September 29, 2026
Buying a high-power low-voltage transformer?
Pay attention to design details that can save you up to PLN 360,000* over its service life.
*Amount calculated for a 1600 kVA transformer.
What are high-power low-voltage transformers and where are they used?
There is no clearly defined threshold for what constitutes "high power." In practice, transformers rated above 400 kVA can generally be classified as high-power units.
A low-voltage transformer is one in which none of the voltages exceeds 1140 V. Voltage configurations are often non-standard and specific to the particular application:
- 800V/400V - transformers used in renewable energy installations, coupling battery energy storage systems (BESS), industrial power networks and photovoltaic farms.
- 690/440V, 580/400V, 400/380V and others - used in marine electrical installations and energy storage systems to provide the required voltage on the DC bus of a frequency converter connected to the low-voltage grid.
- 690/400V - industrial power networks, renewable energy installations and generator sets.
- 690V/2x690V -for supplying 12-pulse rectifiers.
Is there an upper power limit?
Yes. The limit is determined by the capabilities of the winding machines. At Elhand, the maximum rating is 5000 kVA.
How can you tell whether the transformer being offered is high quality?
Key parameters and design features to compare:
- Core no-load losses
- Winding load losses
- Core construction
- Winding construction
Why are no-load losses important?
No-load losses occur whenever the transformer is energized, regardless of the load. This is energy converted into heat - energy you cannot use but still pay for every day.
These losses depend, among other factors, on the material used for the core laminations. Unfortunately, many manufacturers use the cheapest materials available in order to minimize the transformer purchase price. An uninformed customer may buy a cheap transformer only to pay dearly for that decision in electricity costs from the very first year of operation.
Elhand tip: Ask your prospective supplier what grade of electrical steel is used to manufacture the core. Choose a supplier that offers a sound compromise between no-load losses and the transformer purchase price.
How can you identify the electrical steel grade?
Electrical steel can be grain-oriented (GO) or non-grain-oriented (NGO). GO electrical steel offers significantly better magnetic properties than NGO steel.
Another important parameter is the specific core loss of the electrical steel.
For example, M140 electrical steel has a specific loss of 1.4 W/kg, while M530 has a specific loss of 5.3 W/kg - approximately 380% higher.
Elhand tip: We use M140 grain-oriented electrical steel. Compare this with the materials offered by your suppliers.
What determines load losses?
The formula for load losses is: Pk = I² × R
Load losses depend on the square of the current flowing through the winding and on the winding resistance. Losses can therefore be reduced by lowering the winding resistance.
However, reducing winding resistance requires increasing the conductor cross-sectional area, which in turn increases the transformer price. Manufacturers may therefore increase winding resistance rather than reduce it. As a result, the customer buys a slightly cheaper transformer but soon pays more in electricity costs.
Do all manufacturers specify losses in the same way?
According to EN 60076, manufacturers are required to specify load losses at the reference temperature applicable to the relevant insulation class. This is because winding resistance - and consequently the losses - increases with operating temperature.
- Class F – reference temperature: 120°C
- Class H – reference temperature: 145°C
Unfortunately, not all manufacturers follow this requirement, and some specify losses at a temperature lower than the reference temperature corresponding to the insulation class.
A transformer with losses of 10 kW at a reference temperature of 145°C will have losses of only 6.6 kW at 20°C. It looks better in a quotation. In operation, it is the same machine.
Elhand tip: Check whether the quotation specifies the reference temperature for the stated losses. If it does not, ask the manufacturer to confirm it.
The difference between aluminium and copper windings
Copper is a significantly better electrical conductor than aluminium. The electrical conductivity of copper is approximately 60-65% higher than that of aluminium. Therefore, an aluminium winding requires a larger conductor cross-section to carry the same current as a copper winding.
The softening temperature of copper is approximately 350°C, compared with 200°C for aluminium. This is an important parameter in the design process.
Copper, however, has one major disadvantage - it is expensive. For this reason, aluminium strips and wires are commonly used in low-power low-voltage transformers.
A properly designed aluminium-wound transformer should have:
- A winding operating temperature below the temperature limit of the insulation class.
- Uniform winding temperature distribution with no hot spots in the windings.
- Adequate dynamic withstand capability against short-circuit forces.
- Winding temperature during a 2-second short circuit below the permissible limit: 350°C for copper and 200°C for aluminium.
