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Instrument Transformers

The instruments and relays associated with protective and control devices are usually connected in the secondary circuits of instrument-voltage and instrument-current transformers of a-c power circuits that operate at voltages in excess of a few hundred volts. Consider a 3-phase generator rated at 22000 v between terminals with an output of 100000 kva. The line-neutral voltage of such a generator is 22000 / = 12700 v and the rated line current is 100000000 / ( X 22000) = 2625 amp per phase. Obviously, it would be extremely impractical to apply such values of voltage and current directly to voltmeters, ammeters, wattmeters, relays, and other control devices. For that reason, use is made of instrument potential transformers, which step the voltage down to about 115 secondary v and instrument current transformers, which step the current down to a rated value of 5 amp or less. Potential transformers are usually connected line to neutral in 3-phase installations, although line-to-line connections are not uncommon. The secondaries of instrument-potential transformers and instrument-current transformers are grounded for reasons of safety. Grounding eliminates the hazard of raising the secondary winding to a high potential through capacitance coupling with the primary winding. In addition, as a matter of safety, the secondary circuit of a current transformer should never be opened while under load, because when the secondary is opened there are no secondary ampere turns opposing the primary ampere turns, and all of the primary current becomes exciting current, and this may induce a very high voltage in the secondary winding.

Last Update: 2011-01-05