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2 Functions
2.19
Rate-of-Frequency-Change Protection df/dt (ANSI 81R)
With the rate-of-frequency-change protection, frequency changes can be quickly de-
tected. This allows a prompt response to frequency dips or frequency rises. A trip
command can be issued even before the pickup threshold of the frequency protection
(see Section 2.17) is reached.
Frequency changes occur for instance when there is an imbalance between the gen-
erated and the required active power. They call for control measures on one hand and
for switching actions on the other hand. These can be unburdening measures, such
as network decoupling, or disconnection of loads (load shedding). The sooner these
measures are taken after a malfunction appears, the more effective they are.
The two main applications for this protection function are thus network decoupling and
load shedding.
2.19.1 Functional Description
Measuring Princi-
From the positive-sequence voltage, the frequency is determined once per cycle over
ple
a measuring window of 3 cycles, and a mean value of two consecutive frequency mea-
surements is formed. The frequency difference is then determined over a settable time
interval (default setting 5 cycles). The ratio between frequency difference and time dif-
ference corresponds to the frequency change; it can be positive or negative. The mea-
surement is performed continuously (per cycle). Monitoring functions such as under-
voltage monitoring, checks for phase angle jumps etc. help to avoid overfunctioning.
Frequency In-
The rate-of-frequency-change protection has four stages, from df1/dt to df4/dt. This
crease/ Decrease
allows the function to be adapted variably to all power system conditions. The stages
can be set to detect either frequency decreases (-df/dt) or frequency increases
(+df/dt). The -df/dt stage is only active for frequencies below the rated frequency, or
less if the underfrequency enabling is activated. Likewise, the df/dt stage is active for
frequencies above the rated frequency, or higher, if the overfrequency enabling is ac-
tivated. The parameter setting decides for what purpose the particular stage will be
used.
To avoid a proliferation of setting parameters, the settable measuring window for the
frequency difference formation and the dropout difference are each valid for two
stages.
Operating Ranges
The frequency can be determined as long as there is a sufficiently strong positive se-
quence system of voltages. If the measurement voltage drops below a settable value
U MIN, frequency protection is disabled because precise frequency values can no
longer be calculated from the signal.
Time Delays/Logic
Tripping can be delayed by a set time delay associated with each applied time stage.
This is recommended for monitoring of small gradients. When the time delay expires,
a trip signal is generated. After pickup dropout the tripping command is immediately
reset, but not before the minimum command duration has expired.
Each of the four frequency change stages can be blocked individually by binary input.
The undervoltage blocking acts on all stages simultaneously.
118
7UM61 Manual
C53000-G1176-C127-3
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