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NC2 0 - Manual - 01 - 2015
FUNCTION CHARACTERISTICS
Undervoltage - 27
Preface
Two operation thresholds, independently adjustable (U<, U<<) with adjustable delay (t
U
<, t
U
<<).
The fi rst one may be programmed with defi nite or inverse time, while the second threshold operates
with defi nite time characteristic.
Both thresholds may be temporarily disabled by MMI command.
Each threshold may be separately enabled or disabled.
The fi rst threshold trip (U<) may be inhibited by start of the second threshold (U<<).
Operation and settings
The fundamental frequency of the input voltages (U
L1
, U
L2
, U
L3
) are employed.
Each of three voltages compared with the setting values (U<, U<<).
The start and trip logic may be selected OR or AND.
With OR selection, a start is issued when at least one of the three voltages goes down the adjust-
able threshold (START); with AND selection, a start is issued when all the three voltages go down
the adjustable threshold.
After expiry of the associated operate time (t
U
<, t
U
<<) a trip command is issued; if instead the volt-
ages exceed the threshold, the element is restored.
The fi rst threshold (U<) may be programmed with defi nite or inverse time according the following
characteristic curve:
t=0.75 t
U<inv
/ [1-(U/U<
inv
)]
Where:
t: operate time
U: relay input voltage
U<
inv
: threshold setting
t
U<inv
: operate time setting
For the inverse time characteristic, following data applies:
The operate time setting is referred to an input voltage equal to 1/4 of the pickup value.
Asymptotic reference value (minimum pickup value): 0.9 U<
Minimum operate time: 0.1 s
Range where the equation is valid: 0 ≤ U/U<
inv
≤ 0.9
The fi rst undervoltage element can be programmed with defi nite or inverse time characteristic by
setting the U<Curve parameter (DEFINITE, INVERSE) available inside the Set \ Profi le A(or B) \
Undervoltage-27 \ U< Element \ Setpoints menu.
Each element can be enabled or disabled by setting ON or OFF the U< Enable parameter inside
the Set \ Profi le A(or B) \ Undervoltage-27 \ U< Element \ Setpoints menu and/or the State parameter
inside the Set\Profi le A(or B) \ Undervoltage-27 \ U<< Element \ Defi nite time.
The operating logic (AND or OR) is adjustable inside the Set \ Profi le A(or B) \ Undervoltage-27 \ Com-
mon confi guration menu by means the Logic27 parameter; the allowed settings are OR (at least
one voltage lower than threshold) or And (all three voltage must be lower than threshold).
•
•
•
•
t-int-F27.ai
U
U<< U<
0.9U<
t
U
<<
t
t
U
<
General operation time characteristic curve for the undervoltage elements - 27
TRIP
t-int-F27.ai
U
U<< U<
0.9U<
t
U
<<
t
t
U
<
General operation time characteristic curve for the undervoltage elements - 27
TRIP
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