Wednesday, April 23, 2014

Current Transformer Applications and their connections

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.

3-phase 4-wire connections

These connections are most popular in electrical system for connections of energy meters where there are three phase energy meters needed to be installed.