Jean-Claude DUART, DuPont, (Switzerland), jean-claude.duart@che.dupont.com
Thomas LIM, DuPont, (Singapore), thomas.lim@sgp.dupont.com
Richard P. MAREK, DuPont, (USA), richard.p.marek@usa.dupont.com
Radoslaw SZEWCZYK, DuPont, (Poland), radoslaw.szewczyk@pol.dupont.com
Over the last several years, key transformer material shortages have occurred more and more frequently.
Coincidentally, the costs have risen dramatically as well and are likely to continue in this upward spiral for some time.
One of the most effective tools capable of forcing significant reduction in amount of materials used is implementation of high-temperature insulation in the design of transformers.
This paper explores the influence of the high-temperature insulation on materials used in transformer design.
Applicable standards and typical insulation systems are discussed. Solutions for raw materials reduction are presented and key parameters of several design examples are analysed.
High-temperature insulation, Aramid fibre, Hybrid insulation system, Semi-helical winding
The size of a particular transformer and amount of materials used are driven by lots of factors, like dielectric clearances required to ensure proper insulation level, appropriate conductor size to meet limits for losses, or necessity of providing sufficient cooling for the windings. Improvement in any of these areas could bring benefits in form of reduction of size and weight of the equipment. There is not much progress observed in dielectric withstand of the commonly used solid or liquid insulation materials, so there is little chance to reduce dielectric clearances in designed transformers. Similarly, the copper and aluminum are still the only materials good for use as current leading conductors. However, some developments were made in terms of the thermal characteristics of insulation materials used in transformer industry.
The first step was made some 50 years ago, when thermally upgraded cellulose was developed for transformer insulation. During last 40 years materials with even higher temperature capability have been developed. For solid insulation the aramid fiber has been introduced as well as high-temperature resistant enamels. Among insulating fluids, silicones and ester fluids have become more and more popular.
Traditional design of liquid-filled transformers has been based for many years on cellulose solid insulation combined with mineral oil and most of the accumulated world experience in transformer design is based on these materials. However, high-temperature insulation systems based on solids and fluids that can withstand higher operating temperatures are getting popular.
Sometimes, high ambient temperatures, space or weight limitations force designers to look for new solutions. Then, designs with higher temperature rises can be proposed as an alternative for conventional designs. For example, aramid fiber with thermal class of 220°C allows operating of the equipment at higher temperature rise for windings without any negative impact on the insulation life. There has been many transformer for different special applications built during last years including traction and rectifier transformers, transformers for mobile substations, or more recently, transformers for wind farm applications. High-temperature materials have been also used for upgrading of retrofitted transformers.
Over the last several years, key transformer material shortages have occurred more and more frequently.
Coincidentally, the costs have risen dramatically as well, and are likely to continue in this upward spiral for some time.
Prudence and good ecological conscience would therefore dictate conservation of these materials. At the same time, more efficient designs are being driven by various governing bodies in most industrialized countries and, more recently, in many emerging countries, too.
Use of high-temperature insulation in the design of transformers can be one of the most effective tools capable of forcing significant change is the use of key transformer materials. The traditional approach generally results in higher losses as a consequence of the high temperature and intended reduced material content. Recently, more moderate approaches have been introduced to the market.
In some cases the losses of a conventional unit can be matched or even reduced by re-engineering the design to take advantage of the thermal capability of the hightemperature material.
An IEC Technical Specification was published in late 2004 as Part 14 of IEC 60076 [1], in response to a growing interest in transformers with higher operating temperatures.
It was developed to meet the international need for additional guidance in the specification and design of liquidimmersed power transformers, using either high-temperature insulation or combinations of high-temperature and conventional insulation. Four insulation systems are defined in this document, both to provide design guidance and to improve communication between the manufacturer and the user [2]. Each system is described below.

A homogenous insulation system is a uniform composite of solid and liquid insulations that have the same thermal capability. The typical liquid-immersed transformer, for example, is composed of cellulose solid insulation and mineral oil liquid insulation. Both components have approximately the same thermal capability. For conventional transformers, this temperature is defined by the maximum hot spot temperature of 98°C, corresponding to the 105 (A) thermal class.
