INSTRUMENT TRANSFORMERS 



571 



RESISTANCE AND INDUCTANCE OF THE CURRENT COILS OF TYPICAL ALTER- 

 NATING-CURRENT INSTRUMENTS 



Current Transformers. It is important that when the trans- 

 former is being operated the secondary circuit always be kept 

 closed. If it is opened, there will be no demagnetizing effect 

 due to the secondary, and as the primary current is fixed by the 

 load on the line, the flux will rise to a high value. This will 

 increase the iron losses to such an extent that the insulation may 

 be injured by the heat so that at some subsequent time it may 

 be punctured by a moderate voltage, or perhaps burned out. 

 Again, the voltage at the secondary terminals will be large, and 

 the secondary insulation may be injured. There is also the possi- 

 bility of disagreeable, if not fatal, shocks. 



Opening the secondary circuit when the transformer is being 

 operated may alter both the ratio and the phase angle, for the 

 circuit opening may occur when the iron is fully magnetized. In 

 the subsequent use of the instrument, the iron will not be put 

 through its normal hysteresis cycle, and the exciting current will 

 therefore be altered. At low loads, the alteration may amount 

 to several per cent. For the same reason, direct current, used 

 for the purpose of calibrating the instruments, must never be 

 sent through either the primary or secondary of the transformer. 

 Under ordinary operating conditions, these changes in the mag- 

 netic state of the core will persist, since, in instrument trans- 

 formers, the magnetic circuit is unusually good. The core may 

 be demagnetized in the usual manner, an alternating current 

 being sent through the primary, and gradually decreased from 

 its full-load value to zero, the secondary being open. 



Owing to the necessity of having considerable insulation be- 

 tween the primary and the secondary coils and of having the 

 terminals of the coils widely separated there may be a pronounced 

 stray field in the neighborhood of current transformers, 

 the case with the type shown in Fig. 346A. Instruments, if not 



