Appendix A: Trip Curves & Equations
56 Siemens Energy & Automation, Inc.
Figure A.9 Definite Inverse Curve (SEA8)
A.4 I-Squared-T Curve
The ISGS provides an I-Squared-T characteristic in addition
to the standard inverse curves.
Equation A.4 I-Squared-T Equation
0.01
0.1
1
10
100
11050
9.9
7.0
5.0
3.0
1.5
0.5
0.1
Multiples of Pickup
Time to Trip (Seconds)
Time Dial
Trip Characteristic
Reset Characteristic
T = time to trip, in seconds
= multiple of pickup setting
D = time dial setting, 0.1 to 9.9 in steps of 0.1
t
r
= reset constant = 7.4
T
50.7D 10.14+
i
i
p
----
2
-------------------------------------=
For
i
ip
-----
1 : T
t
r
D
i
i
p
----
2
1–
-----------------------
=
<
i
i
p
-----
Figure A.10 I-Squared-T Curve
A.5 Custom Protective Curve
The custom curve consists of up to 60 current-time pairs
corresponding to points on the time-current characteristic
curve. Current refers to multiple-of-pickup value (I/Ip) on the
horizontal axis, and time refers to time-to-trip values on the
vertical axis. Each point consists of two values (I/Ip and t),
loaded in order from lowest to highest value of I/Ip via the
SEAbus or local ports. Siemens Wisdom software is required
in order to load a custom curve. Time-to-trip has a range of
0.00 to 655.35 seconds in steps of 0.01 seconds. I/Ip has a
range of 1.1 to 20.00 in steps of 0.01. The first point in the
data set must be I/Ip=1.1, the last point must be I/Ip=20.
Points in between these two limits can be for any values of I/
Ip and t as long as the slope (
∆t/(∆I/Ip)) of the curve
described by the points is between 0 (horizontal) and -
∞ (ver-
tical). For input current in excess of 20 x Ip, the relay will
enter a definite time mode and the curve will be considered
to be flat (constant time) at the time value associated with I/
Ip=20. Once loaded, a custom curve is not adjustable, that is
there is no time dial adjustment.
A.6 Over/Undervoltage Curves
The ISGS provides a moderately inverse overvoltage and a
moderately inverse undervoltage protection defined by the
equation in Equation A.5 and Equation A.6. Their charac-
teristics are provided in Figure A.11 and Figure A.12.
Multiples of Pickup
Time to Trip (Seconds)
Time Dial
1000
100
10
1
0.1
0.01
1
10
50
9.9
7.0
5.0
3.0
1.5
0.5
0.1
A
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