Research On Boundary Protection Algorithm For Hvdc Transmission Line

High voltage direct current transmission has the advantages of low cost and strong controllability.It has been widely used in the remote optimization of renewable energy sources and the interconnection of asynchronous power grids.Transmission line fault will be directly related to the security of the whole system.Therefore,the line protection is one of the research focuses in HVDC transmission system.Firstly,the frequency characteristics of DC line and its boundary are analyzed in this paper.The boundaries of different topologies have a strong attenuation effect on the tuning components.The line also has a certain attenuation effect on high frequency components and the attenuation increases with the increase of the length of the line.However,the low frequency component is hardly affected by the attenuation.Secondly,the formation and non-stationary characteristics of fault transient are analyzed.As a new time-frequency analysis method,the synchrosqueezing wavelet transforms,which is a new member of the family of wavelet transform,is introduced.It can separate the dense modal response and has the advantages of strong anti-interference ability and low requirement of mother wavelet selection.It’s suitable for the analysis of non-stationary signals.Thirdly,a new method is proposed for the protection of DC line boundary based on the difference of the amplitude of the transient tuning voltage between the internal and external faults.The criterion of boundary unit is constructed by using the ratio of the tuning voltage component and the low frequency voltage component extracted by the synchros-queezing wavelet transforms.The recognition algorithm of internal and external fault is realized.The criterion and wavelet algorithm of starting unit and fault pole identification unit are also constructed.Then,based on the analysis of possible adverse effects on boundary protection and protection rejection of internal fault due to attenuation of high frequency components caused by the long DC line,a boundary protection using two terminal transient for DC transmission line is researched.If the fault is close to the rectifier side,the internal and external faults of the line can be identified by the protection of the inverter side.Conversely,the internal and external faults can be distinguished by the protection of rectifier side.Finally,in order to test the principle and wavelet algorithm of two kinds of line boundary protection,a large number of different fault simulations are carried out by using the power system electromagnetic transient calculation software PSCAD/EMTDC.The results show that the two kinds of boundary protection can accurately identify the internal and external fault of the line,and have a certain resistance to transition resistance.