Instrument Transformers

Instrument transformers:

Instrument transformers are high accuracy class electrical devices used to isolate or transform voltage or current levels. The most common usage of instrument transformers is to operate instruments or metering from high voltage or high current circuits, safely isolating secondary control circuitry from the high voltages or currents. The primary winding of the transformer is connected to the high voltage or high current circuit, and the meter or relay is connected to the secondary circuit.

Instrument transformers may also be used as isolation transformers that secondary quantities may be used in phase shifting without affecting other primary connected devices

Types of Instrument transformers:

  1. Current Transforme.
  2. Voltage Transformer

Current Transformer:

Current transformers (CT) are a series connected type of instrument transformer. They are designed to present a negligible load to the supply being measured and have an accurate current ratio and phase relationship to enable accurate secondary connected metering.Current transformers are often constructed by passing a single primary turn (either an insulated cable or an uninsulated bus bar) through a well-insulated toroidal core wrapped with many turns of wire. This affords easy implementation on high voltage bushings of grid transformers and other devices by installing the secondary turn core inside high-voltage bushing insulators and using the pass-through conductor as a single turn primary.

A current campuses a current transformer with a split core that can be easily wrapped around a conductor in a circuit. This is a common method used in portable current measuring instruments but permanent installations use more economical types of current transformer. Specially constructedwidebandCTs are also used, usually with an oscilloscope, to measure high waveform Sor pulsed currents within pulsed power systems. One type provides an IR voltage output that is proportional to the measured current; another, called aRogowski coil, requires an external integrator in order to provide a proportional output.

The CT is typically described by its current ratio from primary to secondary. A 1000:5 CT will provide an output current of 5amperes when 1000 amperes are flowing through its primary winding. Standard secondary current ratings are 5 amperes or 1 ampere, compatible with standard measuring instruments.

There are three basic types of current transformers:wound, a toroidal sandbar.

  • Wound Current Transformer–The transformers primary winding is physically connected in series with the conductor that carries the measured current flowing in the circuit. The magnitude of the secondary current is dependent on the turns ratio of the transformer.
  • Toroidal Current Transformer–These do not contain a primary winding. Instead, the line that carries the current flowing in the network is threaded through a window or hole in the toroidal transformer. Some current transformers have a “split core” which allows it to be opened, installed, and closed, without disconnecting the circuit to which they are attached.
  • Bar-type Current Transformer–This type of current transformer uses the actual cable or busbar of the main circuit as the primary winding, which is equivalent to a single turn. They are fully insulated from the high operating voltage of the system and are usually bolted to the current carrying device.

Voltage Transformer :

Voltage transformers (VT), also called potential transformers (PT), are a parallel connected type of instrument transformer. They are designed to present a negligible load to the supply being measured and have an accurate voltage ratio and phase relationship to enable accurate secondary connected metering.The PT is typically described by its voltage ratio from primary to secondary. A 600:120 PT will provide an output voltage of 120volts when 600volts are impressed across its primary winding. Standard secondary voltage ratings are 120volts and 70volts, compatible with standard measuring instruments.

There are three primary types of potential transformers (PT): electromagnetic, capacitor, and optical. The electromagnetic potential transformer is a wire-wound transformer. The capacitor voltage transformer (CVT) uses a capacitance potential divider and is used at higher voltages due to a lower cost than an electromagnetic PT. An optical voltage transformer exploits the electrical properties of optical materials

References

1.https://en.wikipedia.org/wiki/Instrument_transformer

  • Instrument transformers are high accuracy class electrical devices used to isolate or transform voltage or current levels. 
  • Instrument transformers are classified as Current and voltage transformers.
  • Current transformers (CT) are a series connected type of instrument transformer. 
  • Voltage transformers (VT), also called potential transformers (PT), are a parallel connected type of instrument transformer.
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