Types of Transformers Used in Power Transmission and Distribution

Electricity leaves a power plant at one voltage, crosses hundreds of kilometres at another, and reaches your wall socket at a third. The equipment that makes each of those jumps possible is the transformer. Without it, India's grid simply could not move power over long distances without losing most of it as heat.

The scale of that work is growing fast. According to Central Electricity Authority (CEA) data, India added 113,013 MVA of AC transformation capacity at 220 kV and above in 2025-26, about 90% of the CEA's target of 126,007 MVA. By July 2026, the national network held roughly 1,486 GVA of AC transformation capacity at 220 kV and above, with a full-year FY27 target of 158,339 MVA.

But not every transformer does the same job. A unit in a 765 kV substation, one on a pole outside a village, and one inside a hospital basement are built very differently. This guide breaks down the main types of transformers used in power transmission and distribution, how each one works, and where each one fits.

How to Classify Transformers: A Four-Lens Framework

Transformers can be grouped in several overlapping ways. One unit can belong to more than one category at the same time. For example, a 400/220 kV interconnecting transformer is a power transformer, a step-down transformer, an auto transformer, and an oil-immersed transformer, all at once.

To keep things clear, this guide uses four lenses:

Let's walk through each lens in turn.

1. Step-Up and Step-Down Transformers: The Basic Function

Every transformer works on the same principle: electromagnetic induction between two coils wound on a shared magnetic core. The voltage ratio depends on the ratio of turns in the two windings.

Secondary Voltage / Primary Voltage = Secondary Turns / Primary Turns

Step-Up Transformer

A step-up transformer has more turns on the secondary winding than on the primary. It raises voltage and lowers current for the same power.

You will find step-up units at power plant switchyards, where generator output (usually between 11 kV and 30 kV) is raised to 220 kV, 400 kV, or 765 kV for transmission. These are called generator step-up (GSU) transformers. The logic is simple: line losses depend on the square of current (I²R), so raising voltage cuts current sharply, and that is what makes long-distance transmission economical. Solar and wind parks use step-up transformers in the same way, lifting inverter or turbine output to the pooling substation voltage.

Step-Down Transformer

A step-down transformer has fewer turns on the secondary. It lowers voltage in stages as power moves closer to consumers.

• At grid substations: 765 kV to 400 kV, 400 kV to 220 kV, and 220 kV to 132 kV or 66 kV.
• At sub-transmission substations: 132 kV or 66 kV down to 33 kV, then 33 kV to 11 kV.
• At the last mile: 11 kV down to 433 V (three-phase), which gives 230 V single-phase supply at homes.

The difference between step-up and step-down transformer design is mostly about direction of use and insulation levels. The physics stays the same.

2. Power Transformers: The Workhorses of Transmission

Power transformers handle bulk power at high and extra-high voltages. They sit at generating stations and transmission substations, typically rated in tens to hundreds of MVA.

Key features:

• Designed for high load factor: They run close to full load for long periods, so they are built for maximum efficiency at or near rated load.
• On-load tap changers (OLTC): Most grid power transformers carry OLTCs to adjust voltage without switching off supply.
• Heavy protection: Buchholz relays, pressure relief devices, differential protection, and online condition monitoring are standard.
• Long lead times: According to a Ministry of Power reply in Parliament in March 2026, delivery timelines run about 11 to 15 months for 220 kV transformers, around 15 months for 400 kV units, and 20 to 24 months for 765 kV transformers. This is why transformer procurement is often on the critical path of a substation project.

Common examples in India's grid:

• Generator step-up transformers at thermal, hydro, and nuclear plants
• Interconnecting transformers (ICTs) at 765/400 kV and 400/220 kV substations
• 220/132 kV and 132/33 kV transformers at state transmission substations
• Converter transformers at HVDC terminals

3. Distribution Transformers: The Last-Mile Link

Distribution transformers step down 33 kV or 11 kV to the 433 V level used by homes, shops, farms, and small industries. They are the most numerous transformers on any network.

They are much smaller than power transformers and are usually rated in kVA rather than MVA. Common sizes include 25, 63, 100, 250, 400, and 630 kVA, going up to 2,500 kVA for larger loads. Because their load swings through the day, they are designed for good all-day efficiency and low no-load (core) losses. Smaller units are pole-mounted, while larger ones sit on plinths or in compact substations and kiosks.

In India, distribution transformers come under the Bureau of Energy Efficiency (BEE) Standards and Labelling programme, which pushes utilities towards lower-loss designs, including amorphous core transformers.

