Complete guide to understanding voltage transformers (potential transformers) for metering and protection
systems
A voltage transformer (VT), also called a potential transformer (PT), is an instrument transformer
used to step down high voltages to a safe, standardized level for metering, protection, and control systems. Unlike
power transformers that transfer energy, voltage transformers are designed for accurate voltage measurement.
This complete guide covers voltage transformer types, working principles, accuracy classes, and
applications in electrical power systems.
secondary voltage (typically 110V or 120V) while maintaining an accurate voltage ratio and phase relationship.
What is a Voltage Transformer?
A voltage transformer is a parallel-connected instrument transformer that provides a reduced replica
of the primary system voltage. The secondary voltage is proportional to the primary voltage by a fixed turns ratio.
Voltage Transformer vs Power Transformer
| Feature | Voltage Transformer | Power Transformer |
|---|---|---|
| Purpose | Measurement & protection | Power transfer |
| Connection | Parallel (shunt) | Series with load |
| VA Rating | Low (25-500VA) | High (kVA to MVA) |
| Accuracy | High precision required | Efficiency focused |
| Core Design | Saturates to protect meters | Large core for power |
Standard Secondary Voltage Ratings
| Standard | Secondary Voltage |
|---|---|
| IEC | 100V, 110V, 100/√3V |
| ANSI/IEEE | 115V, 120V, 69.3V |
Voltage Transformer Working Principle
The voltage transformer operates on electromagnetic induction, same as any transformer:
Basic Voltage Ratio Formula
V1/V2 = N1/N2
Where:
- V1 = Primary voltage
- V2 = Secondary voltage
- N1 = Number of primary windings
- N2 = Number of secondary windings
Working Operation
- Primary winding connects in parallel across line voltage
- High voltage creates magnetic flux in core
- Flux induces proportional voltage in secondary
- Secondary output connects to meters/relays
• Voltage ratio = 13800/120 = 115:1
• Turns ratio = 11500/100 = 115:1
Types of Voltage Transformers
1. Electromagnetic Voltage Transformer
The most common type using conventional wound construction.
| Feature | Description |
|---|---|
| Construction | Wire-wound primary and secondary |
| Voltage Range | Up to 145kV |
| Accuracy | Very high (0.1 to 0.5 class) |
| Application | Metering and protection |
2. Capacitor Voltage Transformer (CVT)
| Feature | Description |
|---|---|
| Construction | Capacitor stack + auxiliary transformer |
| Voltage Range | 72.5kV to 765kV |
| Advantages | Lower cost at HV, carrier coupling |
| Application | EHV substations, PLC communication |
CVT Components:
- C1 (HV capacitor stack)
- C2 (LV capacitor)
- Compensating reactor
- Electromagnetic unit (auxiliary transformer)
3. Optical Voltage Transformer
| Feature | Description |
|---|---|
| Principle | Polarized light rotation (Faraday effect) |
| Advantages | No saturation, wide bandwidth |
| Application | Digital substations, special measurements |
Voltage Transformer Accuracy Classes
IEC 61869-3 Accuracy Classes
| Class | Ratio Error | Phase Angle Error | Application |
|---|---|---|---|
| 0.1 | ±0.1% | ±5 minutes | Precision metering |
| 0.2 | ±0.2% | ±10 minutes | Revenue metering |
| 0.5 | ±0.5% | ±20 minutes | General metering |
| 1.0 | ±1.0% | ±40 minutes | Industrial metering |
| 3P | ±3.0% | – | Protection |
Burden (VA Rating)
| IEC Standard Burdens | VA Rating |
|---|---|
| 10VA | Low burden meters |
| 25VA | Standard metering |
| 50VA | Multiple meters |
| 100VA | Heavy burden applications |
Voltage Transformer vs Current Transformer
| Feature | Voltage Transformer (VT) | Current Transformer (CT) |
|---|---|---|
| Connection | Parallel (across line) | Series (in line) |
| Primary | High voltage, low current | High current |
| Secondary | 110V or 120V standard | 1A or 5A standard |
| Open Circuit | Safe to open secondary | Dangerous – never open |
| Short Circuit | Dangerous | Safe to short secondary |
voltage transformer secondaries can be safely opened. However, VT primary should never be energized with secondary
shorted.
