49
NA11 - Manual - 02 - 2009
FUNCTION CHARACTERISTICS
Phase overcurrent - 50/51
Preface
Three operation thresholds, independently adjustable (
I
>,
I
>>,
I
>>>) with adjustable delay
(
t
>,
t
>>,
t
>>>).
The fi rst one may be programmed with defi nite or inverse time according the IEC and ANSI/IEEE
standard, as well as with rectifi er, I
2
t or EM curve.
The second threshold may be programmable with independent or dependent time according to the
I
2
t curve and the third threshold with independent time.
For each threshold a reset time can be set (
t
>RES
,
t
>>RES
,
t
>>>RES
) useful to reduce the clearing time
for intermittent faults.
The fi rst threshold trip may be inhibited by start of the second and/or third threshold (
I
>>,
I
>>>).
Similarly the second threshold trip may be inhibited by start of the third threshold (
I
>>>).
Operation and settings
Each phase fundamental frequency current is compared with the setting value. Currents above the
associated pickup value are detected and a start is issued. After expiry of the associated operate
time a trip command is issued; if instead the current drops below the threshold, the element it is
restored.
The fi rst threshold (
I
>) may be programmed with defi nite or inverse time according the following
characteristic curves:
Standard Inverse Time (IEC 255-3/BS142 type A or SIT):
t
= 0.14 ·
t
>
inv
/ [(
I
/
I
>
inv
)
0.02
- 1]
Very Inverse Time (IEC 255-3/BS142 type B or VIT):
t
= 13.5 ·
t
>
inv
/ [(
I
/
I
>
inv
) - 1]
Extremely Inverse Time (IEC 255-3/BS142 type C or EIT):
t
= 80 ·
t
>
inv
/ [(
I
/
I
>
inv
)
2
- 1]
Moderately Inverse (ANSI/IEEE type MI):
t
=
t
>
inv
· {0.01 / [(
I
/
I
>
inv
)
0.02
- 1] + 0.023}
Very Inverse (ANSI/IEEE type VI):
t
=
t
>
inv
· {3.922 / [(
I
/
I
>
inv
)
2
- 1] + 0.098}
Extremely Inverse (ANSI/IEEE type EI):
t
=
t
>
inv
· {5.64 / [(
I
/
I
>
inv
)
2
- 1] + 0.024}
Rectifi er (RI):
t
= 2351 ·
t
>
inv
/ [(
I
/
I
>
inv
)
5.6
- 1]
I-squared-t (
I
2
t = K):
t
= 16 ·
t
>
inv
/ (
I
/
I
>
inv
)
2
Electromechanical (EM):
t
=
t
>
inv
· {0.28 / [-0236 · (
I
/
I
>
inv
)
-1
+ 0.339]}
Where:
t
: operate
time
I
>
inv
: threshold
setting
t
>
inv
: operate
time
setting
The second threshold (
I
>>) may be programmable with defi nite or inverse time according to the I
2
t
curve:
t
= 16 ·
t
>
inv
/ (
I
/
I
>
inv
)
2
The third threshold (
I
>>>) with defi nite time.
For all inverse time characteristics, following data applies:
Asymptotic reference value (minimum pickup value): 1.1
I
>
inv
Minimum operate time:
0.1 s
Range where the equation is valid:
[1]
1.1 ≤
I
/
I
>
inv
≤ 20
If
I
>
inv
pickup ≥ 2.5
I
n
, the upper limit is 50
I
n
For all defi nite time elements the upper limit for measuring is 50
I
n
for standard CT inputs or 35
I
n
for
low power CT inputs.
All overcurrent elements can be enabled or disabled by setting
ON
or
OFF
the
I> Enable
,
I>>
Enable
and/or
I>>> Enable
parameters inside the
Set \ Profi le A(or B) \ Phase overcurrent-50/51\
I> Element
(
I>> Element
,
I>>> Element) \ Setpoints
menus.
The fi rst overcurrent element can be programmed with defi nite or inverse time characteristic by
setting the
I>Curve
parameter (
DEFINITE, IEC/BS A, IEC/BS B, IEC/BS C, ANSI/IEE MI,
ANSI/IEE VI, ANSI/IEE EI, RECTIFIER, I2t, EM
) available inside the
Set \ Profi le A(or B) \
Phase overcurrent-50/51 \
I> 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
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t-int-F50-51.ai
I
I
>>
I
>>>
t
>
t
>>
t
>>>
I
>
t
TRIP
General operation time characteristic for the phase overcurrent elements - 50/51
t-int-F50-51.ai
I
I
>>
I
>>>
t
>
t
>>
t
>>>
I
>
t
TRIP
TRIP
General operation time characteristic for the phase overcurrent elements - 50/51
