2015_Doble Paper

BASIC DESIGN PRACTICES for TRANSFORMER COOLING

Vasanth Vailoor & Kevin RileyTrantech Radiator Products Inc.

ABSTRACT

Transformer losses due to electro-magnetic conversions produce heat in the coils and the cores. In medium and large transformers, the heat needs to be dissipated quickly to prevent high temperatures due to thermal resistance. Elevated temperatures accelerate aging of insulation paper. The life of a transformer is measured using the degree of polymerization (DP) of the insulating paper and aging, or the remaining life of a transformer is measured as the inverse of DP. Temperature affects life and this is given by the Arrhenius equation that shows that the life of a transformer is approximately reduced by 50% with every 8ºC rise in temperature. High temperature also accelerates aging of insulating paper in the presence of moisture. Thus elevated temperature is a common enemy of transformers.

INTRODUCTION

Transformers dissipate heat by all three modes of heat transfer – conduction, convection and radiation. All modes of heat transfer occur simultaneously when temperature rises. Depending on the size of a transformer and the density of losses, conduction or convection could be the controlling factor. Radiation usually plays a minor role due to the operating range of temperatures encountered in transformers. Small transformers have a relatively large surface area per unit volume and the temperature rise in the materials used for construction is well within regulatory specifications and justifies considerations of market economy and risk of failure. However, larger transformers with higher electrical load density need additional external cooling systems. Liquid cooled transformers can be designed to meet the electrical loads and also limit the temperatures to safe operating conditions with passive or active cooling systems.

When losses occur in the core and coils of a transformer, the heat must be dissipated to the surroundings with minimal temperature rise. In a liquid filled transformer, the heat or the thermal energy is passed on to the fluid in the cooling system – this is the transformer oil, be it mineral oil, Silicone oils or natural oil esters. The oil in turn must transfer the heat to the ambient, either air or water. Heat transfer occurs either by natural convection due to density changes with temperature or may be enhanced with active components like fans and pumps. Natural convection is the choice of design when reliability is critical and radiators are installed. However, when space around a transformer is constrained, fans and pumps may be used to develop a compact cooling system using radiators or coolers. Coolers typically need less real estate, cost less initially and take up less foot-print space due to their secondary cooling surface that enhances the heat transfer capability but need more attention in the form of regular maintenance. In all such liquid filled transformers with radiators or coolers, convection is the primary mode of heat transfer.

BACKGROUND

Convective heat transfer is governed by Newton’s Law of cooling – in equation form,

Q = U * A * T - - - - - - - - - - (1)

Where Q is the quantity of heat transferredU is the overall heat transfer coefficientA is the heat transfer area andT is the effective temperature difference between the hot and the cold fluid

‘A’, the heat transfer area can be easily measured. The effective temperature difference ‘T’, is the log-mean-temperature (LMTD) given by,

T = ((TTop,Oil – THot,Air) – (TBottom,Oil – TAmbient))/(ln((TTop,Oil – THot,Air)/(TBottom,Oil – TAmbient))) - (2)

© 2015 Doble Engineering Company – Life of a TransformerTM SeminarAll Rights Reserved

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Due to the practical limitations of measuring these parameters accurately, the effective temperature difference is approximated with the maximum temperature, given by,

T ≈ TMax = TTop,Oil – TAmbient - - - - - - - - (3)

This is a reasonably good approximation for transformer cooling systems and is justified due to the relatively small temperature differences involved in this application.

The overall heat transfer coefficient ‘U’ is a characteristic of the heat transfer equipment dependent on heat exchanger geometry and flow characteristics of the fluids exchanging heat. This is a very complex parameter to calculate and so is usually determined by experimentation in transformer applications. Most radiator manufacturers experimentally determine a standard ‘U’ for a normal operating condition and use empirical correction factors for other operating conditions. In addition, a standard ‘U’ must be established for each radiator geometry.

