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4. THERMAL DESIGN
4.1 General
Heat is mainly produced in a transformer due to the passage of load current through the resistance of the winding conductors (load loss), and due to heat production in the magnetic core (no-load loss). Additional but less significant sources of heat include eddy current heating in conductors and support steel structures, and dielectric heating of insulating materials.
Transformer thermal design is aimed at removing the generated heat effectively and economically so as to avoid deterioration of any of the components of the transformer due to excessive temperature. In oil-immersed transformers the core and windings are placed in a tank filled with mineral oil. The oil acts as the primary cooling medium since it is in close contact with the heat-producing components. Dry type transformers, where the windings are resin cast, may be specified for particular low fire risk applications with ratings up to about 10 MVA.
4.2 Temperature Rise
Heat is produced directly in the windings from the I 2 R losses in the conductors. Insulation usually consists of paper tape wound around the conductor.
This gives the required insulation of the conductor from its neighboring turns. The paper tape is saturated with oil since the whole winding is immersed in the bulk of oil inside the transformer tank. This gives insulation from other windings, and from the earthed parts of the transformer structure.
Heat generated in the conductor must firstly be conducted through the paper tape insulation and then into the bulk of oil. From there the heat is conducted and convected away from the winding eventually to be dissipated into the air surrounding the transformer. In order to avoid damage to the insulation, the maximum service temperature must be limited. The basis for 'normal life expectancy' of oil-immersed transformers with oil-impregnated Class 105 (previously defined as Class A) paper insulation is that 'the temperature of the insulation on average shall not exceed 98 degrees C.' In practice, not all parts of a winding operate at the same temperature since some parts are cooled more effectively than others. The part of the winding which reaches the hottest temperature is known as the 'hot spot.' The hot spot location in the winding is not precisely known, although infrared imaging techniques may be used if a fault is suspected. Modern transformers incorporate fiber optic devices to couple the temperature transducers to the recording apparatus in order to obtain sufficient insulation. Direct hot spot thermal fiber optic probes are located at the calculated hot spot position.
Distributed thermal sensor fiber optic probes more accurately map the temperature image of the transformer but in practice they are more difficult to install. Conventional temperature probes are not suitable for direct attachment to conductors which may be at a high voltage above earth. Therefore the 'average temperature' of a complete winding is normally determined by measuring its change in resistance above a reference temperature. Research and development tests have established that the hot spot temperature is about 13 degrees C above the average winding temperature in typical naturally cooled transformers. Measurement of average winding temperature therefore allows the hot spot to be deduced, at least in an empirical way.
When a transformer is unloaded the conductor temperature is virtually the same as the ambient temperature of the air surrounding the transformer.
When load current is passed, the conductor temperature rises above ambient and eventually stabilizes at an elevated value (assuming the load current is constant). The total temperature of the hot spot is then given as:
Hot spot temperature=ambient temperature +average winding temperature rise +hot spot differential The basis of the IEC specification for thermal design, with the transformer at full load, is to assume an annual average temperature of 20 degrees C. On average, over a year therefore, the limit of 98 degrees C is achieved if:
98 degrees C >= 20 degrees C + average winding temperature rise +13 degrees C
Therefore the average winding temperature rise should be #65 degrees C and this forms the basis of the IEC specification for 65 K average winding temperature rise (70 K for cooling-type OD -- see SEC. 4.6).
There are also IEC requirements for the temperature rise of the insulating oil when the transformer is at full load. The specified rise of 60 degrees C ensures that the oil does not degrade in service and is compatible with allowing the average winding temperature to rise by 65 degrees C.
4.3 Loss of Life Expectancy with Temperature
Insulating materials are classified by a statement of the maximum tempera ture at which they can be operated and still be expected to give a satisfactory life span. Operation at a moderately elevated level above the maximum recommended temperature does not result in immediate insulation failure.
