Analysis Of The Grounding Electrode Line Impedance Supervision System And The Study Of New Protection Principle

Compared with AC transmission technology,HVDC transmission technology has obvious advantages in large capacity and long-distance power transmission,so it has a broad application prospect in China.In recent decades,a large number of HVDC projects have been built in China.Grounding electrode system plays an important role in HVDC system since it is used to keep the voltage of the neutral point steady and provide a path for the unbalanced current to ground.With the length of grounding electrode line increases,the probability of the line fault become not negligible.However,the project operation experience shows that the existing electrode line impedance supervision(ELIS)is not perfect,and there is a risk of protection rejection in some cases.Therefore it is of great theoretical and engineering significance to analysis the performance of electrode line impedance supervision system and put forward a new protection method from other perspectives.In this paper,the modal analysis method and Karenbauer transformation matrix are used to decouple the grounding electrode line into ground mode and aerial mode network for convenient calculation.By using the modal analysis method,the measured impedance of the ELIS system in the normal operation of the grounding electrode line is analyzed,and the analytical expression of the equivalent impedance of the grounding electrode line measured by the ELIS system under different fault conditions is derived.The analysis shows that the measured impedance of ELIS system is half of the ground mode wave impedance while the line without fault,which is close to a pure resistance.While in the case of single line fault of the grounding electrode line,the fault impedance is related to the aerial mode parameters and ground mode parameters of the grounding electrode line,while in the case of double line fault of the grounding electrode line,the fault impedance is only related to the ground mode parameters of the line.In this paper,the influence of different factors on the adaptability of ELIS system is analyzed by theoretical derivation and PSCAD simulation.Theoretical analysis and simulation show that the transition resistance and fault distance will have a certain impact on the ELIS system.In case of high transition resistance or long-distance fault,the ELIS system may refuse to operate.If the length of the grounding electrode line is an integral multiple of the half wavelength of the injected signal aerial mode,there will be some special fault points on the grounding electrode line,after the single line ground fault or single line break fault occurs at these special fault points,the change of equivalent impedance of the grounding electrode line is very small,and the ELIS system will inevitably refuse to operate.This paper points out that it is necessary to select the injection signal frequency of ELIS system according to the length of grounding electrode line and the actual line parameters in order to avoid special fault points.The effective protection distance of ELIS system can be increased by reducing the attenuation constant of the line by properly reducing the frequency of the injected signal.Through theoretical and simulation analysis,it is found that the voltage spectrum of the first terminal of the grounding electrode line will change after the fault occurs.In this paper,a new method of grounding electrode line protection is proposed,which obtains the different frequency band components of the first terminal voltage of the grounding electrode line by using wavelet packet analysis,and take the amplitude change of the multi frequency live voltage as the protection criterion.PSCAD simulation shows that the proposed protection method can effectively identify the fault on the grounding electrode line,which is less affected by the transition resistance,and does not maloperation in the case of DC line fault.