Instrument Transformers – Basic Operating Principles

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Instrument transformers are used for measuring and control purposes. They provide currents and voltages proportional to the primary, but there is less danger to instruments and personnel.

There are two distinct classes of instrument transformers: the potential transformer and the current transformer. 
Potential transformers (PTs) are used to step down high voltage while current transformers (CTs) used to step current down. The function of a PT is to accurately measure voltage on the primary, while a CT is used to measure current on the primary.

Potential Transformer
Potential (voltage) transformers have primary and secondary windings on a common core:
Schematic of a Potential Transformer & Symbol in electrical circuit
Standard potential transformers are single-phase and are usually designed so that the secondary voltage maintains a fixed relationship with the primary voltage. Potential transformers are used with voltmeters, wattmeters, watt-hour meters, power-factor meters, frequency meters, synchroscopes and synchronizing apparatus, protective and regulating relays, undervoltage and overvoltage trip coils of circuit breakers.

Generally, a potential transformer is designed to be connected in parallel with the lines to transform and step down the line voltage to 115 or 120 volts for metering or relay operation. They are typically rated 50 to 200VA (volt-amperes) at 120 secondary volts. The secondary terminals should never be short circuited because a heavy current will result, which can damage the windings.

Current Transformers
A current transformer transforms line current into values suitable for standard protective relays and instruments.  The primary of a current transformer has a  few turns, while the secondary may have a great many turns which results in the stepping down of current as shown in the schematic below:

Current Transformer schematic & Symbol in electrical circuit
Current transformers are used with ammeters, wattmeters, power factor meters, watt-hour meters, compensators, protective and regulating relays, and trip coils of circuit breakers. The secondary of current transformers are typically rated 5Amps.
Most times, current transformers have several taps on the secondary winding to adjust the range of current possible to measure on the primary.

Note if the secondary of a current transformer is opened, an extremely high voltage is induced in the secondary which is dangerous to personnel and can destroy the current transformer. For this reason, the secondary of a current transformer should always be shorted before removing a relay from its case or removing any other device that the current transformer operates. This protects the current transformer from overvoltage.