Table of Contents
1. Introduction
Instrument transformers are fundamental components of electrical power systems that scale down high-voltage and current levels to measurable, standardised levels for meters, protection relays, and control equipment. In medium voltage (MV) systems, they enable safe, accurate, and reliable measurement of electrical quantities required for metering, protection, monitoring, and control.
Instrument transformers are broadly classified into two categories:
- Current Transformers (CTs)
- Voltage Transformers (VTs), also known as Potential Transformers (PTs)
As modern power grids evolve with increasing penetration of renewable energy sources, distributed generation, and smart grid technologies, the role of instrument transformers has become increasingly critical. Infrastructure expansion, grid modernisation, smart substations, and renewable energy projects drive market demand for MV instrument transformers.
This article presents a comprehensive technical overview of principles, types, standards, selection criteria, and applications of instrument transformers used in MV systems, typically operating between 1 kV and 36 kV.
2. Fundamental Principles and Functions
2.1 Basic Operating Concept
Instrument transformers are precision electromagnetic devices that reduce high primary voltages and currents to standardised secondary values—typically 110 V for voltage and 1 A or 5 A for current. Unlike power transformers, they are measurement devices, where accuracy, linearity, and reliability take precedence over efficiency or power transfer.
2.2 Primary Functions
- Metering: Inputs for revenue meters and energy accounting
- Protection: Feeding protective relays for fault detection and isolation
- Monitoring: Power quality analysis and system diagnostics
- Isolation: Electrical separation between HV circuits and LV equipment
- Control: Supplying control and indication circuits (in specific VT designs)
3. Medium Voltage Application Context
Medium voltage systems form the interface between transmission and low-voltage distribution networks. Typical MV voltage levels include 6 kV, 10 kV, 11 kV, 22 kV, and 33 kV.
Instrument transformers in this range must satisfy the following engineering requirements:
- High accuracy across a wide operating range
- Reliability in indoor and outdoor environments
- Compliance with international standards (IEC / IEEE / ANSI)
- Cost-effectiveness for large-scale deployment
- Minimal maintenance over long service life
4. Current Transformers (CTs)
4.1 Principle of Operation
A current transformer consists of a primary winding connected in series with the power circuit and a multi-turn secondary winding connected to measuring or protective devices. The turns ratio relates the primary and secondary currents:
where is primary current, is secondary current, and , are the number of secondary and primary turns, respectively.
4.2 Types of CTs Used in MV Systems
- Ring-Type CT: Primary conductor (cable or busbar) passes through the core
- Bar Primary CT: Integral primary bar
- Wound Primary CT: Used for low primary currents
- Core Balance CT (CBCT): Sensitive earth fault detection
5. Conventional Protection Classes
Class 5P
- Composite error ≤ 5% at Accuracy Limit Current (ALF × In)
- Used for general overcurrent & earth fault protection
🔹 Class 10P
- Composite error ≤ 10% at ALF × In
- Suitable where very high accuracy is not critical
🔹 Accuracy Limit Factor (ALF)
ALF=Rated Primary Current/Accuracy Limit Current
Example:
- CT: 1000/1 A, 5P20
- ALF = 20
- Must maintain ≤5% error up to 20 × 1000 A = 20,000 A
👉 These are suitable for electromechanical and static relays
6. CT Accuracy Classification for Special Protection (IEC 60044-6)
Protection CTs are classified based on their transient performance under fault conditions, including DC offset and saturation characteristics.
6.1 TPS (Transient Protection Stabilised)
- Intended for steady-state protection performance
- CT parameters (knee-point voltage, excitation current, resistance) explicitly specified
- No standardised transient performance
- Common applications:
- Transformer differential protection
- Restricted earth fault (REF) schemes
Key feature: Very high stability; transient behaviour engineered case by case
(Often referred to as Class PS / PX CTs)
6.2 TPX (Limited Transient Overshoot)
- Controlled transient overshoot within specified limits
- Low remanent flux (≤10%)
- Maintains accuracy for a defined time before saturation
Typical applications:
- Transformer differential protection
- Generator protection
- Busbar protection with numerical relays
6.3 TPY (High Remanence)
- Designed for high DC offset conditions
- High remanent flux capability (up to 80%)
- Suitable for high resistance earthed (HRG) systems
Typical applications:
- Earth fault protection in HRG systems
- Industrial MV networks with long cable feeders
6.4 TPZ (Very High Stability)
- Optimised for low-resistance erthed systems
- Very low transient error
- Fast flux reset under severe fault conditions
Typical applications:
- Sensitive earth fault protection
- Differential protection in LRG systems
7. Instrument Security Factor (ISF)
A critical parameter for CT design is the Instrument Safety Factor (IFS), which protects connected metering devices from high fault currents. The safety factor is then calculated as:
- Ipl is the primary current at which secondary error reaches 10%
- Typical metering CTs have ISF ≤ 5, often ISF = 1.5
- A lower ISF ensures early saturation during faults, protecting meters
8. CT Accuracy Classes for Metering (IEC 60044-1 / IEC 61869-2)
| Class | Accuracy Range | Typical Application |
| 0.2S | ±0.2% at 20–120% In, ISF = 1.5 | Revenue metering |
| 0.5S | ±0.5% at 20–120% In, ISF = 1.5 | Commercial metering |
| 0.2 | ±0.2% at 20–120% In | Precision monitoring |
| 0.5 | ±0.5% at 20–120% In | General metering |
| 1.0 | ±1.0% at 50–120% In | Indication only |
Selection rule:
Revenue metering requires 0.2S or 0.5S, while system monitoring may use 0.5 or 1.0 class CTs.
