Provided is a determination method of differential protection suitable for an electricity transmission line with multi-terminal T-connection. The determination method includes: sampling and calculating all phase currents, and calculating a maximum fault component current Δİ.sub.max in phase current amplitudes of each phase current, a differential current I.sub.cd in the each phase current, a resistive current I.sub.res in the each phase current and a vector sum Δİ.sub.Σ of fault component currents except Δİ.sub.max in the phase current amplitudes of the each phase current; and comparing the maximum fault component current Δİ.sub.max in phase current amplitudes of each phase current and a threshold setting value I.sub.max0, determining a value of a resistive coefficient K corresponding to the phase current amplitudes of the each phase current according to a comparison result, and performing differential determination on the differential current I.sub.cd and the resistive current I.sub.res in phase current amplitudes of the each phase current according to a differential determination formula: if the differential current I.sub.cd of the target phase satisfies the above differential determination formula, it is determined that a fault occurs in an area of the target phase.

Provided is a determination method of differential protection suitable for an electricity transmission line with multi-terminal T-connection. The determination method includes: sampling and calculating all phase currents, and calculating a maximum fault component current Δİ.sub.max in phase current amplitudes of each phase current, a differential current I.sub.cd in the each phase current, a resistive current I.sub.res in the each phase current and a vector sum Δİ.sub.Σ of fault component currents except Δİ.sub.max in the phase current amplitudes of the each phase current; and comparing the maximum fault component current Δİ.sub.max in phase current amplitudes of each phase current and a threshold setting value I.sub.max0, determining a value of a resistive coefficient K corresponding to the phase current amplitudes of the each phase current according to a comparison result, and performing differential determination on the differential current I.sub.cd and the resistive current I.sub.res in phase current amplitudes of the each phase current according to a differential determination formula: if the differential current I.sub.cd of the target phase satisfies the above differential determination formula, it is determined that a fault occurs in an area of the target phase.

Embodiments of the present invention disclose a current source converter differential protection method, including: sampling, by a relay protection device, secondary side currents of current transformers on two sides of a protected current source converter, to obtain incomer-side three-phase currents, and outgoer-side three-phase currents; rectifying the incomer-side three-phase currents and the outgoer-side three-phase currents, to obtain an incomer-side input current and an outgoer-side output current which are equivalent; converting the incomer-side input current and the outgoer-side output current according to a current ratio of the current transformers on the two sides, to acquire a transient differential current and a transient restraint current; acquiring a differential current and a restraint current according to the transient differential current and the transient restraint current; and achieving differential protection according to the differential current and the restraint current. The embodiments of the present invention further disclose a relay protection device correspondingly.