Why the answer is never the same twice
Search for the number of transformer types and you will be told two, three, four and seven, all confidently. None of them are wrong. They are classifying by different things. Two types if you are splitting by construction, core and shell. Three if by phase or by voltage role. Four if by application. The number is an artefact of the question, not a fact about transformers.
By what it does to voltage
Step up raises it, step down lowers it, and isolation leaves it alone while separating the windings. This is the classification most people mean, and on a real transformer it is a matter of which winding you energise rather than a different machine.
By where it sits in the system
Power transformers in substations, above about 5 MVA, moving transmission voltage to distribution voltage. Distribution transformers at the customer end, below about 2500 kVA, producing the building voltage. Instrument transformers, current and potential, which measure rather than supply. This is the classification that matters commercially, because it determines who builds it and what it costs.
By cooling and insulation
Liquid filled, where oil or ester both insulates and carries heat away, and dry type, cooled by air with solid insulation. This decides where a unit can be installed more than any other single property, and it is usually the first question a supplier asks.
By mounting
Pad mounted on a concrete pad at ground level for underground distribution. Pole mounted for overhead. Vault or submersible for below grade urban installations. Unit substation bolted directly to switchgear. Same electrical job, different packaging.
By construction
Core type, where windings surround the core, and shell type, where the core surrounds the windings. Five legged wound core is the standard for three phase pad mounted distribution units because it handles unbalanced and single phase loading properly. This is the classification manufacturers care about and buyers almost never need.
Common questions
How many types of transformers are there
There is no single correct number, because different sources classify by different properties. By voltage action there are three, step up, step down and isolation. By system role there are three, power, distribution and instrument. By cooling there are two, liquid filled and dry type. By mounting there are four or more. The useful question is which classification matters for what you are buying.
What are the three main types of electrical transformers
Most commonly power transformers in substations, distribution transformers serving customers, and instrument transformers for metering and protection. Some sources instead mean step up, step down and isolation, which classifies by what the unit does to voltage rather than where it sits.
What is the difference between a power transformer and a distribution transformer
Size, position and duty. Power transformers sit in substations, are usually above 5 MVA and run near full load. Distribution transformers sit at the customer end, are below about 2500 kVA and are energised continuously but loaded intermittently, so they are designed for low no load loss.
What type of transformer is a pad mounted transformer
A liquid filled distribution transformer in a pad mounted enclosure. Liquid filled describes the cooling and insulation, distribution describes its role in the system, and pad mounted describes how it is installed.
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