| Transformers are indispensable key electrical equipments in power system,which play the role of energy transfer and voltage conversion.Whether they are safely operated or not is directly responsible for the continuous stability of the power system.Transformer inrush current heavily influences the performance of differential protection.It matters a lot to study the characteristics of the inrush current and to seek a protection scheme which takes both reliability and sensitivity into account.The major work and achievements of this paper is as follows:(1)For inrush current produced during transformer no-load closing status,the decay rate of k0 is adopted in this paper to distinguish between inrush current and internal fault current under no-load closing status.The new method is based on the fact that the attenuation time constant of the current changes a lot under empty charge status and that with internal faults.Simulation results by MATLAB and misoperation datas from the field indicate that the proposed method can not only reliably block the protection when the transformer is in the ultra-saturation condition,but also provide a faster operation speed under slight internal faults,which is a beneficial complementary to the conventional second harmonic criterion.(2)Aimed at recovery inrush current when an external fault is removed,in this paper first is reviewing the advantages and disadvantages of the three-slope characteristic and single-slope characteristic;then examining the operation trace of protection under various operating conditions,finding major differences among each condition.By doing so and associated with field experience,a novel scheme based on dual-characteristic percentage restraint curve is presented.The new scheme puts three-slope characteristic in service all the time while puts single-slope characteristic in service for a limited period after the protection is enabled.Simulation tests on RTDS show the validity of the proposed scheme.(3)A new identification method of transformer no-load closing state is put forward.The significance of the method lies in that if the protection is recognized as a non-empty state of the transformer after starting,the locking condition should be relaxed to speed up the protection,while if it is recognized as the empty state of the transformer,the more stringent locking measures are used.The proposed method is both simple and reliable,and is easy to implement. |
