loses synchronism during the network fault. A sufficiently fast trip of the utility circuit
breaker of the power station can avoid these risks.
The lower the three-phase symmetrical voltage of the network is, the higher is the
probability that the generator loses the synchronism. The positive sequence voltage
is also available during asymmetrical faults. It is an appropriate criterion for detecting
the risk of loss of synchronism than, for example, the lowest phase-to-phase voltage.
Analyzing the loss of synchronism of a generator is rather complicated and requires
a model of the generator with its prime mover and controllers. The generator may be
able to operate synchronously even if the voltage drops by a few tens of percent for
some hundreds of milliseconds. The setting of PSPTUV is thus determined by the
need to protect the power station from the risks of the islanding conditions since that
requires a higher setting value.
The loss of synchronism of a generator means that the generator is unable to operate
as a generator with the network frequency but enters into an unstable condition in
which it operates by turns as a generator and a motor. Such a condition stresses the
generator thermally and mechanically. This kind of loss of synchronism should not
be mixed with the one between an island and the utility network. In the islanding
situation, the condition of the generator itself is normal but the phase angle and the
frequency of the phase-to-phase voltage may be different from the corresponding
voltage in the rest of the network. The island may get a frequency of its own relatively
fast when fed by a small power station with a low inertia.
PSPTUV complements other loss-of-grid protection principles based on frequency
and voltage operation.
Setting the PSPTUV operation
The magnitude of positive phase sequence voltage U
1
at the fault point U
1f
can be
calculated for different types of faults with the formulae:
three-phase short circuit
GUID-27F4A2D7-C8B7-4A9D-A915-D59D4B92CB6E V1 EN (Equation 4)
two-phase short circuit
U
Z Z
Z Z Z
f
f k
f k k
1
2
2 1
0 5=
+
+ +
≈ .
GUID-C2E64AF0-0776-42A9-8BF7-5EE12C5A113E V1 EN (Equation 5)
two-phase earth fault
U
Z Z Z
Z Z Z Z Z Z
f
k k f
k k k f k k
1
2 0
1 2 0 1 2
3
0 33
3
=
+
+ + +
≈
( )
( )( )
.
GUID-6CE59803-DC3E-4B32-A4D4-71F00C5F2B21 V1 EN
(Equation 6)
Section 5 1MRS756378 D
Protection functions
172 REF615
Application Manual
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