10kV S13/S20/S22-MRL 3D Wound Core Distribution Transformer

The choice between ONAN (Oil Natural Circulation and Air Natural Cooling) and ONAF (Oil Natural Circulation and Air Forced Cooling) cooling methods for Kete oil-immersed 3D wound core transformers is corely based on the matching between the transformer’s heat dissipation requirements and actual heat dissipation conditions. Specifically, it can be comprehensively judged from the following 5 key dimensions:

①Transformer Rated Capacity and Heat Generation from Losses

This is the most fundamental basis. The heat generated by a transformer mainly comes from copper losses (load losses) and iron losses (no-load losses). The larger the capacity, the higher the losses and the stronger the heat dissipation demand:

·Small-capacity transformers (low loss):

When the rated capacity is small (e.g., ≤500kVA for 10kV class, ≤1000kVA for 35kV class), the heat generated by losses is small. Heat can be dissipated through the natural convection of transformer oil (hot oil rises, cold oil falls) and the natural convection between the casing/radiator and air, without the need for additional cooling equipment. Therefore, ONAN is preferred.

·Medium and large-capacity transformers (high loss):

When the capacity exceeds the above range (e.g., ≥800kVA for 10kV class, ≥1600kVA for 35kV class), losses increase significantly, and the natural heat dissipation rate cannot match the heat generation. This may cause the oil temperature to exceed national standard limits (usually top oil temperature rise ≤55K or 60K). In such cases, fans must be used to force air flow and accelerate heat dissipation, so ONAF is chosen.

②Operating Load Characteristics

The actual load rate and operation mode of the transformer directly affect heat dissipation requirements:

·Low load rate or intermittent operation:

If the transformer operates at light load for a long time (load rate < 50%) or only runs during short-term peak periods (e.g., rural distribution transformers), even if its capacity is slightly larger, the actual heat generation is low. The natural heat dissipation capacity of ONAN can meet the demand, and there is no need to waste energy on fans. Thus, ONAN is selected.

·High load rate or continuous full-load operation:

For transformers in industrial parks, urban core distribution networks, and other equipment that operate at full load for a long time or have large load fluctuations (needing to cope with short-term overload), heat generation remains high. ONAN cannot meet the heat dissipation requirements, so ONAF’s forced air cooling via fans is necessary. Even during overload, fans can enhance heat dissipation capacity (usually supporting 10%-20% short-term overload). Hence, ONAF is chosen.

③Installation Environment and Heat Dissipation Conditions

Environmental factors directly affect the efficiency of natural heat dissipation, and cooling methods need to be adjusted according to scenarios:

·Well-ventilated and low-temperature environments:

If the transformer is installed in an open outdoor area, high-altitude regions (low air density but good ventilation), or cold areas, the efficiency of natural convection heat dissipation is high. Even if the capacity is close to the critical value, ONAN can be prioritized.

·Poorly ventilated and high-temperature environments:

If installed in enclosed distribution rooms, basements, tropical regions, or industrial environments with heavy dust/oil pollution, natural heat dissipation is hindered. Even with a slightly smaller capacity, oil temperature may rise excessively due to poor heat dissipation conditions. In such cases, upgrading to ONAF is necessary to compensate for environmental defects through forced air flow by fans.

④Cost and Maintenance Requirements

There are significant differences in initial costs and operation/maintenance costs between the two cooling methods:

·Advantages of ONAN:

It has no fans or control circuits, resulting in a simpler structure. The initial procurement cost is low (5%-10% lower than ONAF of the same capacity), and there is no need for motor maintenance, making long-term operation and maintenance costs almost zero. It is suitable for scenarios sensitive to cost and with limited maintenance resources (e.g., rural power grids, small user transformers).

·Disadvantages and necessity of ONAF:

It requires additional configuration of fans, thermostats, and power circuits, leading to higher initial costs. Fan motors need regular inspection (dust cleaning, bearing lubrication), involving certain maintenance workload and energy consumption (fan power is usually tens to hundreds of watts). However, for medium and large-capacity transformers or those with insufficient heat dissipation conditions, these costs must be borne to ensure safe operation. Thus, ONAF is a “necessary choice” rather than a “cost-priority choice.”