ABB RELION 620 Series Technical Manual page 710

Section 4
Protection functions
4.6.1.5
704
The combined start duration START_DUR indicates the maximum percentage ratio of
the active protection modes. The values are available via 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
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
1MRS757644 F
620 series
Technical Manual