Introduction of Instrument Transformer

An instrument transformer is a device used in electrical power systems to measure high voltage and current levels by stepping them down to safer, lower values. This allows for precise monitoring, metering, and protection of the power system without exposing instruments and personnel to the high energy levels present in the primary circuit. There are two main types of instrument transformers: current transformers and voltage transformers , also known as potential transformers.

Current transformers are designed to produce a current in their secondary winding proportional to the current flowing in their primary winding, but at a much lower, safer level. Voltage transformers, on the other hand, provide a reduced voltage level in their secondary winding that is proportional to the primary voltage.

Types of Instrument Transformer

Current Transformers (CTs)

Current transformers (CTs) are designed to measure high currents by converting them into smaller, manageable currents suitable for measurement and monitoring. The primary winding of a CT typically has very few turns and is made of thick wire to handle high current levels, and it is connected in series with the circuit being measured. The secondary winding, which has many more turns, is connected to measuring instruments like ammeters or power meters.

Key Features:

• The primary and secondary sides of a current transformer (CT) both handle current, with the voltage being nearly zero

• CTs operate with their secondary winding in a nearly short-circuited state

• The secondary winding of a CT must never be open-circuited

Voltage Transformers (VTs)

A voltage transformer (VT) operates on the same principle as a standard step-down transformer but with a more precise transformation ratio. The primary side of a VT is connected to high voltage, while the secondary side is connected to instruments such as voltmeters or power meters. VTs convert high voltage to a standard secondary voltage (e.g., 100V or lower) for protection, measurement, and instrumentation purposes. They isolate high voltage from electrical personnel, ensuring safety. Unlike current transformers, VTs have high-impedance secondary circuits, and the secondary current is determined by the circuit's impedance.

Key Features:

• Transforms high primary voltages into standard, lower secondary voltages with precise ratios

• Provides safe measurement by isolating high voltage circuits

• Achieve phase relationship maintenance

Advantages of Instrument Transformer

Instrument transformers provide a lower secondary voltage, reducing insulation requirements for measuring instruments and making measurement operations safer.

Instrument transformers isolate the power supply line from the measuring instruments.

Measuring instruments can be connected from the instrument transformer to other locations using wires, enabling remote measurement.

Instrument transformers standardize the secondary voltage or current (usually 110V or 5A), facilitating the interchangeability of measuring instruments.

Precautions

1. For current transformers (CTs), never leave the secondary circuit open while the primary is energized, as this can result in dangerous high voltages and potential damage to the transformer and connected instruments.

2. The secondary winding and the core of the transformer should be properly grounded.

3. For voltage transformers (VTs), the secondary load is not too low to avoid excessive current draw, which can lead to inaccuracies and potential transformer damage.

4. Pay attention to the correct polarity and phasing of the connections, especially when connecting CTs and VTs in protective relay circuits.

FAQs

Can instrument transformers be used for direct current (DC) measurements?

No, instrument transformers are designed for alternating current (AC) measurements. Special devices, such as Hall effect sensors or shunt resistors, are required for DC measurements.

What are instrument transformers used for?

Instrument transformers have two main uses: one is to expand the range of AC electrical instruments, and the other is to isolate high voltage and high current and convert them into low voltage and low current, which are then used as signals for relay protection, automatic devices and control circuits.