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2.6
Rate-of-Change-of-Frequency (81R)
For some applications, operation of 81 (Under/Over-Frequency) elements can take an unacceptably long time.
e.g. where the frequency changes gradually and so takes time to reach the Under or Over-Frequency pick-up
level; after which the element delay to trip will then begin. For such applications it may be more desirable to use
an 81R Rate of Change of Frequency (ROCOF) element.
ROCOF is applied to both load shedding and loss-of-mains (LOM) detection. In both instances, it may be
necessary to have fast tripping times. For load shedding it may be necessary to restore voltage and frequency
levels to regulated levels within a regulated timeframe. For LOM, it may be necessary to disconnect a source of
generation before the mains network is restored e.g. by auto-reclosure thereby avoiding synchronising problems.
Where load has reduced, creating an excess of generated power, frequency will rise and a positive df/dt curve will
result. This would be the case where a local load has become 'islanded' and the local generation is higher than
that required to supply it. Reductions in 'downstream' load levels, caused by switching or tripping events, should
not be sufficient to cause the 81R elements to operate since the connection to the mains supply will ensure
voltage and frequency regulation and the effect on the local generation should not be great.
If the User is concerned about vector shifts below 3°, then the delay setting should used to delay operation of the
81R element. A delay of 300ms will ensure that the 81R element will not operate for a vector shift of any size.
2.6.1
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, therefore
recommends an 81R pick-up 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 pick-up as 0.125 Hz/s, a delay in the order of 300ms is recommended.
The second 81R element can be set to a higher pick-up level, but with a corresponding reduction in operate time.
2.6.2
Load Shedding
Frequency broadly decays in an exponential curve during a load shedding situation, eventually stabilising 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 operational
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.
©2014 Siemens Protection Devices Limited
Figure 2.6-1 Use of ROCOF elements
Chapter 7) 7SR158 Applications Guide
Chapter 7) Page 11 of 24
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