However, it will result in shortened life span. The law due to Arrhenius gives the estimated life span as:
Loss of life expectancy=A+B/T
A and B are empirical constants for a given material
T is the absolute temperature in K
For the particular characteristics of Class A transformer insulation, the Arrhenius law results in a halving of life expectancy for every 6 degrees C above the temperature for normal life. (Conversely, life expectancy is increased for a temperature reduction, but this effect can only be applied in a limited way since life spans beyond about 40 years would be influenced by factors other than temperature alone.) The Arrhenius effect allows for periods of operation with the insulation above its specified 'normal life' temperature provided these periods are balanced by periods of lower temperature where the life is above normal. This effect may be utilized in normal transformer specifications since, in operation, the hot spot temperature will fluctuate both with variations in ambient temperature and variations in loading level.
In the summer the ambient temperature may rise to 40 degrees C so that the hot spot at full load would rise to 40+65+135+18 degrees C. In the winter, however, at say 0 degrees C, the hot spot at full load would only reach 0+65+13=78 degrees C.
So long as the annual average temperature was not above 20 degrees C, the overall life expectancy would remain normal due to the additional life gained in the winter counterbalancing the increased loss of life in the summer.
The Arrhenius effect can also be applied to allow overloading of a transformer. Consider, for example, a 24 hour period in which the transformer is loaded to 75% of its rating for all but 2 hours when it is loaded to 120% of its rating. The period at 120% load has no overall detrimental effect on the life of the transformer since the increased loss of life in the 2 hours of over load is balanced by the slower-than-normal ageing at 75% load.
The overloading with no loss in life described above can be extended one step further to cover emergency conditions when a definite loss in life is tolerated in order to meet an abnormal, but critical, system operational requirement. Thus, a transformer could be operated at, for example, 200% normal load for, say, 2 hours with an additional loss in life of, say, 5 days in that 2 hours (assuming other parts of the transformer such as bushings or tap changer contacts, to which the effect does not apply, can also accept a 200% load for this time).
Refer to IEC 60354 for a more comprehensive guide to oil-filled transformer overload values and durations, and IEC 60905 for dry type transformers.
4.4 Ambient Temperature
Since ambient temperature has an important influence on transformer performance and internal temperature such environmental details must be included in the transformer enquiry specifications. The IEC reference ambient temperature is given in four components as follows:
Maximum: 40 degrees C
Maximum averaged over a 24 hour period: 30 degrees C
Annual average: 20 degrees C
Minimum: 225 degrees C
In some parts of the world the first two values may not be exceeded but the annual average is often above 20 degrees C. In Middle East desert areas the first three temperatures may all be exceeded by 10 degrees C.
If any of the IEC reference ambient temperatures are exceeded by the site conditions the permitted internal temperature rises are adjusted to restore the basic thermal equation for normal life expectancy. For example, if the annual average temperature was 25 degrees C instead of 20 degrees C, the permitted average winding rise is reduced to 60 degrees C to restore the 98 degrees C total hot spot tempera ture. Note that the correct annual average temperature to use when specifying transformers is a 'weighted' value given as follows:
Ta1 =20log10 + 1/N X [1—Σ--N x 10^Ta/20]
Where
Ta1 =weighted annual ambient temperature
Ta=monthly average temperature
N=month number
The weighted value is designed to take proper account of the Arrhenius law.
4.5 Solar Heating
The sun provides an additional source of heat into the transformer which must be taken into account in tropical climates. The additional temperature rise of the oil in the transformer will be small, typically 2 degrees Cor3 degrees C, for most transformers. It is only for small transformers, such as pole-mounted units, where the exposed surface area is large compared with the volume, that the effects become significant. In these circumstances, it may be necessary to subtract 5 degrees C or even 10 degrees C from the permitted winding temperature rise at full load in order to maintain normal life expectancy. Even for large transformers where the effect of solar heating on internal temperature rise is negligible, the manufacturer should be advised of exposure to tropical solar radiation since the operation of other components such as temperature gauges, electronic control modules, and gaskets, may be adversely affected.
FIG. 21 Cooling arrangements.
