How do ONAN and ONAN/ONAF transformers compare?
Oil natural – air natural (ONAN) and oil natural – air forced (ONAF) variants belong to the most common oil-filled transformer cooling designs. This post compares both of them with regards to dimensions, losses, sound level, reliability or equipment price. Welcome to ONAN/ONAF transformer cooling.
Transformer cooling in general
Precondition of a reliable operation of any electric equipment is an effective cooling system. In liquid immersed transformers the liquid has two basic functions:
- Electric insulation
- Internal cooling
Thus, it is being called insulation liquid or (sometimes) also cooling liquid. With regards to cooling the purpose is to transport the heat from the place of generation (windings and core) to the outer surface.
ONAN transformers use the natural convection. The oil naturally circulates inside the transformer and is naturally cooled by the ambient air. Such method is called oil natural – air natural, in short ONAN. The effectiveness of the cooling depends on the ambient temperature and the cooling surface. The power density of the transformer normally increases with the rated power. When the transformer is rated for lower power, the surface of the main tank is sufficient to remove the heat. As the power rating and power density go up, the cooling surface needs to be increased. That is the purpose of cooling radiators. Each radiator is a stack of cooling fins mounted at the outside of the main tank. Radiators dramatically increase the total cooling surface.
In order to increase the cooling effect, the next step is to use ONAF cooling. Transformer oil still naturally circulates inside the transformer, but the radiators are blown by forced air by means of axial cooling fans. These fans are optimized for long lifetime and low noise.
The flow of the forced air can be either horizontal or vertical. Both variants are used as seen in Figure 2.
Principle of ONAN/ONAF cooling
ONAN/ONAF transformer has a set of fans which may be put into service as desired at high loading [1].
The cooling fans (ventilators) are traditionally of fix-speed type (on/off). These types of fans are often driven by asynchronous motors with external rotor (i.e. design with stator inside). However, variable speed cooling fans are used as well. They become popular due to their high efficiency and even lower noise at reduced speed. Variable speed fans are driven by EC motors. Cooling fans are controlled dependent on transformer temperature.
a) Control based on top oil temperature
b) Control based on top oil temperature and winding temperature
ONAN versus ONAN/ONAF cooling
In fact, the comparison is ONAN versus ONAN/ONAF. That is because almost every air-forced cooled liquid immersed transformer can be operated with natural cooling up to certain degree of loading (typically 60-70%, often subject to end user specification). Several real-world ONAN/ONAF designs are listed in Table 1. The last column shows the ONAN continuous rating in percentage of the rated ONAF capacity.
Table 1: ONAN/ONAF rating of converter transformers (real examples)
1. Dimensions
Transformers in medium and high-power range use radiators for cooling. The radiator is a type of oil to air heat exchanger. The required surface of radiators depends on the design temperature gradient (ΔT) or temperature of cooling medium respectively, amount of heat losses and the flow rate of cooing medium. Therefore, by using air forced cooling the surface of radiators can be reduced for given amount of heat losses.
2. Losses and loss capitalization
Forced cooling (ONAF) is more effective than natural cooling when it comes to evacuation of heat load.
a) Reduce the surface of the radiators (as mentioned above)
b) Design the transformer with higher losses (cost optimized design)
The optimization process shall find a suitable compromise between transformer cost and the losses along with their capitalization over the life cycle.
3. Sound level
As expected, ONAF transformer produces somewhat higher sound level emissions. However, it shall be noted that the transformer cooling fans are of low-speed design. Combining low-speed operation with aerodynamically optimized design of the blades the additional sound level is rather unsignificant. In addition, the cooling fans are switched off at lower loads.
4. Reliability
User may be concerned about the reliability of forced cooling. However, as already mentioned, the cooling fans have low speed design, require minimum maintenance and their bearing lifetime is very long. In the rare case of failure of a single fan the other cooling fans still provide considerable cooling capacity.
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5. Purchase price
ONAN/ONAF transformer is more cost effective than ONAN transformer. Exception is the lower power range where the benefits of forced cooling may be negligible making that option unattractive. On the other hand, in high-power range the cost benefit becomes more obvious.
Summary
ONAN and ONAF cooling are the most widespread cooling methods of liquid-immersed power transformers including those in variable frequency drive systems. ONAN cooling uses natural oil circulation and natural air convection. The solution is extremely simple and robust. There are no moving parts and thus minimum maintenance. ONAF cooling benefits from reinforced heat exchange in the radiators by blowing forced air through them. The cooling fans have been perfected over the decades with respect to aerodynamics, long life, silent operation and low power consumption. ONAN/ONAF transformer can inherently operate with reduced capacity even in case of a complete loss of forced air cooling. Moreover, very large power transformers most probably require forced cooling due to higher power density and less cooling surface per MVA (growth rules explained in [2]).
References
[1] IEC 60076-2 Power transformers – Part 2: Temperature rise for liquid-immersed transformers, ed. 3.0, Geneva, Switzerland, 2011
[2] Why do larger drive systems achieve better efficiency (online), https://mb-drive-services.com/why-do-larger-drives-achieve-better-efficiency/
