Research On Online And Real-time Evaluation Method For The Measurement Error Of Capacitor Voltage Transformer

Capacitor voltage transformer is the measurement device transforming high voltage signal into low voltage signal.Because of long-term running under harsh environment,some CVTs’ measurement error fluctuates that results in inaccurate energy metering.In order to assure fair trade in electricity,power companies calibrate CVT every 4 year using the periodic calibration method recommended by the standard JJG 1021-2007 Instrument Transformer in Power System.It uses highly accurate voltage transformer to calibrate CVT after disconnect CVT from grid.Because it is difficult to stop the substation and the calibration procedures are complicated,calibrating all CVTs in grid every 4 year is nearly impossible.Thus,there are some CVTs exceeding permitted measurement error running in grid.It is hard to calculate the electricity incorrectly metered by these CVTs.So,evaluating the measurement error of CVTs in real time makes great significance to fair trade in large number of electricity and focused by researchers in electromagnetic measurement field.The cyber-physics fusion method attracts great interests these days because it can obtain information from numorous data that have physical relation.It organizes CVTs physically related in a substation as a group,and develops the constraint from the cyber-physics relation.Then,it establishes evaluation model following the constraint to evaluate single CVT’s measurement error in the group.However,previous researches foucus on single mode problem and develop a single-mode method which divides CVTs in several groups using a certain way.The single-mode method cannot divide group dynamically as the CVT’s measurement error changes.Therefore,it can only solve specific problem.The method works not well in complex conditions such as automatic voltage control(AVC),unbalanced load and multiple CVTs’ faults.In order to improve these shortcomings,a self-adaption and multi-mode evaluation system is proposed to evaluate CVT’s measurement error online without standard voltage transformer.Firstly,two single-mode methods are developed in this paper to improve effectiveness and adaptability of online evaluation method while dealing with complex conditions of primary voltage adjusting and unbalanced load.Combining these two methods,this system divides CVTs in one substation into several groups following the rule rigid constraint prior to flexible constraint and establishes multi-mode evaluation model containing initial mode and abnormal mode.By dividing evaluation group flexibly and switch model adaptively,this system ultilize these two methods to realize long-term online evaluation for CVT’s measurement error.These evaluation methods and system have been piloted in a substation and the effectiveness has been verified.The research work is listed as follows:1)An online evaluation method dealing with multiple non-Gaussian variables is proposed.It divides three CVTs measuring three-phase voltage as a group and analyses non-Gaussian distribution of voltages under AVC.Then it reveals that three-phase voltage contain several non-Gaussian variables and establishes evaluation model for multiple non-Gaussian variables based on three-phase voltage balance constraint.Next,the independent component analysis is studied to solve this model to solve CVT’s measurement error evaluation in the substation with AVC.Theoretical analysis and experiment in laboratory indicate this method has better performance in error detection than the existing principal component analysis and also meet the requirement of CVT with 0.2 accuracy class.2)An online evaluation method based on coherence between CVTs on same phase is proposed.It divides CVTs measuring the same phase voltage signal as a group and reveals the coherence between CVTs connecting on the same phase.Then,it establishes evaluation model for same phase CVTs.Besides,a nonparametric method with higher sensitivity is developed to solve this problem.Since it is a strong constraint and do not involve non-Gaussian variable voltage unbalance factor,the evaluation model as well as its solution is simple.Theoretical analysis and simulation result indicate this method meets requirement of CVT with 0.2 accuracy class and the nonparametric method is of higher sensitivity than the traditional parametric method.3)Combining two single-mode evaluation methods above,a self-adaption and multimode evaluation system that can divide evaluation group dynamically is proposed.It divides all CVTs of same voltage level in one substation into several groups following the rule that rigid constraint prior to flexible constraint and establishes initial mode evaluation model.Then,it reveals the principle that different CVTs have different influence on the constraints of the group.Next,it redivides CVTs into groups by substitution and elimination faulty one in the group in case that the constraint is disobeyed.Besides,it establishes evaluation model for abnormal mode and switch models when the evaluation mode changes.Because it divides evaluation groups dynamically based on CVT’s measurement error changes rather than permanently,it can evaluate CVT’s measurement error after the error changes which can not be accomplished by the single-mode method.The simulation indicates that the evaluation problem of CVT in substation containing single fault and multiple faults is solved by this system.4)An evaluation platform for measurement error of CVT is developed.During its operation in a substation,a measurement error fault is detected and subsequent calibration verifies this fault.The application indicates the online evaluation method proposed in this paper is effective and can give power companies valuable advice in equipment maintenance.This paper proposes a self-adaptive and multi-mode evaluation method to evaluate measurement error of CVTs in one substation.Thus,it assures the accuracy of electricity metering and relieves the dispute in energy trade.Since cyber-physics fusion method does not rely physical model of CVT,it can also be applied to voltage transformer of different principle.