The correct design and operation of the cooling system in power transformers is very important to maintain the health of the transformer. The design and type of cooling are usually governed by the application of the power transformer and are influenced by power output, location, and environmental conditions. The cooling system thus in turn affects the design of the transformer.  The primary purpose of the cooling system is for the efficient removal or transfer of energy created by the heating effect of losses within the power transformer (components like windings, core, and structures), out into the environment.

The environmental cooling medium is usually air and/or water. The following are the different types of cooling configurations that may be used in oil-filled transformers [1, 2]:

ON	Oil Natural
ONAN	Oil Natural Air Natural
ONWF	Oil Natural Water Forced
OFAN	Oil Forced Air Natural
OFAF	Oil Forced Air Forced
OFWF	Oil Forced Water Forced
ODAN	Oil Directed Air Natural
ODAF	Oil Directed Air Forced
ODWF	Oil Directed Water Forced

Naturally oil-cooled method is usually used on smaller rated transformers and generally within 30 MVA, where the heat generated from the core and windings are large enough to allow for natural convection circulation of oil to cool the transformer.  The principle of natural convection arises when hot oil rises and the cold oil falls in an enclosed area resulting in the organic circulation of oil. The cooling surface area can be increased by providing cooling tubes or fins to improve the efficiency of cooling.

Transformers may also have forced air cooling using fans to allow for external air circulation. As the rating of transformers increases (usually greater than 60 MVA) the internal oil circulation becomes more of a requirement to allow for quicker transfer and removal of heat generated allowing the transformer to operate within the designed temperature rise. In this case, forced oil circulation is used by employing oil pumps in the oil flow circuit.

Water cooling is usually used in Hydro plant applications where the transformers are located underground. The water cooling forms an open-loop system with the oil circulation being a closed loop. To prevent contamination of the oil with water the coolers are usually designed with either double finned or having an intermediate closed loop low-pressure water circulation. Leak detection is usually provided. These configurations are presented in the diagrams below.

Failure Modes in the Cooling System

Cigre A2.49 [3] has highlighted the following Failure modes for the cooling systems. These are predominantly due to the wearing out of cooling system components. Other problems may be due to installation problems, especially after maintenance activities. When this happens the two basic functions of the cooling system i.e. oil circulation and heat exchange are affected.

The failure of cooling fans usually affects the load of the transformer. Usually, there are spare fans but if more than one fan is out of service there may be a need to reduce load to maintain the designed temperature rise of the transformer.  

Sometimes when maintenance work is carried out on the coolers it is possible for the fans being replaced to be connected in the wrong direction resulting in inefficient cooling due to recirculation of warm air causing elevated oil temperatures.

The failure of cooling pumps is another major problem, especially of forced oil systems. This affects the flow and circulation of oil in the transformer windings and core resulting in ineffective heat transfer to the external environment. Pump failure usually results in elevated oil temperatures. Again it can happen that after maintenance work pumps are connected in the wrong direction resulting in reduced heat transfer efficiency which affects the general cooling of the transformer.

Failure of the control circuit of the cooling system plays can affect the operation of the cooling systems where insufficient fans and pumps from the coolers are activated for the relevant loading. This will cause elevated oil temperatures.

In cooler radiators, a high level of particles and sludge formation may block cooling ducts, piping and flow paths. This affects the oil flow and reduces the cooling efficiency. Oil-water heat exchangers can also be blocked on the waterside due to deposits or corrosion causing decreased cooling efficiency.

Another common problem is when radiator valves are left in a closed position. This prevents oil from circulating within the radiators causing major heating within the transformer. The low viscosity of oil can also affect oil movement in the convection process, especially through the winding cooling ducts. The viscosity of the oil is affected by dissolved particles and oil aging by-products and is dependent on oil temperature.

Leaks are a significant problem in cooling systems usually at the interface points to the tank and ancillary components. These may be due to the effects of corrosion or aging of the insulation material like gaskets.

