what is spill current of current transformer and how to check the C.T. polarity if labelling is missing?

Answer 1

Current transformers are used in power systems for measurement purposes, to reduce high voltage primary currents to a manageable value, say 0 to 1 or 5 amp. Their secondary winding always works shorted. Spill current is the amount of current it cannot detect. IOW, the primary current must magnetize the core before a secondary current can be detected.

Polarity can be checked only against another labeled CT.

1) Connect their secondaries in parallel. Keep them shorted.
2) Connect both CTs to an adequate AC current source.
3) Briefly unshort them.

If you get a big spark when you unshort them, you know they are in parallel with the same polarity,

or

measure the current through the CTs.
a) If the current is very low, they are connected in parallel with the same polarity
b) If the current is high, their polarities are crossed.

Answer 2

If a C.T. saturates, it will start generating harmonics to the line frequency. Some of the harmonics may contain enough energy to produce current that will trip protection relays attached to the C.T. -- even though the line current flowing through the C.T. is at a 'safe' (low) level.

Make sure your C.T.s are sized correctly so they don't saturate.

As for the polarity -- get a 1.5V battery, a piece of wire and an oscilloscope. Run the wire through the C.T., and short out the battery (temporarily) while you look at the output of the C.T. on an oscilloscope.

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