Calculation of a dry-type transformer: exact method and practical examples 

Calculation of a dry-type transformer is not a formality, but a critical stage in the design of an electrical network. We saw how errors at the calculation stage led to overheating in the workshop of a foundry near Voronezh, to an emergency shutdown of a UPS in a data center near Moscow, and to a failure of the backup power system in a hospital in Kazan. In each case, there was one reason: the calculation was performed using a simplified method, ignoring real operating conditions - dust, altitude, load cycling, power factor.

What breaks the “standard” calculation

The standard calculation of a dry-type transformer according to GOST R 52736–2023 or IEC 60076–11 begins with the rated power. But in practice this is not enough. We are recording three system errors:

• Ignoring temperature conditions: at +40°C ambient, the permissible load is reduced by 12–18%, especially for transformers with temperature class F (155°C)
• Incorrect assessment of harmonics: modern power supplies (frequency converters, UPS, LED lighting) generate up to 15% of the 5th and 7th harmonic current. This causes additional losses in the windings and core - up to 25% above the nominal
• Neglect of the simultaneity factor: in real networks, 100% load on all outputs occurs less often than once every three years. Calculation without taking this into account leads to an overestimation of power by 30–40%

At Wenzhou Qiaonasen Electrical Equipment Co., Ltd., we use an adapted calculation method that includes correction factors for each of these components. It works on facilities from 25 kVA to 2500 kVA - including the QNS-DRY series forced air cooled, double insulated H/F transformers.

Practical calculation: step by step

Let's take an example from a real project - replacing an oil transformer with a dry one in an office building in Yekaterinburg. Initial data:

• Maximum rated load: 630 kVA
• Average daily load: 42%
• Power factor: 0.88 (after installing capacitor banks)
• Harmonic level: THD(I) = 9.3% (Fluke 435 analyzer)
• Altitude: 260 m
• Ambient temperature: +35°C (maximum in summer)

Step 1. Determine the base power: 630 kVA × 1.15 (reserve for future development) = 725 kVA.
Step 2. Apply harmonic correction: at THD(I) = 9.3% coefficient K_h= 1.12 → 725 × 1.12 = 812 kVA.
Step 3. Consider the temperature: at +35°C coefficient K_t= 0.96 → 812 × 0.96 = 779 kVA.
Step 4. We check the simultaneity coefficient: for office load it is 0.78 → 779 × 0.78 = 608 kVA.

Bottom line: instead of 1000 kVA, you can use a 630 kVA class F transformer. This reduced capital costs by 22%, reduced cabinet dimensions by 35% and made it possible to place the equipment in an existing room without reconstruction. Calculating a dry transformer here is not mathematics, but an engineering solution with an economic consequence.

Where people make mistakes most often - and how to avoid them

Some people think: “If the power matches, then it’s suitable.” But this is only true under ideal conditions. In practice we face other limitations:

• Dimensions and weight:A 1250 kVA free-cooling transformer can weigh 2100 kg and require a reinforced floor. When space is limited, we choose models with fans - they are 18% more compact with the same power
• Noise:Level L Dry Type Transformers_w> 65 dB is not installed in administrative buildings. We check the noise level at 100% load - not at idle, as many do
• Moisture protection:IP00 is the indoor standard. But if the transformer is mounted in a basement or outdoors under a canopy, a minimum of IP23 is required. We always specify the protection class in the technical specifications

One of our clients ordered 800 kVA for the production of food additives. Three months later I discovered corrosion on the terminals. The reason was the lack of coating of the windings with epoxy resin with increased resistance to aggressive vapors. After replacing it with the QNS-DRY-S model with IP54 protection, the problem disappeared.

Conclusion: calculation of a dry transformer is a dialogue with reality

Correct calculation of a dry-type transformer does not guarantee success in itself. It creates the basis for reliability, safety and economic efficiency. It avoids overpaying for excess capacity, prevents premature wear of equipment and reduces the risk of downtime. At Wenzhou Qiaonasen Electrical Equipment LLC, we don’t just select a model from a catalog. We analyze the load schedule, measure the spectrum of harmonics, take into account the climatic features of the region and even check the condition of ventilation in the room. Because a transformer is not a “box with wires”, but a life support element of the entire electrical network. Calculating a dry-type transformer is the first decision that determines whether the system will last ten years or require replacement in two. That is why we do it not according to a template, but according to the fact.