5.2 Slope & Angle Calculations
The excitation diagram shown in Figure 4 depicts the terms used in the following section. The
graph plots the logarithm of the current on the horizontal axis and the logarithm
of voltage on the
vertical axis. The blue region is the knee point with a slope
s
KP
. It is calculated with Equation 5.2.a
and transferred to the angle [
σ
KP
] using Equation 5.2.b. The CT under test is saturated in the red
region above the knee point where
s
SAT
<
s
KP
. In the green region, below the knee point, the CT
under test is in the linear operating state with
s
LIN
>
s
KP
.
Chapters 5.3.1 and 5.3.2 describe the determination of the saturation condition and the knee points
of the CT under test. The secondary voltage and current values of two subsequent measurements
are used in order to calculate the slope and the angle of the point on the excitation curve.
Each measurement
k leads to a pair of values (I
k
, U
k
). The horizontal
x-value is log
I and the
vertical
y-value is log
U. The slope
s between two subsequent measurements
n and
n+1 is
calculated using the slope properties of the log-log graph:
5.2.a) Equation:
s
= Δ
y
Δ
x
= Δ
log
U
Δ log
I
=
log
U
n
+1
−log
U
n
log
I
n
+1
−log
I
n
=
log
U
n
+1
U
n
log
I
n
+1
I
n
90225-1.30 Instruction Manual CT-T1
Page 18 of 62
Figure 4: Excitation diagram
The angle
σ can be calculated by using Equation 5.2.b):
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