SEG Electronics GmbH Technical Manual WIC1
40 DOK-TD-WIC1E, Rev. M
5.11 Selection of the CT transformation voltage ratio
Selection of the CT suitable for the WIC1 depends on the rated current of the transformer to be protected and is
calculated acc. to the following formula:
Example:
SN = 1600 kVA
UN = 10 kV
- I
N
= 92.5 A
According to the table in 4.3 “CTs for the WIC1”, the CTs WIC1W3AS1 and WIC1W4AS1 are suitable for this rated
current.
Further conditions that have to be taken into account with respect to the selection of the CTs.
Short-circuit transient characteristic:
The WIC1 can measure a short-circuit current of a circuit up to 20 times of the upper rated CT current.
This means for the selected CT:
o WIC1W3AS1: 112 A × 20 = 2240 A, or
o WIC1W4AS1: 224 A × 20 = 4480 A.
If the short-circuit current of a circuit is approx. 3 kA, for example, because of the location, then the
preference should be given to the “W4” CT, so that the current can be measured cleanly by the WIC1.
Operating current:
Furthermore please pay attention to the operating current of the protection object. The operating cur-
rent should be in the rated CT current range. The WIC1 system can be loaded continuously up to 2.5
times of the upper rated CT current. This has no effect on the measurement of the overload. But due to
the limitation of the measurement to 20 times the upper rated CT current, the short-circuit protection is
limited.
With parameter Is the calculated rated current is preset in the protection relay.
According to Table 5.1, IS currents of 88 A can be set for both CT types in relay versions WIC1-2 and WIC1-3.
In the example shown below, the transformer can be operated with 1.1 times the rated current for 10 s. The setting
value for I> is calculated as follows:
WIC1-1:
I> = 1.1 x Is with Is = 92.5 A
WIC1-2 and WIC1-3:
By means of the switches, however, only 1.15 x Is or 1.2 x Is can be set for parameter I>. Therefore it is up to the
user to choose either 1.15 x Is or 1.2 x Is
The different rated primary currents of all standard transformer sizes at various rated mains voltages are compiled
in chapter 7.2.
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