Cooling systems are usually classified as radiators or coolers. Coolers have a high ratio of secondary cooling surface or fins to primary cooling surface compared to radiators.

It is also to be noted that the life of transformer, determined by insulation aging is most critical at the hot spot temperature. However, this parameter is relatively difficult to measure due to lack of access and sometimes, even to determine its location. So the hot spot temperature is estimated and modeled with great effort. But the top oil temperature is a good reliable indicator of the hot spot temperature depending on transformer oil channel design, oil flow velocity and oil characteristics like viscosity and heat capacity.

The choice of active and passive cooling systems is abundant. Active systems with fans and pumps are smaller in size but need additional power for operations and are favored when space around the transformer is at a premium. Passive systems, on the other hand, need more space but need no external power and maintenance needs are considerably less – and it comes with an upfront price tag. Due to a high fin density or secondary cooling surface, coolers are more compact in size and initially less expensive. Depending on the operating environment, over time, the fins can accumulate dust and debris that reduces the heat transfer coefficient due to thermal resistance or fouling and need careful maintenance to maintain its thermal performance.

Another significant difference between active and passive coolers is their temperature profile at part load operations as shown in Figure 1.

Figure 1


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DESIGN APPROACH

Due to the inherent nature of manufacturability, the design approach to the choice of radiators and coolers are different.

Since the oil must flow through the transformer and the radiator, it is preferable to minimize the number of connectors and potential sources of leaks in the joints. So the radiator is made to fit the available dimension provided on a transformer. However when an oil pump is used to enhance thermal dissipation by increasing oil flow in a radiator or a cooler, it would be preferred to have vibration isolators between the transformer and the cooling system. Due to the additional connectors available, the length of a radiator or the dimensions of a cooler is less constraining and gives the designer the opportunity to optimize cost by selecting standard sizes.

In passively cooled radiator applications, with natural convection on the air and oil side, the radiator is made to fit the I/O (inlet and outlet) ports available on the transformer. Thus once the number of plates is designed and manufactured, the heat dissipation capacity is influenced by only one variable parameter - the temperature gradient between the two fluids - air and the transformer cooling oil.

In actively cooled systems, air flow and/or oil flow rates may be changed independently. Thus such actively cooled systems are rated at manufacturer’s standard operating condition, which may be different from the user’s standard or recommended operating conditions. This could potentially cause the same cooling system to be rated differently by the manufacturer and the user, unless other independent operating conditions are also specified.

Additional modifications are necessary when different transformer fluids are used due to differences in viscosity, other thermal characteristics and permissible operating conditions that may change in the near future.

HARDWARE CONSIDERATIONS

There are many types of hardware considerations when calculating needed heat dissipation in various transformers. The most common approach is the simplest approach which is through natural convectional flow or an ONAN method. This application can be uprated or enhanced by adding fans in which case the method changes to an ONAF configuration. In taking away the heat from the radiator plate surface by means of forced air this will increase heat dissipation. As we move into the next category of cooling methods we must employ pumps in order to accelerate oil circulation therefore increasing heat dissipation.

The OFAF w/radiators method is the most flexible system as it can provide a triple rating. By this system ratings of an ONAN, ONAF and OFAF can be provided based upon transformer demand. OFAF w/cooler systems provides a means of significant cooling when space is limited or directed flow rates within the core winding design require constant flow and efficient cooling.

OFWF systems utilize water flow to cool the transformer oil. As it is known that the ambient temperature of water is less than atmospheric air in the same weather this is the most efficient form of cooling but with generally shorter life spans and higher maintenance.

Any system coded as an OD application regardless of whether the oil is of the thermosiphon or pumped movement is a system that is designed to circulate oil along pre-determined paths with the transformer’s main winding.

When considering any of these systems for new installation or replacement it is necessary to understand the historical and current requirements of your transformer in order to meet desired cooling efficiencies. What worked as original factory specifications for and older model transformer may not meet the current


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needs or loads for a replacement or retrofit application. Environmental and footprint conditions play the biggest factor in how new transformer cooling systems should be approached when cooling efficiency needs are established. In order to help identify these various cooling system types listed on your transformer a listing of code letters has been provided in Table 1.

