EMR-5000
IM02602012E
81U
Δ
R – Underfrequency and DF/DT
With this setting, the frequency element supervises the frequency and the absolute frequency difference during a
definite time interval.
In the selected frequency parameter set 81[X], an underfrequency pickup threshold f<, a threshold for the absolute
frequency difference (frequency decrease) DF and supervision interval DT can be set.
81O
Δ
R – Overfrequency and DF/DT
With this setting, the frequency element supervises the frequency and the absolute frequency difference during a
definite time interval.
In the selected frequency parameter set 81[X], an overfrequency pickup threshold f>, a threshold for the absolute
frequency difference (frequency increase) DF and supervision interval DT can be set.
Working Principle
(Please refer to block diagram on next page.)
The frequency element supervises the three phase voltages »
VA«, »VB« and »VC«. If any of the three phase voltages
is below 15% Vn, the frequency calculation is blocked. According to the frequency supervision mode set in the Device
Planning (81UR & DF/DT or 81OR & DF/DT), the evaluated phase voltages are compared to the set frequency pickup
threshold and the set frequency decrease or increase threshold DF.
If in any of the phases, the frequency exceeds or falls below the set pickup threshold and if there are no blocking
commands for the frequency element, an alarm is issued instantaneously. At the same time the timer for the
supervision interval DT is started. When, during the supervision interval DT, the frequency still exceeds or is below the
set pickup threshold and the frequency decrease/increase reaches the set threshold DF, a tripping command will be
issued.
Working Principle of DF/DT Function
(Please refer to f(t) diagram after the block diagram.)
Case 1:
When the frequency falls below a set f< threshold (81U) at t1, the DF/DT element energizes. If the frequency difference
(decrease) does not reach the set value DF before the time interval DT has expired, no trip will occur. The frequency
element remains blocked until the frequency falls below the underfrequency threshold f< (81U) again.
Case 2:
When the frequency falls below a set f< threshold (81U) at t4, the DF/DT element energizes. If the frequency difference
(decrease) reaches the set value DF before the time interval DT has expired (t5), a trip command is issued.
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