Research On Characteristics Of Brake-Closing Travelling Wave And Transfomer Sympathetic Inrush And Relevant Protections

Travelling wave-based protection is a perfect protection principle not only for it is free from the transition resistance and fault location theoretically, but also for it has an ultra high speed operation characteristic. Many travelling wave-based protection principles have been presented. Most of these protection principles aim at the transmission line faults under normal conditions taking no account of the influence of circuit breakers closing, especially the influence of asynchronous closing on the validity of protection actions. In order to improving travelling wave-based protection and impelling its practicality, an in-depth analysis to the characteristics of travelling waves caused by breakers closing, together with new method for the travelling wave closing protection, is essential.Recently, a chain of differential relays mal-operation events due to transformer sympathetic inrush when a nearby transformer is energized have been reported. But up to now, the investigations about sympathetic inrush mostly focus on its generation and change characteristic. However, the factors considered are to some extent different from power system practice. Moreover, the inherent reason for the mal-operation mentioned has not been cognized thoroughly. Therefore, it is urgent to have a further research on sympathetic inrush, and to present new measure to improve transformer differential protection after mastering the new rules of sympathetic inrush.The paper studies the two aspects described above. By analyzing characteristics of travelling wave caused by breakers closing, a novel travelling wave protection method accounting for breakers asynchronous closing is presented. On the other hand, by researching new change rules of sympathetic inrush and inherent reason for the mal-operation of transformer differential relays, a novel method which can not only identify sympathetic inrush correctly but also reduce the mal-operation probability is presented.In order to obtain exactly and quickly the polarity and magnitude relationships between forward travelling waves and backward travelling waves under various conditions such as different trouble spots, different fault types and different transition resistance, a new method to calculating the reflection coefficient and the inter-refraction coefficient of every mode under different closing sequence of three-phase breakers, is proposed. Analysis results show that forward travelling waves have the same polarity as backward travelling waves if faults are at the middle of lines. However, if faults are at the end of lines, the polarity relations between forward travelling waves and backward travelling waves depend on fault type, transition resistance and trouble phase closed first or not. In the case of big transient resistance and fault phase breaker closed firstly, or in the case of small transient resistance and fault phase breaker closed secondly, the polarity of forward travelling waves is different from that of backward travelling waves. Thus, the polarity relations are the same with those caused by closing unload lines. As a result, the existing methods will be invalid.A new closing protection accounting for breakers asynchronous closure is presented. The new closing protection is valid not only to the faults at the middle of lines but also to the faults at the end of lines. The new method base on the theory that the reflect coefficient of current travelling wave is–1 if the transmission line is fault-free. Whereas the reflect coefficient of current travelling wave is not equal to–1 if switching into faulted line. Thus, the new method can ensure that the relays will operate correctly in any case theoretically.The transformer sympathetic inrush and its change are influenced by many factors. However, the change rules of sympathetic inrush are not mastered completely because the sympathetic inrush between two different capacity transformers and transformer on load are not analyzed in detail. Aiming at those problems, the characteristics of series and parallel sympathetic inrush are compared, and the effects of load on sympathetic inrush are analyzed as a keystone. As a result, the new change rules of sympathetic inrush are explored which are as the base to recognizing inherent reasons for the mal-operation of transformer differential relays caused by sympathetic inrush.In order to discover the real reason for mal-operation of transformer differential relays caused by sympathetic inrush, the paper analyzes the second harmonic proportion of sympathetic inrush and excitation inrush firstly. Then, the effects of CT saturation on the second harmonic proportion and dead angle of sympathetic inrush are studied. Finally, the influences of Y-delta transform by CTs are analyzed. The results show that the second harmonic proportion and dead angle of sympathetic inrush are similar to excitation inrush. But the decay of sympathetic inrush is more slowly and sympathetic inrush will lead to CT saturation more easily. Consequently, the sympathetic inrush cannot be measured correctly, and transformer differential relays would mal-operate.To confirm the inherent reason for mal-operation of transformer differential relays caused by sympathetic inrush and obtain a countermeasure, the influences of CT saturation on the second harmonic proportion and dead angle of differential current are investigated. Furthermore, the possibility of differential relays mal-operation caused by sympathetic inrush is analyzed accounting for power system practice. The investigation shows that parallel sympathetic inrush has a little effect on differential relays mal-operation whereas the series sympathetic inrush is easy to lead differential relays mal-operation. Finally, a new method for identifying sympathetic inrush is proposed to block differential relays correctly and avoid mal-operation caused by sympathetic inrush.