Parameter: Direction
•
Default setting: Both
Each 81R function element can be individually set to operate on positive frequency change rate, negative
frequency change rate or frequency change rate independent of direction.
Parameter: Delay
•
Default setting: 0 s
This is the time delay setting for the DTL element. This should be set to suit the individual application.
Element Inhibits
In the event of an inhibit being applied by a binary input or logic signal, an element that is picked up will be
reset. Reset of the inhibit will allow restart of the operating delay.
Frequency df/dt measurement is blocked when all phase-phase voltages fall below 30 V. To allow the voltage
to stabilize this under voltage inhibit will reset 300 ms after all line voltages recover to levels above 30 V.
The 81R function is blocked for voltage vector shifts > 3°. To ensure against operation at lower values of vector
shift (i.e. an equivalent ROCOF of 20.8 Hz/s) a time delay of 300 ms can be applied to the 81R element (i.e.
this is in excess of the 81R operate time).
Settings Example
Loss of Mains (LOM)
ROCOF’s of up to 0.1 Hz/s can be expected on power networks due to normal system operations. The G.59
standard, defining the requirements for connecting significant distributed generation to a mains supply, there-
fore recommends an 81R pickup setting of 0.125 Hz/s. However frequency does not decay in a linear manner
and for this reason it is inadvisable to set the 81R element to be too fast. A short period of rapid decay may be
uncharacteristic of the system condition but sufficient to trip off the element. Not allowing for this complex
decay characteristic is one of the main reasons that 81R protection has gained a reputation for being unstable.
At such a low level of 81R pickup as 0.125 Hz/s, a delay in the order of 300 ms is recommended.
The second 81R element can be set to a higher pickup level, but with a corresponding reduction in operate
time.
Load Shedding
Frequency broadly decays in an exponential curve during a load shedding situation, eventually stabilizing at a
reduced level. However predicting the rate of decay is a complex issue. The most significant factors being the
scale of the overload and the amount of inertia (resistance to change) in the network. A number of opera-
tional conditions will have an effect and these will rarely be constant over a number of load shedding events.
For this reason a detailed knowledge of the system is essential for arriving at suitable settings.
Where other protection elements are used as part of the load shedding scheme, such as 27 undervoltage or 81
underfrequency, it is advisable that the 81R elements are set so that they contribute to the overall scheme
coverage. Operate times should be long enough so that any previous loading shedding event has had an
opportunity to take effect. There will be a delay between a load shedding command being sent and the
resulting reduction in load during which frequency may continue to fall. A delay of 300 ms to 400 ms will be
sufficient to allow for all cases.
In particular it must be remembered that automatic load shedding represents a highly unusual system condi-
tion and can be viewed as an emergency, last-ditch measure. For this reason it should never be set too sensi-
tively or too fast-acting, inadvertent loss of load having a potentially catastrophic effect on the power
network.
The provision of programmable logic in the relays means that protection features can be easily configured to
provide quite complex control functionality. For example, following a load shedding event, an 81 Under/Over-
frequency element can be configured to provide load restoration once the frequency has recovered for a
minimum duration. With the order of load restoration being defined in sequential logic.
Protection and Automation Functions
5.28 81R Frequency Protection – "df/dt"
352 Reyrolle 7SR5, Overcurrent Protection Device, Device Manual
C53000-G7040-C014-1, Edition 11.2019
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