02 Sep, 2026
I. Transformer Selection
① Classification by voltage level:
1000 kV, 750 kV, 500 kV, 330 kV, 220 kV, 110 kV, 66 kV, 35 kV, 20 kV, 10 kV, 6 kV, etc.
② Classification by insulation and heat-dissipation medium: dry-type transformers and oil-immersed transformers. Dry-type transformers can be further divided into SCB epoxy-resin cast dry-type transformers and SGB10 non-encapsulated Class-H insulated dry-type transformers.
③ Classification by core structure and material: laminated silicon-steel transformers, wound-core silicon-steel transformers, and amorphous-alloy-core transformers.
④ Classification by energy-saving design series: SJ, S7, S9, S11, S13, S15.
⑤ Classification by number of phases: single-phase transformers and three-phase transformers.
⑥ Classification by capacity: in China, transformer rated capacities are currently based on the R10 preferred-number series, i.e. multiples based on the tenth root of 10, such as 50 kVA, 80 kVA, 100 kVA, 125 kVA, 160 kVA, 200 kVA, 250 kVA, 315 kVA, 400 kVA, 500 kVA, 630 kVA, 800 kVA, 1000 kVA, 1250 kVA, 1600 kVA, 2000 kVA, 2500 kVA, 3150 kVA, 4000 kVA, 5000 kVA, etc.
Selection of Transformer Capacity
Standard transformer capacities include 200 kVA, 250 kVA, 315 kVA, 400 kVA, 500 kVA, 630 kVA, 800 kVA, 1000 kVA, etc.
A transformer should not normally operate under overload conditions; therefore, its capacity should be calculated according to the actual operating load.
Example: if the actual load is 230 kW, transformer operating efficiency is about 0.9, and the load power factor can reach 0.85 or above, the required transformer capacity is:
S = P / (COSφ × η) = 230 / (0.9 × 0.85) = 300.65, so a 315 kVA transformer may be selected.
The maximum permissible short-circuit current of a distribution transformer is 18–25 times the transformer rated current, and the duration must not exceed 0.25 seconds.
Whether the transformer should be installed in a high-voltage distribution room mainly depends on environmental factors, such as whether there is heavy external dust, corrosive substances or gases, or persistently high ambient temperatures. If none of these special conditions exists, installation on a transformer platform is also acceptable, provided adequate safety measures are implemented around the transformer.
For a three-phase power transformer rated 10/0.4 kV and 630 kVA, select the fuse-link currents for the high- and low-voltage sides.
For a 10/0.4 kV, 630 kVA three-phase power transformer, the rated currents are:
High-voltage rated current: Ie = Se / (1.732 × U1e) = 630 / (1.732 × 10) = 36.37 A;
Low-voltage rated current: Ie = Se / (1.732 × U2e) = 630 / (1.732 × 0.4) = 909.33 A;
The high-voltage-side fuse link is generally selected at 1.5 times the rated current: 36.37 × 1.5 = 54.6 A.
The low-voltage-side fuse link is generally selected at 1.5 times the rated current: 909.33 × 1.5 = 1365 A.
Generally, the reactive-power compensation capacity of a distribution transformer is about 20%–40% of transformer capacity. For a 630 kVA distribution transformer, the compensation capacity is approximately 120–240 kvar. Accurate calculation is relatively complicated, and because loads often change, the calculated value has limited practical significance. Designers commonly estimate at 30%, i.e. select 200 kvar as the maximum compensation capacity, which is also the installed capacity.
Low-voltage metering configuration for a 630 kVA transformer:
The transformer secondary rated current is Ie = S / (1.732 × Ue) = 630 / (1.732 × 0.4) = 909 A; a 1000:5 current transformer should be selected.
A transformer selection margin of 30% of the total capacity is recommended.
Transformer capacity is a unit of power (apparent power), expressed in VA or kVA. It is the product of the RMS AC voltage and RMS AC current, calculated as S = UI. The rated transformer capacity is indicated on its nameplate (as shown in the original figure).
Selecting a transformer requires determining the required capacity, normally according to the actual load of the electrical system. For a power-supply system, transformer capacity is selected according to the calculated load S. For a single transformer supplying temporary power (such as at a construction site) with a stable load, the load factor is generally taken as about 85%.
