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Transformer cooling – The impact of oil viscosity on the performance

by By: Dipl.Ing. Gerfrid Newesely, technical consultant for Nynas

The cooling behaviour of oil filled transformer is influenced by several factors including the type and volume of insulating oil, the surface and design of the radiators, the availability of oil pumps or air fans, the loading mode, etc.

When comparing various mineral oils for heat dissipation efficiency, it is important to evaluate oils of similar quality to have a fair comparison. Referring to the CSA-C50-97 standard for example, comparing a class A oil with a class B one will normally demonstrate that the class A provides better performance. When comparing oils meeting the same standards, many of the factors are constant figures (Heat transfer coefficient, Heat Capacity, Thermal conductivity). With these parameters fixed, one need to look at the other properties of the insulating oil that can differentiate their performance for heat transfer.

If the transformer design is fixed, and if it is a transformer with natural oil flow (thermosyphon circulation without pump) and natural air circulation without fans (by IEC terms this is called ONAN Oil Natural Air Natural [1]), then it is mainly the physical properties of the insulating oil that plays the most important role. This can be understood as a low viscosity oil will allow for a higher circulation speed of the oil that results in a better efficiency of the cooling system.

a) Flow speed: It is true that the flow speed in various part of a given transformer will not be easy to calculate but nevertheless, in a given design, it is safe to assume that the oil flow pattern will be similar and governed by the oil viscosity at a given temperature. This has been verified in many equipment and is accepted as basic design data. The flow speed can be evaluated by the formula for the "frictional resistance" [2] that is based on the laws of Bernoulli and Newton:


Where
p = pressure
n = kinematic viscosity
l,d = dimensions of the tube
g = specific weight
w = oil speed in the tube
g = gravity constant

When further developed this formula gives for v:



A part of the term is constant (at a certain temperature), therefore:



Or in words: The lower the viscosity, the higher the circulation speed of the oil which equates in a higher quantity of heat being dissipated.

b) Heat exchange factor: This has an important influence on cooling. The heat exchange to the oil happens on the surface between the winding and the oil. This factor as well is improved with lower oil viscosity. The Reynolds Number is a basic engineering parameter that is used in the evaluation of the flow profile of a liquid. The Reynolds (Re) indicates whether the fluid, in this case the insulating oil, has a laminar or a turbulent flow characteristic:

Reynolds number [2]:

Or in words:
High flow speed and low viscosity give a high Reynolds Number.
If this value is:
Re < 2300: flow is laminar
Re > 2300: flow is turbulent
Laminar flow means that the boundary layer between the winding and the oil is not disturbed and is thick. This boundary layer of oil insulates and impedes the heat transfer from the surface of the winding to the oil. In a turbulent flow situation, this layer is disturbed and this allows for other parts of the oil to contact the surface. Therefore turbulent flow gives better heat exchange factors.

High quality transformer oils are formulated to have low viscosity (with observation of all given security limits as for example Flash Point, see Specification, [3]).

In the Standards for insulating oils (IEC 60296 [4], ASTM 3487 and others) there is an upper limit for viscosity which is 12 mm2/sec at 40°C. High quality transformer oils are normally in the range of 7- 8 mm2/sec at 40°C and even with such low viscosity, they still meet the requirement for flash point (<135°C) from the same standard.
Benefits and value:
There is the question whether and in which way these better cooling properties of a low viscosity transformer oil could be utilised for the optimisation of design of the cooling system of a transformer.

a) This could be used for the refill of older units where the surface of the insulated windings is clogged by some sludge that is hindering the heat exchange on the heat exchange surface thus decreasing the cooling properties of the system. This lower heat exchange capacity can be compensated by oil with low viscosity that gives higher oil flow speed and therefore should result in better dissipation of the heat generated.
b) Another option would be to reduce the cooling surface of radiators (or reduction of the number of radiators) when such low viscosity oil is used. This allows for a reduced manufacturing cost of the transformer and can also allow for a more compact design.
Experience:
A Canadian transformer manufacturer was interested to reduce the number of radiator banks on a 1500 kVA ONAN transformer from 3 to 2 (reduction of cooling surface). The use of a high quality naphthenic oil was considered along with a more expensive CDP parafinic insulating oil (also known as synthetic isoparafin insulating fluid). Both oils meet the CSA-C50-97 standard for Class A oil.

