Combined Instrument Transformer Working Principle

In modern power systems, accurate measurement of electrical current and voltage is essential for metering, protection, monitoring, and control. However, the current and voltage in transmission and distribution networks are often far too high to be connected directly to conventional measuring instruments, protective relays, or monitoring equipment. Instrument transformers are therefore used to safely convert high electrical quantities into standardized, measurable signals.

The two most common types of instrument transformers are current transformers (CTs) and voltage transformers (VTs). A CT converts a high primary current into a smaller secondary current, while a VT reduces a high system voltage to a standardized secondary voltage. These reduced signals can then be supplied to meters, relays, monitoring systems, and other electrical equipment.

A combined instrument transformer integrates both current transformer and voltage transformer functions into a single physical unit. Instead of installing separate CTs and VTs, a combined unit can provide both current and voltage measurement signals from one compact device.

Understanding the combined instrument transformer working principle is important for engineers and power system professionals because it explains how current and voltage can be measured simultaneously while maintaining electrical isolation and measurement accuracy.

1. What Is a Combined Instrument Transformer?

A combined instrument transformer is an electrical measurement device that incorporates a current transformer and a voltage transformer into one integrated unit. It is designed to provide both current and voltage signals for measurement, metering, and protection applications.

In a conventional power system installation, CTs and VTs may be installed separately. The CT is connected in series with the primary circuit to measure current, while the VT is connected between the high-voltage circuit and ground or between phases to measure voltage. A combined instrument transformer performs both functions within the same overall housing and insulation system.

2. Main Components of a Combined Instrument Transformer

Although the exact design depends on the manufacturer and application, a combined instrument transformer generally consists of several key components.

Current Transformer Section

The CT section is responsible for measuring the current flowing through the primary power circuit.

Primary conductor:

The primary conductor carries the actual system current. Depending on the design, it may be a conductor passing through the magnetic core or an integrated primary winding.

Magnetic core:

The magnetic core provides the magnetic path required for transformer operation. It is designed according to the CT’s measurement or protection requirements and plays an important role in determining accuracy and saturation characteristics.

Secondary winding:

The secondary winding produces a reduced current proportional to the primary current. Standard secondary ratings commonly include 1 A or 5 A, depending on the requirements of the connected metering or protection equipment.

Voltage Transformer Section

The VT section is responsible for reducing the system voltage to a safe and standardized level.

Primary winding:

The primary winding is connected to the high-voltage system. It is designed with suitable insulation to withstand the system voltage and transient electrical stresses.

Secondary winding:

The secondary winding provides a lower voltage for meters, relays, monitoring equipment, and control systems. Common secondary voltage ratings include 100 V and 110 V, although other ratings may be available depending on regional standards and application requirements.

Insulation System

Because the transformer may be installed in medium- or high-voltage systems, insulation is a critical part of the design. The insulation system separates high-voltage components from grounded structures and low-voltage secondary circuits.

Depending on the application, insulation may involve materials such as epoxy resin, oil, paper, porcelain, composite materials, or other specialized insulating systems.

Shared Components

A combined instrument transformer also includes components that support both the CT and VT sections.

Housing and enclosure:

The enclosure protects internal components from mechanical damage and environmental conditions.

Mounting structure:

The mounting structure provides mechanical support and allows the transformer to be installed securely in substations, switchgear, or other electrical equipment.

Terminal box:

The terminal box provides connections between the transformer secondary circuits and external metering, protection, or monitoring equipment. Proper terminal identification and wiring are essential for correct polarity, phase relationships, and system operation.

3. Combined Instrument Transformer Working Principle

The combined instrument transformer working principle is based on electromagnetic induction. However, the CT and VT sections perform different electrical functions and operate through different connection methods.

Current Measurement Principle

The CT section measures current based on the transformer principle.

When current flows through the primary conductor, it produces a magnetic field around the conductor. This magnetic flux passes through the CT magnetic core and induces a current in the secondary winding.

The relationship between primary and secondary current is approximately determined by the turns ratio:

Ip / Is ≈ Ns / Np

where:

• Ip = primary current
• Is = secondary current
• Np = number of primary turns
• Ns = number of secondary turns

For example, if a CT has a ratio of 1000/5 A, a primary current of 1000 A corresponds approximately to a secondary current of 5 A under rated conditions.

The secondary current is much smaller and can be safely connected to measuring instruments and protective relays. Typical CT secondary outputs are 1 A or 5 A.

It is important to note that a CT secondary circuit should not normally be left open while current is flowing through the primary circuit. An open CT secondary can generate dangerously high voltage. Appropriate short-circuiting and protection arrangements are therefore required during installation and maintenance.

Voltage Measurement Principle

The VT section operates on the transformer principle as well, but its primary purpose is to reduce voltage rather than current.

The VT primary winding is connected to the high-voltage system. The applied primary voltage produces magnetic flux in the transformer core, which induces a proportional voltage in the secondary winding.

The approximate voltage relationship is:

Vp / Vs ≈ Np / Ns

where:

• Vp = primary voltage
• Vs = secondary voltage
• Np = primary winding turns
• Ns = secondary winding turns

For example, a VT designed for a high-voltage system may reduce a primary voltage of several kilovolts to a standardized secondary voltage such as 100 V or 110 V.

This lower voltage signal can then be supplied to energy meters, voltage monitoring equipment, protective relays, automation systems, and other control devices.

Simultaneous Measurement Function

The key feature of a combined instrument transformer is that the CT and VT sections can operate simultaneously and independently within the same integrated device.

The CT section provides a current signal, while the VT section provides a voltage signal. These signals can be connected to the same metering or protection system.

For example, a power monitoring system may receive both the current and voltage signals from a combined instrument transformer. The monitoring system can then use these measurements to calculate electrical parameters such as power, power factor, energy consumption, and system loading.

The two measurement functions do not mean that current and voltage are directly combined electrically. Instead, they are integrated mechanically and structurally into one instrument transformer assembly while maintaining the necessary electrical insulation and isolation between circuits.

4. Combined Instrument Transformer vs Separate CT and VT

The choice between a combined instrument transformer and separate CT and VT units depends on the system configuration, voltage level, installation environment, measurement requirements, and maintenance strategy.

A combined design is often attractive when compactness and integration are priorities. Separate CTs and VTs, however, can be preferable when the system requires highly independent configuration, replacement, maintenance, or specialized measurement characteristics.

The selection should therefore not be based solely on physical size. Engineers should evaluate rated voltage, rated current, accuracy class, burden, insulation level, frequency, environmental conditions, protection requirements, and applicable technical standards.

Conclusion

The combined instrument transformer working principle is based on the integration of two essential measurement functions: current transformation and voltage transformation. The CT section uses electromagnetic induction to convert high primary current into a standardized secondary current, typically 1 A or 5 A. At the same time, the VT section reduces high system voltage to a standardized secondary voltage, commonly 100 V or 110 V.

Although the CT and VT sections perform different electrical functions, they can operate simultaneously within the same integrated transformer assembly. This enables power systems to obtain both current and voltage signals from a single compact device for metering, protection, monitoring, and control.

Selecting the right combined instrument transformer requires careful consideration of the electrical system and application. Rated voltage, rated current, transformation ratio, accuracy class, burden, insulation performance, environmental conditions, and protection requirements all need to be evaluated before selection.