2) Leakage reactance:
Leakage reactance tends to increase ratio error. Therefore, the two windings, primary and
secondary should be close together to reduce the secondary winding leakage reactance. Use
of ring shaped cores around which toroidal windings are uniformly distributed also leads to
low values of leakage flux, leakage reactance.
3) Turns compensation:
The ratio error can be considerably reduced by using turns compensation which is almost
used in practice. This involves making secondary turns one or two less than that the number
which would make turns ration equal to nominal ratio.
We have, actual, transformation ratio :
R = n + Iw / Is
Thus if we make the "nominal ratio" equal to the turns ratio the actual transformation ratio
becomes more than the nominal ratio. Now if we reduce the turns ratio and keep the
nominal ratio equal to the earlier value, the actual transformation ratio will be reduced.This
would make actual transformation ratio nearly equal to the nominal ratio.Let us make it
clear with the help of an example.
For example, in a 1000/5A current transformer of the bar primary type is of single turn
primary, the number of turns on the secondary would be either 199 or 198 instead of 200.
The phase angle error, however, will remain almost unaffected by a slight change in the
number of turns.
4) Primary winding current ratings:
Whatever equipment a current transformer is feeding, it is desirable that the ratio of
exciting current to primary current should be small. This means that the ratio of excitation
mmf to primary winding mmf should be low. It is difficult to achieve this condition if the
latter quantity (primary winding current or mmf) is small, at low values of Ip ( or Is) , the
exciting current component Im and Iw are greater proportion of Ip and therefore, the errors
are greater.
An improvement in performance is always obtained by increasing the primary winding mmf.
5) Use of shunts:
If the secondary winding current is too large, it may be reduced by a shunt placed across the
primary or the secondary winding. This method makes an exact correction only for a
particular value and type of burden. It also reduces phase angle error.
6) Wilson compensation method:
Reduction of one or two turns of the secondary winding, no doubt, reduces the ratio error,
but it has no effect on the phase angle error. Also, this method is too coarse a method for
ratio adjustment and therefore we must use a method which exercises a finer control, say
which is equivalent to reduction to a fraction of a turn. A compensated type of design was
given by S. Wilson of the General Electric Company. This method gives finer adjustments.
It employs a few turns of wire called auxiliary secondary turns passed through a hole in the
core and connected in series with the secondary winding. A short-circuited turn is placed
around one position of the core to improve the phase relationships.
7) Two stage design:
This design utilizes a second current transformer to correct the error in secondary current
of first transformer. This method, in general, is applicable to an energy meter because a
second coil is needed in the meter to carry the error-correcting current unless an auxiliary
transformer is used.
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