Research On The Transient Characteristic Of High-impendence Transformer With Current Transformer And Its Impact On Protection

With the rapid development of electric power technology in China,the system capacity is bigger than before,which results in the short circuit current become higher and higher.So it is important and significant for the stable and safe operation of power system to limit the short circuit current,especially the short circuit current at the low voltage side of transformer.Compared with the other methods,using high-impedance transformer is easier to operate and more stable,and it can save lots of space and costs.So high-impedance transformer becomes the most popular way to limit system short circuit current and is widely used in our system.High-impedance transformers have lots of structures,while 500 kV and 220 kV power system almost use the high-impedance transformer which put the high-voltage winding closest to the core and the transformer which put reactor in series with the low-voltage winding.However,when the high-impedance transformer which put the high-voltage windings closet to the core is energized without load,there happened many line zero sequence current protection mal-operations which is at the high voltage side transformer.The occurrence rate of zero sequence current protection mal-operations is very high,and its reasons have not been found,which seriously threatens the safe and stable operation of power grid.So it is emergent to study the inrush current characteristic of high-impedance transformer,the transient characteristic of current transformers(CT),the sympathetic interaction between high-impedance transformer and current transformer,and its impact on protection.In order to reveal the reasons which cause the zero current protection operations when the high-impedance transformer which put the high voltages winding closet to the core is energized,the inrush current characteristic of high-impedance transformer is analyzed in this paper.The difference of structures and electric parameters between high-impedance transformers and traditional transformers are analyzed.Therefore,the different inrush current characteristic of different structure transformers is studied,and the cause of accidents is revealed.In this paper,simulation models of different transformers which can discretionarily distribute the transformer impedance at all side is build based on PSCAD.The comparison with the actual recording data shows the accuracy of the model.Then,the magnetizing inrush current and sympathetic inrush current characteristic of different transformers is analyzed as well.The analytical analysis of current transformer transient characteristics is very difficult because there are many factors have influence on it,so it is effective to study CT characteristics by the simulation model.The most commonly used CT model are J-A model and Lucas model,while there are few research on Lucas model and the model methods of practical CT based on Lucas theory.So the difference between J-A theory and Lucas theory are analyzed firstly.A simulation modeling method of practical CT based on Lucas model is put forward in this paper.Different dynamic simulation tests verify the accuracy of the model.The difference between J-A model and Lucas model in different saturation states are analyzed and the chosen and use of these two models is advised in this paper.Aiming at there have none method to avoid the zero protection mal-operation,the impact of different transformers transient characteristics on CT is analyzed and the sympathetic interaction between different transformer and CT and its impact on protection is analyzed too.Combined simulation with actual recording data,the inrush current characteristics of high-impedance transformer which put the high voltage windings closet to the core is summarized.At last,choosing the optimal three phase or single phase switching angle according to the three phase remanence,add a resistance in the energizing circuit,improve the zero sequence current protection criterion are put forward to restrain the inrush current of high-impedance transformer and avoid the protection mal-operation.At the end of this paper,the main research work and the results are summarized,and the directions of further research work are prospected.