Blocking mode" which selects the blocking mode. The BLOCK input can be
controlled by a binary input, a horizontal communication input or an internal signal
of the IED program. The influence of the BLOCK signal activation is preselected
with the global setting Blocking mode.
The Blocking mode setting has three blocking methods. In the "Freeze timers"
mode, the operation timer is frozen to the prevailing value. 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.
4.5.5.5 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.
1YHT530004D05 D Section 4
Protection functions
615 series 491
Technical Manual
To Next Page
Loading...
Need help?
General
