4.6.1.5 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.
Section 4 1YHT530004D05 D
Protection functions
500 615 series
Technical Manual
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