INSTRUMENT TRANSFORMERS 



577 



Application of Corrections for Ratio and Phase Angle. It is 

 customary to express the results of tests of instrument trans- 

 formers in the form of curves similar to those in Fig. 350. Ref- 

 erence to the curves having the proper load characteristics will 

 give the ratio and phase angle to be used in any particular tests. 



When power measurements are made, besides the ratios, there 

 are to be considered : 



1. The phase displacement in the potential transformer. 



2. The phase displacement in the current transformer. 



3. The phase displacement in the potential coil of the watt- 

 meter. 



These phase relations may be indicated as in Fig. 356. 



at Sec.of Poten.Trane. 



,;. :-t"><l. Phasr diagram for power measurement using instrument 



transformers. 



The phase angle, 7, of the potential transformer is shown as a 

 lead, though F 2 may either lag behind or lead V\ according to cir- 

 cumstances. If 7 is a lag angle, its algebraic sign is to be reversed. 



The wattmeter gives an indication proportional to the mean 

 product of the current in its two coils, or to cos 0'. The apparent 



power factor of the load is 



, _ Wattmeter reading 

 Volt-amperes 



The true power-factor angle is 



and the true power factor is 

 cos 6 = cos (0' + fi 



e P - 7) == 



True watts 

 Volt-amperes 



'. True watts = - , X (wattmeter reading) = 

 cos 9 



CUs v . i 



[1 - tan 0' tan (/3 + B P - 7)] X (Reading), approximately. 



37 



