| VSC-HVDC has the characteristics of small damping and low inertia and therefore the fault developes very fast under line fault.Hence,it is very essential to identify the fault quickly and reliably for stable system opearation.Currently,VSC-HVDC line protection mainly refers to the more mature line protection methods in LCC-HVDC.However,there are still many problems in the application of the LCC-HVDC line protection method in VSC-HVDC.The essence of the problem is mainly reflected in that VSC-HVDC proposes more stringent requirement in protection speed and sensitivity.Meanwhile,the reliability and selectivity of the protection should be considered.Therefore,VSC-HVDC has higher requirements for the line protection speed,the ability of weak fault feature extraction,anti-interference ability and the division of faults in different regions.In addition,the transmission line in VSC-HVDC not only has a large span,but also works in a complex environment,which appears as a hybrid line composed of multiple section of overhead line and underground cable or submarine cable.This situation brings great difficulties to line inspection.Therefore,how to accurately locate the line fault is important to fault recovery and post-maintenance.For the traveling wave(TW)based fault location,which is relatively mature in theoretical basis and engineering experience,TW dispersion has more significant impact on long-distance line fault location.The serious attenuation distortion of fault wave head leads to the difficulty of wave head calibration.Especially in the case considering mixed line fault,high resistance fault and noise interference,the ability of head recognition is obviously insufficient,which eventually leads to the poor accuracy and reliability in TW fault location.In this context,this paper deeply analyzes the transient characteristics and TW propagation characteristics in VSC-HVDC line.Taking time-frequency analysis based fault feature extraction algorithm as the starting point,the paper proposes the corresponding line protection principle and fault location method for the key problems in line protection and TW fault location.The research results include:Firstly,VSC-HVDC line protection method based on single-ended transient current is proposed.The transient process of current under line fault is analyzed in depth.The fault characteristics are extracted by using wavelet transform modulus maxima and the protection criterion with priority is constructed according to the fault severity,which improves the sensitivity and rapidity of protection under metallic fault.The fault area is identified based on the transient current fault characteristics at different outlets of the local converter station,which effectively solves the problem of maloperation caused by serious external fault.Then,the transient current characteristics under lightning disturbance and different line faults are analyzed and compared.The wavelet packet decomposition energy entropy based pilot protection is proposed.Given the information theory and entropy in thermodynamics,the transient current distribution pattern in the whole band is revealed.The protection criterion based on the energy distribution uncertainty of transient current in the wavelet domain is proposed,which improves the distinction of different faults and the protection sensitivity under the single pole to ground fault with high impedance.Besides,based on the transient current characteristics difference between line faults and lightning disturbance within the medium and high frequency range,an auxiliary criterion using multiple band energy ratio under wavelet packet decomposition is proposed to differ non-fault lightning strikes,and thus effectively improve the protection robustness against lightning disturbance.Secondly,based on the difference of the waveform similarity between forward TW and backward TW at two sides of the protection line,waveform similarity based pilot protection using improved Hausdorff distance is proposed.Based on the traditional Hausdorff distance,the Euclidean distance calculation method is changed from exhaustive one to ordered one,which reduces the calculation amount and improves the protection speed.Then a set of auxiliary criteria based on the ratio of reverse shortest Euclidean distance and dynamic variation is introduced to screen the effective shortest Euclidean distance,which effectively improves the protection tolerance to noise interference,data abnormality and asynchronous data.The similarity between the forward TW and backward TW is calculated by using the improved Hausdorff distance,which can not only improve the fault resistance withstanding ability of protection but also ensure the protection selectivity under the serious external fault.Thirdly,based on the propagation characteristics of fault TW in dc line,TW fault location method based on adaptive layered denoising principle is proposed for hybrid line.Based on the original denoising function of stationary wavelet transform,the de-noising mode is adjusted by double adjusting factors to enhance the singularity of mutation point in reconstructed signal.The layered denoising threshold and the modified factor based on the peak to sum ratio of detail coefficient are proposed to adaptively de-noise TW signal,which can effectively retain the fault characteristics while suppressing noise.The denoised signal is processed by singular value decomposition based on Hankel matrix to identify wave head.It not only avoids the influence of phase shift on the wave head calibration in time-frequency data processing method,but also greatly enhances the singularity of the wave head when the fault TW arrives.Thus,the impact from TW dispersion and high impedance fault on the head calibration can be overcome,and accuracy and reliability of TW fault location are improved for hybrid line.Finally,given that the fault location reliability is poor when the communication fails,a single-ended TW fault location method based on singular entropy optimization and variable mode decomposition(VMD)is proposed as backup support.The objective function based on singular entropy theory is proposed to optimize the number of intrinsic modal functions(IMF)and the secondary penalty factors in VMD to obtain the most singular decomposition results.TW front is calibrated by peak point in Teager energy spectrum of decomposition result,which improves the ability of extracting weak fault features and the accuracy and reliability of wave front calibration given high impedance fault and serious wave dispersion.In addition,the correlation between the fault location and RLH in the first wave head is analyzed.Then,the arrival time difference of continuous wave head is predicted to classify the continuous wave fronts.Finally,the wave velocity is adjusted based on the central frequency of IMF,which further improves the accuracy of single-ended TW fault location.Aimed to improve the protection performance for VSC-HVDC,focusing on line fault identification and TW fault location.A variety of line protection and TW fault location methods based on time-frequency analysis algorithm are proposed.The effectiveness of these methods is verified by electromagnetic transient simulation.The fault feature extraction algorithm and noise suppression principle proposed in this paper can not only enhance the protection performance in VSC-HVDC,but also improve the accuracy and reliability of TW fault location.Besides,it provides reference for the research of system operation status monitoring and internal fault diagnosis of converter. |
PDF Full Text Request