9. Voltage Transformers (VTs / PTs)
Voltage or Potential transformers step down MV system voltages to standard secondary values (typically 110 V). The voltage ratio is given by:
9.1 Types of VTs
- Electromagnetic VT: Most common in MV switchgear
- Capacitive VT (CVT): Predominantly used in EHV systems
- Single-phase / Three-phase VT
- Draw-out VT: Facilitates maintenance in metal-clad panels
9.2 VT Accuracy Classes (IEC 60044-2)
- Metering: 0.2, 0.5 with maximum ratio error 0.2% & 0.5% respectively
- Protection: 3P, 6P with maximum ratio error 3% & 6% respectively
VTs must maintain accuracy during system voltage variations and transient conditions.
10. Insulation Levels and Standard Voltage Classes
| Rated Voltage (kV) | System Voltage (kV) | Frequency Withstand (kV) | Impulse Withstand (kV) |
| 12 | 10–12 | 28 | 75 |
| 24 | 22–24 | 50 | 125 |
| 36 | 33–36 | 70 | 170 |
11. Selection Criteria for MV Instrument Transformers
Metering Applications
- Verify required accuracy class
- Confirm burden capacity (VA)
- Ensure compliance with metering regulations
- Validate accuracy over full operating range
Protection Applications
- Select CT class based on earthing philosophy
- Ensure adequate knee-point voltage
- Verify secondary burden and relay compatibility
- Confirm thermal and short-time ratings
12. Installation and Safety Considerations
12.1 Current Transformer Safety
Open Secondary Risk:
Never leave the secondary of an energised CT open-circuited. An open secondary prevents the normal opposing magnetising force from the secondary current, causing:
- Excessive magnetic flux in the core
- High voltage development at secondary terminals (potential 1,000+ V)
- Core overheating and potential fire risk
- Permanent core damage (magnetisation)
Mitigation:
- Always maintain a continuous secondary circuit
- Permanently ground one end of the secondary circuit
- Install secondary short-circuit bars across CT terminals when devices are disconnected temporarily
- Use proper terminal grounds at protective relay panels
12.2 CT Secondary Ground
A single solid ground connection on the secondary winding is essential for:
- Safety of personnel working with the circuit
- Prevention of core saturation during fault conditions
- Mitigation of overvoltage transients
- Accurate protection operation
12.3 Installation Environments
Indoor Installations:
- Epoxy-cast and oil-immersed designs are both suitable
- Controlled temperature and humidity conditions
- Protection from direct weather exposure
- Integration into metal-clad or compartmented switchgear units
Outdoor Installations:
- VPI and modern epoxy-cast designs preferred for moisture resistance
- UV-resistant coatings for long-term durability
- Pole-mounted configurations are increasingly popular, particularly in rural and remote distribution networks
- Enhanced sealing against environmental ingress
12.4 Thermal Considerations
MV instrument transformers must be rated for the maximum ambient temperature and duty cycle. Typical thermal considerations:
- Continuous rated current corresponds to a specific ambient temperature (usually 40°C)
- Short-time thermal ratings define maximum allowable current for defined durations (e.g., 31.5 kA for 1 second)
- Cooling methods differ; oil-immersed units may require natural air circulation, while epoxy units dissipate heat through their surfaces
13. Standards and Compliance
Key international standards for instrument transformers include:
- IEC 60044 / 61869 series
- IEEE / ANSI C57 series
Manufacturer technical references are widely used in industry practice.
14. Emerging Trends
With changing trends in the application of instrument transformers, new developments are taking place in line with the following:
- Electronic and digital instrument transformers
- Integration with IEC 61850 and smart substations
- Increased deployment in solar, wind, and BESS plants
- Eco-friendly insulation and maintenance-free designs
15. Conclusion
Instrument transformers are indispensable elements of modern MV power systems, forming the foundation for accurate measurement, dependable protection, and safe operation. With rapid growth in renewable energy integration and grid automation, their performance requirements continue to rise.
Correct selection based on application, accuracy, insulation, and transient behaviour, combined with strict adherence to standards and safety practices, is essential for long-term system reliability and personnel safety.
External References
Ferroresonance causes and mitigation
Disclaimer:
This article is compiled from publicly available standards, industry references, and practical experience, with the intent to simplify technical concepts for practising engineers. Readers are advised to consult applicable standards, manufacturer documentation, and project-specific specifications before implementation.