Example: if the calculated electrical load of a construction site is 86.06 kVA, the calculated transformer capacity is 100 kVA, so a 100 kVA transformer can be selected according to the standard capacity series.
It should also be noted that the capacity of a single transformer should preferably not exceed 1000 kVA. For larger loads, several transformers may be operated in parallel. Parallel operation requires the same transformation ratio, the same vector group, the same short-circuit voltage, etc. Load sharing must also be considered; generally, the ratio of the largest unit capacity to the smallest should not exceed 3:1.
II. Selection Between Dry-Type and Oil-Immersed Power Transformers
1. Insulation and Heat Dissipation
The insulation medium of a dry-type transformer is air or a solid material such as epoxy resin, and heat is dissipated by natural ventilation or fans.
Its advantages are cleanliness and the absence of oil-leakage risk. It is suitable for locations with high fire- and explosion-prevention requirements, such as shopping malls, hospitals, and metro stations.
Its disadvantage is relatively low heat-dissipation efficiency. In high-temperature environments, forced-air cooling may be required.
An oil-immersed transformer uses insulating oil around the windings; the oil serves both as insulation and as a heat-transfer medium.
Oil has a high heat-storage capacity and can withstand long-term high-load operation. Therefore, oil-immersed transformers dominate large-capacity and high-voltage applications such as ultra-high-voltage transmission and power plants.
However, oil leakage or faults may cause fire or environmental pollution, so an emergency oil-collection pit is required. Such transformers are generally installed outdoors.
2. Safety and Environmental Protection
Because dry-type transformers contain no oil, they inherently offer advantages in fire safety and pollution prevention, making them particularly suitable for densely populated areas or locations with strict environmental requirements.
For example, a data center that had previously experienced downtime due to leakage from an oil-immersed transformer eliminated this category of risk after switching to dry-type transformers.
Although oil-immersed transformers have an oil-leakage risk, insulating oil effectively protects the windings and provides stronger resistance to short circuits and overloads.
For example, under extreme weather conditions, an oil-immersed transformer may continue stable operation during short-term overload, whereas a dry-type transformer may require emergency derating.
3. Typical Applications
Three reasons to choose a dry-type transformer:
• Limited space: compact size and light weight, suitable for basements, rooftops, and other confined areas.
• Safety first: preferred for fire-restricted areas such as hospitals and laboratories.
• Low maintenance: no oil replacement or oil testing is required, reducing long-term operating cost.
Three typical applications for oil-immersed transformers:
• Outdoor, high-capacity projects: voltage levels above 110 kV and capacities above 10 MVA, such as substations and industrial parks.
• High-temperature, high-load operation: strong heat dissipation, suitable for steel mills, chemical plants, and other 24-hour high-load applications.
• Limited budget: procurement cost is about 30%–50% lower than that of dry-type transformers, suitable where initial investment is the main consideration.
4. Maintenance and Cost
Although dry-type transformers have a higher purchase price (about 30%–50% more than oil-immersed units of the same capacity), maintenance is simple—routine work mainly consists of dust removal—and service life can reach 20–30 years.
Oil-immersed transformers require periodic oil-quality testing and filter replacement, resulting in higher maintenance costs. However, their service life can be longer (20–40 years), and their overload capability is excellent, making them suitable for demanding applications.
III. Selection of Copper and Aluminum Transformer Windings
Transformer windings are the core conductive components and have traditionally been made mainly of copper. Since the 1960s, however, rising copper prices and lower aluminum prices have led to the widespread use of aluminum-wound transformers as a lower-cost alternative.
1. Performance Comparison
[Performance-comparison content in the original document is presented as a figure/table and contains no extractable text.]
2. Selection of Copper and Aluminum Windings
Choose a copper-wound transformer:
• High-load, long-duration operating conditions.
• Applications with high requirements for service life and stability.
• Space-constrained applications requiring compact equipment.
Choose an aluminum-wound transformer:
• Short-term projects with limited budgets.
• Low-load, intermittent-use applications.
• Applications sensitive to equipment weight.
