ABB Relion REQ650, Applications Manual

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between the actual generation and the load demand. High fundamental frequency in a
power system indicates that the available generation is too large compared to the
power demanded by the load connected to the power grid. SAPTOF detects such
situations and provides an output signal, suitable for generator shedding, HVDC-set-
point change and so on. SAPTOF is very sensitive and accurate and can also be used
to alert operators that frequency has slightly deviated from the set-point, and that
manual actions might be enough.
8.2.3 Setting guidelines
All the frequency and voltage magnitude conditions in the system where SAPTOF
performs its functions must be considered. The same also applies to the associated
equipment, its frequency and time characteristic.
There are two specific application areas for SAPTOF:
1. to protect equipment against damage due to high frequency, such as generators,
and motors
2. to protect a power system, or a part of a power system, against breakdown, by
shedding generation, in over production situations.
The overfrequency start value is set in Hz. All voltage magnitude related settings are
made as a percentage of a settable global base voltage parameter
UBase . The UBase
value should be set as a primary phase-to-phase value.
Some applications and related setting guidelines for the frequency level are given
below:
Equipment protection, such as for motors and generators
The setting has to be well above the highest occurring "normal" frequency and well
below the highest acceptable frequency for the equipment.
Power system protection, by generator shedding
The setting must be above the highest occurring "normal" frequency and below the
highest acceptable frequency for power stations, or sensitive loads. The setting level,
the number of levels and the distance between two levels (in time and/or in frequency)
depend very much on the characteristics of the power system under consideration. The
size of the "largest loss of load" compared to "the size of the power system" is a critical
parameter. In large systems, the generator shedding can be set at a fairly low frequency
level, and the time delay is normally not critical. In smaller systems the frequency
START level has to be set at a higher value, and the time delay must be rather short.
8.3 Rate-of-change frequency protection SAPFRC
1MRK 505 291-UEN A
Section 8
Frequency protection
169
Application manual