ABB REF615 Applications Manual page 180

Section 5
Protection functions
174
The disadvantage of the fast operation of the PSPTUV operation is that it issues a trip
even if the fault is on one of the medium voltage lines fed by the power station. This
is an undue trip since such a fault would not lead to an islanding condition. This
drawback is smaller than the benefit of having a reliable loss-of-grid protection.
Sometimes it is possible to improve the selectivity. If the PSPTUV operation is
located in the same substation as the overcurrent protection functions protecting
medium voltage lines. It is possible to block the PSPTUV operation with the start
signals of the overcurrent function blocks.
Setting examples for the PSPTUV operation
The purpose of the examples is to illustrate the setting principle of the PSPTUV
operation. The calculations are more complicated in practice, especially if there are
other generators having an impact on the voltages at the generator in question. Fault
calculation tools or network simulators are recommended to be used in complicated
studies.
If there are several generators in the network, the voltages during the fault at the
measuring point of the relay depend on the number of generators actually connected
to the network when the fault occurs. One option is to use a fixed setting that leads
to the highest U value during the fault. In this case, the function block will always
1
trip when it is required to, but there may also appear unnecessary operations. When
the maximum selectivity is pursued, the distribution management system can be
utilized for updating the settings as the switching state of the generators or the network
changes. The two setting groups of the function block can also be utilized for the
same purpose.
Example 1.
Example 1 presents a case where a radial line leads from the substation A to the
substation B. Tripping this line creates an island fed by the power station. If single-
phase autoreclosures are applied in single-phase earth faults, only two-phase or three-
phase short circuits will then lead to a loss-of-grid situation. The setting value for the
Start value setting is calculated based on a two-phase short circuit at the farthest circuit
breaker of the potential island and this point is at the substation A.
The equivalent circuit is applied in the calculation. The U
at a two-phase short circuit is U
source voltage can be approximated as 1.1 p.u. The U
generator terminals during the fault is calculated with the formula:
U U 1 1 . U
( )
= + −
1 G 1 f 1 f
X
kG
GUID-0833A49D-71B3-40DC-B920-5B88882C3A03 V1 EN
X short circuit reactance between the power station and the fault reduced to the
kG
voltage level of the power station
= 0.5 p.u. The transient U
1f
X
kG
0 5 . ( . 1 1 0 5 . )
= + −
'
X X
+
d
1MRS756378 D
value of the faulty point
1
value of the generator
1
voltage appearing at the
1
X
kG
'
X
+
kG d
Application Manual
(Equation 8)
REF615