Study On Line Protection Based On Parameter Identification Method

The security and stability of power system operation cannot be guaranteed without relay protection, and the transmission line is the most important part, hence, the proper performance of the line protection is the most important insurance for system stability and equipments security. Distance relay is a protective device which can indicate the distance between the fault point and the protective device, since the distance is an non-electrical quantity, the distance protection will not be influenced by the power system operation. Therefore, the distance relay is an important protective device. However, currently, the distance relay is realized by impedance measurement, which means the complexity of operational characteristics makes the realization of the protective device very difficult, especially regarding to the settings of protective zone. Moreover, the fault of single phase to earth via high impedance and the transient component of fault current are also important but difficult within the consideration of the line protection.In order to realize a better line protection, it is necessary to adopt some new theory and technology to investigate. Nowadays, the fault location method has been deeply and widely investigated, in the meanwhile, the development of electronic technology, especially computer technology offers support of hardware and technology, and with the up-to-date identify technology, the accuracy of the distance measurement is highly increased. Thus, if the accuracy of distance measurement, which can be fulfilled online, is high enough, then it can be the new generation of distance protection principles.This paper proposes an idea which is using the algorithm of one pole distance measurement with differential equation based on a lot of references.During the study of distance measurement algorithm, the advantage of sequence component which is not influenced by loads and oscillations is utilized to propose an algorithm completely using differential equation for distance measurement based on instantaneous sequence component, and it also presents the equation to obtain the instantaneous sequence component. Moreover, this paper analyzes both the advantages and disadvantages of the algorithm of model-parameter identification for distance measurement, and based on that, it proposes a distance measurement algorithm by the combination of the algorithm of instantaneous sequence component and model-parameter identification for distance measurement. This algorithm using the proper settings of tripping values and the calculated value of transition resistance makes the distance measurement adapted both to metal earthing fault and high impedance earthing fault.Regarding to the influences caused by single phase earthing fault via high impedance to the accuracy of distance measurement, this paper proposes an one-polar algorithm in time-domain, based on the estimation of the current-phase difference. This algorithm can automatically compensate the distance measurement errors based on the value of transition resistance. The core of the algorithm is built on the equation sets of transition resistance, fault distance and the difference in current phase angles, which is indicated by the parameters of sequence systems. It considers the transition resistance and fault distance as the unknown variables, and uses the least square method to obtain the parameters, so that the fault distance can be measured precisely.In order to eliminate the cut-off errors caused by using the difference equation instead of differential equations, and reduce the influence of random errors caused by signal sampling of the practical protective devices, this paper investigates the digital process method of protective signal. After comparing the advantages and disadvantages of various signal processing algorithms, this paper proposes a sine wave adaptive filtering algorithm and a Sinusoidal Approximation algorithm. The former one can sample the fundamental component of the sine input-signal; the latter one is the functional approximation to the original signal, and it can be applied to instantaneous signals of fault at early time, which is theoretically based on the instantaneous amplitude and phase angle of the instantaneous varying signal. The auto-adapt filtering algorithm and sine-approximation algorithm both can use functions to fit the discrete wave-form data, so that the derivation can be used directly to the fit-function, which will not cause cut-off errors.Regarding to the requirement of line protection investigation, the digital simulation of the transmission line and dynamic simulation model are established, and the typical line fault conditions are simulated, with recording the voltage and current wave-form data. The analysis to distance measurement algorithm by using simulation and dynamic modeling data has obtained positive results.