How to Specify a Special Transformer: RFQ Checklist

A special transformer specification is a technical document, and it works when it defines the problem and leaves the solution open. Specifying the problem is the hard part. Specifying the solution as well tends to make the unit more expensive or, occasionally, impossible to build.

Start From the Application, Not the Product

Before any parameters, write down what the transformer has to do. What the load is, how it behaves, where the unit will sit, and what happens if it goes out of service.

What the transformer supplies, and whether that process tolerates a voltage dip.
The load type: motors, rectifiers, furnaces, drives, heating, or mixed.
The operating environment, including anything that will be there for twenty years.
The installation constraints that cannot be engineered away.
Any regulatory, classification, or customer-standard requirement that applies.

Engineers who have built the same type of unit before will spot requirements that a specification written from a template misses. An early conversation shortens the process rather than lengthening it.

Electrical Specification

This section is the core of the document, and the parameters below are the ones that must be either stated or explicitly left to the manufacturer.

Rated power in kVA, with the basis on which it was calculated.
Primary and secondary voltage, with tolerances. Nominal alone is not enough.
Frequency and phase.
Vector group, or the constraint that determines it.
Short circuit impedance, as a percentage or as a limit.
Voltage regulation acceptable across the load range.
No-load and load loss, if the project is working to an efficiency target.
Insulation class and temperature rise limit.
Dielectric test levels and any partial discharge limit.
Tap range, number of steps, and whether tap changing is off-circuit or on-load.
Rated current, which matters most on the secondary of a high current design.

Impedance is the parameter most often specified without realising what it does. It limits fault current, which reduces duty on the switchgear downstream, and it also increases voltage drop under load. Both effects are real and they pull in opposite directions, so the value is a compromise to be set against the system fault level rather than a maximum to be maximised.

Mechanical and Environmental Specification

Overall dimensions and maximum weight, including the access route to the installation point.
Mounting arrangement and any seismic or vibration requirement.
Enclosure type and IP rating.
Cooling method, and the cooling medium actually available on site.
Terminal arrangement and cable entry direction.
Ambient temperature range, altitude, humidity, dust, and corrosive exposure.
Noise limit, if the unit is near occupied space.

Two items on that list cause more rework than the rest combined. Weight, because a transformer that cannot be lifted into position is of no use however well it is designed. And cooling medium, because specifying forced water cooling on a site with no water supply is not a design problem, it is a site survey problem.

Standards, Testing, and Documentation

State which standard the design and testing are to follow: IEC 60076 and GB 1094 are the common alternatives, and some projects work to IEEE or to a customer specification. Where more than one applies, say which takes precedence.

Then set out the test programme.

Routine tests, which run on every unit.
Type tests, which demonstrate the design meets the standard and are usually evidenced by a previous test report on the same design.
Special tests, agreed for the project, which may include a temperature rise test on the actual unit or a short circuit withstand test.
Witness testing, where the customer or their inspector attends.
The test report format and the documentation language.
Certification requirements, and who is responsible for obtaining them.

Where certification is required, whether CE marking, UL or CSA listing, or a classification society approval, it is worth deciding at the specification stage rather than after the design. Some approvals constrain the design itself, and discovering that late is expensive.

Keep the Specification Buildable

The temptation on a difficult project is to over-specify: fix the ratio, the impedance, the losses, the temperature rise, the dimensions, the weight, and the cooling method all at once. Those parameters are linked. A specification that pins all of them leaves no room for the design to satisfy any of them.

The more useful approach is to mark each parameter as either a requirement or a preference. Requirements are fixed; preferences are where the design has room to move, and knowing which is which lets the manufacturer propose something that meets the intent rather than fighting the wording.

What to Send Us

A specification document, a datasheet, a drawing, or a photograph of the nameplate on the unit being replaced. Anything in that list gives us a starting point, and we will come back with what still needs confirming rather than quoting around the gaps. See the custom transformer range or contact us.

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