ABB RELION REX640 Technical Manual page 882

Protection functions
be controlled by a binary input, a horizontal communication input or an internal
signal of the protection relay's program. The influence of the BLOCK signal activation
is preselected with the global setting
The
mode, the operation timer is frozen to the prevailing value, but the OPERATE output
is not deactivated when blocking is activated. In the "Block all" mode, the whole
function is blocked and the timers are reset. In the "Block OPERATE output" mode,
the function operates normally but the OPERATE output is not activated.
4.5.6.6 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.
4.5.6.7 Signals
882
Blocking mode setting has three blocking methods. In the "Freeze timers"
Blocking mode .
1MRS759142 F
REX640
Technical Manual