The Back-up Power Swing Protection With Intelligent Check of Regime Parameter Changes

Due to the simplicity of implementation the back-up function of the power swing protection (PSP), which checks the changes in the current in the unstable power swings (UPS), is widely used. The RMS value of current changes from maximum to minimum in UPS. The maximum value is reached when the angle between power systems is close to 180°, the minimum value - is close to 0°. The duration of the power swing cycle is measured by detecting current RMS value successive crossing of upper and lower thresholds. During stable power swings the power system maintains stability, and oscillations of current RMS value are decay. PSP distinguishes stable power swings from an unstable one by measuring the duration of the slip cycle. As known, the duration of the stable power swings cycle is a lot more than the duration of the unstable power swings cycle. The well-known PSPs that use the measurements of current have a common flaw, which appears at the beginning of a loss of stability. With a long process of loss of stability, the current RMS value can cross the threshold long before the occurrence of unstable power swings. In this case, the duration of the first slip cycle may exceed the setting, and PSP will exclude the crossing point of the RMS current value from consideration. This will lead to the skipping of the first slip cycle and to delaying in the unstable power swings eliminating. The last is dangerous with an increase in the scale of the emergency, the blackout of the power system, and its long restoration. The back-up PSP with the intelligent check of the change of RMS current value is considered. The purpose of intelligent control is to increase the reliability of identifying the first slip cycle. It is achieved by starting the measuring duration of the slip cycle only after the beginning of unstable power swings. The intelligent check of current RMS value changes eliminates the misoperation of back-up PSP in unstable power swings with protracted first slip cycle.