81R Frequency Protection – "df/dt"
Overview of Functions
Frequency df/dt protection (also known as “rate of change of frequency protection”) can be used in place of 81
(Under/Overfrequency) elements when they can take an unacceptably long time. e.g. where the frequency
changes gradually and so takes time to reach the Under or Overfrequency pickup level; after which the
element delay to trip will then begin. For such applications it may be more desirable to use an 81R df/dt func-
tion element.
[dw_7SR5_function81ROverview, 1, en_US]
Figure 5-52 Use of ROCOF Elements
ROCOF is applied to both load shedding and loss-of-mains (LOM) detection. In both instances, it may be neces-
sary to have fast tripping times. For load shedding it may be necessary to restore voltage and frequency levels
to regulated levels within a specific timeframe. For LOM, it may be necessary to disconnect a source of genera-
tion before the mains network is restored e.g. by auto-reclosure thereby avoiding synchronizing 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.
The 81R function is more sensitive than the vector shift (78R) function to system frequency/voltage vector
movements. The minimum 81R setting = 0.05 Hz/s. This corresponds to a vector shift of 0.007°. The maximum
81R setting = 10 Hz/s corresponding to a vector shift of 1.44° at 50 Hz (minimum 78R setting = 2°).
Structure of the Function
The frequency df/dt 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 A81RPFRC* in IEC 61850.
The function requires VT inputs, it monitors the primary system using the 3 phase voltage inputs e.g. VT1/2/3.
The df/dt protection can be set to operate on positive frequency change rate, negative frequency change rate
or frequency change rate independent of direction.
5.28
5.28.1
5.28.2
Protection and Automation Functions
5.28 81R Frequency Protection – "df/dt"
350 Reyrolle 7SR5, Overcurrent Protection Device, Device Manual
C53000-G7040-C014-1, Edition 11.2019
To Next Page
Loading...
