Current transformer
Instrument transformers isolate measurement or protection circuits from the high voltage of the primary system.
In the illustration on the right, 'I' is the current in the primary, 'B' is the magnetic field, 'N' is the number of turns on the secondary, and 'A' is an AC ammeter.
Current transformers typically consist of a silicon steel ring core wound with many turns of copper wire, as shown in the illustration to the right.
Window-type current transformers are also common, which can have circuit cables run through the middle of an opening in the core to provide a single-turn primary winding.
[1] Current transformer shapes and sizes vary depending on the end-user or switch gear manufacturer.
Low-voltage single ratio metering current transformers are either a ring type or plastic molded case.
Along with voltage leads, revenue-grade CTs drive the electrical utility's watt-hour meter on many larger commercial and industrial supplies.
High-voltage current transformers are mounted on porcelain or polymer insulators to isolate them from ground.
In the United States, the National Electrical Code (NEC) requires residual current devices in commercial and residential electrical systems to protect outlets installed in "wet" locations such as kitchens and bathrooms, as well as weatherproof outlets installed outdoors.
The burden in a CT metering electrical network is largely resistive impedance presented to its secondary winding.
This means a CT with a burden rating of B-0.2 will maintain its stated accuracy with up to 0.2 Ω on the secondary circuit.
These specification diagrams show accuracy parallelograms on a grid incorporating magnitude and phase angle error scales at the CT's rated burden.
Items that contribute to the burden of a current measurement circuit are switch-blocks, meters and intermediate conductors.
This problem can be reduced by using thicker cables and CTs with lower secondary currents (1 A), both of which will produce less voltage drop between the CT and its metering devices.
The introduction of electronic power and energy meters has allowed current phase error to be calibrated out.
To obtain non-standard ratios or for other special purposes, more than one turn of the primary cable may be passed through the ring.
Where a metal shield is present in the cable jacket, it must be terminated so no net sheath current passes through the ring, to ensure accuracy.
Ring-type transformers usually use dry insulation systems, with a hard rubber or plastic case over the secondary windings.
Current transformers, especially those intended for high voltage substation service, may have multiple taps on their secondary windings, providing several ratios in the same device.
The Rogowski coil uses this effect and requires an external integrator in order to provide a voltage output that is proportional to the measured current.
In a switchyard, live tank current transformers have a substantial part of their enclosure energized at the line voltage and must be mounted on insulators.
Live tank CTs are useful because the primary conductor is short, which gives better stability and a higher short-circuit current rating.
[1] A high-voltage current transformer may contain several cores, each with a secondary winding, for different purposes (such as metering circuits, control, or protection).