Since the overwhelming history and expertise of the transformer industry relates to this type of transformer, the
IEC 60076-14 document defines this insulation system as the standard reference. This standard system is labeled the
“conventional” insulation system and consists of mineral oil as the liquid insulation and non-thermally upgraded insulation as the solid insulation. By reference then, all other insulations capable of continuous operation at temperatures above that of either mineral oil or non-thermally upgraded cellulose (105°C), are considered “high-temperature” insulations. An insulation system composed of hightemperature liquid and high-temperature solid insulation or conventional liquid and high-temperature solid insulation is considered to be a high-temperature insulation system.
This approach was also a practical necessity, since there is no generally accepted test method for determining the rises than these maximums are reasonable and expected due to the wide range of application and design flexibility.
Average winding temperature rise values in increments of 5 degrees are typical.
The remaining three defined insulation systems are related in that they all use mineral oil or the equivalent as the dielectric coolant and all use a combination of hightemperature and conventional solid insulation. The differences lie in the degree of usage for the hightemperature insulation. Table 2 compares the maximum temperature limits for insulation systems using conventional fluid. Table 2 also emphasizes that there are two distinctly different maximum hot spot temperature limits that must be addressed and mapped in the thermal analysis.
The mixed insulation system uses high-temperature solid insulation adjacent to the winding conductors, which are located in the hotter regions of the winding. This includes conductor insulation and, if necessary, spacers, strips and cylinders in direct contact with these conductors. Cellulose material is then used in the rest of the winding and other lower temperature areas, where thermal class 105 (A) limits are met. The mixed insulation system uses the least amount of high-temperature insulation and is essentially used to augment the capability of the conventional insulation system.
The technique is used when normal average winding temperature rises are needed, but it is desirable to exceed thermal capability of an insulating liquid. In fact, the stated capabilities of the fluids listed in the document are specifically noted as “generally accepted” values and the table heading is “Estimated maximum operating temperature
(°C)”. Accordingly, the liquids were divided into three basic categories that also define the maximum “recognized” thermal capability. Given the lack of definition in the standards, the categories and limits were based on the normal hot spot temperatures.
As an example, with many rectifier transformer designs, it is necessary to reduce the average winding temperature due to excessive hot spot temperatures. This reduces the average winding temperature rise and also increases the size of the unit. By using a mixed insulation system, it is possible to operate closer to a normal average winding application history as a starting point. Table 1 compares the maximum temperature limits for these three basic combinations. The average winding temperature rise is clearly defined as a maximum value. Lower temperature temperature rise and then merely protect the hot spot locations with higher temperature insulation.
Table 1: Maximum temperature limits of homogenous insulation systems
Insulation Type Limit Description Mineral Oil Ester
Liquid Maximum Top [°C] 100 130
Solid
Maximum Top Rise [K]
Maximum Hot Spot [°C]
Maximum Aging Hot Spot [°C]
Average Winding Rise [K]
60
118
98
65
90
190
170
115
Table 2: Maximum temperature limits of mineral oil insulation systems compared to conventional
Insulation Type
Liquid
Conventional Solid
High-Temperature Solid
Limit Description
Maximum Top [°C]
Maximum Top Rise [K]
Maximum Hot Spot [°C]
Maximum Aging Hot Spot [°C]
Maximum Hot Spot [°C]
Maximum Aging Hot Spot [°C]
Average Winding Rise [K]
Conventional
N/A
N/A
65
Mixed
100
60
118
98
Semi-Hybrid
150
130
65
130
110
75
Silicone
155
115
220
200
130
Hybrid
170
150
95

An application where high-temperature materials are used only for conductor insulation is defined as a semi-hybrid insulation system. When applied to all windings, the goal is usually longer life and better reliability. In some cases, lower cost or reduced losses are the incentive, where hightemperature insulation may be used on only one winding.
Generator step-up transformers, furnace transformers and other high load factor applications are all good candidates for a semi-hybrid design. Another suitable application applies to transformers designed for use in high ambient temperature conditions.
The hybrid insulation system uses high-temperature solid insulation adjacent to all winding conductors, including all conductor insulation, spacers, strips and cylinders in direct contact with the winding, and cellulose materials in lower temperature areas where thermal class 105(A) limits are met. The hybrid insulation system uses the largest percentage of high-temperature insulation from the three mineral oil systems. Table 2 also indicates that this system has the highest thermal limits of the three, which fits well with high overloading, small size and high power density.
Historically, medium power mobile transformers used as emergency substation replacements were some of the first applications to take advantage of the hybrid insulation system. This technique was an excellent match for the high power densities needed for this type of unit. The hybrid insulation system has also been used in the repair industry for many years. This approach often provides additional capacity and quick delivery at an attractive price. More recently, new units have been specified to provide improved reliability, higher overload capacity, or to supply more capacity in the same available space. These applications must be designed to specifically take advantage of the insulation thermal capability.
