ABB RELION REX640 Technical Manual page 925

1MRS759142 F
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
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.
4.6.1.6 Application
The frequency protection function uses the positive phase-sequence voltage to
measure the frequency reliably and accurately.
The system frequency stability is one of the main principles in the distribution
and transmission network maintenance. To protect all frequency-sensitive electrical
apparatus in the network, the departure from the allowed band for a safe operation
should be inhibited.
The overfrequency protection is applicable in all situations where high levels of the
fundamental frequency of a power system voltage must be reliably detected. The
high fundamental frequency in a power system indicates an unbalance between
production and consumption. In this case, the available generation is too large
compared to the power demanded by the load connected to the power grid. This
can occur due to a sudden loss of a significant amount of load or due to failures
in the turbine governor system. If the situation continues and escalates, the power
system loses its stability.
The underfrequency is applicable in all situations where a reliable detection of a
low fundamental power system voltage frequency is needed. The low fundamental
frequency in a power system indicates that the generated power is too low to meet
the demands of the load connected to the power grid.
The underfrequency can occur as a result of the overload of generators operating
in an isolated system. It can also occur as a result of a serious fault in the power
system due to the deficit of generation when compared to the load. This can
happen due to a fault in the grid system on the transmission lines that link two
parts of the system. As a result, the system splits into two with one part having the
excess load and the other part the corresponding deficit.
The frequency gradient is applicable in all the situations where the change of
the fundamental power system voltage frequency should be detected reliably. The
frequency gradient can be used for both increasing and decreasing the frequencies.
This function provides an output signal suitable for load shedding, generator
shedding, generator boosting, set point change in sub-transmission DC systems
and gas turbine startup. The frequency gradient is often used in combination
with a low frequency signal, especially in smaller power systems where the loss
of a large generator requires quick remedial actions to secure the power system
integrity. In such situations, the load shedding actions are required at a rather high
frequency level. However, in combination with a large negative frequency gradient,
the underfrequency protection can be used at a high setting.
REX640
Technical Manual
Blocking mode setting has three blocking methods. In the "Freeze timers"
Blocking mode .
Protection functions
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