81 Frequency Protection – "f>" or "f<"
Overview of Functions
Frequency protection is used to:
•
Initiate load shedding – selective disconnection of load to match system capacity due to loss of genera-
tion or under/overvoltage (for example, wind power supply, system faults or plant failure)
•
Monitor the permissible frequency range and provide alarms for values outside of normal limits
•
Protect equipment (for example, plant components, machines, etc.) against damages caused by overload
due to underfrequency or overfrequency
Abnormally low frequency in power systems are typically caused by mismatch between generating capacity or
transmission capacity and connected load due to disconnection of generation or transmission circuits due to
faults or error. The balance can be restored by automated disconnection of less critical loads to allow the
system to stabilize with critical load connection maintained.
Abnormally high frequency can be caused by problems with generation regulation control systems in combi-
nation with tripping of load or due to islanding of a local generation system which is disconnected from the
main grid and its normal load due to system faults.
Structure of the Function
The
Frequency Protection
function has group dependent settings. These settings can be different in
each settings group – group numbers (Gn) 1 to 4.
This function corresponds to the logical node
A81PTUF*
and
A81PTOF*
in IEC 61850.
Each element can be individually selected to measure frequency from the 3 phase voltage inputs e.g.
VT1/2/3.or the 4th voltage input e.g. VT4.
Each threshold frequency protection element can be set to operate as either an underfrequency or an overfre-
quency element.
Each element has a settable hysteresis band to avoid intermittent operation for frequency near to setting.
If 3 phase voltage is selected the phase to neutral voltage is used to measure frequency.
The voltage selection algorithm uses VT1 input when it is within voltage limits. When VT1 input falls out of
limits the device checks VT2 and VT3 inputs and uses one of them if it is within limits. The frequency measure-
ment is blocked if the applied fundamental frequency voltage at the relay terminals is less than the minimum
voltage threshold.
Voltage Selection
When the
81 Input Selection
is set to VT1/2/3 the largest magnitude phase to neutral voltage is selected
as the source for frequency measurement based on fundamental frequency DFT measurement. When a phase
is selected it will continue to be used unless the voltage falls below the minimum threshold of 30 Vrms using
fundamental frequency DFT. If the selected voltage falls below the minimum threshold, the source will change
to the highest phase to neutral voltage from the other 2 phase to neutral values. The frequency measurement
will be continuous and any element that is picked up will be unaffected.
5.20
5.20.1
5.20.2
Protection and Automation Functions
5.20 81 Frequency Protection – "f>" or "f<"
288
Reyrolle 7SR5, Transformer Protection Device, Device Manual
C53000-G7040-C015-1, Edition 11.2019
