Alstom MiCOM P546 - 1.38 Independent rate of change of frequency protection [81 R]; 1.38.1 Basic functionality

P54x/EN OP/La4 Operation
(OP) 5-100 MiCOM P543, P544, P545 & P546
OP
1.38 Independent rate of change of frequency protection [81R]
In the load shedding scheme below, it is assumed under falling frequency conditions that by
shedding a stage of load, the system can be stabilized at frequency f2. For slow rates of
decay, this can be achieved using the underfrequency protection element set at frequency f1
with a suitable time delay. However, if the generation deficit is substantial, the frequency will
rapidly decrease and it is possible that the time delay imposed by the underfrequency
protection will not allow for frequency stabilization. In this case, the chance of system
recovery will be enhanced by disconnecting the load stage based upon a measurement of
rate of change of frequency and bypassing the time delay.
This element is a rate of change of frequency monitoring element, and operates
independently from the under and over frequency protection functions. A timer is included to
provide a time delayed operation and the element can be utilized to provide extra flexibility to
a load shedding scheme in dealing with severe load to generation imbalances.
Since the rate of change monitoring is independent of frequency, the element can identify
frequency variations occurring close to nominal frequency and therefore provide early
warning to the operator on a developing frequency problem. Additionally, the element could
also be used as an alarm to warn operators of unusually high system frequency variations.
P4008ENc
fn
f
1
f2
Slow decay
Rapid decay
Frequency
Time
Figure 71 Rate of change of frequency protection
1.38.1 Basic functionality
The
relay provides four independent stages of rate of change of frequency protection
(df/dt+t). Depending upon whether the rate of change of frequency setting is set positive or
negative, the element will react to rising or falling frequency conditions respectively, with an
incorrect setting being indicated if the threshold is set to zero. The output of the element
would normally be given a user-selectable time delay, although it is possible to set this to
zero and create an instantaneous element.
An Independent setting is available for calculating the rate of change of frequency
measurement, df/dt Avg. Cycles over a fixed period of either 6 or 12 cycles. This provides
the ability to de-sensitize the frequency based protection element against oscillations in the
power system frequency. The 12-cycle averaging window setting improves measurement
accuracy, but slows down the protection start time following fault inception. The maximum
fault detection start time following fault inception can be approximated as follows:
Fault Detection Delay Time (cycles) = 2 X M + 1
Where M = No. of frequency averaging cycles df/dt.Av. Cycles