Research On Voltage Balancing And Fault Detecting Technology Of Supercapacitor

With the development of new energy technology, inorder to absorb the random and temperaly connected wind and solar source,energy storage system is becoming an indispensable part of microgrid.The supercapacitor energy storage system based on the power electronic technology has bright prospect with the advantage of fast response speed, easy arranagement,low maintenance cost and optimal cost-effectiveness ratio.Howerver.because supercapacitor has the disadvantage of low energy density and low voltage of cell,the supercapacitors have to be integrated in large scale for application in power grid. Tehnology problems are emerged in large scale integration process. The first problem is voltage balance.This problem is caused by disperation of parameters.This problem influences the reliablity and integrated scale of energy storage system.The second problem is fault detection.This problem is how to select fault capacitors before parameter identification with lower sampling rate and resolution. The third problem is online capacitance measurement and parameter identification.This problem is how to measure the capacitance and indentificate the parameters of supcapacitor while the capacitance is variable.This paper researched the three key problems of supercapaciotor energy storage system. The control strategy of charging process was also designed in this paper. The main reseaching results were shown below:First, this paper presented a novel voltage equalizer named parrell connected capacitor equalizer. The analysis method based on enhanced discrete time domain method was proposed. The analysis method used surfaces of state variables to analysis the equalizer circuit.The simulation experiments were carried out to verify the effect of voltage equalizer and the accuracy of analysis method.A voltage equalizer prototype was made to validate the computation and the simulation result.The numerical computation results were verified by simulation experiment and prototype experiment.The numerical computation, simulation experiment and prototype experiment showed that the circuit has evident effect of voltage equlazation in the states of charge, discharge and static. The simulation result indicated that the effect of equalization has weak ties with series connected supercapacitor amount.The experimental results demonstrated that the proposed circuit had low requirement of sampling resolution and computation burden for voltage balancing process. The proposed circuit solved the voltage euqlazation problem better than current methods.Second, a new fault detection method based on k-means cluster was proposed in this paper. This is the fist time to apply the cluster method in field of supercapacitor fault detection. This method detected fault basing on the voltage signal data of cell.The k-means cluster method was applied to analysis the sampling feature vectors extracted from voltage of supercapacitor cells.In order to find the optimal k value,a new validity index named Euclidean distance index was proposed.The fault subsets were detected in the cluster resultThe result provided that the abnormal group of supercapacitors can be detected by the proposed method in various situations.The first advantage of the proposed method is that the identification of capacitor parameters is avoided.The second advantage is that this method has low dependency on precise of sensor’s data. Because of the advantages above,the proposed method is convenient for engineering implementation.Third, this paper proposed a capacitance measurement method and a parameter identification method for supercapacitor. Through the measuring equations infered by this paper,the instant values of variant capacitance were measured indirectly base on voltage and current data.The least square method in limited memory based on proposed measuring equations was used to identify the parementers of capacitor function. The experiments of simulation were designed to verify the method.The experiment results support that the proposed method can identify the capacitance function paramenters in random charging and discharging state while the capacitance is linear function or quadratic function.The capacitance value can be indentified preicesely through parameter identification.The meauring precsion of proposed method is improved comparing with existing piecewise linearization method.The adapablity to nolinear variance of the proposed method is obviously improved.Fourth, the dynamic segmentation charging control strategy and online fault detection system were designed.The control state machine for charging control was introduced.The software process of online fault detection system was designed.