Toshiba GR200 Series Instruction Manual page 30

Line differential protection ied

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Figure 1.1-4 Six zones in Mho characteristic

The reach of the zone1 element

The user shall notice that the reach of the zone1 element is usually set to approximately 80 to

90% of the length of the line (the reach of the zone1 should not include the busbar at the remote

end. That is, the zone1 element shall not operate for a fault occurring beyond the remote

busbar). Accordingly, if a fault occurs out of the zone1 and lies behind the remote busbar, the

zone2 element starts to operate to clear the fault, but its operation will be retarded by the

comparison of the operation of the zone1 element†.

†Note: Operation of the carrier command protection function (DISCAR or DEFCAR) is

practical when the user wishes to clear instantly the fault that occurs in that area.

Measuring error and its influence

In the case of a fault with resistance, the reactance component seen by the relay at the terminal

sending power is smaller than the actual value and it tends to overreach. On the contrary, the

reactance component seen by the relay at the terminal receiving power is greater than the

actual value and it tends to underreach. The line impedance has different values in different

phases. When its average value is used for the relay setting, underreaching will occur in a

phase with greater impedance than the average value. In the case of fault resistance, its

impedance is greater for earth faults where the fault is grounded via a steel tower or tree

rather than a phase fault consisting of arc resistance only. Therefore, measuring errors about

the earth fault relay are generally greater than the errors about the phase fault relay. The

Z1S–Z5S in Mho characteristic with blinders

6F2S1914 (0.49)

Z4S-Mho.Reach

Z5S-Mho.Angle

GRL200 (Soft 033 & 037)

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