5.3.3. Turns Ratio
The turns ratio TR of a CT is usually given as the ratio of the rated primary current I
H
versus the
rated secondary current I
X
(see Equation 5.3.3a))
5.3.3.a) Equation:
TR
=
I
H
I
X
In the measurement setup the secondary winding is negligibly loaded only by the voltage
measurement of the CT-T1. Figure 6 shows a Turns ratio measurement setup
If the measured voltage U
X
is below the knee point voltage (The method of determining this voltage
is described in Chapter 5.3.2.) of the CT under test, the energy in the primary winding is equal to
the energy in the secondary winding and Equation 5.3.3.b) would be true.
5.3.3.b) Equation:
U
X
⋅I
X
=U
H
⋅I
H
The Combining of Equations 5.3.3.a) and 5.3.3.b) leads to the method (see Equation 5.3.3c)), used
to measure the turns ratio by the CT-T1.
5.3.3.c) Equation:
TR
=
U
X
U
H
5.3.4. Polarity
The polarity is determined simultaneously with turns ratio measurement by measuring the phase
shift of the two time variant voltages (u
x
(t) and u
H
(t)). It is considered positive if the phase is around
0° and the turns ratio displayed on the device is positive; otherwise, if the phase is around 180°,
the polarity is considered negative and the turns ratio is negative as well.
90225-1.30 Instruction Manual CT-T1
Page 21 of 62
Figure 6: CT Turns ratio measurement setup
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