Protective Elements
f< and df/dt – Underfrequency and Rate of Change of Frequency
With this setting the frequency element supervises if the frequency falls below a set pickup threshold and if the
frequency gradient exceeds a set threshold at the same time.
In the selected frequency parameter set f[X], an underfrequency pickup threshold f<, a frequency gradient df/dt and
a tripping delay can be set.
Whereby:
Positive df/dt = the frequency element detects an increase in frequency
Negative df/dt = the frequency element detects a decrease in frequency and
Absolute df/dt (positive and negative) = the frequency element detects both, increase and decrease in frequency
f> and df/dt – Overfrequency and Rate of Change of Frequency
With this setting the frequency element supervises if the frequency exceeds a set pickup threshold and if the
frequency gradient exceeds a set threshold at the same time.
In the selected frequency parameter set f[X], an overfrequency pickup threshold f>, a frequency gradient df/dt and a
tripping delay can be set.
Whereby:
Positive df/dt = the frequency element detects an increase in frequency
Negative df/dt = the frequency element detects a decrease in frequency and
Absolute df/dt (positive and negative) = the frequency element detects both, increase and decrease in frequency
Working Principle f< and df/dt | f> and df/dt
(Please refer to the block diagram on next page)
The frequency element supervises the three voltages (depending on if the voltage transformers are wired in Star or
Delta connection »
VL12«, »VL23« und »VL31« oder »VL1«, »VL2« und »VL3«).
If any of the three phase voltages is e.g. below 15% Vn, the frequency calculation is blocked (settable via parameter
»
V Block f«). According to the frequency supervision mode set in the Device Planning (f< and df/dt or f> and dt/dt),
the evaluated phase voltages are compared to the set frequency pickup threshold and the set frequency gradient
(df/dt) threshold.
If in any of the phases, both - the frequency and the frequency gradient exceed or fall below the
set thresholds and if there are no blocking commands for the frequency element, an alarm is issued instantaneously
and the tripping delay timer is started. When the frequency and the frequency gradient still exceed or are below the
set threshold after the tripping delay timer has elapsed, a tripping command will be issued.
406
MRU4
DOK-HB-MRU4-2E
