Dry Type Power Transformer Selection for High-Rise Buildings: A Complete Fire Safety & Sizing Guide

High-rise buildings, due to their complex structures, high population density, large electricity consumption, and high requirements for power supply reliability, have imposed strict requirements on the selection of core power distribution equipment. Dry type power transformer, with their core advantages such as oil-free, flame-retardant, and small size, have become the preferred equipment for power distribution systems in high-rise buildings. This article will provide a professional selection guide for engineering design and project procurement from two core dimensions: fire safety and capacity selection.

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Ⅰ. Fire Safety: Why Must High-Rise Buildings Use Dry-Type Transformers

High-rise buildings have the highest requirements for fire prevention among all application scenarios. According to Article 4.3.5 of the "Code for Electrical Design of Civil Buildings" (GB51348-2019), this is a **mandatory provision**: The transformers installed in civil buildings must be selected from dry-type transformers, gas-insulated transformers, or non-flammable liquid-insulated transformers.

The underlying logic of this mandatory regulation is clear safety considerations: The oil-immersed transformers contain a large amount of flammable insulating oil. In case of inter-turn short circuits or overload faults, it may generate gas and cause fires or even explosions. In contrast, dry-type transformers use air or epoxy resin pouring as the insulating medium, and their oil-free design fundamentally eliminates the risks of oil leakage and oil-related fires. In high-rise buildings with large crowds and difficult evacuation, choosing dry-type transformers is a fundamental guarantee for the safety of lives and property.

Selection and implementation key points:

- If installed in critical areas such as fire protection distribution rooms and core computer rooms, fire-resistant dry-type transformers should be given priority. Their windings, cores, and other components have undergone fire-resistant reinforcement treatment.

- For places with high fire requirements, the combustion performance grade of the transformer should reach F1.

- Insulation materials should have flame retardant or non-flammable properties. F-class (allowing the highest temperature of 155℃) and H-class (180℃) insulation materials are the conventional choices. Among them, H-class materials have a greater heat resistance margin and are particularly suitable for enclosed distribution rooms with poor ventilation.

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Ⅱ. Capacity Selection for Dry Type Power Transformer: How to Precisely Match the Load Requirements of High-Rise Buildings

The capacity selection of dry-type transformers directly affects the reliability of power supply and economic operation. It requires comprehensive consideration from both the load calculation and configuration strategy perspectives.

1.Load calculation and capacity determination

The first step in selecting the transformer capacity is to calculate the total calculated load of the building. The electricity load of high-rise buildings is complex, covering lighting, air conditioning, elevators, water supply and drainage, fire protection systems, intelligent security systems, etc. The capacity should be determined based on the calculated load, that is, the rated capacity of the transformer should be no less than the calculated load.

The distribution transformers for high-rise buildings are generally selected with the Dyn11 connection group. This is because it can effectively suppress the third harmonic and reduce the zero-sequence impedance, which offers significant advantages in improving the sensitivity of single-phase short-circuit protection.

Key points for equipment selection:

- It is recommended to reserve a 15%-20% load margin during the selection process to avoid prolonged overload operation during peak hours.

- The long-term working load rate of the transformer should not exceed 85% to ensure economic operation.

- When there are primary and secondary loads (such as fire protection systems, elevators, emergency lighting), the regulations require the installation of at least two transformers. When one of them fails, the remaining transformers should be able to meet the power needs of all primary and secondary loads.

2. Upper limit of single-unit capacity and redundancy configuration

The capacity of a transformer is not necessarily the larger the better. The regulations have clear limitations on the capacity of a single unit.

When the voltage on the low-voltage side is 0.4kV, the capacity of a single transformer should not exceed 1250kVA; and when there is only one transformer, it should also not exceed 1250kVA.

For super high-rise buildings or projects with particularly concentrated loads, it is recommended to adopt a separate operation mode of multiple transformers, such as two 800kVA or three 500kVA transformers. This can not only meet the total load demand but also achieve load distribution and power supply redundancy.

3. Rational Utilization of Overload Capacity

Dry-type transformers have a certain short-term overload capacity, but they need to be used scientifically. Under natural air cooling (AN) conditions, the transformer can operate continuously at the rated capacity for a long time; with an optional forced air cooling system (AF), the output capacity can be temporarily increased by 50%, but this is an emergency measure and should not be used as a long-term operation mode.

Selection and implementation key points:

- For places with significant differences in load during day and night (such as air conditioning and lighting), the overload capacity of dry-type transformers can be appropriately utilized during the selection process. However, a complete temperature monitoring system must be equipped.

- When the transformer is operating under overload conditions, the winding temperature must be monitored in real time. The alarm temperature for F-class insulated transformers is 130°C, and the trip temperature is 150°C.

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Ⅲ. Model Selection Quick Reference Table

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Ⅳ. Conclusion

The selection of dry-type transformers for high-rise buildings focuses on balancing the three goals of **fire safety, power supply reliability, and energy conservation and economic efficiency**. The mandatory "dry-type" requirement in the regulations ensures the safety baseline, while the capacity selection is the key to ensuring long-term economic operation. In actual projects, it is recommended to combine the specific load characteristics of the building, the space conditions of the distribution room, and the requirements of the local power supply department to comprehensively determine the final solution.

Jiangsu Unita Electric Equipment Co.，Ltd. (JSZONMA) can supply you with suitable products for your project according to your requirements. If you need more information, please feel free to contact us.