| With the progressive development of power system, the electric power grid’s scale is bigger and bigger, the grid structure has become increasingly complex. The requirements for protection devices have become more sophisticated. Transmission lines is an important power system electrical components, single-phase ground fault is the most common transmission line faults.Firstly, the typical transmission line protection principle has been summarized analysis, focuses on the principle of the protection of the comparison voltage phase method, distance relay adopting the positive sequence voltage as polarization, zero-sequence reactance relay and its improvements.Fourier algorithm is the most commonly used algorithms in digital protection devices. Full-wave Fourier algorithm is influenced by the decaying DC component when extracting fundamental component from the fault current. In order to solve the problem, a novel full-wave Fourier algorithm of filtering decaying DC component is proposed and the derivation of the decaying DC component parameters calculation is given. Firstly, the initial amplitude and decay time constant of the decaying DC component can be calculated with the sampling values in one cycle of the fault current, and then the corrected sample values can be got by using the sampling values minus the decaying DC component. The fundamental component can be got by the revised sampling values with full-wave Fourier improved algorithm. The static model simulation signals and PSCAD/EMTDC simulated signals are applied to test the performance of the algorithm separately. The simulation results illustrate that the proposed algorithm can effectively reduce the impact of decaying DC component. Compared with traditional algorithms, the proposed algorithm only needs one cycle of sampling data. The fundamental component is calculated accurately.This paper proposes a new algorithm of distance protection. First, according to the fault branch Kirchhoff s voltage law equation, the measured impedance expression is obtained. According to geometric distribution characteristics of measured impedance, fault supplementary impedance and effective fault impedance in impedance complex plane. Phase angle of fault supplementary impedance caused by transition resistance is calculated with phase relationship between measured negative sequence current at relay and fault current. Effective fault impedance from fault point to relay is obtained. When used at sending side can correctly distinguish within and outside the protection zone; when used at receiving side, if system state is not close to "equal phase point" after the failure, ground fault out of the protection zone with high impedance will not malfunction, ground fault within protection zone will reliable operation. If system state is close to "equal phase point" and via low transition resistance fault within protection zone can be removed reliably. Protection performance is better than traditional Phase Comparison Distance Relay.In this paper, the zero-sequence differential impedance protection principle based on bilateral information is proposed. The definition of zero-sequence differential impedance and the criterion of zero-sequence impedance differential protection principle are given. The zero-sequence differentia impedance is equal to the opposite value of the sum of both sides system zero sequence impedance, which is in the third quadrant, when the fault happens within the protection zero. The zero-sequence differentia impedance is equal to the transmission line zero sequence impedance, which is in the first quadrant, when the fault happens out of the protection zero. Some models are built in PSCAD to verify the correctness of the zero sequence differential impedance protection principle. Experimental results show that the criterion is not influenced by the fault resistance, and can be applied in the line with or without shunt reactor and weak feeding system, and the strict synchronization of the sampled data is not needed. |
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