The Asymmetric Issues In Power Systems With Un-transposed Transmission Lines And Its Solution

Along with the concentrated load in the developed area,the rare transmission corridor,and the widely used technology of two-circuit on the same tower,the 220k V stations are densely covered in the specific small area.The asymmetrical parameter caused by short un-transposed transmission Line lead to an asymmetrical 3-phase current and voltage in normal operation system.The asymmetric problem of untransposed transmission line will not be so critical if the system load is heavy. With the expanding of the power system,complex structure of the grid and diverse operation mode,more and more untranspoesd lines get asymmetric problem when the load is not so heavy.The asymmetric problem caused by the asymmetric 3-phase transmission lines parameter leads to the failure of relay protection.The traditional relay calculation is based on the hypothesis of 3-phase symmetry,The larger negative-sequence component and zero-sequence component that decomposed from the asymmetric phase voltage and current lead to the relay malfunction and the failure of auto-reclosing in some specific circumstances.During the power system restoration,the negative-sequence component will cause multi-generator tripping,generator synchronization failure and so on,it's very likely caused large-scale power failure.For the common 3-phase asymmetric problem in power system,this paper firstly present a 3-phase power flow calculation model that based on phase component.Take a real 220kV ring transmission system as example,use the model and the CIM(Current Injection Method) to simulate the phenomenon,make the verification on this model through the comparion of the CIM result and the EMTP result. Secondly,analyse the asymmetric problem and proposed the solution.Thirdly,based on the two steps before,design a typical double-circuit grid to verify the correctness of this model and the solution,then propose a universal solution about the asymmetric problem.