Load loss measurement

[prc]

In power transformers when load loss is measured by three wattmeter method ,it is normally seen that the watt meters will be showing different readings even up to 1:3 variation. Some time one of the meter will be showing negative reading.The current and voltage on three lines are same.Can any one explain how it is happening? Any papers on web on the subject ?

 

[dpc]

I don't know what you'll find in a web search, but there are certainly a lot of old papers on IEEE Transactions on transformer loss testing.

The main problem is that the test is done with one winding short circuited. In this condition the current supplied to the transformer is nearly totally reactive and the wattmeters are trying to find a needle of real power in a haystack of vars.

ANSI C57.12.90 covers transformer testing.

 

[busbar]

One reason for variations in individual phases of a 3ø transformer is that each phase’s coils are wound on physically separate “legs” of a 3-to-5-leg magnetic core. The center assembly electrical characteristics will differ from the outside windings, especially when measurements are “buried in the mud;” id est, relatively small compared to a transformer’s [nameplate] rated-load numbers.