Application - ABB RELION REX640 Technical Manual

Section 4
Protection functions
4.5.6.6
798
The timer calculates the start duration value START_DUR, which indicates the
percentage ratio of the start situation and the set operation time. The value is available
in the monitored data view.
Blocking logic
There are three operation modes in the blocking function. The operation modes are
controlled by the BLOCK input and the global setting in Configuration/System/
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
protection relay's 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, 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.

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
1MRS759142 C
REX640
Technical Manual