With the changing environmental conditions becoming more prevalent, elevated ambient temperatures may affect the heat transfer from the transformer to the environment. This is also a major problem with transformers located in enclosed areas (buildings) when the HVAC system fails. Also, very low temperatures (at zero degrees Celsius or below) can affect water cooling where the water can freeze.  

Sometimes, in the summer months, high inlet cooling water temperatures can affect the cooling capability of the coolers. This may occur if the transformer is not properly designed for the environmental conditions.

Inspections and Maintenance

It is very important to have an intense inspection and maintenance program to identify failures beforehand so that these can be proactively resolved without affecting the performance.

Infrared scanning is an important and simple tool that can be used to provide a relative difference in surface temperature enabling areas of elevated temperature to be easily identified. Infrared scanning is usually done when the transformer is on load. It is usually most effective when done on a transformer that has been returned to service where problems like a closed radiator valve are left closed.

Temperature monitoring is a standard monitoring that is provided on most transformers. The prime purpose is to measure the energy within the transformer and any abnormal conditions can be easily picked up especially with regards to the cooling system as it easily affects the average temperature of the transformer. The top oil temperature usually represents the inlet temperature of the coolers and when compared to the outlet temperature a differential can be established. This can then be compared between coolers.

Cigre Working Group A2.27 recommendations for Condition Monitoring Facilities recommended that the following temperatures should be available for condition monitoring [4]:

• Top oil – measure of the temperature of the oil at the top of the tank
• Bottom oil – measure of the temperature representing oil entering the bottom of the windings usually the cooler outlet temperature
• Cooler inlet oil – can be taken same as Top oil temperature
• Cooler outlet oil – measurement taken from the cooler outlet oil. In some transformer designs the bottom oil measurement can be used
• Cooling medium at inlet to coolers – a measurement representative of the temperature of the cooling medium (normally air or water) at the inlet to the coolers. In the case of an air temperature measurement, the sensor should be mounted in the shade. Air ambient temperature can be used if this measurement is not available. For the water cooling medium a sensor or thermometer pocket should be included at both the cooler inlet and outlet
• Ambient temperature – Monitor the ambient temperature if the transformer is located in an enclosed area (building). This temperature must be alarmed for immediate investigation when the alarm value is triggered.

The Cooler Performance Index: D Temp (Inlet – Outlet) can then be derived by finding the difference between the Inlet and Outlet temperatures. The difference (delta) can then be compared between coolers or designed values, if available. Temperature deltas more than 30% between coolers should be investigated further.

Oil flow is another important monitoring parameter. Forced oil (OF, OD) cooling systems have designed flow rates to provide the accepted temperature rise. Modern transformers usually have oil flow indicators or switches confirming oil flow. Analogue oil flow meters provide actual flow rates which can be trended. These can also be compared between coolers and any flow anomalies can be easily identified. Oil flow provided pumps can be compared with about 80% of oil pump nameplate or in pump manual (considering 20% oil flow reduction due to oil path hydraulic resistance).

Routine visual inspection of the transformer oil-cooling loop components should be performed as regularly as prudent but should not exceed 12-month frequency. All abnormal conditions must be rectified as soon as possible. This can be a non-expensive way of proactively picking up problems. Fans must be routinely energized to verify proper operation, especially standby fans.

Sludge building up, especially on older transformers can affect the flow of oil in the windings and piping. This must be monitored routinely by doing oil tests such as colour/appearance, acidity, dielectric dissipation factor (DDF), acidity, and interfacial tension (IFT), which can provide indications of sludge components before visible sludge occurs.

References

1.	IEC 60076-2	Power transformers – Part 2: Temperature rise for liquid-immersed transformers
2.	C57.12.00-2015	General Requirements for Liquid-Immersed Distribution, Power, and Regulating Transformers
3.	Cigre WG A2.49	Condition Assessment of Power Transformers
4.	CIGRE, Technical Brochure 343	Recommendations for Condition Monitoring and Condition Assessment Facilities for Transformers