| Flexible HVDC transmission has the advantages of no commutation failure,independent control of active and reactive power and real-time power tracking.These advantages provide technical support for the construction of multi terminal DC power grid and the promotion of large-scale consumption of renewable energy in the future.DC line protection is the key to ensure the safe and stable operation of flexible DC transmission system.Aiming at the problems of insufficient withstand transition resistance and anti-interference ability in the existing traveling wave protection,this paper improves the reliability.The specific work contents and results are as follows:Firstly,the most serious fault condition of flexible DC transmission line,that is,the fault current in case of inter pole short circuit fault,is analyzed,and the reasons why the protection needs to distinguish the fault in a short time are given.Then the coupling characteristics of DC line,the propagation characteristics of traveling wave and the frequency variation characteristics of line parameters are analyzed,and the frequency domain expressions of voltage traveling wave under different fault types are given,It lays a foundation for the follow-up protection research.Secondly,a single ended protection method for flexible DC transmission line based on principal component analysis and support vector machine is designed according to the difference of traveling wave characteristics of internal and external line modulus traveling wave: 1)The PCA dimensionality reduction preprocessing is carried out on the internal and external fault training samples,and the feature matrix for internal and external faults is constructed;2)SVM is used to preprocess the feature points of internal and external fault training samples in the feature space formed by PCA dimensionality reduction,and the hyperplane equation dividing the internal and external faults is obtained,which is used as the protection setting criterion;3)The feature matrix obtained by data preprocessing is used to project the linear modulus fault traveling wave data to form characteristic points,and the internal or external fault will be correctly distinguished by the hyperplane division area to which the feature point belongs.In case of internal positive grounding fault,the polarity of traveling wave line modulus and zero modulus is the same,and the waveform and the area formed by the zero axis contain a large number of overlapping parts.In case of internal negative grounding fault,the polarity of traveling wave line modulus and zero modulus is opposite.The amplitude of traveling wave zero modulus in case of internal pole-to-pole short-circuit fault tends to be close to 0.According to this feature,the ratio of zero modulus integral to linear modulus integral is used as fault pole selection criterion.Finally,the Zhangbei four terminal flexible DC power grid is used to simulate and verify the designed protection scheme,and the sampling frequency of the protection algorithm is 10 k Hz.The simulation data show that the protection scheme can identify the fault and select the correct fault pole within 1.5ms,and the protection has good rapidity and sensitivity;The transition resistance value of high resistance grounding fault at the far end of the protection can reach 300Ω,the protection can withstand the noise interference with signal-to-noise ratio of 20 d B,and the protection reliability is good;The number of training samples that may affect the protection action results and different current limiting inductance values on both sides of the line are simulated and analyzed.The simulation results are within the setting range,and the protection has good adaptability. |
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