(a) Tank surface only (c) Separate cooler banks (b) Radiators on tank
FIG. 22 225/21 kV, 35/40 MVA, ONAN/ONAF transformer at Coquelles substation,
France.
4.6 Transformer Cooling Classifications
In the simplest cooling method, the heat conducted to the oil from the windings and core is transmitted to the surrounding air at the tank surface. In practice, only the smallest distribution transformers, for example 10 kVA pole mounted, have enough tank surface area to dissipate the internal heat effectively (see Fig. 21a). As the transformer size increases, the surface area for heat dissipation is deliberately increased by attaching radiators to the tank. A 1,000 kVA hermetically sealed transformer with radiators is shown in FIG. 21b. A 200 kVA pole-mounted transformer with radiator tubes is shown in Fig. .13 ( Section 17). As the transformer rating increases still further the number of radiators required becomes too large all to be attached to the tank and separate cooler banks are used as indicated in FIG. 21c.
In the cooling method described above, no moving parts are used. As the oil is warmed inside the tank, the warmer oil rises to the top of the tank and into the tops of the radiators. As the oil cools, it falls to the bottom of the radiator and then back into the bottom of the tank. This sequence then repeats itself, giving a 'natural' circulation of cooling oil.
Increased cooling efficiency is obtained by fitting fans to the radiators to blow cooling air across the radiator surfaces. FIG. 22 shows cooling fans on a 225/21 kV, 35/40 MVA, ONAN/ONAF transformer.
A further increase in efficiency is achieved by pumping the oil around the cooling circuit, thereby boosting the natural circulation. The oil is forced into closer contact with the winding conductors to improve the heat extraction rate. In practice, baffles and cooling ducts direct the oil into the heat producing areas.
The IEC cooling classification codes allow the desired type of cooling to be simply specified. The codes indicate the primary cooling medium, that is the medium extracting the heat from the windings and core, and the secondary cooling medium, that is the medium which removes the heat from the primary cooling medium. The type of cooling method (how it is circulated) can also be specified. The following codes are used:
Kind of cooling medium -- Code (letter)
- Mineral oil O
- Water W
- Air A
- Non-flammable oil L
- Kind of circulation
- Natural N
- Forced F
- Forced directed liquid D
The coding method is to specify, in order, the primary cooling medium, how it is circulated; the secondary cooling medium, how it is circulated.
For example, an oil-immersed transformer with natural oil circulation to radiators dissipating heat naturally to surrounding air is coded as ONAN.
Adding fans to the radiators changes this to ONAF, and so on.
Notice that a dry type transformer, with heat dissipation directly (but naturally) to the surrounding air uses only a two letter code, namely AN.
4.7 Selection of Cooling Classification
Choosing the most appropriate method of cooling for a particular application is a common problem in transformer specification. No clear rules can be given, but the following guidance for mineral oil-immersed transformers may help. The basic questions to consider are as follows:
1. Is capital cost a prime consideration?
2. Are maintenance procedures satisfactory?
3. Will the transformer be used on its own or in parallel with other units?
4. Is physical size critical?
4.7.1 ONAN
This type of cooling has no mechanical moving parts and therefore requires little, if any, maintenance. Many developing countries prefer this type because of reliability, but there is an increasing cost penalty as sizes increase.
4.7.2 ONAF
A transformer supplied with fans fitted to the radiators will have a rating, with fans in operation, of probably between 15% and 33% greater than with the fans not in operation. The transformer therefore has an effective dual rating under ONAN and ONAF conditions. The transformer might be specified as 20/25 MVA ONAN/ONAF. The increased output under ONAF conditions is reliably and cheaply obtained.
Applying an ONAN/ONAF transformer in a situation where the ONAF rating is required most of the time is undesirable since reliance is placed on fan operation. Where a 'firm' supply is derived from two transformers operating in parallel on a load-sharing basis the normal load is well inside the ONAN rating and the fans would only run in the rare event of one transformer being out of service. Such an application would exploit the cost saving of the ONAF design without placing too much emphasis on the reliable operation of the fans. Note that fans create noise and additional noise mitigating precautions may be needed in environmentally sensitive areas.
