Undervoltage Protection - ABB REU 610 Technical Reference Manual

REU 610
5.1.4.3.
26
Voltage Relay
Technical Reference Manual
The settable drop-off/pick-up ratio enables the voltage protection to
satisfactorily work with a voltage regulator, for instance, such as a tap
changer. The tap changer step is often 1.67%, which is less than the
drop-off/pick-up ratio of voltage relays. This can cause a situation
where the voltage protection remains active even though the tap
changer already has changed the voltage.
Overvoltage protection based on negative phase-sequence
voltage
When the calculated negative phase-sequence voltage value U
value of the stage U >, the stage generates a start signal after a 50 ms' start time.
2
When the set operate time at definite-time characteristics or the calculated operate
time at IDMT characteristics elapses, the stage generates a trip signal. The stage is
reset in 70 ms after the calculated negative phase-sequence voltage value has fallen
below the set start value of the stage.
The negative phase-sequence voltage is calculated as follows:
+ 2 ×
U a U
=
U 12 23
2s
3
a = 1∠ 120°
2
a = 1∠ -120°
The negative phase-sequence value is scaled to the amplitude of the measured
voltage. On a network with a reverse phase order, the calculated negative phase-
sequence value has the same amplitude as the measured voltage signal.
Stage U > can be set to be blocked when one of the measured phase-to-phase
2
voltages falls below 0.15 × U
Stage U > has a fixed drop-off/pick-up ratio of 0.96.
2

Undervoltage protection

The undervoltage protection can be based on either conventional voltage
measurement or the calculated positive phase-sequence voltage.
The low-set undervoltage stage U< is based on conventional voltage measurement.
The low-set stage can also be used for alarm purposes.
The high-set undervoltage stage U<< can be set to be based on either:
Conventional voltage measurement ( U<< mode selected) or
*
Calculated positive phase-sequence voltage ( U
*
+ ×
a U
31
. The selection is made in SGF4.
n
1MRS755769
exceeds the set start
2s
(1)
< mode selected).
1