Voltage Transformer Connections
Single Phase Connections
| Connection | Application |
|---|---|
| Phase-to-Ground | Grounded systems, residual voltage |
| Phase-to-Phase | Ungrounded systems |
Three Phase Connections
| Configuration | VTs Required | Application |
|---|---|---|
| 3 Single-Phase VTs | 3 | Full three-phase measurement |
| Open Delta | 2 | Phase-to-phase voltage only |
| Grounding Transformer | 3 | Ground fault detection |
CVT Applications
Capacitor Voltage Transformers serve dual purposes:
1. Voltage Measurement
- Revenue metering at EHV
- Protection relay inputs
- System voltage monitoring
2. Power Line Carrier (PLC)
- Couples high-frequency signals to transmission line
- Enables communication between substations
- Used with line traps (wave traps)
Electromagnetic VT vs CVT Comparison
| Feature | Electromagnetic VT | Capacitor VT (CVT) |
|---|---|---|
| Voltage Range | Up to 145kV | 72.5kV to 765kV |
| Accuracy | Higher (0.1 to 0.2 class) | Lower (0.2 to 0.5 class) |
| Transient Response | Excellent | Slower (capacitor effects) |
| Cost at HV | Very expensive | More economical |
| PLC Capability | No | Yes (built-in) |
| Size at 400kV+ | Very large | Compact |
| Best Application | High-speed protection | EHV metering, communication |
Voltage Transformer Selection Guide
Key Selection Factors
| Factor | Consideration |
|---|---|
| System Voltage | Must match or exceed line voltage |
| Accuracy Class | 0.2 for revenue metering, 3P for protection |
| Burden | Total connected load must not exceed rated VA |
| Insulation Level (BIL) | Match system BIL requirements |
| Indoor/Outdoor | Environmental protection requirements |
| Connection Type | Phase-to-ground or phase-to-phase |
Voltage Range Selection
| System Voltage | Recommended VT Type |
|---|---|
| Up to 36kV | Electromagnetic VT (resin-cast or oil-filled) |
| 36kV to 145kV | Electromagnetic VT (oil-filled) |
| 145kV to 765kV | Capacitor Voltage Transformer (CVT) |
| Digital substations | Optical VT or electronic VT |
🏭 Factory Experience:
- Ferroresonance Prevention: When using electromagnetic VTs with underground cables, always
install ferroresonance suppression devices. Cable capacitance can cause destructive oscillations. - Burden Matching: For accurate metering, ensure total connected burden is between 25% and
100% of rated burden. Under-burdened VTs may have higher errors. - CVT Transient Response: CVT output may deviate during fault conditions due to capacitor
discharge. For high-speed protection, use electromagnetic VTs or digital CVT designs. - Secondary Grounding: Always ground one terminal of VT secondary for safety. Follow IEC
61869-3 or IEEE C57.13 grounding requirements.
Voltage Transformer Testing
Factory Tests
| Test | Purpose |
|---|---|
| Ratio test | Verify voltage ratio accuracy |
| Polarity test | Confirm terminal markings |
| Winding resistance | Check winding condition |
| Insulation resistance | Verify insulation quality |
| Power frequency withstand | Test dielectric strength |
Frequently Asked Questions
What is a voltage transformer used for?
A voltage transformer (VT) is used to step down high system voltage to a safe, standardized level
(typically 110V or 120V) for metering, protection relays, and control systems. It provides electrical isolation
between high voltage and measuring instruments.
What is the difference between a voltage transformer and a potential transformer?
There is no difference – voltage transformer (VT) and potential transformer (PT) are the same device. VT is the IEC
term while PT is commonly used in North America. Both terms refer to instrument transformers that measure voltage.
Why is a CVT used instead of a conventional VT?
Capacitor voltage transformers (CVTs) are used at voltages above 100kV because they are more economical than wound
electromagnetic VTs. CVTs also enable power line carrier communication coupling.
What happens if you short circuit a voltage transformer secondary?
Short-circuiting a VT secondary creates a dangerous condition. Unlike CTs, VT secondaries should never be shorted as
it creates effectively a short circuit on the primary system, causing extremely high current and potential failure.
What is the accuracy class of a voltage transformer?
Voltage transformer accuracy classes (per IEC 61869-3) include 0.1, 0.2, 0.5, 1.0, and 3P. Class 0.2 is typical for
revenue metering while 3P is used for protection applications.
Related Articles
- Current Transformer (CT):
Complete Guide - CT Accuracy Class Explained
- Power
Transformer vs Distribution Transformer - What is a Transformer?
- Types of Transformer:
Complete Classification Guide
References
- IEC 61869-3 – Instrument Transformers: Inductive
Voltage Transformers - IEEE C57.13 – Standard Requirements for
Instrument Transformers - Hitachi Energy – Voltage
Transformers
Disclaimer: This information is provided for educational purposes. Always follow applicable standards and
manufacturer guidelines when specifying or installing voltage transformers.