issue a tripping command and isolate the faulted power system element. In this paper, we discuss other enhancements in the design of power-swing blocking and out-of-step tripping functions that improve the security and reliability of the power system. We also demonstrate the behavior and performance of the new power-swing blocking element through use of electromagnetic transient program (EMTP) and relay simulations. INTRODUCTION Power systems under steady-state conditions operate typically close to their nominal frequency. A balance between generated and consumed active power exists during steady-state operating conditions. Power system faults, line switching, generator disconnection, and the loss or application of large blocks of load result in sudden changes to electrical power, whereas the mechanical power input to generators remains relatively constant. These system disturbances cause oscillations in machine rotor angles and can result in severe power flow swings. Depending on the severity of the disturbance and the actions of power system controls, the system may remain stable and return to a new equilibrium state experiencing what is referred to as a stable power swing. Severe system disturbances, on the other hand, could cause large separation of generator rotor angles, large swings of power flows, large fluctuations of voltages and currents, and eventual loss of synchronism between groups of generators or between neighboring utility systems. Large power swings, stable or unstable, can cause unwanted relay operations at different network locations, which can aggravate further the power-system disturbance and cause major power outages or power blackouts. A power-swing blocking (PSB) function is available in modern