Fault Location Techniques For Non-Effectively Earthed System Based On Transient Signals

Fast development of economy and improvement of living standards lead to requirements to higher power supply reliability. In the countries which the power market reform has been carried out, supply reliability is the important indicator of power company performance supervised by the State Electricity Regulatory Commission. In this situation, the development and application of power line fault location technique is paid more attention to by power utilities, which intend to shorten time of fault searching and repairing, and improve the supply reliability and customer satisfaction.In China, most MV distribution networks (6-66kV) employ non-effectively earthed neutrals. In such systems, when a single phase to earth (SPE) fault occurs, the faulted system can be permitted to keep service for 1-2 hours without removing the fault, which reduces the probability of more serious accidents. Unlike effectively earthed systems, the fault current caused by SPE fault in non-effectively earthed systems is very small, and the steady characteristics are inconspicuous. So the fault location issue for SPE fault in non-effectively earthed systems has not being well solved for a long time. The existing techniques have many shortages, such as low sensitivity, inaccurate mathematic model, especially in the aspect of inadequate utilization of transient signals. Therefore new fault location techniques need to be developed in order to meet the requirement of non-effectively earthed systems.The algorithm and application technique of fault distance estimation for SPE fault is researched in this paper. With analysis to the fault characteristics of SPE fault, the fault distance equation can be gotten based on the measured transient voltage and current. Then the fault distance can be gotten by solving the equation.According to the above key thought, a series of corresponding researches were carried out. At last a new fault location estimation algorithm is proposed, which is suitable to the SPE fault in non-effectively earthed neutral network. The result of ATP simulation and static simulation has proved its accuracy and practicality. The detailed research works include:(1)The transient and steady characteristics of SPE fault in non-effectively earthed neutral network are analyzed, and the transient signals are researched in detail. From the analysis, the steady fault signal is not suitable to fault estimation; especially for the compensated system, the methods based on the steady signals are invalid totally. But the amplitude of transient signals is bigger and has an obvious variety, and in the transient analysis, the Peterson coil can be neglected, which is convenience to the fault location estimation of non-effectively earthed neutral network.(2)The traditional equivalent circuits of power lines are analyzed and a newΓ-mode equivalent circuit is proposed. The accuracy ofΓ-mode circuit is proved by analyzing its two-port external characteristics and impedance-frequency characteristics. The employing ofΓ-mode equivalent circuit can reduce the net nodes and simplify the fault equation. The analyzing results proved thatΓ-mode equivalent circuit can equal medium length power lines in the selected frequency band. The adoption ofΓ-mode equivalent circuit lays a solid foundation for SPE fault mathematical model.(3)The SPE fault can be decomposed to aerial mode component and earth mode component by Karrenbauer transformation. And based onΓ-mode equivalent circuit, the modal network of SPE fault can be built with the boundary condition. Further the fault location estimating equation can be gotten. The equation shows the relationship of variables (voltage, current) at bus end, faulted line parameters and earth resistance.(4)The Laplace fault equation is discretized in frequency domain using bilinear transformation, and then the corresponding difference equation in domain can be gotten. If the fault differential equation is transformed to difference equation directly, the estimation accuracy is worse. Therefor the differential equation in s domain is transformed to Laplace equation in first, and then the Laplace equation is transformed to z equation by bilinear transformation. At last the difference equation in time domain is gotten with z transformation definition, which the difference equation is the fault estimation equation including voltage, current, faulted line parameters and earth resistance. During the transformations, frequency alias is avoided and the accuracy of fault difference equation is improved.(5)The overdetermined equations of SPE fault are solved by the least square method. If only few samplings is used to solve the fault equation, the calculation error is bigger and the result is instable. In this paper the least square estimation is used to solve the fault overdetermined equations, which is formed by plentiful samplings. The least square estimation needs not any prior knowledge, which is the best approach to solve the fault transient problems.(6)Simplify the fault estimation equation by the transforming from faulted line parameters estimation to fault location estimation. To overcome the shortage of complex and low-accuracy of faulted line estimation equation, the faulted line parameters is transformed to the product which multiplying the fault distance and line parameters per unit length using the distributed parameters. So the fault location equation can be gotten. This simplification reduces the calculation of the least square estimation and improves the estimation accuracy.(7)The results of ATP simulation and static simulation have proved the accuracy of the proposed fault location estimation algorithm. With building the simulation model and setting simulation parameters, different conditions of SPE fault is simulated. The results proved the accuracy and practicality of this fault location algorithm.(8)With comparison and examination to each estimation items of results, the high-accuracy item is chosen as the last distance result. The fault location estimation equation includes two distance items. From the comparison of energy and interference immunity, the high-accuracy item can be gotten as the result.The proposed algorithm has the high reliability, sensitivity and applicability to transient signals. And it is free from the Peterson coil and fault position, and can estimate the fault distance with half cycle or more short sampling length. This fault location estimation method is low cost without any additional devices and can be realized easily, which can satisfy the requirement of field engineering.But when the earth resistance is much higher, the estimation accuracy decreased relatively, which should be improved in future.The proposed distance estimation technical makes the important progress in fault location field of SPE fault, and its application has obviously significance for the improvement of supply reliability.