Research On Technologies Of Shunt Capacitor Operation Condition Monitoring Based On Disturbance Data Analysis

With the expansion of power system and the increase of user load,the demand on reactive power is growing rapidly.In order to ensure the stable operation and improve the power quality of distribution network,a large number of shunt capacitors are used in the network to compensate reactive power,improve power factor and provide voltage support.However,energizing capacitor may produce transient disturbances such as overvoltage and impulse current,which may bring harmful impacts to the distribution system,especially to the user equipment.Therefore,monitoring the capacitor energizing condition on line is helpful to identify the transient disturbance source and take the targeted suppression measure.Moreover,the switching condition of a capacitor(energized or de-energized)is closely related to the node voltage and the reactive power flow of the distribution system.Monitoring the switching condition of each capacitor can grasp the changing rules of voltage and reactive power,and provide technical and decisive support for planning and optimizing the distribution grid operation.In addition,in order to prolong the lifetime,shunt capacitors are usually designed with internal fuses.With the aging and deterioration of the insulation dielectric,the capacitor elements will be broken down one by one under the overvoltage.After a certain amount of elements broken down,the relay protection will switch off the capacitor,which results in that the capacitor can not continue to provide voltage support and reactive power compensation for the distribution network.If the protection fails to switch off the capacitor,the fault current will cause the shell deformation and rupture and event lead to explosion in serious cases.Therefore,monitoring the capacitor health condition on line can help to provide effective support for improving intelligent capacitor operation and maintenance,and ensure the economy and safety of distribution network operation.Both capacitor switching operation and capacitor element breakdown fault will cause obvious transient disturbances of voltage and current in distribution network,so detecting,identifying and mining the transient disturbances caused by these two inducements can realize the online monitoring of capacitor operation condition(switching condition and health condition).Due to the large number and scattered distribution of capacitors,it is difficult to install communication devices on each capacitor for the monitor purpose.Therefore,this paper proposes to acquire the capacitor operation condition information and realize the condition monitoring by detecting,identifying and analyzing the transient disturbances of the real-time waveforms monitored by the key nodes of the network.For the transient analysis of capacitor energizing,considering that it is difficult for the three-phase capacitor switches to exactly synchronously close in practice,based on the three-phase simplified circuit for capacitor energizing,the transient characteristics of capacitor asynchronous energizing under the influence of neutral point operation mode are analyzed and compared with the characteristics of synchronous energizing.Moreover,the effect of the three-phase circuit asymmetry on the characteristics is investigated.Correspondingly,the characteristic extraction method in the combination of time domain,frequency domain and time-frequency domain is proposed,the disturbance identification algorithm based on SimpleMKL-SVM is studied and the capacitor location method is investigated in order to realize the on line monitoring of capacitor energizing condition.The simulation and experiment results have verified the transient characteristics of capacitor asynchronous energizing,the effect of three-phase circuit asymmetry as well as the feasibility of capacitor energizing condition monitoring method.For the capacitor switching condition monitoring,in order to realize the accurate location of switching capacitor with a few monitor devices and to meet the demand on monitoring both energizing and de-energizing condition,this paper proposes to monitor the capacitor switching condition by using the unique transient changes of three-phase reactive powers caused by the capacitor asynchronous switching in the setting scheme.The coded method by setting a unique combination of a three-phase switching order and time interval is provided for different kinds of capacitors in the distribution networks with different neutral point operation modes.Moreover,the decoding method for the transient changes of reactive power is also provided,which includes the detection method for the transient change based on the threshold and the false alarm probability as well as the determination method for the operation mode,order and time interval of the coded capacitor.In addition,the practical implementation is elaborated,and the applicability is analyzed.The simulation and experiment results verify that this proposed monitoring method only requires one waveform monitor located at the substation to monitor the operations(energizing or de-energizing)of downstream distribution capacitors without being impacted by the switching interference of other reactive power compensation devices as well as the transformer measurement error and the background noise,and satisfies the power quality requirement of three-phase voltage unbalance.For the capacitor health condition monitoring,considering that the weak steady-state voltage and current changes caused by the capacitor element breakdown fault are easy to be disturbed and difficult to measure accurately,the transient disturbance caused by the element breakdown fault is proposed to monitor the capacitor health condition.Based on the three-phase simplified circuit for element breakdown fault,a transient response analysis of the fault in the time domain and frequency domain is used to establish the transient characteristics.Correspondingly,the characteristic extraction method in the combination of time domain,frequency domain and time-frequency domain is studied,and the multiple-kernel SVM algorithm based on trace-margin is adopted to identify the transient disturbances caused by the fault.A method for determining the faulty capacitor unit is proposed based on the position of the waveform monitor.Under the worst case analysis,a scheme for assessing the health condition is investigated.The simulation and experiment results verify the transient characteristics of element breakdown fault and the feasibility of capacitor health condition monitoring.This paper realizes the on-line monitoring of capacitor operation condition by detecting and identifying the transient disturbances related to the operation condition from the real-time waveforms monitored by the key nodes of the network,which can provide a reasonable and effective research idea and method for the related research about the on-line monitoring of the power equipment operation condition.