What Is a Potential Transformer? Working Principle, Accuracy, and Selection Guide

What exactly is a potential transformer?

A potential transformer (PT), also known as a voltage transformer (VT), is an instrument transformer that reduces a high primary voltage to a standardized, much lower secondary voltage. Its secondary winding feeds measuring instruments, protective relays, synchronizing devices, and control circuits; it is not designed to deliver power.

The distinction matters more than the name. A power transformer transfers large amounts of energy, while a PT reproduces voltage faithfully. A typical PT handles only a burden of 10 VA to 75 VA, yet it must hold ratio error and phase error inside strict limits so that revenue metering, protection coordination, and switching decisions are based on an accurate picture of the network.

How a potential transformer works

A PT operates on the same electromagnetic induction principle as any transformer. Its primary winding is connected in parallel across the line whose voltage must be measured. The alternating flux in the core induces a proportional voltage in the secondary winding. With a turns ratio of 100 to 1, an 11,000 V primary produces 110 V at the secondary at rated voltage. Three design points separate a PT from an ordinary power transformer:

• Parallel connection. The primary sees full system voltage but draws only a small excitation current, so the PT itself consumes almost no energy.
• Accuracy before efficiency. The core operates at low flux density to stay linear across the working voltage range, and the windings are arranged to reduce leakage reactance and ratio error.
• Safe, standardized output. The secondary is commonly rated at 100 V, 110 V, or 120 V and must be solidly grounded. As a safety rule, a PT secondary must never be short-circuited, because the resulting fault current would overheat and destroy the windings.

Key components of a potential transformer

A PT is compact compared with a distribution transformer, but its construction is demanding. The insulation must withstand full line voltage while the magnetic circuit maintains a precisely accurate ratio.

Core and windings

The core is built from high-permeability grain-oriented silicon steel laminations and runs at deliberately low flux density to reduce saturation and waveform distortion. The primary winding is designed for the full insulation level of the system and often includes tappings for ratio adjustment. The secondary winding is placed close to the primary to keep leakage reactance low, which directly improves phase accuracy.

Insulation system

For medium-voltage networks up to 35 kV, PTs are usually cast in epoxy resin or built with oil-paper insulation. Epoxy-cast PTs are common indoors and inside switchgear because they resist moisture and require little maintenance. Outdoor high-voltage installations more often use oil-immersed or gas-insulated designs.

Bushings and enclosure

Porcelain or silicone-rubber bushings bring the primary terminals out to the line connection, while the secondary terminals sit in a low-voltage terminal box. The enclosure rating and creepage distance must suit the installation site, whether that is indoor switchgear, an outdoor substation, or a coastal area with high pollution levels.

What is a potential transformer used for?

In a substation, almost every protection and metering function depends on the voltage image that the PT supplies. Its main applications are:

• Voltage and energy measurement. Voltmeters, energy meters, power analyzers, and power quality monitors read the scaled secondary voltage and multiply it by the PT ratio.
• Protection. Under-voltage, over-voltage, directional, and distance relays use PT inputs to detect faults and abnormal conditions.
• Synchronism checking. Before two feeders or generators are paralleled, a PT on each side lets the synchronizing relay compare voltage magnitude and phase angle.
• Ground-fault detection. Three phase-to-neutral PTs with a broken-delta secondary winding produce a residual voltage that indicates an earth fault, even in non-effectively grounded networks.
• Voltage control and load shedding. Distribution automation schemes use PT signals for transformer tap changing, capacitor bank switching, and low-voltage load shedding.

The choice of secondary connection, whether wye, delta, or broken delta, is a protection-engineering decision, and the same winding logic appears across the rest of the network. A useful starting point is the three-phase transformer connection guide, which explains how these configurations behave in practice.

Because relays and meters are built for standardized inputs, PTs are produced with well-defined secondary voltage levels, as shown in Table 1.

Potential transformer vs current transformer: what is the difference?

