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 2767 Automatic Instrument Transformer Test Set
The 2767 automatic current and voltage transformer test set is a modern, fully automatic instrument 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. Its measurement ranges for current/voltage errors, phase displacement and current/ voltage excitation are fully up to international standard requirements.
- Current and voltage transformer measurements with a single instrument.
- 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.
- Plain-language display of error messages on a 2 x 16 character dot-matrix.
- Short measurement times, and dynamic averaging.
- High accuracy, and low inherent burden.
- RS 232 interface for connection for external printer or for computer connection.
- Optional IEEE488 GPIB interface.
- Specifications conform to the standards and recommendations of ANSI/IEEE C57.13 and IEC 60044.
- Interactive parameter entry simplifies operation.
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.
- Current or voltage power supply
- Reference current comparator or reference electronic voltage divider
- Burden to simulate the measuring instruments (voltage burdens and current burdens) connected to the test object
- Measuring bridge to measure the vector differences between the reference device and the test object