What is a Potential Transformer (PT)? Why is it essential in Power Systems?

November 3, 2026

Power systems operate at voltages ranging from a few kilovolts (kV in distribution networks to several hundred kilovolts in transmission grids.

1). Meters,

2). Protection relays and

3). Control devices

are designed for low, standardized voltages.

Connecting them directly to a high-voltage (HV) line would be unsafe for personnel and would require instruments with impractically large insulation.

To solve this, engineers use an instrument transformer called the Potential Transformer (PT) which also known as a Voltage Transformer (VT).

A PT steps the high system voltage down to a safe, standardized secondary voltage while keeping the secondary voltage accurately proportional to the primary voltage, in both magnitude and phase for measurement and protection.

What is a Potential Transformer?

A Potential Transformer is an instrument transformer that reduces a high primary voltage to a lower and standard secondary voltage through electromagnetic induction.

The reduced voltage is supplied to:

Measuring instruments (voltmeters, wattmeters, power-factor meters)
Protection relays (overvoltage, undervoltage, distance, directional and differential schemes)
Energy meters (tariff and revenue metering)
Control, synchronizing and SCADA equipment

Standard Secondary Voltages

The most common rated secondary voltages are 110 V and 63.5 V depending on the application and connection:

Secondary Voltage	Meaning	Typical Use
110 V	Line-to-line (phase-to-phase) rating.	PTs connected between two phases (or) three-phase PTs.
63.5 V (110 / √3 ≈ 63.5 V)	Line-to-neutral (phase-to-earth) rating.	Single-phase PTs connected phase-to-earth in three-phase systems.

Single-phase PTs connected phase-to-earth are typically rated as (Primary / √3) / (110 / √3) V.

For Example:

11 kV / √3 : 110 V / √3

In North American practice, 120 V and 69.3 V (120 / √3) secondaries are common and 110 V and 63.5 V are the values used in the application.

Working Principle of Potential Transformer

A PT works on the same principle as any transformer: mutual induction (Faraday’s law of electromagnetic induction).

It operates in the following sequence:

Primary connection: The primary winding with many turns is connected across the high voltage (HV) line (in parallel with the system, i.e., shunt connection).
Flux production: The primary current sets up an alternating magnetic flux in the laminated iron core.
Induction: The core links this flux with the secondary winding which has far fewer turns that is inducing a proportionally lower EMF.
Output: The secondary supplies an isolated, safe and proportional low voltage signal to meters and relays.

For an ideal transformer the relationship between voltages and turns is:

V₁ / V₂ = N₁ / N₂

Where

V₁ & V₂ – Primary and Secondary Voltages,

N₁ and N₂ – Numbers of Primary and Secondary Turns

Since N₁ is much greater than N₂ and V₂ is much smaller than V₁.

Key Operating Characteristic

A PT operates very close to no-load because the connected instruments draw only a small current.

Its secondary must never be short-circuited: a short circuit would draw a very large current that is causing excessive heating and likely damaging the PT. This is the opposite of a Current Transformer (CT) whose secondary must never be left open.

Construction of Potential Transformer

Component	Description
Core	Laminated high-grade silicon steel (or similar) to reduce eddy-current and hysteresis losses and to keep magnetizing current low.
Primary winding	Large number of turns of fine conductor that is connected across the HV line and insulated for the full system voltage.
Secondary winding	Fewer turns of thicker conductor that is delivering the standard low voltage (e.g., 110 V or 110/√3 V). May have more than one winding (e.g., a measuring winding and a residual/open-delta winding).
Insulation	Oil, resin (cast epoxy), paper (or) SF₆ gas depending on voltage level and design.
Terminals & housing	Primary and secondary terminals, enclosure (porcelain (or) composite insulators at higher voltages) and an earthing terminal.
Protection devices	HV fuses on the primary side (common at medium voltage) & MCB/fuses on the secondary side.

Types of Potential Transformers

1). Electromagnetic Potential Transformer

The conventional wound-type PT described above.

It is widely used from low and medium voltages up to roughly 132 kV; beyond that, it becomes bulky & costly because of the insulation required.

2). Capacitor Voltage Transformer (CVT)

A CVT uses a capacitor divider to reduce the line voltage to an intermediate level followed by a smaller electromagnetic transformer to reach the standard secondary value.

It is more economical at extra-high voltages (commonly 132 kV and above) and can also serve as a coupling device for power-line carrier communication.

