Toshiba GR200 Series Instruction Manual

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6F2S1915 (0.46)GRZ200 (Soft 031 & 032)- 26 -𝑍1 = 𝑉𝑏 − 𝑉𝑐𝐼𝑏 − 𝐼𝑐(2.1-6)As shown above, the positive-sequence impedance is used when determining settings forphase faults.Figure 2.1-1 Phase-to-phase fault(ii) Single-phase-earth faultFigure 2.1-2 shows the power system in the event of a single-phase-earth fault. It is verydifficult to measure precisely the distance to the fault point for a single-phase-earth fault. Thisis because the impedance of the zero-sequence circuit including the earth return is generallydifferent from the positive-sequence impedance. Therefore, the faulted phase voltage is notsimply proportional to the faulted phase current.Figure 2.1-2 Single-phase-earth faultIt is necessary to analyze the impedance seen by the relay in the event of a single-phase-earth fault according to the method of symmetrical components. Figure 2.1-3 shows anequivalent circuit for a single-phase-earth fault based on the method of symmetricalcomponents. If we define the positive-sequence, negative-sequence and zero-sequence voltagesas being V1F, V2F and V0F, then the voltage at the relaying point of each symmetrical circuitwill be given by the following equations based upon the assumption that the positive-sequenceZsVFVFIbVcIc ZmVaVbZSVaFIbVcIcVaVbZGIa