What is Instrument Transformers? Types, Working Principle &Construction Instrument Transformers

Instrument Transformers

Instrument Transformers are special-purpose transformers used in electrical power systems to measure high voltages and currents safely and accurately, and to provide electrical isolation between high-voltage circuits and measuring/protection equipment.

They step down current or voltage from high values to standardized, low, measurable values suitable for meters, relays, and other instruments.

“Instrument transformers” are used for measuring voltage and current (where direct measurement might not be Easy/Economical) in electrical power systems.
Generally in electrical power system voltage or current is too large to measure conveniently directly. So these are scaled down to a standardized lower value by using instrument transformers.

Types of Instrument Transformers

Instrument Transformers are classified into two types

Current Transformer (CT)
Voltage Transformer (VT)

Current Transformer (CT)

Purpose: Steps down high current to a standard low value (typically 5 A or 1 A).

Working Principle:

Working Principle The working principle of a Current Transformer is based on the principle of mutual electromagnetic induction, similar to a conventional power transformer. When a high alternating current flows through the primary winding, it produces an alternating magnetic flux in the core. This alternating flux links with the secondary winding and induces an electromotive force (EMF) in it, according to Faraday’s law of electromagnetic induction.

Since the secondary winding is connected to a closed circuit (the burden, which is the load of the measuring instruments or relays), the induced EMF drives a current through it. The ratio of the primary current () to the secondary current () is approximately inversely proportional to the ratio of the number of turns in the primary winding () to the number of turns in the secondary winding ():

Ip / Is ≈ Ns/ Np

This relationship is known as the turns ratio. By having a large number of turns in the secondary and a small number of turns in the primary, a high primary current can be stepped down to a low, standardized secondary current.

A crucial aspect of CT operation is that the secondary winding should never be open-circuited while the primary is energized. If the secondary is open, there is no secondary current to produce a counter-magnetic flux. This leads to an unopposed primary magneto motive force, causing the core to saturate. The rapidly changing flux induces a dangerously high voltage across the open secondary terminals, which can be a safety hazard and can also damage the CT’s insulation.

Construction

The construction of a Current Transformer is similar to a standard transformer but with some key differences tailored to its specific function. Primary winding has few turns (often just one), secondary winding has many turns. The main components include:

Core: The core is typically made of high-permeability silicon steel laminations to minimize energy losses. It is designed to have a low reluctance path for the magnetic flux. The shape of the core can be toroidal (ring-shaped), rectangular, or in the form of a wound strip.
Primary Winding: The primary winding is connected in series with the high-current line being measured. It consists of a very small number of turns, often just a single turn. In some types, like the “bar-type” CT, the primary conductor itself acts as the single-turn primary winding as it passes through the core’s central window.
Secondary Winding: The secondary winding has a much larger number of turns compared to the primary. It is wound around the core and is connected to the measuring instruments or protective relays. The standard secondary current ratings are typically 1A or 5A.
Insulation: Proper insulation is provided between the primary and secondary windings, and between the windings and the core, to withstand the high voltage of the primary circuit. The entire assembly is often enclosed in a robust casing, which can be made of resin, oil-immersed paper, or other insulating materials depending on the voltage level and application.

Advantages

Current Transformers offer several significant advantages in electrical systems:

Safety: They provide galvanic isolation between the high-voltage primary circuit and the low-voltage secondary circuit, ensuring the safety of personnel and equipment connected to the secondary side.
Accurate Measurement: CTs provide a highly accurate and linear transformation of current over a wide operating range, which is crucial for precise metering and effective protection.
Standardized Output: The secondary current is standardized to 1A or 5A, allowing for the use of standard, low-range measuring instruments and relays, regardless of the primary current magnitude.
Wide Range of Applications: They are versatile and can be used for a variety of applications, from revenue metering to complex protection schemes.
Reliability and Durability: CTs are generally robust and have a long operational life with minimal maintenance requirements.
Ease of Installation: Many types of CTs, especially the window or split-core types, are easy to install without the need to disconnect the primary conductor.

Disadvantages

Despite their numerous benefits, Current Transformers also have some limitations:

Saturation: Under very high fault currents, the magnetic core of a CT can saturate. This leads to a non-linear relationship between the primary and secondary currents, causing inaccuracies in measurement and potentially affecting the performance of protective relays.
Burden Sensitivity: The accuracy of a CT is dependent on the connected burden (the impedance of the secondary circuit). A burden higher than the rated value can lead to significant errors.
Open Circuit Hazard: As mentioned earlier, an open-circuited secondary winding can result in extremely high voltages, posing a serious safety risk.
Cost: High-precision and high-voltage CTs can be expensive.
Size and Weight: High-voltage CTs can be large and heavy, requiring substantial mounting structures.

