ABB RELION 620 Series Technical Manual page 674

Section 4
Protection functions
4.5.7.5
4.5.7.6
668
Application
PSPTUV can be applied for protecting a power station used for embedded generation
when network faults like short circuits or phase-to-earth faults in a transmission or a
distribution line cause a potentially dangerous situations for the power station. A
network fault can be dangerous for the power station for various reasons. The
operation of the protection can cause an islanding condition, also called a loss-of-
mains condition, in which a part of the network, that is, an island fed by the power
station, is isolated from the rest of the network. There is then a risk of an autoreclosure
taking place when the voltages of different parts of the network do not synchronize,
which is a straining incident for the power station. Another risk is that the generator
can lose 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 a more 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 can 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 can be different from the corresponding
voltage in the rest of the network. The island can have 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 the
frequency and voltage operation.
Motor stalling and failure to start can lead to a continuous undervoltage. The positive-
sequence undervoltage is used as a backup protection against the motor stall
condition.
Signals
Table 629: PSPTUV Input signals
Name Type
U SIGNAL
1
BLOCK BOOLEAN
Default Description
0 Positive phase sequence voltage
0=False Block signal for activating the blocking mode
1MRS757644 F
620 series
Technical Manual