The Sever Problems Of Research About Transformer Differential Protection In Practical Application

Abstraction: With a hundred development of the power system , it's very popular for long distance transmission with super-voltage and large capacity ,then the large transformer is very important in the power system. This paper discussed the following three problems from the practice: When by-pass bus breaker instead of 220kV transform breaker, it is proposed the methods of solve the problem of the dead area ; the selection the type of CT which is used as transformer differential protection in the 500kV transform.( 1 ) This paper first analyzed the character of the wave of inrush which was produced by the transformer put in no-load and voltage resumed by the excising the extra-contingency ,then pointed out the deficiencies of several restraint principles which was used in the transformers differential protection in the practice. According to the requirement of " against accident mothed" made by the National Grid Company, this paper proposed a project of configured of differential protection which was used a principle of a secondary harmonics restraint and a waveform symmetry principle,and made both of them complemented in the practice.(2)This paper proposed a problem of solving the dead areafrom the practice when by-pass bus breaker instead of 220kV transform breaker. It pointed out this protection dead area was very danger for the practice work, so it should be attach importance to it. Through the analyzed this problem from the theory and take into account the practice, this paper proposed a method of solving the protection dead area.(3) About the he selection the type of CT which is used as transformer differential protection in the 500kV,when CT have different type or large difference capacity iron-cored,they will have different saturation degrees in the course of short circuit contingency or transitory state with extra-contingency; even unsaturation because of great difference excitation character, there will product a large unbalance current. In the practical work, CT installment in the transform work under the different voltages , there transient character always not same as each other, so in the condition of the extra-contingency there will have unbalance current in the differential protection loop of transform. 500KV transform differential protection is act in the course of transient of short circuit, so it requires CT can work in all kind of transient work condition. CT with iron-cored have good transient work character, if transform use iron-cored CT ,it is very benefit to differential protection, but iron-cored CT is very expensive, this paper proposed a selection principle of iron-cored CT as a transform differential protection used in 500KV transform.