Application - ABB RELION 620 Series Technical Manual

1MAC504801-IB E
4.3.1.5
620 series ANSI
Technical Manual
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
trip timer is frozen to the prevailing value, but the TRIP 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 TRIP output" mode, the function operates normally but the TRIP
output is not activated.

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
Section 4
Protection functions
325