Research On Equivalent Modeling Of Geomagnetic Induced Current And Its Influence On Distance Protection

Power Systems are subjected to possible changes caused by Geomagnetic Induced Current (GIC) induced from magnetic storm. Many times of geomagnetic storm violations at home and abroad show that distance protection may result in a blackout because of the influence of GIC. This paper studies the equivalent modeling of GIC and its effect on distance protection and explores new prevention measures for the mal-operation of distance protection. The research results have important theoretical and practical significance to prevent geomagnetic storm impact on the safety of super and ultra-high voltage grid projects.Based on typical waveform feature of GIC, a low frequency sinusoidal voltage source is applied as the equivalent source model of GIC and the mechanism of GIC magnetic bias is analyzed. Introducing the concept of time scale, the influence on transformer excitation characteristics from different stages of GIC function is analyzed and the distribution characteristics of GIC in transformer is discussed. Then it comes to a conclusion that compared with HVDC-type DC magnetic bias, the magnetic bias caused by GIC shows the feature of unsustainablity and uncertainty, which may result in cyclical and asymmetry transformer saturation. Because of GIC’s low frequency AC properties, it also may pass through the coupling transformer winding.Based on the analysis distance protection setting principle and operating characteristics, the effect on distance protection of each section of GIC is discussed. The periodic transformer saturation caused by GIC will cause fluctuation of measured impedance, thus having impact on transmission line protective relaying, especially distance protection backup for line-to-transformer type connection. An simulation model with three circuit transmission lines and two generators is built on MATLAB platform. Considering post-fault over load, the performance of distance protection zone III backup under the affect of GIC with different magnitude and frequency is simulated and the affect on electrical characteristics of AC system especially impedance measurement is analyzed in detail. Analysis on the implementation principle of conventional overload criterions shows that the operation accuracy will be affected in the case of GIC.To prevent such a mal-operation, a new method based on cosine voltage and the content of second order harmonic is proposed. The new scheme is reliable in the case of GIC and its effectiveness is verified.