Study On On-line Diagnosis Of Insulation Degradation Of High-voltage Cable In Coal Mine

Power cable as an important energy transmission element of coal mine power supply system,if it can not monitor its insulation status in real time,it will easily lead to unbalanced operation of the system,which will affect the reliability and safety level of coal mine power supply.In the mines where gas and coal dust accumulate,the electric spark generated by the breakdown of the cable insulation can easily cause an explosion and cause casualties.Therefore,from the perspective of coal mine power supply reliability and safe production,it is required to monitor the insulation status of the power cable in the coal mine power supply system in real time.However,for the 6/10kV three-core mining cable,its leakage current is small when the insulation drops,the signal characteristics are not obvious,and it is difficult to accurately diagnose the insulation state.Although there are many diagnostic methods for existing cable insulation status,the practical application of these methods is not satisfactory.In particular,there is a lack of application experience in mining cables.Based on an in-depth analysis of the degradation mechanism of cable insulation and the partial discharge of void,this paper establishes an improved partial discharge model,presents a method for denoising partial discharge pulse signals,and studies the diagnostic methods using the line model and the integrated features of the line voltage and current signals.In order to faithfully simulate the process of partial discharge of cable insulation,this paper takes the partial discharge of void in the insulating medium as the research object,and establishes an improved partial discharge model of air gap.The partial discharge causes the ionization of the medium around the void,and the accumulation of partial discharge leads to the growth of the void and the growth of the carbonization channel in the medium around the void.According to the principle of partial discharge in air gaps,several key parameters that determine partial discharges are studied.It is pointed out that the growth of the void and the carbonization channels will lead to changes in the insulation properties of the void and the surrounding insulating medium,and it is determined to be variable.The equivalent resistance and equivalent capacitance simulate changes in the insulation properties of the void and its surrounding medium.It is determined that the change in insulation performance of the void and its surrounding medium is simulated with variable equivalent resistances and equivalent capacitances.The initial voltage and the extinguishing voltage of the partial discharge were determined using the initial conditions and the extinguishing conditions of the streamer discharge in the void.The variable equivalent resistance and equivalent capacitance are controlled by the number of partial discharge in the improved partial discharge model,and the flow process of the simulation model is given.The simulation results show that the amplitude and the transient waveforms of the partial discharge signal obtained by the improved PD model are closer to the measured PD signal.Effectively eliminating the noise interference in PD signal helps to improve the accuracy of PD monitoring,and is the key problem to be solved in the practical application of PD monitoring method.This paper first analyzes the measurement method of PD signals,studies the amplitude-frequency characteristics of partial discharge signals before and after the noise,and then studies the basic principles of variational modal decomposition and wavelet thresholding.According to the nonlinear and non-stationary characteristics of partial discharge signals,a denoising method based on variational modal decomposition and wavelet threshold reconstruction is proposed,and the implementation flow of this method is given.The value of two important parameters(the number of intrinsic modalities and penalty factor)in the process of variational modal decomposition is determined,and the selection of wavelet basis,decomposition scale and threshold estimation function in the wavelet threshold is determined.In comparison with several existing denoising methods,such as variational modal decomposition method,empirical mode decomposition method,empirical mode decomposition autocorrelation method,and wavelet threshold method,it is verified that,for low SNR of PD signal,the denoising effect based on variational mode decomposition and wavelet threshold reconstruction method is best.In addition,the denoising of the partial discharge signal obtained by the experiment was performed,which further verified the denoising ability of the proposed method.A reasonable line model can improve the accuracy and efficiency of the cable insulation diagnosis technology,and is the basis and key of theoretical confirmation and simulation to verify the insulation state diagnosis method.The paper analyzes the electrical parameters of the cable and the three parameters models of the line,namely distributed parameter model,?