As stated before, the use of high temperature materials can reduce key transformer materials in the transformers. With the high prices of raw materials it may have a positive impact on the price of the equipment. However, it was not the economics which made it attractive to use high temperature insulation systems. Reduction of materials needed to build the transformer is necessary to meet strict requirements related to low weight or limited size of the transformer in some applications.
The typical case for reduced weight of raw materials are transformers for mobile substations, where equipment of specific power rating must be placed on the road platform.
High temperature insulation of windings allow to operate the equipment at the temperature higher than conventional transformers. This means that much more heat can be generated in windings. Cross section area of conductors may be reduced, which then reduces the size of entire active part and the tank, too. It must be emphasized that high temperature materials allow reduction of cross sections of conductors to much more extend than conventional materials and pushing the current densities to real extremes, sometimes to 7-8 A/mm
2
, which in consequence gives great results in weight reduction. Tables 3 and 4 show characteristics of one example of a mobile transformer in comparison to conventional designs having the same power rating or having the same weight [3].
Table 3: Comparison of hybrid mobile transformer and conventional design of the same power rating
Rated power [MVA]
Weight [kg]
No-load loss [kW]
Load loss [kW]
Avg winding rise [K] hybrid
45
44 100
11.9
752
95 conventional
(same power)
45
57 500
10.0
225
65
Table 4: Comparison of hybrid mobile transformer and conventional design of the same weight hybrid conventional
(same weight)
Rated power [MVA]
Weight [kg]
No-load loss [kW]
Load loss [kW]
Avg winding rise [K]
45
44 100
11.9
752
95
31.5
44 100
8.0
110
65
Another case where weight of the transformer matters is a pole type transformer. Power rating of the equipment is limited in this case by the weight of the unit which could be installed on the pole. Properties of high temperature materials offer potential increase of rated power up to 30% with the size and weight kept at the given level, or if the same power rating is maintained, the weight can be reduced up to 30%. More detailed analysis of reduction in different materials is presented on the case of 300 kVA transformer used in Korean grid. Table 5 shows comparison of this transformer as a conventional design and as a high temperature execution.
Table 5: Summary of material savings in high temperature design vs. conventional design of
300 kVA, 22.9 kV transformer installed in Korean grid conventional high temp.
Avg wdg rise [K]
Top oil rise [K]
No-load loss [W]
Load loss [W]
Core steel [kg]
HV conductor [kg]
65
65
692
4 186
380
91
95
65
650
5 472
372 (-2%)
80 (-12%)
LV conductor [kg]
Liquid [l]
Insulation [kg]
68
555
14.2
59 (-13%)
500 (-10%)
11.7 (-18%)
In case of wind turbine application, where transformer is located inside the tower, weight is not as critical as the dimensions. However, transformer specification not always allows to increase the losses of the equipment. An important aspect here is also poor cooling of transformer located on the bottom of the tower. An example of the design with

dimensions reduced significantly and losses reduced at the same time is shown in table 6 [4]. In this design winding temperature rise is increased to 110 K but top oil temperature is still kept at the reasonably low level of 70 K.
Amount of oil in this transformer is reduced by 31% comparing to conventional design. This is especially important in this case, as the more expensive silicone fluid is used here.
Table 6: Comparison of conventional design and high temperature design of Bio-SLIM® transformer
(2.3 MVA, 20 kV) conventional Bio-SLIM®
Avg wdg rise [K]
Top oil rise [K]
No-load loss [W]
Load loss @ 75°C [W]
Length [mm]
Width [mm]
Height [mm]
Liquid weight [kg]
Total weight [kg]
55
50
2 350
18 000
2 085
1 150
2 150
1 210
6 000
110
70
2 350
16 000
2 160
760
2 125
840
5 040
Another specific application of high temperature insulation technology and material savings offered by it are transformers exposed to high short term overloads. High overloads can generate excessive temperature in windings even if their duration is short. This can lead to significant loss of insulation life and endanger reliability of insulation system. To avoid excessive temperature rises in the conventional way, transformers can be designed for rated power close to the value of overload. As a result, the transformer can be quite much oversized relative to its normal continuous loading.