4.7.3 OFAF
Forcing the oil circulation and blowing air over the radiators will normally achieve a smaller, cheaper transformer than either ONAF or ONAN.
Generally speaking, the larger the rating required the greater the benefits.
However, the maintenance burden is increased owing to the oil pumps, motors and radiator fans required. Application in attended sites, with good maintenance procedures, is generally satisfactory. Generator transformers and power station inter-bus transformers will often use OFAF cooling.
4.7.4 ODAF/ODWF
These are specialized cooling categories where the oil is 'directed' by pumps into the closest proximity possible to the winding conductors. The external cooling medium can be air or water. Because of the design, operation of the oil pumps, cooling fans, or water pumps is crucial to the rating obtainable and such transformers may have rather poor naturally cooled (ONAN) ratings. Such directed and forced cooling results in a compact and economical design suitable for use in well-maintained environments.
4.8 Change of Cooling Classification in the Field
Transformers may be specified with future load requirements in mind such that the design may allow for the future addition of oil pump or air fan equipment. As loads increase in distribution systems and transformers become overloaded, a relatively cheap increase in rating can be obtained by converting ONAN transformers to ONAF by fitting radiator fans. The manufacturer should always be consulted with regard to fan types and number, and the actual rating increase for a particular transformer.
If considered in the initial design specification it may be practical to fit oil circulation pumps to obtain a higher OFAF rating at some future date as the load demand increases. The rating increase is dependent on the internal design of the cooling circuit. Fitting oil pumps to a transformer not having such cooling ducts only supplements the natural oil circulation past the bulk of the winding assembly, and has very little improvement in cooling efficiency.
4.9 Capitalization of Losses
Although transformers are very efficient machines increasing attention is paid to minimizing the cost of losses in electrical systems over the lifetime of the plant. A transformer manufacturer can build a lower loss transformer if required but this usually results in the use of more materials, or more expensive materials, with the end result of a higher initial purchase cost.
Even so, the total cost of buying and operating the transformer over a life of, say, 25 years can be less for an initially more expensive, but low loss, trans former. Refer to Section 22, Sub-Sec. 22.2 for an introduction to financial and economic assessments.
When a number of manufacturers have been asked to bid for a particular transformer contract, a choice can be made on the basis of total cost, that is the capital cost, plus the cost of supplying the losses over an anticipated life span. To assign a cost to the losses can be an elaborate procedure. Note that the basis for costing losses must be advised to the manufacturer at the time of inviting quotations in order that the manufacturer can optimize capital cost and the cost of losses to give a competitively priced transformer. In most cases, the consultant or electrical supply utility will simply specify separate capitalizing factors for the load and no-load losses and typical figures for UK transmission transformers are no-load loss capitalization rate ₤4,000/kW and load loss capitalization rate ₤650/kW.
The transformer manufacturer will then simply arrive at the capitalized price as:
Capitalized cost=selling cost+4,000 x no-load losses (kW) +650 x loadloss (kW)
It is sometimes useful for the supplier to provide alternative designs (e.g. a high loss and low loss design) to illustrate the variation in prime and capitalized costs as a function of transformer losses. The methods of capitalizing losses have been the subject of numerous studies. Very precise calculations are not considered to be justified, since the accuracy of the results may only be of the same order as the assumptions made regarding the evolution of the parameters.