The PT is often mentioned together with the current transformer (CT) because both are instrument transformers: the PT measures voltage, while the CT measures current. They cannot be swapped, and connecting either one incorrectly creates a serious hazard.

The operating rule to remember: a PT behaves like a small voltage source, so its secondary must never be short-circuited; a CT behaves like a current source, so its secondary must never be open-circuited. Both secondary circuits must always be grounded to keep personnel and equipment safe.

Accuracy classes, burden, and what to check before buying a PT

The value of a PT is only as good as its accuracy under real service conditions. Two parameters dominate the specification: accuracy class and burden.

Accuracy class

Standards such as IEC 61869-3 define metering classes (0.2, 0.5, 1.0, 3.0) and protection classes (3P, 6P). A class 0.5 metering PT, for example, limits voltage error to 0.5 percent at rated burden, while protection classes allow wider error but must keep reliable performance during transient overvoltage.

Burden

Burden is the load connected to the secondary, expressed in volt-amperes (VA). A relay, a meter, and the interconnecting wiring may together present 25 VA, so the PT must have a rated burden at least equal to that value. If the connected burden rises beyond the rated burden, ratio error and phase error grow beyond the class limit.

Practical selection checklist

• Voltage class and insulation level. Match the system highest voltage, typically 12 kV, 24 kV, or 40.5 kV for medium-voltage networks.
• Ratio and connection. Confirm whether the PT is line-to-line, line-to-neutral, or broken-delta connected, and specify the correct secondary voltage.
• Accuracy class and rated burden. Verify that the metering class satisfies the utility or project requirement and that the rated burden covers all connected devices.
• Voltage factor. In non-effectively grounded systems, the PT must normally withstand 1.9 times rated voltage for 30 seconds, which demands a different core design from that used in solidly earthed networks.
• Environment. State indoor or outdoor construction, enclosure protection rating, and creepage distance based on the site pollution level.

PTs in the distribution network

A PT never replaces a power transformer. On a 10 kV or 20 kV distribution feeder, one or more PTs supply revenue meters, protection relays, and remote terminal units, but the actual power transfer depends on a separate distribution transformer. Mixing up the two roles is one of the most common specification mistakes.

For utilities and industrial users, the workhorse of power distribution remains the oil-immersed transformer. Low-loss 10 kV and 20 kV oil-immersed distribution transformers in the S11-S22 series are routinely paired with PT-based metering panels in substations and distribution rooms. The boundary between the two duties is covered in the practical guide on power transformer vs distribution transformer selection.

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Indoor and fire-sensitive locations

When the transformer must be mounted inside a building, hospital, mall, or industrial workshop, dry-type construction avoids the fire and leakage risks of oil-filled equipment. Many projects choose epoxy resin cast dry-type transformers from 800 kVA up to 25,000 kVA because the cast resin windings withstand humidity, dust, and partial discharge over long service intervals.

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Compact and box-type solutions

In tight distribution points, the PT, metering cubicle, switchgear, and transformer can share one housing. For feeder pillars, rural networks, and commercial enclosures, an American box-type combined transformer puts incoming protection, metering, and the distribution transformer in a single weatherproof cabinet, shortening installation time and simplifying maintenance. If you are reviewing the whole assembly around the PT, the comprehensive guide to high-voltage switchgear explains how voltage transformers, breakers, and bushings work together in the same panel.

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Putting the potential transformer in its place

A potential transformer is a small but essential link between the high-voltage network and the low-voltage instruments that supervise it. It steps voltage down safely, holds the ratio and phase within tight limits, and provides the isolation that protects people and control gear.

When you specify a PT, check the ratio, accuracy class, burden, insulation level, and secondary connection; select a unit rated for the system maximum voltage and earthing arrangement; and respect the secondary circuit rules, never short-circuit a PT secondary, never open-circuit a CT secondary, and always ground the secondary side. And if the project actually needs the transformer that carries the load, remember that a PT is the eyes of the protection system, not a replacement for the power transformer itself.