Study Of Method For Fault Detection Of HVDC Transmission System Electrode Lead

While the electrode lead of a HVDC transmission system only plays the role of earthing and conducting the unbalanced current of the two poles when the system is in balanced bipolar mode,the fault of it will be still a deterrent to the security, reliability and stability of the system. The rapidity of the Differential Time Domain Reflectometry(DTDR) is confined due to the interval of the side to side pulse is confined to be longer than the attenuation time of the pulse which will travel several times between the electrode and the convertor,though the method has the advantage of graeat reliability and accurate positioning compared to other method method for fault detection of electrode lead.The improvement of the rapidity of the method is the navel question to be solved in this dissertation.The principle of the method and factors that effect the rapidity of the method is analyzed,and corresponding improvements are suggested.The rapidity of the method is confined by that the pulse will travel several times between the electrode and the convertor which is caused by totally reflection of the pulse at the end of the electrode line. The transmitting and reflection of the pulse on the line is studied and an appropriate coupling of an resistance with an inductor is suggested to eliminate the reflection,which enables the interval time of the side to side pulse could be shorter than the attenuation time of the pulse.Further improvement of the rapidity is limited by the time the pulse takes traveling once between the electrode and the convertor. The transmitting distortion of the bipolar rectangular pulse is studied with the aid of wavelet transform,on the PSCAD/EMTDC model of the Differential Time Domain Reflectometry method for fault detection of electrode lead.It is found that the pulse width of the bipolar rectangular pulse is kept by the high frequency component around the leading edge and the zero crossing of the pulse.Because of that,the real fault location can be screened out according to the relation of equality between the width of the injected pulse and the width of the fault signal which could be extraced from the high scale of the wavelet transform of the fault signal.It is suggested that the pulses be emitted periodically with the period setted to be the the time the pulse takes traveling once between the electrode and the convertor and within every period pulses with different width are emitted at different moment and every period contains the same pulses.By means of that,the interval time will be no more limited by the time the pulse takes traveling once between the electrode and the convertor,further improvement of the rapidity is produved as a result.The correction of the suggestion to improve the rapidity of the method is confirmed using the PSCAD/EMTDC model of it.