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Determination of power swing locus (contd..) |
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If  , then the power swing locus on the R – X is an arc of the circle. (See fig 24.4(b)). |
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It can be easily shown that |
It is also clear from fig 24.4 (b), that the location of the electrical center is dependent upon the
 ratio. Appearance of electrical center on a transmission line is a transient phenomenon. This is because, during unstable transient,  is not stationary. As the rotor angles separate in time electrical center arises during out-of-step condition. When  , the rotors are said to have slipped a pole. However, once past , the corresponding phasors start coming closer to each other. Thus, electrical center vanishes after sometime. When = 0, another transient point, the rotor is said to have slipped by 2 - poles. |
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The voltage profile across the transmission system at the point of occurrence of electrical center is shown in fig 24.5.
At the electrical center, the voltage is exactly zero. This means that relays at both ends of the line perceive it as a bolted three phase fault and immediately trip the line. Thus, we can conclude that existence of electrical center implies (1) system instability (2) likelihood of nuisance tripping of distance relay. |
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