Electrical Testing of Instrument Transformers

In the world of electrical measurements, instrument transformers are indispensable equipment that forms the foundation for many critical devices that fall under the category generally known as instruments such as voltmeters, ammeters, watt-meters, protection relays, etc. Their primary purpose is to change currents and voltages from one magnitude to another and to isolate the measuring circuit from the primary circuit for safety.

Instrument transformers can divide into two basic categories: current transformers (CT) and voltage transformers (VT). Current transformers are most commonly window type (or bushing type), where the primary of the transformer is an incoming or outgoing conductor(s) which is pulled through it. Bar-type and wound CTs are also available, which are connected in line (series) with bus bars or cable terminations bolted on both sides. Voltage transformers also come in variations such as electromagnetic, capacitor, and optical type and are parallel-connected to the circuit to be measured.

Std IEEE C57.13 and IEC 60044 have established standardized requirements for classifying instrument transformers according to accuracy and burden as well as test procedures applicable to current and voltage transformers. Certainly, instrument transformers must have high accuracy in order to ensure proper operation of the devices connected to them.

Instrument transformers for metering purposes must have high accuracy to ensure precise billing while those used for protection purposes must function accurately to ensure the proper operation of protective equipment in the event of a fault, to guarantee maximum safety of supply. Therefore, it is essential to inspect and test them at regular intervals.

Haefely CITAS 2771 & VITAS 2772 Instrument Transformer Analyzing Sets

The CITAS 2771 and VITAS 2772 are modern, fully automatic instruments for fast, accurate measurement of instrument transformer errors. Increasingly severe quality control requirements call for a higher degree of operating comfort and absolute reliability of the measuring equipment employed. Based on the latest design technology, this instrument sets new standards of quality, reliability, convenient operation and simple maintenance. The measurement ranges for current/voltage errors, phase displacement and current/ voltage excitation are fully up to international standard requirements.


  • Current and voltage transformer measurements.
  • Fully automatic measurement and digital display of current/voltage ratio errors, phase displacement, test current and voltage.
  • Measurements are possible on instrument transformers of practically any primary and secondary current and voltage ratings.
  • Test & standard transformers may have different ratios.
  • A cost-effective solution: – Different transformers can be tested using one single standard transformer – Different standard and test transformer ratios are matched without the use of an external divider.
  • Controls Haefely electronic current and voltage burdens
  • Optically decoupled from computer by fiber optic data connection
  • Scope function – waveshapes of the measured signals shown in real time
  • Datalogger – readings can be collected and represented graphically to see deviations
  • Compact, reliable, and EMC hardened design, IP50

Typical Test Setup

In order to perform an accuracy test on CTs and VTs there are several components to be included in the test setup.

Are you interested in Instrument Transformer Test Sets?

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(703) 365-2330 or hvsales@hvtechnologies.com