Comprehensive guide to potential transformers for metering and protection in power systems
A potential transformer (PT) is an instrument transformer used in power systems to step down high
voltages to a safe, standardized level for meters and protective relays. The term “potential transformer” is
commonly used in North America, while “voltage transformer” (VT) is the IEC standard terminology.
This complete guide covers potential transformer construction, working principle, accuracy classes,
wiring configurations, and applications in metering and protection systems.
voltage of 120V (ANSI) or 110V (IEC) while maintaining accurate voltage ratio and phase relationship for measuring
instruments.
What is a Potential Transformer?
A potential transformer (also called voltage transformer or VT) is a parallel-connected instrument
transformer that provides an accurate replica of the primary system voltage at a safe, reduced level. PTs are
essential components in:
- Revenue metering systems
- Protective relay inputs
- Synchronizing equipment
- Power quality monitoring
PT vs VT: What’s the Difference?
| Term | Standard | Region |
|---|---|---|
| Potential Transformer (PT) | IEEE/ANSI C57.13 | North America |
| Voltage Transformer (VT) | IEC 61869-3 | International |
Both terms refer to the same device. For more details on VT types and applications, see our comprehensive voltage
transformer guide.
Potential Transformer Construction
The construction of a potential transformer includes:
| Component | Function |
|---|---|
| Primary Winding | High voltage side, many turns of fine wire |
| Secondary Winding | Low voltage side (120V or 110V output) |
| Magnetic Core | Silicon steel laminations for low losses |
| Insulation | Oil or resin for HV isolation |
| Terminals | H1, H2 (primary); X1, X2 (secondary) |
Standard Secondary Voltage Ratings
| Standard | Secondary Voltage | Application |
|---|---|---|
| ANSI/IEEE | 120V, 115V, 69.3V | North America |
| IEC | 110V, 100V, 100/√3V | International |
Working Principle of Potential Transformer
The potential transformer operates on electromagnetic induction:
Voltage Ratio Formula:
Vp/Vs = Np/Ns
Where:
- Vp = Primary voltage
- Vs = Secondary voltage (typically 120V)
- Np = Number of primary winding turns
- Ns = Number of secondary winding turns
When 4200V is applied to primary, 120V appears at secondary.
Types of Potential Transformers
1. Electromagnetic Potential Transformer
| Feature | Description |
|---|---|
| Voltage Range | Up to 145kV |
| Accuracy | 0.3 to 1.2 class (IEEE) |
| Construction | Wound type with magnetic core |
| Application | Metering and protection |
2. Capacitor Coupled Voltage Transformer (CCVT)
| Feature | Description |
|---|---|
| Voltage Range | 69kV to 800kV |
| Components | Capacitor divider + auxiliary transformer |
| Advantages | Cost-effective at EHV, PLC capability |
| Application | Transmission substations |
PT Accuracy Classes (IEEE C57.13)
| Class | Ratio Error | Application |
|---|---|---|
| 0.3 | ±0.3% | Revenue metering |
| 0.6 | ±0.6% | Standard metering |
| 1.2 | ±1.2% | Indication, protection |
PT Burden Classes (IEEE C57.13)
| Class | VA Rating | Power Factor |
|---|---|---|
| W | 12.5 VA | 0.10 |
| X | 25 VA | 0.70 |
| Y | 75 VA | 0.85 |
| Z | 200 VA | 0.85 |
Errors in Potential Transformers
Ratio Error
The difference between ideal and actual voltage ratio:
Where Kn = Rated ratio, Vs = Secondary voltage, Vp = Primary voltage
Phase Angle Error
The angular displacement between primary voltage and reversed secondary voltage. Phase errors affect power and energy
measurements.
| Error Type | Cause | Effect |
|---|---|---|
| Ratio Error | Winding resistance, core losses | Voltage measurement error |
| Phase Error | Winding reactance, magnetizing current | Power factor, energy errors |
PT Wiring Configurations
Three-Wire Delta Service
Common in medium voltage industrial applications:
- 2 PTs required (open delta connection)
- Phase B grounded at secondary
- Measures line-to-line voltages
Four-Wire Wye Service
Standard for utility metering:
- 3 PTs required (wye-wye connection)
- Neutral grounded
- Measures line-to-neutral voltages
PT vs CT Comparison
| Feature | Potential Transformer (PT) | Current Transformer (CT) |
|---|---|---|
| Measures | Voltage | Current |
| Connection | Parallel (shunt) | Series |
| Standard Output | 120V or 110V | 5A or 1A |
| Open Secondary | Safe | Dangerous |
| Short Secondary | Dangerous | Safe |
For detailed CT information, see our Current Transformer Complete
Guide and CT Burden
Calculation Guide.
🏭 Factory Experience:
- Fusing: Always install primary fuses on PT circuits. Use current-limiting fuses rated for
the system voltage. - Secondary Grounding: Ground X2 terminal for safety. Follow local codes for grounding
conductor size. - Burden Check: Calculate total connected burden before installation. Include wire resistance
for long cable runs. - Ferroresonance: When removing one PT from a bank of three, install a loading resistor to
prevent ferroresonance with cable capacitance.
Frequently Asked Questions
What is a potential transformer used for?
A potential transformer (PT) is used to step down high voltage to a safe level (typically 120V) for
metering, protection relays, and control circuits. It provides electrical isolation between the high voltage system
and measuring instruments.
What is the difference between PT and VT?
There is no technical difference – PT (potential transformer) and VT (voltage transformer) are different names for
the same device. PT is the IEEE/ANSI term used in North America, while VT is the IEC term used internationally.
What is a standard PT ratio?
Common PT ratios include 4200:120 (35:1), 14400:120 (120:1), and 69000:120 (575:1). The ratio depends on the system
voltage, with 120V being the standard secondary voltage in North America.
Can you short circuit a PT secondary?
No, you should never short circuit a PT secondary. Unlike CTs, shorting a PT creates a short circuit on the primary
system, causing extremely high current that can damage or destroy the PT.
What is the accuracy class of a PT?
IEEE C57.13 defines PT accuracy classes as 0.3, 0.6, and 1.2. Class 0.3 PTs are required for revenue metering, while
Class 1.2 is acceptable for indication and protection applications.
Related Articles
- Voltage Transformer (VT):
Complete Guide - Current Transformer (CT):
Complete Guide - CT Accuracy Class Explained
- CT Burden Calculation
Guide - What is Power Transformer?
References
- IEEE C57.13 – Standard Requirements for
Instrument Transformers - IEC 61869-3 – Instrument Transformers: Inductive
Voltage Transformers - Flex-Core – Potential Transformers Technical
Resources
Disclaimer: This information is provided for educational purposes. Always follow applicable standards and
manufacturer guidelines when specifying or installing potential transformers.