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34
SME2-IS - Manual - 06 - 2021
FUNCTIONAL CHARACTERISTICS
— Residual overvoltage - 51N
Foreword
This function has two adjustable thresholds with time-independent characteristic.
Each protection threshold can be enabled or disabled.
A constant reset time can be set for each threshold, to reduce the time for eliminating intermittent
faults (while being no less than the protection start time).
Measurement criterion
The protection employs the fundamental component of the residual current I
E
calculated as the vec-
tor sum of the phasors of the three phase currents measured at the amperometric inputs.
Operating logic
and regulations
The residual overcurrent protection compares the fundamental component of the residual current
(I
E
) with the set thresholds (I
E
>, I
E
>>); violating a threshold starts (START) the threshold itself and the
corresponding timer (t
E
>
def
, t
E
>>
def
). If the threshold violation persists, when its tripping time expires,
the threshold trips (TRIP), otherwise the threshold is reset.
Each protection threshold can be enabled or disabled,
[1]
with the following defaults:
• The rst threshold (51N.S1) is disabled with I
E
>
def
= 0.500 I
n
(250 A) and tripping time t
E
>
def
= 0.50 s
• The second threshold (51N.S2) is enabled with I
E
>>
def
= 1.000 I
n
(500 A) and tripping time t
E
>>
def
=
0.10 s
The second harmonic restraint block is active for both thresholds.
A constant reset time can be set for each of the two thresholds t
E
>
RES
and t
E
>>
RES
Note 1 If the threshold is disabled (Enable = OFF) the trip command is not available but it still contributes to the internal logic
E
L1
L2
L3
I
E
I
E
>>
t
E
>
t
E
>>
I
E
>
t
Tripping characteristic of the three thresholds of the residual overcurrent protection - 51N
TRIP
IE> Start
IE> Trip
t
E>
t
E>
RESET
INPUT
t
E>RES
t
E>RES
t
E>RES
t
Timers for the rst residual overcurrent protection threshold - 51N
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