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Two step definite and inverse time-
delayed residual
overcurrent protection (TEF1 and TEF2)
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The directional/nondirectional residual current functions are intended for solidly
earthed multiphase systems. The function has two measurement steps, one low current
step with definite or inverse-time-delay and one high current step with definite time-
delay.
For TEF1 input I4 is used to measure I
two functions are identical. References will therefore only be made to TEF1 in the fol-
lowing description, but they also apply for TEF2.
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In case of single-phase earth-faults, the primary fault resistance varies with the network
conditions, the type of fault and location of the fault. In many cases, the fault resistance
is much higher than the resistance that can be covered by an impedance-measuring dis-
tance protections. This can be the case with a phase to earth fault to a tower with large
tower footing resistance.
Earth faults with high fault resistance can be discovered by measuring the residual cur-
rent
I
=I
+I
N
L1
L2
The inrush current can cause unwanted tripping of the earth-fault overcurrent relay
when energizing a directly earthed power transformer. The earth-fault overcurrent pro-
tection is therefore provided with second harmonic restraint, which blocks the operation
if the residual current (I
In some cases, it is possible to improve the selectivity by adding a settable minimum
operate current (I
>Low) and a minimum operate time (tMinInv) to the inverse char-
N
acteristic. These functions are included in the earth-fault protection modules.
To minimize the operate time, in case of closing the circuit breaker to a fault, the resid-
ual overcurrent protection module is provided with a switch-onto-fault logic, which can
be activated at breaker closure. The tripping time will temporarily be reduced to 300 ms.
and for TEF2 I5 is used. Except from this the
N
) contains 20% or more of the second harmonic component.
N
&KDSWHU
&XUUHQW
(Equation 49)
103
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