Wednesday, 20 July 2016

Why CT Secondary Shall Never be Kept Open?

The most important precaution which shall be taken care while working with a CT is that its secondary should never be kept open. In this post we will discuss this aspect of CT.

As we know that Current Transformers (CTs) are always used with secondary winding connected with Ammeter, Relays or Wattmeter Coils. A precaution which shall always be taken is that Never open the secondary winding circuit of a Current Transformer while its Primary Winding is energized.

If the secondary winding circuit of a CT is kept open then it will lead to severe consequence to the personnel opening the CT secondary and to the CT itself. The question arises why?
To understand this, first we should know the basic difference between a Power Transformer and a CT. The basic difference between a Power Transformer and a CT is that, in Power Transformer the primary current is the reflection of the secondary current by N1I1 = N2I2 while in CT the primary current is dependent on the load current or line current as Ct is connected in series with the line. So primary current of CT (assuming constant line current) is constant irrespective of whether the secondary of CT is connected with burden or not.

During normal operation of CT, the primary and secondary winding produces mmf which by lenze’s law opposes each other. As the secondary mmf is slightly less than the primary mmf, the net mmf is small. This net mmf is the working / magnetizing mmf of the core of CT.

Now, in case secondary winding is kept open then secondary current will be zero while the primary current of CT will remain same. Therefore the opposing mmf of secondary will no longer exist. Hence the net mmf is due to primary current only i.e. N1I1 which is very large. This large mmf will produce large flux in the core and will saturate the core. Again, due to large flux in the core the flux linkage of secondary winding will be large which in turn will produce a large voltage across the secondary terminals of the CT. This large voltage across the secondary terminals will be very dangerous and will lead to the insulation failure and there is a good chance that the person who is opening the CT secondary while primary is energized will die due to shock.

Also, because of excessive core flux, the hysteresis and eddy current loss will be very high and the CT will get overheated. As CT is oil filled, because of overheating, the oil of CT will get boil and start to vaporize. Because of vaporization of CT oil, the CT housing will get pressurized and blast. This blasting will lead to fire and smoke. Again, it is not the end here but due to smoke, the nearby lines will trip due to earth fault which may trip the Power Generating Station.

So, see how a small mistake causes a series of dangerous consequences. 

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