Applications:

Measurement: Supplying reduced current to ammeters, watt meters, energy meters.
Protection: Operating overcurrent, earth fault, differential protection relays.
Control: Input to SCADA and automation systems.
Testing: Used in calibration and testing of electrical equipment.

Key Point: The secondary should never be open-circuited when energized—dangerous high voltage can appear.

Types of current transformers

Based on Construction

a) Wound Type CT

Construction: Primary winding has more than one turn wound on the core, similar to a normal transformer.

Use: For low-current measurement where primary current is not too high.

Example: Laboratory testing, precision metering.

b) Bar Type CT

Construction: The primary winding is a single solid bar (busbar) passing through the core.

Use: For high-current measurements in busbars or switchgear.

Feature: Very robust and used in substations.

c) Toroidal (Window) Type CT

Construction: No separate primary winding; the conductor carrying the current passes through the CT’s window.

Use: Retrofitting on existing conductors without disconnecting the circuit.

Example: Residual Current Devices (RCDs), Earth Fault Protection.

Based on Application

A) Measuring CT

Purpose: For operating measuring instruments (ammeters, energy meters).

Accuracy Classes: 0.1, 0.2, 0.5, 1.0, etc.

Burden: Low (few VA).

B) Protective CT

Purpose: For operating protective relays during abnormal or fault conditions.

Accuracy Classes: 5P, 10P (Protection class).

Feature: Can handle high saturation currents during faults.

C) Special Purpose CT

Examples:

Summation CT: Combines currents from several circuits.
Differential Protection CT: For generator, transformer, or motor differential protection schemes.
Split-core CT: For temporary or clamp-on measurements.

Potential Transformer (PT)

(Also called Voltage Transformer, VT)

Purpose: Steps down high voltage to a standard low value (e.g., 110 V or 63.5 V).

Construction:

Similar to a power transformer but optimized for accuracy at low loads. The key components include:

Core: The core is constructed from high-quality, low-loss silicon steel laminations. This minimizes magnetizing current and reduces core losses, which is essential for maintaining high accuracy. Both core-type and shell-type constructions are used.
Primary Winding: This winding consists of a large number of turns of fine wire and is connected in parallel with the high-voltage circuit whose voltage is to be measured or monitored.
Secondary Winding: This winding has significantly fewer turns compared to the primary winding. It is wound on the same core as the primary and provides the stepped-down voltage. The standard secondary voltage is typically 110 V or 120 V.
Insulation: Due to its connection to high-voltage lines, insulation is a critical aspect of PT design. High-quality insulation materials like oil-impregnated paper, varnish, and porcelain are used between the windings, and between the windings and the core, to withstand the high potential.
Bushings: To connect the high-voltage primary winding to the power line, porcelain bushings are used. These provide a well-insulated and secure terminal point. For safety, one end of the secondary winding is always grounded.

Working Principle

The working principle of a Potential Transformer is based on mutual electromagnetic induction, the same principle that governs a standard transformer.

When the primary winding is connected across the high-voltage line, an alternating current starts to flow through it. This current produces an alternating magnetic flux in the core. According to Faraday’s Law of Electromagnetic Induction, this changing magnetic flux induces an electromotive force (EMF) in both the primary and secondary windings.

Since the secondary winding is connected to a high-impedance load (like a voltmeter or the voltage coil of a relay), it draws a very small current. The voltage transformation ratio is determined by the turns ratio of the primary () and secondary () windings:

By having a large number of turns on the primary and a small number of turns on the secondary, the PT accurately steps down the high voltage to a safe and measurable level at the secondary terminals.

Based on their design, there are two main types of Potential Transformers:

Conventional Wound Type (Electromagnetic): These are similar to standard transformers with primary and secondary windings on a magnetic core. They are highly accurate and used for a wide range of voltages.
Capacitor Voltage Transformer (CVT) or Capacitor Coupled Potential Transformer (CCPT): Used for very high voltages (above 100 kV), these transformers use a series of capacitors in a potential divider arrangement to initially step down the voltage to an intermediate level (e.g., 10-20 kV). This intermediate voltage is then further stepped down by a smaller, conventional electromagnetic transformer.