2.4OVERCURRENT ELEMENTS
2.4.1 PHASE TIME OVERCURRENT ELEMENT (51P)DIFFERENTIAL UNITS (87-1) (87-2)
A Phase Time Overcurrent unit is provided in the relay. There are three possibilities for the curve selection, depending on
the selected model.
IEC curves comply with the criteria specified in the IEC255-4 Standard and British Standard BS142
ANSI curves comply with ANSI C37.90 standards
IAC curves simulate the behaviour of GE IAC relays.
A time dial can be applied to any of these curves to optimize co-ordination with other devices in the net. Additionally, there
is a possibility to define a User’s Curve, which can be used for both three-phase overcurrent and ground elements.
In order to avoid undesired trips, time overcurrent elements will not pickup till a value of 1.05 * Set value.
Due to design criteria, actual pickup level has been INTENTIONALLY set to +5% over the set value to ensure that even in
the worst case scenario the relay will NEVER TRIP under the intended setting.This meets IEC international standards.
2.4.1.1 IEC CURVES
The general formula for IEC/BS142 curves is as follows:
Where:
D =Time Dial setting (set in the relay by user).
V = I / I
pickup setting
> 1.05
2.4.1.2 ANSI CURVES
The general formula for all ANSI curves is as follows:
Where:
M =Time Dial setting (set in the relay by user).
V = I / I
pickup setting
> 1.05
CURVE NAME A P Q B K
Mod Inverse (IEC Curve A) 0.14 0.02 1 0 0
Very Inverse (IEC Curve B) 13.5 1 1 0 0
Extremely Inverse (IEC Curve C) 80 2 1 0 0
CURVE NAME A B C D E
Inverse 0.0274 2.2614 0.3000 -4.1899 9.1272
Very Inverse 0.0615 0.7989 0.3400 -0.2840 4.0505
Extremely Inverse 0.0399 0.2294 0.5000 3.0094 0.7222
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