Distribution transformer health has a direct link to aggregate technical and commercial (AT&C) losses. Overloaded or poorly maintained units fail often, interrupt supply, and waste energy. That is why feeder segregation, transformer metering, and distribution transformer monitoring are core parts of loss reduction programmes like the Revamped Distribution Sector Scheme (RDSS).

4. Instrument Transformers (CT and PT): The Grid's Measuring Eyes

Meters and protective relays cannot connect directly to a 220 kV line or a 2,000 A busbar. Instrument transformers scale those values down to safe, standard levels that measuring devices can handle. There are two main types.

Current Transformer (CT)

• Connection: In series with the line.
• Function: Steps current down to a standard secondary value, usually 1 A or 5 A.
• Uses: Energy metering, protective relaying (overcurrent, differential, distance protection), and SCADA data.
• Accuracy classes: Metering cores use classes such as 0.2, 0.2S, or 0.5. Protection cores use classes such as 5P, 10P, or PS.
• Safety rule: The secondary of a CT must never be left open while the primary is energised. An open secondary can develop dangerously high voltages and damage the CT.

Potential Transformer (PT) or Voltage Transformer (VT)

• Connection: In parallel (shunt) across the line.
• Function: Steps voltage down to a standard secondary level, typically 110 V line-to-line.
• Uses: Voltage metering, energy metering, synchronisation, and voltage-based protection.
• Capacitive Voltage Transformers (CVTs): At 220 kV and above, CVTs are widely used. They are more economical at high voltages and can also carry power line carrier communication signals.
• Safety rule: A PT secondary must never be short-circuited.

Accurate CTs and PTs matter for more than protection. They form the base of revenue metering at substations and feeders, so their accuracy directly affects how much energy a utility can account for and bill.

5. Auto Transformers: Economical at High Voltages

A conventional transformer has two separate windings that are electrically isolated from each other. An auto transformer uses a single continuous winding, with part of it shared between the primary and secondary circuits.

How it works: Some power is transferred by induction (like a normal transformer) and some passes directly through the shared winding by conduction. This makes the unit smaller, lighter, and more efficient than a two-winding transformer of the same rating.

Where auto transformers shine:

• When the voltage ratio is close to 1: The savings in copper and core material are largest when input and output voltages are close, such as 765/400 kV or 400/220 kV.
• Interconnecting transformers: Most 400/220 kV ICTs in Indian substations are auto transformers, as are the large single-phase banks used at 765/400 kV substations. The CEA's standard specifications for transformers, now mandatory for utilities, list standard auto transformer ratings such as 500 MVA at 765/400 kV and 315 MVA at 400/220 kV.
• Other uses: Voltage regulators, motor starting, and variable laboratory supplies (variacs).

Trade-offs to keep in mind:

• No electrical isolation between primary and secondary, so a fault on the high-voltage side can affect the low-voltage side.
• Lower impedance means higher fault currents, which must be considered in protection and switchgear design.
• Not economical when the voltage ratio is large, such as 33 kV to 433 V. Distribution transformers are therefore almost always two-winding types.

Many grid auto transformers also carry a third, delta-connected tertiary winding. It helps stabilise voltages during unbalanced loads and can supply local auxiliary loads or reactive compensation.

6. Oil-Immersed vs Dry-Type Transformer: Choosing the Right Construction

The final lens is construction: how the windings are insulated and how heat is removed. This is often the most practical decision for project owners, especially for distribution and indoor applications.

Oil-Immersed Transformers

The core and windings sit inside a steel tank filled with insulating oil. The oil insulates the windings and carries heat away to radiators.

Cooling methods range from ONAN (oil natural, air natural) and ONAF (oil natural, air forced) to OFAF and ODAF for very large units. Mineral oil is the standard insulating fluid, though natural ester oils are increasingly used where higher fire safety and biodegradability are needed.

Oil-immersed units handle the highest voltages and ratings, offer good overload capacity and long service life, and cost less per kVA. The trade-off is fire and spill risk. Outdoor installations need oil soak pits, fire walls, and fire protection, and the oil needs periodic testing such as breakdown voltage (BDV) and dissolved gas analysis (DGA).

Dry-Type Transformers

The windings are insulated with solid materials and cooled by air, with no liquid in the system. The two common forms are cast resin transformers (CRT) and vacuum pressure impregnated (VPI) transformers.

• Cooling methods: AN (air natural) and AF (air forced, using fans).
• Strengths: Low fire risk, no oil leaks, minimal maintenance, safe for indoor and populated spaces.
• Limitations: Higher upfront cost, generally limited to medium voltages and moderate ratings, more sensitive to dust and moisture when installed outdoors.
• Typical locations: Hospitals, metro stations, airports, high-rise buildings, malls, data centres, and industrial plants with strict fire codes.

