Functions
2.26 Rate-of-Frequency-Change Protection df/dt (ANSI 81R)
SIPROTEC, 7UM62, Manual
C53000-G1176-C149-7, Release date 03.2010
195
2.26.2 Setting Notes
General
The rate-of-frequency-change protection is only effective and accessible if during the configuration address
145 df/dt Protect. has been set accordingly. The user can select between 2 or 4 stages. The default
setting is 2 df/dt stages.
Address 4501 df/dt Protect. serves to switch the function ON or OFF or to block only the trip command
(Block relay).
Pickup Values
The setting procedure is the same for all stages. In a first step, it must be determined whether the stage is to
monitor a frequency rise at f>f
N
or a frequency drop at f<f
N
. For stage 1, for instance, this setting is made at
address 4502 df1/dt >/<. The pickup value is set as an absolute value at address 4503 STAGE df1/dt.
The setting of address 4502 informs the protection function of the applicable sign.
The pickup value depends on the application and is determined by power system conditions. In most cases, a
network analysis will be necessary. A sudden disconnection of loads leads to a surplus of active power. The
frequency rises and causes a positive frequency change. A failure of generators, on the other hand, leads to a
deficit of active power. The frequency drops and leads to a negative frequency change.
The following relations can be used as an example for estimation. They apply for the change rate at the begin-
ning of a frequency change (approx. 1 second).
Definitions:
f
N
Nominal Frequency
ΔP Active power change
ΔP = P
Consumption
– P
Generation
S
N
Nominal apparent power of the machines
H Inertia constant
Typical values for H are:
Example:
f
N
= 50 Hz
H = 3 s
Case 1: ΔP/S
N
= 0.12
Case 2: ΔP/S
N
= 0.48
Case 1: df/dt = -1 Hz/s
Case 2: df/dt = -4 Hz/s
The default settings are based on the above example. The four stages have been set symmetrically.
for hydro-electric generators (salient-pole machines) H = 1.5 s to 6 s
for turbine-driven generators (cylindrical-rotor machines) H = 2 s to 10 s
for industrial turbine-generators H = 3 s to 4 s
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