3). Classification by Construction and Connection

By phases: Single-phase (phase-to-phase or phase-to-earth) and three-phase PTs.
By installation: Indoor and outdoor types.
By insulation: Oil-immersed, cast resin (dry type) & gas-insulated types.
By purpose: Measuring PTs and protection PTs, or combined-purpose units with separate windings.

Voltage Ratio

The basic relationship is:

PT Ratio = Primary Voltage ÷ Secondary Voltage

Solved Example

11 kV / 110 V PT

Ratio = 11000 V ÷ 110 V = 100 : 1

When the primary voltage is 11000 V, the secondary delivers 110 V.

Conversely, a secondary reading of 105 V corresponds to a primary voltage of 105 x 100 = 10,500 V.

Additional Examples

PT Rating	Ratio Calculation	Result
11 kV / 110 V	11000 ÷ 110	100 : 1
33 kV / 110 V	33000 ÷ 110	300 : 1
132 kV / 110 V	132000 ÷ 110	1200 : 1
(11 kV / √3) / (110 V / √3)	6,351 ÷ 63.5	≈ 100 : 1 (the √3 factors cancel)

Note: the “nominal ratio” (rated primary ÷ rated secondary) is not exactly equal to the actual ratio under load, because of ratio and phase-angle errors.

Main Functions of PT

Accurate voltage measurement: Supplies voltmeters and transducers with a scaled, proportional voltage.
Protective relay operation: Provides voltage signals for overvoltage, undervoltage, distance, directional, earth-fault and synchro-check relays.
Energy metering: Feeds the voltage input of revenue and tariff meters together with CTs.
Power quality monitoring: Enables monitoring of voltage magnitude, sags, swells, harmonics and unbalance.
System synchronization: Supplies voltage and phase information for synchronizing generators (or) feeders with the grid.
SCADA and automation: Provides standardized voltage inputs for remote monitoring and control systems.
Electrical isolation: Separates HV circuits from low-voltage instruments and personnel.

Why PT is Important?

Advantages	Explanation
Safe voltage levels	Operators and equipment only deal with about 110 V not kilovolts.
High measurement accuracy	Defined accuracy classes ensure reliable readings and fair billing.
Reliable relay protection	Protection relays receive a faithful reproduction of system voltage during normal and fault conditions.
Electrical isolation	Faults or surges on the HV side are kept away from instruments and control panels.
Standardized outputs	Common secondary values allow interchangeable meters, relays and control equipment from different manufacturers.
Cost reduction	Instruments need only low voltage insulation, so they are smaller & cheaper.

Accuracy, Errors and Accuracy Classes

Sources of Error

Ratio (voltage) error: The difference between actual ratio and nominal ratio, caused by winding resistance, leakage reactance and magnetizing current. It is expressed as a percentage.
Phase-angle (phase displacement) error: The angle by which the secondary voltage phasor (reversed) differs from the primary voltage phasor. It is expressed in minutes / centiradians) and is important for metering and directional relays.

Errors increase when the connected burden exceeds the rated burden and vary with voltage and burden power factor.

Typical Accuracy Classes (IEC 61869-3)

Purpose	Class	Typical Application
Measurement	0.1, 0.2, 0.5, 1.0, 3.0	0.1 / 0.2: accuracy and revenue (tariff) metering; 0.5: commercial metering; 1.0 / 3.0: indicating instruments
Protection	3P, 6P	Relaying applications where accuracy over a wide voltage range (including fault conditions) matters

Accuracy classes for measurement are guaranteed within a specified range of voltage (80% to 120% of rated voltage) and burden (25% to 100% of rated burden at a power factor of 0.8 lagging).

Protection classes are specified over a wider voltage range (from 5% of rated voltage up to the rated voltage factor times rated voltage).

Always confirm these values against the applicable standard edition.

Burden (VA Rating)

Burden is the load connected to the PT secondary, expressed in volt-amperes (VA) at a stated power factor.

The PT must be selected so that the total connected burden does not exceed its rated burden otherwise accuracy is lost.

Indicative Burden Calculation

Connected Device (illustrative values)	Burden (VA)
Voltmeter	5
Energy meter (voltage circuit)	5
Protection relay (voltage circuit)	10
Total (approximate arithmetic sum)	20

A PT with a rated burden of 50 VA, for example, would carry this 20 VA load within its accuracy class.

Actual device burdens vary, so use the manufacturer data.

Standard rated outputs commonly include values such as 10, 15, 25, 30, 50, 75 and 100 VA.

