96
1MRS751802-MUM
Protection Relay
Technical Reference Manual, Standard Configurations
REX 521
7.1.6.
3U< and 3U<<
Abnormal busbar voltages can be caused by faults in the network or a faulty tap
changer or voltage regulator of a power transformer. The function blocks are
designed for the single-phase, two-phase and three-phase undervoltage protection
whenever the DT characteristic or, as concerns 3U<, the IDMT (Inverse Definite
Minimum Time) characteristic is appropriate. Suppression of harmonics is possible.
Fig. 7.1.6.-1
Function block symbols of 3U< and 3U<<
Three-phase under voltage protection, low-set stage, 3U<
Start voltage
0.10...1.20 x Un
Operate time
0.1...300.0 s
Operation mode
Not in use
Definite time
C curve
Time multiplier
0.1...1.0
Measuring mode
Mode 1 (Phase-to-phase voltages / Peak-to-peak meas.)
Mode 2 (Phase-to-phase voltages / Fundam.freq.)
Mode 3 (Phase-to-earth voltages / Fundam.freq.)
Note! The values below apply when f/fn = 0.95...1.05
Operation accuracy
±2.5% of the set value with Measuring Mode 3
±1,0% of the set value with Measuring Modes 1 and 2
Start time
Injected voltages < 0.5 x start voltage:
internal time < 32 ms
total time < 40 ms
Reset time
40...1000 ms
(depends on the minimum pulse width set for the trip
output)
Reset ratio, typically
1.04
(depends on the value of the
Oper.
hysteresis
parameter, refer to the manual of the
Three-phase Undervoltage Protection function,
1MRS752333-MUM, chapters 2.3.1. and 3.2.4.)
Retardation time
< 60 ms
Operate time accuracy in DT mode
±2,5% of set value
Accuracy class index E in inverse-time mode ± 35 ms or the accuracy appearing when the measured
voltage varies ±2.5%
BS1
BS2
GROUP
RESET
START
TRIP
3U<
BS1
BS2
GROUP
RESET
START
TRIP
3U<<
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