Seg WI Series User Manual

3.3.2.2 CT Type WE2
In case of small values of the primary currents there is a special “E”-labeled CT available
that features a mixed core based on MU metal and o󰊞ers an optimized transmission
characteristic: WIC1WE2AS1.
It is typical of self-powered protection relays such as the WIC1 that small primary currents
cause a non-linear transmission characteristic of the relay.
The “E” CTs improve the accuracy of the whole system if the primary current values are less
than 16 A.
A worse accuracy can often be accepted for an application without a calculation-based
ground current protection, so that the WIC1W2AS1 (i.e. without „E“) represents an
inexpensive alternative. If a higher accuracy is needed, or in case of a required earth
current protection element it is recommended to always use an “E”-type CT (i.e. prefer
WIC1WE2AS1 over WIC1W2AS1), because in the lower operation range this CT type is more
precise than the WIC1W2AS1.
3.3.2.3 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. It can be calculated according to the following formula:
In =
Sn
3
⋅ Vn
☼
Example:
•
Sn = 1600 kVA
•
• Vn = 10 kV•
The resulting nominal current is: In = 92.4 A
Check the table of available CTs in ╚═▷ “2.6.1.3 Order Form of the WIC1-Compatible
Current Transformers” or ╚═▷ “3.3.2.1 Current Ranges of the WIC1-CTs”, and you 󰊟nd
that both W3 and W4 are suitable for this nominal current.
Further criteria can now be applied to pick one out of these suitable 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. (See also the red bars in diagram ╚═▷ Fig. 18.)
This means for the selected CT:
W3: 112 A × 20 = 2240 A, or
W4: 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, so that the current can be
measured cleanly by the WIC1.
• Operating current:•
70 WIC1 WIC1-1.0-EN-MAN
3 Hardware
3.3.2.2 CT Type WE2

Seg WI Series Specifications

General

Brand	Seg
Model	WI Series
Category	Relays
Language	English