-parameter model and T-parameter model.According to the two-port network equation,the relationship between the electrical parameters of the three models and the voltage and current at the ends of the cable is deduced,and the errors between the insulation parameters obtained from the three calculation relations and the actual insulation parameter values are compared.Combining with the actual situation of mine power cables,taking into account the length of the cable,the complexity of the calculation formula,and the accuracy of calculations,the advantages and disadvantages of the three relationships are analyzed,and it is finally determined that taking the relationship obtained from the?model as a cable insulation parameter calculation formula.Using the vector values of the voltage and current at ends of the cable to perform complex calculations,the insulation conductance and dielectric loss tangent of the cable are obtained.A method of diagnosing the cable insulation state based on double-end synchronous measurement is proposed.The voltage and current signals of the ends of the cable is required to be collected with high accuracy in the method of diagnosing the insulation state based on the double-end synchronous measurement.To ensure the accuracy of the algorithm,the wide-area synchronous measurement technology is used to realize the synchronous measurement of the signals at ends of the cable.The structure of the wide-area synchronous measurement technology and the characteristics and key technologies of the synchronous phasor measurement technology are analyzed.GPS is used to realize synchronous timing in the whole network.Taking into account the influence of current transformer accuracy on the accuracy of the double-end synchronous measurement method,real-time on-line insulation diagnosis of long-distance cable and"quasi-real-time"insulation diagnosis of short-distance mining cables are proposed.The maximum error of the algorithm caused by the accuracy of the transformer is analyzed,and the conditions of the double-end synchronous measurement method for real-time on-line insulation diagnosis are given.When the short-distance mining cable is in the no-load maintenance period,the influence of the accuracy of the transformer on the algorithm is analyzed,and the feasibility of the method applied to the“quasi-real-time”monitoring of the short-distance mining cable during the no-load maintenance period is proved.The effects of the 2nd~7th harmonics in the system,GPS timing error and system frequency offset on the double-end synchronous measurement method were analyzed.It is pointed out that harmonics has no effect on the algorithm.At present,the synchronization error of the GPS timing technology is very small,and the influence on the accuracy of the algorithm is also very small.System frequency offset causes voltage and current phasor calculation errors,resulting in a certain error in the calculation of insulation parameters.When the cable insulation is asymmetric,the zero-sequence voltage is generated and the zero-sequence current appears in each cable line.According to the relationship between the zero-sequence component and the zero-sequence insulation parameters,an on-line diagnosis method for cable insulation status based on decision tree method is proposed.The equivalent circuit of the transmission line with symmetrical insulation,single-phase insulation dropping and two-phase insulation asymmetrical descending is analyzed respectively,and the internal equivalent circuit of zero sequence voltage of different insulation descending type lines is formed.The zero-sequence voltage equivalent circuit of the insulation down line is combined with the zero-sequence network of the system to form a new zero-sequence equivalent circuit.The variation rule of zero-sequence voltage and zero-sequence current with zero-sequence insulation parameters of cable is given.The phase relationships between the variation of the zero-sequence current ?~I_j and the phase-to-ground voltage before and after the single-phase insulation drop and the two-phase insulation drop of the cable are analyzed,and it is pointed out that for a single-phase insulated down-conductor cable,the phase-to-ground voltage of the insulation descent and ?~I_j is in phase,and for a two-phase insulated down cable,the phase of ?I _j is between the phase-to-ground voltages of the two phases of insulation drop,and the calculation flow of ?I _j is given.A decision tree for cable insulation status is established,and a detailed determination procedure for insulation status was given.The calculation expression of the zero-sequence insulation parameters of the insulated asymmetrical descending line is different from that of the insulated symmetrical line.At the same time,combining the phase relationship between ?I _j and the phase-to-ground voltages,the decision tree of the cable insulation state is established,and the detailed determination process of the insulation state is given.This method overcomes shortcomings that the double-end synchronous measurement method can not be real-time on-line diagnosis of mining cable,and other methods can not determine the downside phase of mining cable.The expression of the zero-sequence voltage in the ungrounded and arc-suppressed coil grounding systems is analyzed.It is pointed out that the grounding mode of the neutral point of the system does not affect the relationship between the zero-sequence components and the zero-sequence insulation parameters of the lines.The influence of the three-phase imbalance of the load on the decision-making method is studied,when the decision tree is applied to the ground power grid,and the corresponding modification scheme of the decision algorithm is given.The 6kV neutral ungrounded coal mine power supply system is simulated and analyzed.The simulation results show the superiority and feasibility of the decision tree method.Finally,at the end of this paper we summarize the work done,and look forward to the future research directions.