To prove the oils had the required cooling properties, that transformer has been tested using the "Heat run test" following IEEE C 57.12.90-1999 [5], chapter 11 (Temperature rise) equivalent to IEC 60076 part 2 [1]. This test determines the average winding temperature rise of the transformer. If it rises more that 65 C above ambient temperature, overheating of the insulation takes place and that leads to an accelerated ageing of the insulation and, in excessive cases, could damage the transformer.

The comparative test within this Canadian transformer manufacturer was initiated as the supplier of the more expensive parafinic based insulating oil claimed superior heat exchange properties of his oil without proving that statement. But, considering all the above explanations, it becomes clear that this statement was not correct, as the physical properties of both oils were quite similar.
Test set and procedure:
A specific unit was selected for the testing. The transformer was tested first with the parafinic oil, drained and tested again with the naphthenic oil.

The transformer was first filled and put under vacuum. After vacuum treatment the oils were tested for water content and breakdown voltage (ASTM D 877, IEC 60157). After being prepared for the heat run, the transformer was loaded by simulating loading using the short circuit method for more than 24 hours with total losses (no load and load losses) to rise the temperature of windings and oil from start (ambient) temperature to maximum operation temperature at a defined load. During that procedure, all relevant temperatures have been measured and plotted:

• top oil temperature ( by a sensor in the transformer tank near to the oil surface)

• oil temperature radiator top and bottom

• ambient cooling air temperature ( as an average of 3 sensors each in a distance of approx. 1,2 m from the transformer at about half the height of the transformer, therefore approx. 1 m height)

The transformer was protected against air current to avoid disorder of the measurements.

Before starting to load the transformer, the cold resistance of the windings was determined. After that, following the Standard, the transformer was loaded (heated) with total losses until the unit’s temperature did not rise (vary) by more that 1 C during a consecutive period of 3 hours.
This was reached after 27 hours. Then, the load was reduced to rated current for 60 minutes and after that period, the load was switched off to determine the average winding temperature by the resistance method (hot resistance).

The average winding temperature is determined by the equation:

T = R/R0 (Tk + T0 ) – Tk
(Equation 26, chapter 11.3 IEEE C57.12.90-1999, [5])

Where:

T is the temperature (°C) corresponding to hot resistance R,
T0 is the temperature (°C) at which cold resistance R0 was measured
R0 is the cold resistance, measured according to Clause 5, (Ohm)
R is the hot resistance (Ohm)
Tk is 234,5 °C for copper (resp. 225,0 for aluminium)
Final conclusion:
- The result of the calculation was a similar value (below 65 C) with both oils and therefore the test was passed successfully.

- The result with the CDP parafinic insulating oil (synthetic isoparafin) was similar to the one obtained with the naphthenic oil.

- Considering the difference in the procurement of the oils as well as the value associated with the long experience (>40 years) of naphthenic oils in real life applications as opposed to the limited (<10 years) experience in actual application of the newer CDP parafinic oil tested, it was agreed that the high quality naphthenic oil was offering the most competitive alternative.

- On a transformer of the same type but with the full set of 3 radiator banks, a similar heat run test has been performed in the past. The test was passed successfully but with a smaller temperature raise.

- Using low viscosity naphthenic oil allows for more efficient cooling which permits transformers to :

1) Be less intensive on radiator usage
2) Be smaller in size
3) Be less demanding in the amount of oil required
4) Be less expensive to build
5) Operate at lower temperature

Lowering the operating temperature of transformers is of great benefit as it reduces the thermal ageing of the cellulose insulation, therefore extending the life of the transformer.
Literature:
[1] IEC 60076 Part 1 (2000), Part 2 (1993) Power transformers
[2] Dubbel, Taschenbuch für Maschinenbau, Ed.17, Berlin 1990
[3] Nynas, Specification for Insulating oil Nytro 10CX
[4] IEC 60296 Ed.3 (2003), Specification for mineral insulating oil for
transformers and switchgear
[5] IEEE C 57.12.90-1999, Power transformers



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