Current transformers are used for
measurement of current in any circuit. It has same working principle of that
transformer but only used for step down of current.
Current Transformer are used in
measurement circuits , it is advisable to use CT instead of connection ammeter
in the circuit as currents above 20A ammeter size becomes so large which is not
feasible that is why current transformers are used.
Current transformer primary has only one turn. Current
transformer primary is not really a turn but just a conductor. The primary
winding of Current Transformer has very few turns, while the secondary winding
have a many turns depends upon how much the current must be stepped down.
There is extra precaution required during
connections of CT are that secondary of CT should never be kept open which
otherwise leads to very high secondary voltages which ultimately leads to
burning of CT or even explosion of CT. So such conditions should be avoided.
Now let’s discuss about the same by
considering a Transformer, Let’s take a Transformer with one primary turn and
many secondary turns. Now when current flows through primary this will leads to
very voltage induced in secondary of the order of Kilovolts. This is leads to
breakdown of insulation of CT.
Usually Secondary current of CT are
available with 5A rating and Input current may be of any ampere value.
CT Connection for
Transformers:-
It has been found that usually in Star/
Delta connected transformer at Star side CT connections are in Delta and at
Delta Side of Transformer CT connections are done in Star.
This is usually done for the following
reason:-
The delta CT connection circulates the
zero-phase-sequence components of the currents inside the delta and thereby
keeps them out of the external connections to the relay. This is necessary
because there are no zero-phase-sequence components of current on the delta
side of the power transformer for a ground fault on the wye side; therefore,
there is no possibility of the zero-phase-sequence currents simply circulating
between the sets of CTs and, if the CTs on the wye side were not delta
connected, the zero-phase-sequence components would flow in the operating coils
and cause the relay to operate undesirably for external ground faults.
3-Phase 4 Wire connections in Energy Meter
You will
see the 3- Phase 4-wire connections on an Energy meter in which neutrals of
every CT secondary are get grounded.
These
connections are most popular in electrical system for connections of energy
meters where there are three phase energy meters needed to be installed.
