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FUNCTION CHARACTERISTICS
49 NA20 - Manual - 04 - 2022
Negative sequence overcurrent - 46
Preface
Two operation thresholds, independently adjustable ( I
2
>, I
2
>>) with adjustable delay ( t
2
>, t
2
>>).
The first one may be programmed with definite or inverse time according the IEC and ANSI/IEEE
standard, as well as with I
2
t or EM curve.
The second threshold has a definite time characteristic.
For each threshold a reset time can be set ( t
2>RES
, t
2>>RES
) useful to reduce the clearing time for
intermittent faults.
The first threshold trip may be inhibited by start of the second threshold ( I
2
>>).
Operation and settings
The negative sequence current is computed as:
I
2
=( I
L1
+e-
j120°
· I
L2
+e
+j120°
· I
L3
)/3
where e
-j120°
=-1/2-j√3/2, e
j120°
=-1/2+j√3/2.
The negative sequence current is compared with the setting values. Currents above the associated
pickup value are detected and a start is issued. After expiry of the associated operate time ( t
2
>, t
2
>>)
a trip command is issued; if instead the current drops below the threshold, the element is restored.
The first threshold ( I
2
>) may be programmed with definite or inverse time according the following
characteristic curves:
• Standard Inverse Time (IEC 255-3/BS142 type A or SIT): t = 0.14 · t
2
>
inv
/ [( I
2
/ I
2
>
inv
)
0.02
- 1
• Very Inverse Time (IEC 255-3/BS142 type B or VIT): t = 13.5 · t
2
>
inv
/ [( I
2
/ I
2
>
inv
) - 1]
• Extremely Inverse Time (IEC 255-3/BS142 type C or EIT): t = 80 · t
2
>
inv
/ [( I
2
/ I
2
>
inv
)
2
- 1]
• Moderately Inverse (ANSI/IEEE type MI): t = t
2
>
inv
· {0.01 / [( I
2
/ I
2
>
inv
)
0.02
- 1] + 0.023}
• Very Inverse (ANSI/IEEE type VI): t = t
2
>
inv
· {3.922 / [( I
2
/ I
2
>
inv
)
2
- 1] + 0.098}
• Extremely Inverse (ANSI/IEEE type EI): t = t
2
>
inv
· {5.64 / [( I
2
/ I
2
>
inv
)
2
- 1] + 0.024}
• I-squared-t ( I
2
t = K): t = 16 · t
2
>
inv
/ ( I
2
/ I
2
>
inv
)
2
• Electromechanical (EM): t = t
2
>
inv
· {0.28 / [-0236 · ( I
2
/ I
2
>
inv
)
-1
+ 0.339]}
Where:
t : operate time
I
2
>
inv
: threshold setting
t
2
>
inv
: operate time setting
The second threshold has a definite time characteristic.
For all inverse time characteristics, following data applies:
• Asymptotic reference value (minimum pickup value): 1.1 I
2
>
inv
• Minimum operate time: 0.1 s
• Range where the equation is valid:
[1]
1.1 ≤ I
2
/ I
2
>
inv
≤ 20
For all definite time elements the upper limit for measuring is 50 I
n
.
All overcurrent elements can be enabled or disabled by setting
ON
or
OFF
the
I2> Enable
and/or
I2>> Enable
parameters inside the Set \ Profile A(or B) \ Negative sequence overcurrent - 46 \
I2> Element ( I2>> Element) \ Setpoints menus.
The first overcurrent element can be programmed with definite or inverse time characteristic by
setting the
I2>Curve
parameter (
DEFINITE, IEC/BS A, IEC/BS B, IEC/BS C, ANSI/IEE MI,
ANSI/IEE VI, ANSI/IEE EI, I2t, EM
) available inside the Set \ Profile A(or B) \ Negative sequence
overcurrent - 46 \ I2> Element \ Setpoints menu.
The trip of I2 > element may be inhibited by the start of the second element ( I2 >>) by set-
ting
ON
the Disable I2> by start I2>> (
I2>disbyI2>>
) parameter available inside the
Set \ Profile A(or B) \ Negative sequence overcurrent-46 \ I2>> Element \ Setpoints menu.
Note 1 When the input value is more than 20 times the set point , the operate time is limited to the value corresponding to 20 times the set point
t-int-F46.ai
I
2
I
2
>>
t
2
>
t
2
>>
I
2
>
t
TRIP
General operation time characteristic curve for the negative sequence overcurrent element - 46