Fast Underfrequency - GE Multilin 850 Instruction Manual

CHAPTER 4: SETPOINTS

Fast Underfrequency

FASTPATH:
850 FEEDER PROTECTION SYSTEM – INSTRUCTION MANUAL
Frequency variations originate from unbalance conditions between generation and load.
The main reasons for these conditions are given:
• Inadequate load forecast or deficient generation capacity programming.
• Busbars, generator group or interconnection feeders trip.
• System splits into islands.
When the frequency variation is small, the unbalance condition is corrected by the
generator regulator. In the case of big frequency variations, the regulator is not able to
correct itself, and the frequency value decreases which the danger of losing generation
capacity.
If this underfrequency condition is not corrected a general blackout may occur.
In case of a shortage of generation capacity, the only possible way of recovering the
stability of the system is through a selective load shedding scheme. The load
disconnection is done when the frequency goes down below certain thresholds, in order to
provide adequate reaction time for the generators to recover via their speed regulators.
It is important to point out that when the frequency decreases quickly, relay operation
based on the detection of the underfrequency condition may not be enough to recover
stability. In this case the load shedding scheme must also take into account the rate of
change of frequency. This is done by calculating the frequency derivative over time. Loads
are "shed" based not only on an absolute (static) underfrequency threshold, but also on the
dynamic rate of change of frequency.
The Fast Underfrequency element is mainly used in medium voltage and distribution
substations as a selective load shedding scheme. By doing so, frequency recovers stability
and potentially dangerous situations that might affect generators in other parts of the
electrical system are avoided.
The Fast Underfrequency element measures frequency by detecting the consecutive
voltage zero crossings and measuring the time between them. The measured frequency
has a range between 20 to 70 Hz. The out-of-range measurement will be classified as
invalid, which will not affect the behavior of the SET and RESET counters. The fast
frequency is the average value of the measured frequency in a short window. Compared
to the regular metered voltage frequency value, the fast frequency has the faster response
but lesser accuracy.
Path:
Setpoints > Protection > Group 1(6) > Frequency > Fast Underfrequency > Common
Setup
FREQUENCY INPUT
Range: dependant upon the order code
Default: Ph VT Bnk1-J2
This setting provides the selections for the frequency signal source.
MINIMUM VOLTAGE
Range: 0.10 to 1.10 x VT in steps of 0.01 x VT
Default: 0.40 x VT
The setting sets the minimum voltage for all Fast Underfrequency elements operation
specified per times VT. The setpoint prevents incorrect operation if the voltage decreases
below the threshold.
If the 3-phase VT uses a delta connection and FREQUENCY INPUT is set to J2-3VT, the
positive sequence voltage is used as the supervision voltage. In such condition, the true
supervision level is internally changed to 1/sqrt(3) of the user setting since the base of VT
here is the phase-phase voltage.
PROTECTION
4–249