In some special cases, the user may specify some capitalizing formulae to be applied and an example is detailed below. Tenders may be requested by the purchaser that guarantee the losses quoted by the manufacturers. No-load losses should be quoted at a given reference temperature and voltage. Load losses must be carefully quoted at a given rating and tap position. Auxiliary losses (fans, etc.) must be detailed at each level of cooling. For reactors only the total guaranteed losses are normally required. If dNET is the net difference in evaluated cost of losses (given here in UK £) between test and guaranteed losses then:
£NET=(NLt -NLg)EVALNL 1(LLt -LLg)EVALLL 1(ALt -ALg)EVALAL
if £NET . 0,
then £PEN=1.15 x £NET
where:
NLt =tested no-load losses (kW)
NLg =guaranteed no-load losses at time of tender enquiry (kW)
EVALNL =no-load loss evaluation factor (d/kW)
LLt =tested load losses (kW)
LLg =guaranteed load losses at time of tender enquiry (kW)
EVALLL =load loss evaluation factor (d/kW)
ALt =tested auxiliary losses (kW)
ALg =guaranteed auxiliary losses at time of tender enquiry (kW)
EVALAL =auxiliary loss evaluation factor (d/kW)
£PEN=price adjustment (in d) to be deducted from the base price if dNET . 0.
The intent of this approach is to balance the potential estimated savings against the known capital transformer cost, and also to compensate for the uncertainty of predicted parameters such as the cost of money, inflation of energy costs, future electric plant construction costs, predicted load and load growth rates and uncertainty as to the exact lifetime of the transformer.
5. CONSTRUCTIONAL ASPECTS
5.1 Cores
Cores are constructed from an iron and silicon alloy which is manufactured in a way to enhance its magnetic properties. Basic cold-rolled flat alloy sheets known as 'cold-rolled grain-oriented silicon steel' have been used since the 1960s. Since this time improved quality control producing better grain orientation and thinner sheets ('Hi-B steels') have reduced no-load losses by some 15% compared with conventional cold-rolled grain-oriented silicon steel types. The magnetic core is made up from several thin sheets, 0.3-0.23 mm thick, of the core metal. Each sheet has a thin coating of insulation so that there is no conduction path from sheet to sheet. This technique is used to minimize eddy currents in the core metal. (If the core were a solid block of metal, these eddy currents would produce excessive heating.) Surface laser-etched 0.23 mm steels are now also being used by leading transformer manufacturers. This results in a further 15% loss reduction and such treatment may be justified as a result of the electrical supply utility's loss capitalization formulae. It is important for the transmission and distribution systems engineer to specify the flux density in conjunction with the manufacturer before ordering transformers. If the flux density is too high the transformer may go into saturation at the most onerous tap setting. Typical values for modern cold-rolled grain-oriented silicon steel transformers should not exceed 1.7 Tesla (Wb/m2 ) without manufacturers' advice.
Rapidly cooled, thin (typically 0.025 mm thick) amorphous ribbon steel cores with lower magnetic saturation limits of about 1.4 Tesla can reduce the losses in the core compared with cold-rolled grain-oriented silicon steel by up to 75%. Distribution transformers using this material are widely used in North America.
A continuous magnetic circuit is obtained by avoiding air gaps or non magnetic components at joints. Core lamination clamping bolts are no longer used in modern designs. The laminations are held together by the hoop stress of the windings, by fiberglass or banding or by pinching the yoke between external clamps.
5.2 Windings
5.2.1 Conductors and Insulation
Transmission and distribution oil-immersed power transformer windings are usually made of copper to reduce load losses. Winding insulation in oil immersed transformers is a cellulose paper material. High voltage transformers are vacuum impregnated with high quality, extremely clean, hot mineral-insulating oil with a water content less than 2 ppm (parts per mil lion) and air content less than 0.2%. As ratings increase the winding conductors are connected in parallel (to reduce eddy current loss) and transposed (to avoid leakage flux circulating currents).
Aluminum has a higher specific resistance than copper and therefore requires a larger cross-section for a given current rating. It is not therefore generally used in power transformers. However, aluminum has certain advantages over copper when used as foil windings in dry type cast resin distribution transformers. Aluminum has a coefficient of expansion of approximately 24x10^-6 K21 compared with 17x10^-6 K21 for copper, and this is more similar to the resins used. The short-circuit thermal withstand time tends to be greater for aluminum compared with copper in an equivalent design and aluminum foil eddy current losses are lower. Modern designs using epoxy and fiberglass resins, vacuum molded to the conductors, having high thermal conductivity (0.5 W/m K) and extremely high electrical strength (200 kV/mm) generally allowing higher conductor temperatures than Class A. The impulse withstand tends to be lower for dry-type transformers but this depends upon strip or foil winding construction and the resins used.