Table 1

Internal to Transformer

First Letter(Cooling medium)

O Transformer Oil Application

Second Letter(Cooling

mechanism)

N Natural convection through cooling equipment and windings

FForced circulation through cooling equipment, natural convection in windings

DForced circulation through cooling equipment, directed flow in man windings

External to Transformer

Third letter(Cooling medium)

A AirW Water

Fourth letter(Cooling medium)

N Natural convectionF Forced circulation

CONCLUSION

In conclusion, there are many factors to consider when designing or replacing a transformer cooling system. Functionality, design constraints, footprint and future maintenance are just a few. Lowering the operating temperature of the transformer, thereby prolonging the functional life of the insulating paper, is critical. And finally – the cost-economic considerations of the entire transformer system is necessary to design the appropriate cooling system since optimization of a part of a system can rarely give an optimized solution of the whole.


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FIELD APPLICATIONS

Example -1Here is an application where a water cooled cooling system was replaced with a bank of radiators at this aging plant that was on the verge of shutting down due to environmental issues of oil leaks into the river. From the heat run test, an appropriate radiator cooling system was designed to replace the water coolers. The two GSUs had three single phase transformers each. Initially, one transformer cooling package was replaced and after passing the hot summer months, monitored results showed an average of 5.6ºC lower operating temperature than the other transformers (see the bold dark line in Figure 2).

All transformers were refitted with radiator cooling systems by the end of the year (see Figure 3a and 3b).

Before and After – Replacement Cooling System for 1948 GSUs

Figure 3a Figure 3b


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Example -2This is an application where a cooler was replaced with a bank of radiators at the Copper mines. Due to the severe dusty environment, the coolers were getting clogged too frequently and maintaining the cooling system was a nightmare. Again with the help of the heat run test report, appropriate size of a replacement radiator cooling system was designed (see Figure 4a and 4b).

Before and After – Replacement Cooling System for the Mining Project

Figure 4a Figure 4b

At this site, the oil temperature was monitored with high temperature alarms. Prior to installation of the radiator cooling system the high temperature alarm system would trigger frequently but after the replacement there have been no such calls. Note that the additional blowers were abandoned after the replacement.

Example -3Here is another example of a cooling system replacement (Figures 5a to 5d).

Site is identified at a Hydro Facility Proposal for Customer Approval

Figure 5a Figure 5b


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Testing of One Transformer Cooling System Replacement

Replacement on all Transformers Completed

Figure 5c Figure 5d

BIOGRAPHY

Vasanth Vailoor is a Mechanical Engineer from the Indian Institute of Technology with specialization in Heat Transfer and Fluid Dynamics. He has now been with Trantech Radiators as the Engineering Manager for over ten years. Before that he has been associated with industries as diverse as Aviation, Cryogenics and Renewable Energy focused on modeling, simulation and testing. His wide range of interests and depth of knowledge in various topics help bridge diverse technologies leading to simple optimized solutions. He has published over ten papers while still maintaining an active commercial career.

Kevin Riley is a Mechanical Engineer and certified Six Sigma Black Belt. He has been with Trantech Radiator Products for 6 years and is the Quality and Product Development Manager. Kevin has worked in the heat exchanger industry for over 10 years holding positions at Young Touchstone and Trantech within senior engineering and operations for products as diverse as Cuprobraze, Fin and Tube and Plate Radiator technologies. Kevin has also worked with Electric Utility and Generation customers for over 12 years in the Fleet, Genset and T&D sectors. He currently works on cooling systems development and components with OEM and utility customers for new and replacement applications covering all types of transformers and equipment. Before beginning his career in the private sector with Caterpillar, Kevin served as an officer in the United States Navy.


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2015_Doble Paper

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