standard test limit in capacitance test of a transformer

You'll have to obtain that from the transformer manufacturer. I don't believe it's given on the transformer nameplate.

None.

You have to conduct the initial test. The capacitance value will become your reference value.

The succeeding tests after the initial one will be evaluated based on the initial/reference capacitance value.

Remember also that capacitance testing is not a pass or fail type of test like hipot. The increase in value at any given time duration will be the key to diagnosing the health of the transformer.

Capacitance test is used to detect any mechanical damage inside the transformer. This may be done if the transformer was recently transported or a large fault current was detected.

Personally, I would recommend a sweep frequency response analysis to be conducted. It is more powerful than a traditional capacitance test. Its algorithm uses the inductance and capacitance of the transformer core and winding to detect mechanical changes within the transformer.

The capacitance value comes with the insulation power factor test (or dissipation factor test). Usually the capacitance of your specimen should not change through time, not unless the transformer went through a rewind.

There are limits for the insulation power factor test but not for the transformer capacitance.

If you are dealing with a condenser bushing, the capacitance value is usually written on the nameplate of the bushing and you can check your tested capacitance with that on the nameplate. It should not differ much or probably the bushing is bad or it has been serviced with the insulation altered.

Typically, a stand-alone 12kV 200mA insulation power factor instrument would be enough to test most transformers and bushings.