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PAC-T100(Transformer Protection and Control Unit) Percent differential protection(87T) Current percent differential protection acts by instantaneous/definite time according to ratio setting of restraint current and differential current. Ratio property can do second-stage setting by low current area and high current area. Percent differential protection element has 2 harmonics / 5 harmonics constraint element to prevent malfunction by inrush or overexcitation of transformer. Also, when it is wye grounded system outside grounding fault, relay is equiping function that correct ground current that occur. Current percent differential protection element uses wye connection CT basically, but already can accommodate CT for placed phase compensation. Differential Current 50/87 Pickup TRIPPING AREA SLOPE2 SLOPE1 87T Pickup Restrain Current KNEE POINT Figure 3-1 Current instantaneous/percent differential protection operating characteristic W1_IA W1_IB W1_IC W2_IA W2_IB W2_IC I0 comp.. Differential Calculate DIFF_A > 50/87_PKP_A event ------------------------------ W3_IA W3_IB W3_IC 0 T 50/87_OP_A event ------------------------------ SETTING TYPE PHS W1-W2 W1-W3 COMP PHS PHS FUNC BLOCK TION PICKUP DELAY EVENT Figure 3-2 Current instantaneous differential protection operating characteristic 3-36
