When talking about dry type transformers, most people are familiar with 10kV distribution models. However, the 35kV 1600kVA epoxy cast dry type transformer belongs to a higher-grade indoor high-voltage power distribution solution. It is commonly used as a main step-down transformer for customer substations, stepping down 35kV high voltage to 0.4kV or 10kV. It breaks the conventional mindset that 35kV systems mostly rely on oil-immersed transformers. For projects with strict fire safety requirements and transformers housed inside buildings, it has become a preferred option for more and more design institutes.
The mainstream model SCB13-1600/35 has a rated capacity of 1600kVA, with a typical voltage ratio of 35/0.4kV, 50Hz three-phase power supply and Dyn11 vector group. The rated high-side current is approximately 26.4A, while the low-side rated current reaches around 2309A, with short-circuit impedance normally set at 8%. The insulation requirement for 35kV grade is far stricter than that of 10kV dry type transformers. Windings are vacuum cast with epoxy resin, F/H insulation class, and partial discharge level is tightly controlled. This is the key reason why 35kV dry type transformers have much higher manufacturing thresholds. The iron core adopts high-permeability silicon steel sheets, and step-lamination technology reduces no-load loss. Cooling mode is AN natural air cooling, with optional AF forced air cooling to improve short-time overload capacity. It is equipped with temperature control system, cooling fans and metal protective enclosure, enabling real-time winding temperature monitoring, over-temperature alarm and trip protection.
In terms of application scenarios, the 1600kVA 35kV dry type transformer serves as the main step-down transformer on the consumer side. A single unit can supply core power for medium-sized factory main substations, large industrial parks, super high-rise urban complexes, underground substations, mines and rail transit stations. Traditional 35kV schemes mostly adopt oil-immersed transformers, which require separate oil pits, emergency oil storage tanks and fire partition walls, leading to high civil engineering costs. In contrast, 35kV dry type transformers contain no insulating oil, eliminating risks of oil leakage, fire and explosion. They can be installed directly in indoor power rooms and greatly simplify civil fire protection works. Nevertheless, it has clear limitations: much higher procurement cost than 35kV oil transformers of the same rating, larger footprint and higher noise level, so it is not suitable for all remote outdoor substations.
Based on years of project selection and on-site operation experience, I'd like to share my personal viewpoints.
First, never directly apply operation experience from 10kV dry type transformers to 35kV units. The insulation structure of 35kV high-voltage windings is more complex and more sensitive to dust and humidity. Although epoxy casting is flame-retardant, heavy dust and high humidity inside the power room will cause surface contamination and surface discharge during long-term operation. Once partial discharge occurs, repair difficulty and cost are far higher than those for 10kV dry type transformers. Many clients mistakenly believe oil-free dry type equipment allows lower inspection standards, which creates major risks. Apart from regular infrared temperature measurement, partial discharge detection and insulation resistance tests are recommended periodically for 35kV grade. Ventilation and dehumidification systems of the transformer room must operate reliably.
Second, rationally evaluate the full life-cycle cost between oil-immersed transformers and 35kV dry type transformers. Judging merely by equipment purchase price, 35kV dry type transformers are significantly more expensive than oil-immersed counterparts of the same capacity. Choosing 35kV dry type transformers only makes economic sense when the project requires indoor installation, dense occupancy, strict fire codes and no oil storage facilities. For outdoor independent substations with sufficient space and relaxed fire requirements, oil-immersed transformers remain the more economical and mature option. Many projects blindly adopt 35kV dry type transformers, yet the high equipment cost can hardly be recovered over time.
Third, the choice of winding material has amplified impacts on high-voltage 35kV equipment. There is a huge price gap between copper and aluminium windings for 1600kVA/35kV dry type transformers of identical specifications. My opinion is clear: for 35kV indoor main step-down transformers, pure copper foil windings are preferred. Aluminium windings feature higher resistivity, higher temperature rise under equal load, poorer thermal stability and weaker short-circuit withstand capability under high voltage. Once winding failure occurs on high-voltage equipment, power outage loss and maintenance cost are extremely high, far exceeding the price difference of the transformer itself. Aluminium winding transformers are only acceptable for secondary branch loads with tight budget, stable load and low failure loss, and should not be used as main step-down transformers.
Fourth, noise and vibration damping are easily overlooked in early-stage design. The 1600kVA 35kV dry type transformer generates greater electromagnetic vibration from the iron core, plus fan noise, resulting in higher overall noise than 10kV dry transformers of the same capacity. If the transformer room is adjacent to office buildings, dormitories or control rooms, vibration pads and damping bases must be installed at foundation stage, and wall sound insulation should be implemented. Noise retrofitting afterwards has limited room and brings high rework cost.
Fifth, load factor design needs to be conservative. 35kV dry type transformers come with high cost, so owners often tend to push the load to the maximum to reduce equipment quantity. However, for such high-voltage epoxy cast equipment, continuous operation above 85% load factor will significantly accelerate insulation aging. The recommended economic load range is 60%~75%. For critical loads with drastic fluctuation and zero downtime requirement, two transformers in standby mode are recommended instead of a single large-capacity unit operating in isolated mode.
To sum up, the 1600kVA 35kV dry type transformer is not a universal replacement for oil transformers. It is a special safety solution for indoor high-voltage main step-down applications. Its core value lies in eliminating fire hazards caused by oil leakage on high-voltage side, adapting to special scenarios such as urban underground substations and building-integrated substations. Selection should not rely merely on the selling point of "oil-free safety". Site conditions, fire codes, budget, outage loss and maintenance capacity need comprehensive evaluation. With proper winding material selection, good environmental control and periodic high-voltage special tests, this equipment can run stably for decades. If buyers only chase low prices and simplify supporting environment control for the transformer room, failure risks of high-voltage dry type transformers will rise sharply.
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