Distribution Characteristics Analysis Of Geomagnetically Induced Current(GIC) In Three-phase Power Transformer And Its Effects On Differential Protection

As the development of power system, more and more accident, such as the tripping of Transmission line and reactive power compensation equipment, relay maloperation and transformer damage, have been reported to be induced by the GIC, which caused by the geomagnetic storm and injecting through grounding transformer neutral point into the power grid.This paper puts forward four kinds of GIC equivalent model. The reasonableness of the equivalent models has been discussed, based on the two kinds of situation that GIC is treated as DC and that low frequency of GIC is considered. Ultimately a reasonable equivalent model has been given.Usually, GIC is evenly distributed in the three-phase transformer, however in some cases, the particularity of low frequency of GIC will affect its distribution in transformer windings. Based on AC system simulation models considering GIC influence, the distribution of GIC in three-phase transformer under different operating conditions has been studied. Similarities and differences of GIC distribution characteristics in the Wye-wye and Wye-delta connection transformer have also been studied. This research demonstrates that GIC will not distribute evenly in the three-phase transformer when asymmetrical saturation occurs. This research also shows, GIC may flow to Y (â–³) side, due to the low frequency variation of the GIC, which may impact on the reliability of protective relaying.Based on AC system simulation models considering the influence of blocking self-control system, the changes of magnetizing current and flux of CT and transformer have been discussed during a system failure. Studies have shown that GIC wil more likely cause the CT saturation. This paper has also been studied the influence that DC blocking device has on the transformer differential protection when the outside asymmetric ground fault occurs.Research shows that when the outside single-phase ground fault occurs, the exit of the capacitor will reduce the non-periodic component, which will slow down the raising speed of the CT operating point, improving the reliability of differential protection. When the outside two-phase fault occurs, CT may be entering the serious saturation zone, which resulting in transformer differential protection malfunction.