Analysis Of Influence On Zero-sequence Protection And Differential Current Protection In Mixed-voltage Transmission Lines System On The Same Tower With Cross-voltage Fault

With the continuous development of UHV grid,the contradiction between land resources and power grid construction has become increasingly prominent.In order to save the transmission corridor,based on the existing double circuit lines,the transformation and expansion of multi-voltage grade lines has become an inevitable trend.Meanwhile,the complexity of the power grid structure is also increased,which brings difficulties to the fault analysis of the circuit.Especially in the case of cross voltage level fault,the coupling effect between the circuit parameters and the asymmetry of line parameters increased the difficulty of the analysis.More than this,the fault feature which is reflected by the case of cross voltage level fault has a serious impact on the relay protection device which is equipped with the original line,which makes the security of power grid operation under threat.The paper mainly aimed at the analysis of the cross-voltage short circuit fault in mixed-voltage transmission lines system.Then the influence on zero sequence protection and current differential protection of the lower-voltage system were analyzed.In the aspect of fault analysis,the single phase cross-fault is taken as an example,according to the similarities of boundary conditions when cross-voltage fault occurred in the two double-circuit lines,an improved composite sequence network unified the two systems can be obtained too.The principle is simple,concept is clear,and easy to calculate.For zero sequence protection,two chapters were used to discuss the influence on the zero-sequence power direction component and zero sequence pilot protection.Action of zero sequence directional element is mainly affected by the fault location and phase difference of the power supply between the two systems.Zero sequence pilot protection is mainly affected by the phase difference of zero sequence voltage at both ends of the line,and it is concluded that the external faults may cause the malfunction of the protection.For differential current protection,the paper focuses on the analysis of the influence on cross-voltage fault on the protection sensitivity when the load current is considered.It is concluded that when the phase difference of the power supply between the two systems close to zero,the brake capacity may be greater than the amount of action,which causing the protection fail to operate.All the results were tested in a 500 kV/220 kV four-circuit transmission lines build in PSCAD at the end of every chapter.