A simple rule of thumb: For grid substations and outdoor distribution, oil-immersed transformers remain the practical standard. For indoor, urban, and safety-critical locations, dry-type transformers are usually the better choice, even at higher upfront cost.

Other Specialist Transformers in the Grid

Beyond the main types, several special-purpose transformers support modern networks:

• Earthing (grounding) transformers: Provide a neutral point for earthing on systems that don't have one, such as delta-connected networks.
• Converter transformers: Connect AC networks to HVDC converter stations and are built to handle harmonics and DC stresses.
• Phase-shifting transformers: Control the flow of power between parallel transmission paths.
• Inverter duty transformers: Designed for solar plants and battery energy storage system (BESS) installations, where power electronics create harmonics and frequent load changes.

Types of Transformers at a Glance

How to Choose the Right Transformer for a Project

Picking a transformer is not just about voltage. Project teams typically weigh:

• Voltage ratio and rating: Based on load studies and future growth, not just current demand.
• Location and safety: Indoor, outdoor, urban, or remote, which drives the oil-immersed vs dry-type decision.
• Load profile: Steady bulk load suits power transformers; fluctuating local load needs distribution designs with low no-load loss.
• Efficiency and lifecycle cost: Lower losses cost more upfront but save energy for 25 years or more.
• Protection and metering needs: Correct CT and PT ratios and accuracy classes for relays and revenue meters.
• Lead time and logistics: EHV units can take close to two years to arrive, and heavy transport needs route planning.
• Grid integration: Renewable and BESS connections need transformers rated for harmonics and variable loading.

Getting these choices right at the engineering stage avoids costly failures, overloads, and redesigns later.

Building India's Transformer Backbone with SPML Infra

A transformer performs only as well as the substation and network built around it. SPML Infra Limited delivers end-to-end power transmission and distribution solutions across India. Its work spans AIS and GIS substations from 33 kV to 765 kV, transmission lines, HT/LT distribution networks, and smart metering with distribution transformer monitoring. Rural electrification projects, including agricultural feeders and household connections, were delivered under schemes such as DDUGJY and Saubhagya. Battery energy storage systems integrate with its substations to stabilise grids carrying more solar and wind power.

With 40+ years of execution across 20+ states, SPML handles design, procurement, construction, commissioning, and O&M under one EPC framework. Its recognitions include the SAUBHAGYA Award 2018 and the Construction World Global Award 2026 for Fastest Growing Construction Company.

Planning a new substation, a distribution upgrade, or a renewable and BESS connection? Request a project consultation with our power team.

Frequently Asked Questions

What are the main types of transformers used in power systems?

The main types of transformers used in power transmission and distribution are power transformers, distribution transformers, instrument transformers (CT and PT), and auto transformers. They are also classified by function (step-up or step-down) and by construction (oil-immersed or dry-type).

What is the difference between a step-up and step-down transformer?

A step-up transformer has more turns on its secondary winding and raises voltage, usually at power plants for long-distance transmission. A step-down transformer has fewer secondary turns and lowers voltage at substations and distribution points so power can be used safely by consumers.

What is the difference between a power transformer and a distribution transformer?

Power transformers work at high voltages and large ratings (tens to hundreds of MVA) at generating stations and transmission substations, running near full load most of the time. Distribution transformers work at 33 kV or 11 kV and lower ratings (usually in kVA), supplying homes and businesses with a load that varies through the day.

Why are CT and PT called instrument transformers?

Current transformers (CTs) and potential transformers (PTs) scale high currents and voltages down to standard, safe levels such as 1 A, 5 A, or 110 V. This lets meters, relays, and control systems measure and protect the network without direct connection to high-voltage lines.

Why are auto transformers used at 400/220 kV substations?

When the input and output voltages are close, an auto transformer needs less copper and core material than a two-winding transformer of the same rating. This makes it smaller, lighter, cheaper, and more efficient, which is why it is widely used for interconnecting 765, 400, and 220 kV networks.

Which is better: oil-immersed or dry-type transformer?

Neither is better in every case. Oil-immersed transformers suit outdoor substations and high voltages because of lower cost, better overload capacity, and long life. Dry-type transformers suit indoor and safety-critical locations such as hospitals, metros, and high-rises because they carry very low fire risk and need little maintenance.

How long does a power transformer last?

A well-maintained oil-immersed power transformer commonly serves 25 to 40 years. Its actual life depends on loading, insulation condition, oil quality, and how well faults and overheating are managed through protection and condition monitoring.