Some manufacturers offer 95 kV BIL designs whereas IEC 60076-11 requires 75 kV BIL for 12 kV systems.
5.2.2 Two Winding (Double Wound)
This is the basic transformer type with two windings connecting a higher voltage system to a lower voltage system. This type is the normal arrangement for step-down transformers in distribution and subtransmission systems, and for generator transformers.
5.2.3 Three Winding
General
There are situations where, for design reasons, or because a third voltage level is involved, that a third winding is added. The impedance representation of three winding transformers is detailed in SEC. 2.4.
Delta Tertiary
A star/star transformer is often supplied with a third (delta-connected) winding for one or more of the following reasons:
- To reduce the transformer impedance to zero sequence currents and there fore permit the flow of earth fault currents of sufficient magnitude to operate the protection (see SEC. 2.4 and FIG. 7).
- To suppress the third harmonics due to the no-load current in the earth connection when the neutral is earthed. These harmonics have been known to induce disturbances in neighboring low voltage telecommunication cables.
- To stabilize the phase-to-phase voltages under unbalanced load conditions (e.g. a single phase load between one phase and neutral). Without a tertiary winding the current flowing in the uncompensated phases is purely magnetizing and, by saturation, causes deformation of the phase voltages and displacement of the neutral point. The addition of a delta tertiary winding balances the ampere turns in all three phases eliminating such phenomena.
- To enable overpotential testing of large high voltage transformers to be carried out by excitation at a relatively low voltage. This requirement depends upon the transformer manufacturers' test bay capabilities.
However, for such test purposes the tertiary may only need to be of a very low rating and connected only for the factory tests.
- To provide an intermediate voltage level for supply to an auxiliary load where a tertiary winding offers a more economical solution than a separate transformer.
Because of these apparent advantages a general view that such a tertiary winding is essential has flourished. However, this is not the case and the tertiary involves an increase in transformer cost of approximately 6% to 8% with a corresponding increase in losses of some 5%. The cross-section of the tertiary winding is usually determined by fault withstand considerations.
With normal three limbed core type star/star transformers satisfactory earth fault current is normally available. Typical values of zero sequence impedance are as follows:
Single phase transformers 5,000-10,000%
Shell type or five limb core type transformers 1,000-5,000%
Three limb core type transformers 50-100%
Therefore the reduction in zero sequence impedance by the addition of a tertiary winding is not as significant in three limb transformers as in the other units.
The development of low loss cold-rolled grain-orientated steel together with improved core construction methods has reduced the magnetizing cur rents in modern transformers. The delta tertiary should only be specified when very small ripple voltages (.2-3% of the fundamental) or overvoltages of the order of a few percent due to out-of-balance loads cannot be tolerated. However, for a bank of single phase transformers, five limbed core types and shell types with an unearthed primary neutral, a delta tertiary should be specified for low zero sequence impedance and 'trapping' triple harmonics considerations.
5.2.4 Auto-Transformers
The basic transformer principle can be achieved using a single winding (per phase). If a tap is made part way down the winding, this can be the low volt age terminal just as though this were a separate winding.
By eliminating the second winding, an auto-transformer is potentially cheaper than a two winding counterpart. In practice, such cost savings only apply for voltage transformation ratios of up to about 3:1 if adequate power transfer is to be achieved. Thus, for example, a transformer with a voltage ratio of 275/132 kV will be a straightforward auto-transformer choice; a ratio of 275/66 kV would, however, probably favor a double wound arrangement.
Both high voltage and low voltage systems have the same neutral (auto transformers are usually star connected) and this would often be undesirable except in transmission systems where solid earthing of neutrals is common at all voltage levels.