Instrument transformers proportionally scale high voltage and current values for safe use with sensitive instruments. Two types exist: potential transformers for voltmeters and current transformers for ammeters and overcurrent relays. They allow installation of instrumentation in control rooms with low voltage wiring. PTs provide low voltage to voltmeters and CTs measure current. They come in various designs and ratings, including large oil-immersed transformers for primary distribution networks.
Instrument transformers scale or scale large voltage and current values proportionally so that they can be used safely on sensitive instruments and components. These tools and components include ammeters, voltmeters, and overcurrent protection relays. Instrument transformers fall into two distinct categories: potential and current transformers. A potential transformer (PT) is used to provide low voltage to voltmeters and a current transformer (CT) provides current handling valves for ammeters and overcurrent relays. TPs can be located with the high voltage supply in local starting panels or remotely in instrument control panels while TCs are generally located.
High voltage power systems can carry voltages up to 30,000 volts with common distribution voltages exceeding 13,000 volts. Three-phase equipment such as electric motors in industrial plants commonly run on 380 or 500 volts and often draw several hundred amperes in operation. The average voltmeter has a maximum voltage rating of 110 to 150 volts and could never directly measure these voltages. To enable the use of these components, transformers for precision instruments proportionally reduce high voltages and currents to safe values. Instrument transformers also allow installation of all necessary instrumentation in control rooms and monitoring facilities with only low voltage wiring present.
Installation transformers fall into one of two separate categories. The first is the potential transformer which supplies low value voltages to voltmeters. The PT is typically equipped with four terminals; two for high input voltage and two for low instrument voltage. The incoming or primary wires of the terminal are connected in parallel across the high voltage supply. This means it can be located remotely in control rooms, although it can often be mounted in high voltage starter panels where local voltage indication is required.
The second type is the current transformer. This variant of the instrument transformer measures current and is used to drive ammeters and overcurrent protection relays. The typical CT differs from its voltage dependent sibling in that it is connected in series with the supply and typically uses an induced primary supply and not a direct connection. To achieve this, a CT is produced with a hole in the center through which a high-voltage cable runs. The current flowing through the cable induces an electromagnetic field in the primary winding of the CT which is then reduced by its secondary winding and fed to the instruments via the two output terminals of the CT.
Instrument transformers are available in a wide range of designs and ratings and can be custom built to suit all user requirements. The common PTs and CTs are fairly small components, rarely measuring more than a couple of square inches. However, large oil immersed instrument transformers are commonly used to facilitate low voltage measurement in primary distribution network applications.
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