POWER TRANSFORMER TERMS GLOSSARY

The following technical terms apply to transformers.


BIL: An abbreviation for basic impulse level, a dielectric strength test. Transformer BIL is determined by applying a high-frequency square-wave voltage with a steep leading edge between the windings and between the windings and ground.

The BIL rating provides the maximum input kV rating that a transformer can withstand without causing insulation breakdown. The transformer must also be protected against natural or man-made electrical surges. The NEMA standard BIL rating is 10 kV.

Exciting current: In transformers, the current in amperes required for excitation. This current consists of two components: (1) real in the form of losses (no load watts) and (2) reactive power in kvar. Exciting current varies inversely with kVA rating from approximately 10 percent at 1 kVA to as low as 0.5 percent at 750 kVA.

Eddy-current losses: Contiguous energy losses caused when a varying magnetic flux sets up undesired eddy currents circulating in a ferromagnetic transformer core.

Hysteresis losses: Continuous energy losses in a ferromagnetic transformer core when it is taken through the complete magnetization cycle at the input frequency.

Insulating transformer: A term synonymous with isolating transformer, to describe the insulation or isolation between the primary and secondary windings. The only transformers that are not insulating or isolating are autotransformers. Insulation system temperature: The maximum temperature in degrees Celsius at the hottest point in the winding.

Isolating transformer: See insulating transformer. Shielded-winding transformer: A transformer with a conductive metal shield between the primary and secondary windings to attenuate transient noise.

Taps: Connections made to transformer windings other than at its terminals. They are provided on the input side of some high-voltage transformers to correct for high or low voltages so that the secondary terminals can deliver their full rated output voltages.

Temperature rise: The incremental temperature rise of the windings and insulation above the ambient temperature.

Transformer impedance: The current-limiting characteristic of a transformer expressed as a percentage. It is used in determining the interrupting capacity of a circuit breaker or fuse that will protect the transformer primary.

Transformer voltage regulation: The difference between the no-load and full-load voltages expressed as a percentage. A transformer that delivers 200 V at no load and 190 V at full load has a regulation of 5 percent.

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