Differential protection determination method

A determination method of differential protection, wherein the determination method is suitable for an electricity transmission line with multi-terminal T-connection, the electricity transmission line with multi-terminal T-connection comprises a host protection apparatus and a slave protection apparatus, the host protection apparatus is connected to a terminal of the electricity transmission line with multi-terminal T-connection, the slave protection apparatus is connected to remaining terminals of the electricity transmission line with multi-terminal T-connection, wherein the determination method comprises:

• step 1:
• respectively sampling a plurality of phase currents of the host protection apparatus and a plurality of phase currents of the slave protection apparatus, and transmitting a plurality of phase current amplitudes sampled from the plurality of phase currents of the slave protection apparatus to the host protection apparatus;
• step 2:
• calculating, by the host protection apparatus, all phase current amplitudes sampled from the plurality of phase currents of the slave protection apparatus and the plurality of phase currents of the host protection apparatus, obtaining a maximum fault component current Δİmax in a plurality of phase current amplitudes of each phase current of all phase currents, and calculating a differential current Icd in the each phase current, a resistive current Ires in the each phase current and a vector sum ΔİΣ of one or more fault component currents except the maximum fault component current Δİmax in the phase current amplitudes of the each phase current;
• step 3:
• selecting a plurality of target phase current amplitudes from all sampled phase current amplitudes, and in response to determining that the maximum fault component current Δİmax in the target phase current amplitudes is greater than a set threshold value Imax0, determining a target phase angle difference θ1 between a phase angle of the maximum fault component current Δİmax in the target phase current amplitudes and a phase angle of the vector sum ΔİΣ of the fault component currents except the maximum fault component current Δİmax in the target phase current amplitudes, wherein the target phase angle difference θ1 is used for assignment of a resistive coefficient K;
• and determining a set value Kθ of the resistive coefficient K according to an undetermined phase angle difference θ, or in response to determining that the maximum fault component current Δİmax in the target phase current amplitudes is less than or equal to the threshold set value Imax0, determining a fixed value Kset of the resistive coefficient K;
• step 4:
• performing processing of step 3 on the phase current amplitudes of the each phase current of all phase currents, to obtain a value of the resistive coefficient K corresponding to the phase current amplitudes of the each phase current;
• and
• step 5:
• performing differential determination according to the differential current Icd in each phase current, the resistive current Ires in the each phase current and the value of the resistive coefficient K corresponding to the each phase current amplitude obtained in step 4 and using a following differential determination formula:
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• wherein Iset is a setting current value, and wherein in response to determining that the differential current Icd of the target phase satisfies the differential determination formula, a fault is determined to occur in an area of the target phase.