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Two step definite and inverse time-
delayed residual
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overcurrent protection (TEF1 and TEF2)
In order to achieve the most sensitive earth fault protection the non-directional function
can be used. As the residual current is normally very small during normal operation the
setting value can be set very low. In case of small residual currents, due to high resis-
tance phase to earth faults or serial faults, the residual voltage in the system can be very
low. Broken phase conductor can cause a serial fault with no contact to earth, or pole
discrepancy in a circuit breaker or a disconnector. The most common type of serial fault
is pole discrepancy at breaker maneuvers.
As the residual voltage is often very small at high resistance earth faults and serial
faults, any directional element can not be used.
The low current step can have four different types of time-current characteristics; defi-
nite time delay or different types of inverse time delay. By using the inverse time delay
characteristics some degree of selectivity between non-directional residual protection
can be achieved..
Directional earth-fault protection is obtained by measuring the residual current and the
angle between this current and the residual voltage
U
=U
+U
N
L1
L2
(Equation 50)
The current I
lags the polarizing voltage (-U
) by a phase angle equal to the angle of
N
N
the source impedance. In solidly earthed networks, this angle is in the range of 40° to
nearly 90°. The high value refers to a station with a transformer as the source imped-
ance. To obtain maximum sensitivity at all conditions, the forward measuring element
should have a characteristic angle of 65°.
As a general rule, it is easier to obtain selectivity by using directional instead of non-
directional earth-fault overcurrent protection, but sufficient polarizing voltage must be
available.
It is not possible to measure the distance to the fault by using the residual components
of the current and voltage, because the residual voltage is a product of the residual of
current and source impedance. It is possible to obtain selectivity by the use of a direc-
tional comparison scheme, which uses communication between the line ends.
If a communication scheme can not be used, the best selectivity is generally obtained
by using inverse time delay. All relays, in the network, must have the same type of in-
verse characteristic. An earth-fault on a line is selectively tripped if the difference be-
tween the residual current (I
) out on the faulted line and the residual current (I
) out
N
N
104
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