Power measurements are made in high voltage circuits connecting the wattmeter to the circuit through current and potential transformers as shown.
The primary winding of the C.T. is connected in series with the load and the secondary winding is connected in series with an ammeter and the current coil of a wattmeter.
The primary winding of the potential transformer is connected across the supply lines and a voltmeter and the potential coil circuit of the wattmeter are connected in parallel with the secondary winding of the transformer. One secondary terminal of each transformer and the casings are earthed.
The errors in good modem instrument transformers are small and may be ignored for many purposes.
However, they must be considered in precision work. Also in some power measurements these errors, if not taken into account, may lead to very inaccurate results.
Voltmeters and ammeters are effected by only ratio errors while wattmeters are influenced in addition by phase angle errors. Corrections can be made for these errors if test information is available about the instrument transformers and their burdens.
Phasor diagrams for the current and voltages of load, and in the wattmeter coils.