With the advancement of low-loss technology, the upper capacity limit for oil-immersed self-cooled transformers is increasing. Transformers with a rated capacity of up to 40,000 kVA can adopt oil-immersed self-cooling. The advantages of this method include no need for auxiliary power for fans, no fan-generated noise, and the ability to mount radiators directly on the transformer tank or install them collectively nearby. Oil-immersed self-cooled transformers are easy to maintain and can operate continuously at rated capacity. If expandable panel-type radiators are used, the transformer may not require an oil conservator and can be designed as fully sealed, further reducing maintenance. This design is typically applicable to distribution transformers of 2,500 kVA and below.
Air-cooled radiators use fans to alter the temperature difference between the oil entering and leaving the radiator, thereby improving cooling efficiency, reducing the number of radiators needed, and minimizing the footprint. Transformers with a capacity of 8,000 kVA and above may opt for air cooling. However, this introduces fan noise and requires auxiliary power for the fans. When the fans are stopped, the transformer can operate in self-cooling mode, but its output capacity must be reduced to two-thirds of the rated capacity. For tubular radiators, two fans can be installed on each radiator. For panel-type radiators, high-capacity fans can be used for centralized air blowing, or one fan can serve multiple radiator groups.
Several issues should be noted for forced-oil cooling methods:
(1) If the oil pump and fans lose power, the transformer cannot operate, even under no-load conditions. Therefore, two independent power sources should be provided for the coolers.
(2) Submersible oil pumps must avoid stator-rotor rubbing. If rubbing occurs, metal particles entering the windings could cause breakdown accidents. The oil circuit design must prevent the submersible pump from generating negative pressure, as this could draw in air and compromise insulation strength.
(3) Forced-oil cooling results in lower oil surface temperature rise, so the oil surface temperature should not be used to estimate winding temperature rise. This is particularly true for forced-oil water cooling, where the oil surface temperature rise remains very low even when the winding temperature rise approaches the specified limit.
(4) For ultra-high voltage transformers using forced-oil cooling, measures should be taken to prevent oil flow discharge. The oil passage design in the windings should avoid oil turbulence, limit oil flow velocity, select oil with appropriate resistivity, ensure smooth insulation surfaces, and provide sufficient volume in the core for charge dissipation. This helps prevent oil flow electrification from escalating into oil flow discharge. When starting coolers, they should be activated sequentially until the required number is in operation.
(5) When selecting high-capacity coolers, ensure that oil flow does not short-circuit and that cooled oil can reach the windings.
(6) For water coolers, water quality must be considered. Impurities in the cooling water can clog the cooler and reduce heat dissipation efficiency. Water pressure must not exceed oil pressure.
(7) For forced-oil air-cooled transformers installed near partitions, the partitions should be at least 3 meters away from the coolers to avoid obstructing free air movement.
When selecting radiator or forced-oil air-cooling methods, note that if the oil pump is stopped, the transformer can operate at 80% of its rated capacity. If both the pump and fans are stopped, it can operate at 60% of its rated capacity, provided sufficient installation space is available.
