A custom transformer is designed backwards from the duty it has to survive. Rating and voltage ratio set the starting point, but the things that actually shape the design are the load profile, the ambient it sits in, the fault level it has to ride through, and the space it has to fit into.
Requirements Gathering and Analysis
This stage decides the quality of everything that follows. The inputs fall into four groups.
- Electrical. Rated power, primary and secondary voltage with tolerances, frequency, phase, vector group, impedance, and the loss figures the project is working to.
- Duty. Continuous, intermittent, or cyclic; the overload magnitude and duration; the daily operating hours; and whether the load is linear or produces harmonics.
- Mechanical. Overall dimensions and the maximum weight the floor or structure can take, the mounting arrangement, the enclosure or IP rating, and the terminal and cable entry arrangement.
- Environment. Ambient temperature range, altitude, humidity, salt or corrosive exposure, dust, vibration, and any noise limit.
Most of the corrections that appear late in a project trace back to one of these being assumed. Two questions are worth asking early: what happens on the worst day, and what has to be true for this unit to be installed at all.
Electromagnetic Design
The electromagnetic design fixes the core and the windings. Core dimensions follow from the voltage per turn and the flux density the steel can be run at, and the core grade chosen trades no-load loss against cost and size.
Winding design sets conductor cross-section against current density, then divides the winding into parallel paths where the current is high enough that skin and proximity effect would otherwise push the current to the outside of the conductor. Get the transposition wrong and one path carries more than its share, which shows up as a hot winding rather than as a wrong calculation.
Impedance is set at this stage too. It is not a free parameter: it controls fault current, voltage regulation, and the harmonic current the unit passes back to the supply, and changing it moves all three at once.
Thermal Design
Thermal design asks a simpler question than it looks: at the worst duty in the worst ambient, what is the hot spot temperature, and how long will the insulation survive it.
The calculation has to include every source of loss, which means the IĀ²R loss at fundamental frequency plus the eddy losses produced by any harmonic content, plus the stray losses in structural steel around the leads. Where the load is cyclic, the thermal time constant of the winding can absorb a short overload, but only if the overload’s magnitude and duration are known.
Cooling method follows from the heat load and from what the site can support: natural air where the rating allows it, forced air where a fan-cooled rating is acceptable, forced oil with air coolers for large oil-immersed units, and water cooling where air cannot remove the heat.
Mechanical Design
The mechanical design is driven by short circuit forces above everything else. Force between conductors scales with the square of the current, so the fault case, not the running case, sets how heavily the windings are braced and how the leads are supported.
- Windings braced and clamped against the calculated fault forces.
- Low voltage leads kept short and arranged symmetrically to limit the forces between them.
- Structural steel near the leads assessed for stray-loss heating, because a part not designed to carry current can still get hot.
- Transport and lifting loads checked separately, since a rigid cast unit and an open winding react differently to being moved.
Insulation Design and Test Levels
The insulation system is chosen against the system voltage and the impulse levels the supply can deliver. Solid insulation in dry type units is assigned a temperature class, and that class sets the limit the thermal design works against rather than a value the unit will actually run at.
Clearances and creepage distances are dimensioned for the impulse level, and on cast resin units the surface profile is part of the design because it determines how the field distributes across the casting and therefore how low partial discharge stays.
Prototyping and Validation
For a design that departs from anything built before, a prototype is often the cheaper route. It surfaces the problems that calculation does not, particularly thermal behaviour under real loading and mechanical response during a fault test.
Testing divides into routine tests performed on every unit, type tests performed on a design to demonstrate it meets the standard, and special tests agreed for a particular project. Which of these are required is normally settled at the quotation stage, because the test programme affects both price and schedule.
Where the Design Decisions Interact
Impedance affects harmonic current, harmonic current affects loss, loss affects cooling, cooling affects physical size, and size affects the fault forces the structure has to resist. Changing one moves the others, which is why a custom transformer is normally evaluated as a whole rather than item by item, and why a specification that fixes too many parameters at once can make a design impossible rather than demanding.
What to Send Us
- Electrical ratings and the vector group, or the constraint that sets them.
- Duty profile, including peaks and their duration.
- Ambient, altitude, and any corrosive or dusty atmosphere.
- Space, weight, and access constraints at the installation point.
- Applicable standard and the test programme required.
- Required delivery date and destination.
See the custom transformer range, or contact us with the requirement.
Related Products
Related Applications
- Transformers for Industrial Equipment
- Transformers for Marine
- Transformers for Offshore
- Transformers for Steel Industry
- Transformers for Electrochemical Industry
Request a Custom Quote
Need a custom transformer for your application? Send us your technical requirements and our engineering team will provide a custom solution within 24-48 hours.
How this is packaged on real projects
On most sites the unit described here is not installed on its own. It goes into an assembly that includes the incoming switching, protection and outgoing feeders, which you can see on the compact substations, transformer substation packages and industrial power distribution pages.
Frequently Asked Questions
Are there common mistakes to avoid with how a custom transformer is designed?
System voltage and frequency, fault level at the point of connection, the load with its duty, and the site conditions including ambient range and indoor or outdoor placement. With those settled the rest of the specification follows.
What makes a transformer custom rather than standard?
Any requirement that falls outside the standard product range. An unusual voltage ratio, a rating between standard steps, a non-standard impedance, a specific vector group, a mechanical envelope constrained by an existing room, or a combination of several of these.
How long does a custom design take?
It follows the drawing approval, not the other way round. The design is settled from your specification and issued for review; production starts once that drawing is approved. Projects that need to move quickly are usually the ones where the specification is complete at the start.
Which standard applies, and is it certified?
The applicable standard and clause set are agreed against your project specification before production starts. Where a project names IEC 60076 for the transformer, we work to those clauses and the routine test report ships with the unit.
What do you need from us to quote?
The single-line diagram or a description of the application, the voltages, the load with its duty, the site conditions, and any standard the project specifies. Send what you have – if something is missing, it can be worked out together rather than delaying the enquiry.