According to EN 60076-5, the manufacturer is required to ensure short-circuit withstand capability for a duration of 2 seconds. During this period, the windings must not heat up above the permissible temperature. For aluminium windings, this limit is 200°C.
Unfortunately, not all manufacturers treat this parameter as important, which is why aluminium-wound transformers that are not properly designed may fail more frequently.
Elhand tip: Aluminium-wound transformers manufactured by Elhand meet the requirements of the EN 60076 series, ensuring a service life comparable to that of copper-wound transformers.
How is the core of a high-power low-voltage transformer constructed?
The simplest and cheapest solution - and at the same time the poorest in terms of magnetic properties - is a core made from laminations cut at 90° and assembled end-to-end. This is known as Non-step-lap Joint.
A slightly better, although still relatively poor, solution is a core made from 90°- cut laminations assembled with overlapping joints.
The best solution - but also the most difficult and time-consuming to manufacture - is step-lap technology, in which several laminations, typically 5-7, overlap at a 45° angle.
A further development of this technology involves introducing multiple steps so that the core limb cross-section more closely approximates a circular shape.
The difference between a step-lap core and a rectangular core
Think of a transformer core as a highway for magnetic flux. If you were driving a fast-moving truck and had to negotiate a sharp 90° bend, you would probably leave the road. Magnetic flux faces a similar problem when it has to pass from one lamination to another while crossing an air gap along the way.
Under normal conditions, magnetic flux flows throughout the entire volume of the core. When it reaches a joint, the magnetic field lines become concentrated, resulting in a local increase in magnetic flux density. In addition, part of the magnetic flux leaves the core, increasing no-load losses.
Magnetic flux path in a core made from laminations cut at 90°
With cores made from laminations cut at 45°, this problem does not occur. The change in direction of the magnetic field lines is smaller, making the bend that the magnetic flux has to negotiate much smoother. The truck stays on the road and negotiates the bend safely at high speed.
Magnetic flux path in a core made from laminations cut at 45°
In rectangular cores, magnetic flux density is distributed unevenly, resulting in localized core overheating. A 10% increase in magnetic flux density causes a 20% increase in no-load losses. This can also result in higher-order voltage harmonics. A hotter core also means a higher winding temperature and higher losses. Transformers with 90° core joints also generate more noise than transformers with 45° core joints.
Comparison: 1600 kVA dry-type transformer
| Parameter | Rectangular core (90°) | Elhand stepped core (45°) |
|---|---|---|
| No-load losses P0 | 2,8 kW | 2,1 kW |
| Load losses Pcu | 19,0 kW | 16,8 kW |
| No-load current Io | ~7,6% | ~0,3% |
| Hot-spot temperature | ~155°C | 135–140°C |
| Noise level | ~72 dB | ~65 dB |
| Expected service life | 20–25 years | 35–40 years |
Note on noise: The decibel scale is logarithmic. A difference of 7 dB means that the Elhand transformer emits more than four times less acoustic energy than a unit with a rectangular core.
Note on insulation life: According to the Arrhenius relationship, reducing the operating temperature of insulation by 10°C can potentially double its service life. The difference between 155°C and 135°C can translate into an additional 15-20 years of trouble-free operation.
Economic calculation
The difference in total losses between a conventional rectangular-core design and the Elhand stepped-core design is approximately 3 kW.
Assuming an 80% load and 6,000 operating hours per year, this corresponds to:18,000 kWh per year
Industrial applications
At an electricity price of PLN 0.80/kWh, the resulting savings are approximately PLN 14,400 per year. Over 25 years of operation, a single transformer can therefore save: PLN 360,000
Marine applications - theoretical estimate
On board a vessel, electrical energy is generated by generator sets.
Assuming a specific fuel consumption of approximately 200 g/kWh, the same additional 18,000 kWh per yearcorresponds to approximately 3.6 tonnes of additional fuel consumed per year.
Over 25 years of operation, a single transformer may therefore unnecessarily consume more than 90 tonnes of fuel - approximately 106,000 litres of MGO (Marine Gas Oil).
Summary
The design differences between a transformer with a rectangular core and one with a stepped core are rarely apparent in a standard quotation or datasheet.
Their real impact becomes apparent after several years of operation in the form of higher electricity costs, accelerated insulation ageing and failures.
When selecting a transformer, the question should therefore not be: "Which transformer is cheaper to buy?" Instead, ask: "Which transformer will generate lower losses and operate at a lower temperature over the next 25-30 years?"
Author: Elhand Engineering Team
