ABB REF615 Applications Manual page 179

1MRS756378 D
REF615
Application Manual
single-phase earth fault
GUID-8D2E499E-DCFF-4F01-ADA0-53F24B5E32C9 V1 EN
Z , Z and Z respective positive, negative and zero-sequence impedances of the faulted circuit
k1 k2 k0
Z fault impedance
f
The approximations are based on the assumption that the sequence impedances
consist mainly of sequence reactances X
0.
When the function block is to trip in an islanding situation, it has to be set according
to the highest magnitude of U
the network that an island will be formed. The highest U
of the relay appears when the fault occurs near the farthest circuit breaker within the
potential island.
Which type of fault the setting should be based on depends on the operating principle
of protection at single phase-to-earth faults. If a single phase-to-earth fault results in
the single-phase operation of circuit breakers, the fault must involve at least two
phases before an island is formed. The highest U
a loss-of-grid condition appears at a two-phase short circuit.
If a three-phase circuit breaker operation takes place at all faults, including a single
phase-to-earth fault, the setting has to be calculated based on a single phase-to-earth
fault to get the highest value of U
The PSPTUV operation naturally detects the islanding condition only when it is
caused by a fault on the line between the power station and the transmission network.
However, the island is also formed by the switching related in locating an earth fault
in a medium voltage network, for example, the trip of a Buchholz relay of a
transformer. In these cases, no high speed autoreclosures are performed, and there is
thus time to trip the utility circuit breaker of the power station by a frequency
protection function or manually by a remote control.
The task of the PSPTUV operation is to detect such fault situations in the network
that will lead to a loss-of-grid condition. The network is not able to identify the
condition after it has been created. The operate time of the PSPTUV operation
therefore has to be made so short, that it will not drop off after the line protection of
the network has cleared the fault and an island is formed. The fast operate time, for
example, 50 ms, also brings another advantage: the high speed autoreclosure is likely
to be a successful one since the PSPTUV operation de-energizes the faulty line even
if the fault current fed by the power station is too low to start the overcurrent protection
between the power station and the fault, but high enough to maintain the arc.
, X , X
k1 k2 k0
during the fault when the fault is in such an area of
1
value indicating a fault leading to
1
during the fault.
1
Section 5
Protection functions
(Equation 7)
with X = X = X and Z
k1 k2 k0
value at the measuring point
1
=
f
173