Why it is necessary to short secondary terminals of current Transformer before disconnecting the Load?
Why Secondary Terminals of Current Transformer is short circuited
A current transformer (CT) is an instrument transformer in which the secondary current is proportional to primary current. It is used in measuring the current of the high voltage systems.
The Primary winding of current Transformer is connected in series with the load or high voltage lines, Thus current in the primary winding of the current Transformer depends on load current or line current. So, primary current of CT (assuming constant line current) is constant irrespective of whether the secondary of CT is connected with burden or not.
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When alternating current flows in the primary winding of CT, the primary current generates a magnetic field in the core of CT. The generated magnetic field sets up magnetic flux in the core. The magnetic flux links to the secondary winding of CT, thus linked flux induces a voltage in the secondary winding of CT.
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The secondary winding of CT is connected to the burden (measuring instrument or protective relay). If the secondary CT is connected to the burden, the alternating current starts flowing in the secondary winding of the transformer.
The current in the secondary creates opposing magnetic flux in the secondary winding that opposes the main flux created by the primary winding. The net flux in the core is equal to the difference of primary and secondary flux. The flux in the core remains within the rated flux rating of the core if the secondary is connected to the burden.
Now if The secondary winding is kept open with primary winding energized, no current will flow in the secondary, and consequently, no secondary flux will be set up.
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In absence of secondary current, the net flux will be higher than the flux generated in the core when secondary of CT is connected to burden. The higher flux generates a higher voltage in the secondary, and as a result of higher secondary voltage, the insulation of the secondary winding can fail. Also, this high voltage may cause an electric shock to the person working in the feeder. The magnitude of the secondary voltage with open secondary may be kilovolts.
For this reason, CT should be never kept open-circuited. While working on measuring equipment like energy meter, ampere meter, or protection relay, the CT must be short-circuited on the test terminal block
