Influence Of CT Saturation On Direction Element And Countermeasures

As an important protection element,directional element is widely used in directional pilot protection and distance protection of transmission line.The direction element can assure protection operates reliably when internal faults occur,and block protection reliability when external faults occur.However,when a fault occurs in the opposite direction of the protected zone and when CT(Current Transformer)is saturated seriously,the data window of full-cycle Fourier algorithm will contain the saturated data,which results in the calculation result fault current phasor being inaccurate.Therefore,the fault occured in the opposite direction will be judged as a positive directional fault incorrectly by direction element,which will cause directional protection misoperation eventually.To ensure that the directional element can operate correctly and reliably under CT saturation,this thesis studies the measures to deal with CT saturation,and designs an improved scheme of direction element based on recognizing CT saturation accurately.The direction element judges the fault direction by comparing the phase relationship between the voltage and fault current at the relay position.However,CT saturation affects the phase calculation result of the current,which will affect the final judgment result of the directional element.Firstly,this thesis studies the factors that affect CT saturation,such as the fault initiation angle,secondary load and residual flux.In order to greatly improve the simulation efficiency,a new method to precisely control and automatically traversal of fault initiation angle in PSCAD has been designed when studying the effect of fault initiation angle under CT saturation.Then,through the summary and analysis of a large quantity simulation data,the primary cause of direction component misjudgement under CT saturation has been obtained.The data window of full-cycle Fourier algorithm contains the saturated zone data,thus the calculation result of the fault current phasor is incorrect and the directional protection misoperates.The secondary current after CT saturation contains saturated zone and linear zone simultaneously.The calculation results which are obtained only by the data in linear zone are correct,so the data in linear zone can be used to solve the problem of directional element misjudgment.This thesis focuses on the two key issues,that is the method of accurately identify the range of linear zone and the method of obtain the discriminant results of directional elements fastly by the data of linear zone.Hower,because the existing CT saturation recognition methods are difficultly to satisfy the requirement of CT saturation fast recognition and the accurate recognition of linear zone at the same time,this thesis proposed an improved scheme of direction element based on the Hausdorff distance algorithm,which can recognize CT saturation fastly and linear zone accurately.Firstly,Hausdorff distance algorithm is used to accurately identify the state of CT saturation and the range of linear zone.Secondly,the short-window fast algorithm is launched in the linear zone to obtain the correct result of direction element.Finally,the correct result obtained in the linear transition period is remembered during saturated zone,until the fault current enters the linear zone again.The proposed scheme obtained the correct result of the directional element only by using the data of linear zone.The simulation results show that the proposed scheme can still operate correctly under CT saturation,which has solved the problem that the direction element operates incorrectly under CT saturation.The performance of proposed scheme is fully verified in protection devices.This thesis studies the hardware structure of the protection device,and designs the main program of the proposed protection scheme.The software flows of the main program include the module of starting-up judgement,the module of Hausdorff distance calculation and the module of short-window fast algorithm calculation.The proposed scheme has been successfully implemented in the device after writing and debugging the programs.The test results show that the directional element in the device can still judge the fault direction correctly under CT saturation,and the proposed scheme can improve the ability to cope with CT saturation when a fault occurs in the opposite direction.