Toshiba GR200 Series Instruction Manual page 49

Line differential protection ied

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Differential protection characteristics (DIFL)

Characteristic of differential (Id) and restraint (Ir) current plane

The DIFL element has dual percentage-restraint characteristics. Figure 2.2-1 shows two

characteristics on the plane between differential current (Id) axis and restraining current (Ir)

axis. The Id is referred as a vector sum of the phase currents at all terminals. The Ir is referred

as a scalar sum of the phase currents at all terminals. The DIFL function is operated if a cross

point between the Id and the Ir moves into the hatched area of Figure 2.2-1. The DIFL function

can removes charging phase-current using the function of charging current compensation (See

section 2.2.1(v))

The characteristic A shows weaker restraint; this characteristic ensures sensitivity

against low-level faults. Conversely, the characteristic B has stronger restraint. That is, the

DIFL characteristic varies its sensitivity depending on the Ir.

Suppose that a fault occurs out of a line and the CT is saturated by the external fault.

The DIFL function can be operated falsely because the differential current due to the increases

erroneously. This is because either by the CT saturation or the CT transient error. Accordingly,

the slope of the characteristic B is steeper than the slope of the characteristic A; as a result,

the increasing Id current does not give rise to the erroneous DIFL operation owing to the CT

saturation or the CT transient error.

However, the DIFL operation in the characteristic B can block this false operation

although the DIFL operation depends on the magnitude of restraining current in the

characteristic B.

Small current region

Figure 2.2-1 DIFL element on Ir-Id plane

6F2S1914 (0.49)

Large current region

GRL200 (Soft 033 & 037)

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