Instrument Transformers
Instrument transformers accurately measure and monitor power voltage as it is transmitted proportionally through the primary and secondary coils. The three most common uses of instrument transformers are: transforming currents or voltages from very high values to measurable and easily handled values for instruments and relays, insulating metering circuits from primary high voltage systems, and providing standardization of current and voltage levels for instruments and relays. In addition, although instrument transformers are often used in the collection of data from measurement and protective equipment, such as meters, control devices, burdens, power supplies and software, they can also be used to provide electrical energy to protective relays. Even though instrument transformers are used for numerous applications, the main function of instrument transformers is to step down voltages or currents that are too large to be conveniently or safely used by an instrument to low standardized values that are much more easily handled by the instrument. Since instrument transformers can be used for a variety of purposes, they are beneficial for wide-ranging industries such as electronics, transportation, utilities, aerospace, defense, petroleum, instrumentation and telecommunications.
The basic design of instrument transformers is a primary winding connected to a primary power system, and the secondary winding to protective and measuring equipment. There are two main types of instrument transformers: instrument current transformers and instrument voltage transformers. Instrument current transformers are used to step down electrical currents so that the current in the secondary coil is proportional to the current flowing in the primary coil. This type of transformer can be further broken down into three additional types: window type, in which the power conductor is passed by means of a hole in the center of the transformer; bar type, in which the power conductor is mounted to a one-turn bar that is a part of the transformer; and brushing type, in which a toroidal core and winding is slipped over the insulating bushings of transformers or circuit breakers and similar equipment. Instrument voltage transformers, also referred to as potential transformers, produce a secondary voltage that is largely proportional to the primary voltage; however, the secondary voltage differs in phase by an angle that is close to zero for a suitable direction for the connections. Instrument voltage transformers can be used alone, but are often used in combination with instrument current transformers. There are also several classes of instrument transformers including: commercial class, used for low power current monitoring; and ANSI class, used for power monitoring requiring high accuracy and low phase angle error.