Phasor Diagram of Potential Transformer (PT)

The phasor diagram of a Potential Transformer (PT) illustrates the relationship between primary and secondary voltages, winding impedance drops, excitation current, and phase angle error.

Phasor Diagram Parameters:

Is – Secondary current
Es – Secondary induced EMF
Vs – Secondary terminal voltage
Rs / Xs – Secondary winding resistance / reactance
Ip – Primary current
Ep – Primary induced EMF
Vp – Primary terminal voltage
Rp / Xp – Primary winding resistance / reactance
Kt – Transformation ratio (primary-to-secondary turns ratio)
Io – Excitation current
Im – Magnetising component of excitation current
Iw – Core-loss component of excitation current
Φm – Main magnetic flux
β – Phase angle error

The main magnetic flux (Φm) is taken as the reference phasor.

The primary current (Ip) is vector sum of excitation current (Io) and reversed secondary current referred to primary side (Is/Kt).

The applied primary voltage (Vp) supplies the induced EMF (Ep) and compensates for the primary winding resistance and leakage reactance drops (IpRp and jIpXp).

Through mutual electromagnetic induction, the primary induced EMF generates the corresponding secondary induced EMF (Es).

After accounting for secondary winding resistance and leakage reactance drops (IsRs and jIsXs), the resulting secondary terminal voltage is Vs.

The phase angle error (β) represents the angular displacement between the primary and reversed secondary voltage phasors referred to the same transformation ratio.

It is an important accuracy parameter in potential transformer measurement applications.

Typical Applications of Potential Transformer

Power generation plants.
Transmission substations.
Distribution networks.
Industrial power systems.
Switchyards.
Protection and metering panels.
High-voltage testing laboratories.

In each case, the PT provides the voltage reference required for monitoring, billing, protection and control.

Comparison: Potential Transformer vs Current Transformer

Difference between Potential Transformer and Current Transformer

Feature	Potential Transformer (PT)	Current Transformer (CT)
Measures	Voltage	Current
Connection to line	In parallel (shunt) with the circuit	In series with the circuit
Operating condition	Near no-load	Near short-circuit
Secondary standard rating	110 V (or 110/√3 V)	1 A or 5 A
Secondary must never be	Short-circuited	Open-circuited
Primary winding	Many turns, fine conductor	Few turns (sometimes a single bar), heavy conductor
Secondary winding	Fewer turns	Many turns

Installation and Safety Considerations

Secondary earthing: Connect one point of the secondary winding to earth to prevent the secondary from floating at a dangerous potential if insulation fails.
Secondary protection: Protect the secondary circuit with fuses or a miniature circuit breaker to guard against secondary short circuits.
Primary fuses: Medium-voltage PTs commonly have HV fuses on the primary side.
Short-circuit avoidance: Never short-circuit the PT secondary.
Polarity: Correct polarity and phase sequence are essential for wattmeters, energy meters and directional or distance relays.
Voltage factor: Follow standard voltage-factor ratings (for example, 1.2 continuous and 1.5 or 1.9 for 30 seconds, depending on system earthing) so that the PT withstands temporary overvoltages during earth faults.
Ferroresonance: Follow ferroresonance guidelines for the PT’s connection and loading, particularly in ungrounded or isolated-neutral systems.

Checklist of Potential Transformer (PT)

Always verify the following before selecting (or) installing a PT:

Parameter	What to Verify
Rated primary and secondary voltage	Match the system voltage and the connected instruments (e.g., 11 kV / 110 V).
Accuracy class	Choose the class suited to the purpose: metering, protection or both.
Burden (VA rating)	Ensure total connected burden stays within the rated burden.
Frequency	Rated frequency (50 Hz or 60 Hz) must match the system.
Insulation level	Rated insulation (highest system voltage, power-frequency withstand and lightning impulse withstand) must suit the installation.
Earthing requirements	Verify system earthing (effectively earthed or not), the primary terminal earthing arrangement and secondary earthing.

Conclusion

A Potential Transformer steps down high system voltage to a safe, standardized secondary value (commonly 110 V line-to-line or 63.5 V phase-to-neutral) with an accurately proportional ratio.

It is connected in parallel with the circuit, works near no-load, and provides isolation between HV circuits and instruments.

It supports measurement, metering, protection, synchronization, power quality monitoring and SCADA.

Correct selection of voltage rating, accuracy class, burden, frequency, insulation level and earthing is essential for safe and reliable operation.

Example: an 11 kV / 110 V PT has a ratio of 100 : 1.