Instrument transformers are used to step-down current or voltage to measurable values. They provide standardized, useable levels of current or voltage in a variety of power monitoring and measurement applications.

There are two basic types of instrument transformers:

• Current transformers: are used to step-down current in a predictable fashion with regard to both current and phase. They are often used as inputs to current-powered instruments.
• Voltage transformers: produce a secondary voltage that is substantially proportional to the primary voltage, but that differs in phase by an angle that is approximately zero for an appropriate direction of the connections. Often, voltage transformers are used as potential transformers or in combination with current transformers.

Some instrument transformers have a waterproof case or are rated for outdoor use. Others meet U.S. military specifications (MIL-SPEC) or standards for electric metering from the American National Standards Institute (ANSI) or the International Electrotechnical Commission (IEC).

Specifications

There are several classes of instrument transformers. Commercial class devices are well-suited for low power current monitoring.

ANSI class devices are designed for power monitoring which requires high accuracy and minimum phase angle; Multi-ratio transformers are used in three-phase systems to measure current or voltage.

Split core transformers include a hinge and locking snap that allows attachment without interrupting the current-carrying wire.

Wound primary transformers have primary windings that usually consist of more than one turn. By contrast, toroidal or donut transformers do not include an internal primary winding.

Bar current transformers are toroidal transformers in which a bus bar is permanently inserted through the window of the toroid.

PC-mounted instrument transformers have a small footprint and are used to measure current or voltage on computer boards. Devices that detect under current or over-currents are also available.

Noncontact current transformers can measure current waveforms without coming into electrical contact with the circuit.

Applications

Instrument transformers are used mainly in measurement and protective equipment. They are also used to provide energy to protective relays.

Accuracy, the degree to which instrument transformers produce a current or voltage that is proportional to monitored value, varies among devices. Highly accurate current transformers are used to measure turn-on, turn-off and conducting performance in power semiconductor switches.

Current transformers are also used in the design of switch-mode power supplies, the development of power electronics, and lightning simulation tests for surge arrestors.

In addition, current transformers are used in capacitor discharge surge testing, the development of motor controllers and electronic variable speed drives, the monitoring of laser tube and diode pulse currents, current injection, the measurement of harmonics in main power cables, and in electromagnetic compatibility (EMC) testing.

Standards

ANSI C12.11 - Instrument transformers for revenue metering, 10kv bil through 350kv bil (0.6 kv nsv through 69 kv NSV).

BS EN 50482 - Instrument transformers - three-phase inductive voltage transformers having um up to 52 kv.

CSA C60044-1 - Instrument transformers - part 1: current transformers.

IEC 60044-7 - Instrument transformers - part 7: electronic voltage transformers (also see iec 60044 set).

MIL-I-1361 - Instrument auxiliaries, electrical measuring: shunts, resistors, and transformers.

References

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Black Box | CARLO GAVAZZI Automation Components | GE Digital Energy