Figure 1: Example of daily loading of rectifier transformer in railway network
Analysis made for rectifier transformers working in Polish railway network showed that actual continuous loading of some transformers might be only 26% of the rated power of the transformer [5]. This results from highly variable loading of transformer showed on the Fig. 1. The transformer is fully loaded only when the train passes section of the system supplied by this particular transformer.
In such a case transformer must be significantly oversized. It uses its rated power only by a short time within the day and after this it consumes relatively high no-load losses resulting from big dimensions of the core. The solution can be designing of transformer equivalent to lower rated power but with higher overload capability. Lower rating of transformer means less materials and smaller equipment to be installed on site. Together with reduced no-load losses it makes such a solution worth consideration. Higher overload capability can be achieved by implementation of high temperature insulating materials, e.g. solid insulation based on aramid fiber in windings area. When hybrid insulation system is chosen, the rated average winding rise can be raised up to
95 K, and aramid paper allows significant overheating of the insulation without extended loss of life being a concern.
Table 7 presents a comparison of conventional transformer being used and high temperature design proposed as a replacement (normal loading during the day is 1,74 MVA).
Table 7: Comparison of conventional and hybrid design for rectifier transformer exposed to high short term overloads conventional hybrid
Avg wdg rise [K]
No-load loss [W]
Full load loss [W]
Total loss @ 26% load [W]
Core weight [kg]
Cu weight [kg]
Oil quantity [l]
Total weight [kg]
55
5 466
40 993
8 315
4 676
3 114
3 449
12 367
95
4 688
61 496
8 960
4 003
2 464
2 847
10 367
Similar study performed for 160 MVA autotransformers installed in Polish National Grid (PSE) also proved potential for material savings. Re-engineering of this autotransformer to the cellulose-aramid hybrid could give together with some other modifications up to 25% reduction in raw materials needed and reduce total owning cost of the transformer, although the load losses would be higher that in original design [6].
The use of high temperature solid and liquid insulating materials in homogenous insulation systems may bring benefits in terms of material savings when entire transformer is designed to operate at the temperature higher than conventional. Then, cooling system may be reduced significantly, which results in less steel used for the transformer tank and less insulating fluid contained in it.
However, for insulation systems using mineral oil, the liquid temperature must be kept at the standard level and only winding design and full utilization of higher thermal capacity of solid insulation gives potential for material savings.
The most natural material saving method is reducing the cross section area of conductors. Higher temperature gradients of windings can be taken by insulation of better temperature class, and the same rated power can be packed in smaller volume of active part. It may happen that conductor cross section area may be reduced to the extend

where the mechanical withstand of the winding for the short circuit forces is the limit. This approach, however, leads straight to the higher losses in windings. For general use substation transformers it can be justified economically when total owning cost of the equipment is considered, which includes purchase price, evaluated losses cost and installation costs. Then, higher losses cost can be compensated by lower original equipment price (less materials) and lower cost of installation, as the space required for transformer may be significantly.
In many cases, however, the losses of the transformer are fixed and reduced cross section of conductors is not an option. Then, savings may be achieved by reduction of cooling ducts within the windings. Windings insulated with high temperature solid insulation material do not require as much cooling as conventional ones. In some designs cooling ducts may be eliminated partially or even completely from specific windings.
An example of the technique based on closing cooling ducts within the transformer winding is a semi-helical winding design. It is an alternative for traditional helical disc winding used commonly for low voltage windings of power transformers, and it is a case of semi-hybrid insulation system.
The idea is based on the number of horizontal cooling surfaces really needed for cooling of the winding. Normally, each disc is cooled by two surfaces (top and bottom), where oil flow is enabled by the radial spacer separating adjacent discs (Fig. 2a). The radial duct spacer height may vary depending on radial build of the winding, and may reach up to 5 mm. For low voltage winding consisting of 80 discs this gives 400 mm of axial space filled with spacer material. The dielectric stress is no concern in case of low voltage discs.
Hence, the only reason for disc-to-disc spacing is cooling.
However, if cooling could be reduced due to high temperature insulation materials used on the conductor, some of the cooling ducts could be closed. In semi-helical concept every second cooling duct is collapsed and regular spacing is replaced with just a thin separator, e.g. 1 mm pressboard, preventing turn insulation from mechanical damages (Fig. 2b). a) b)
Figure 2: Concept of semi-helical winding a) helix, b) semi-helix
Replacing regular spacers with thin separators improves significantly winding’s filling factor. Saved space can be used either for reducing the radial build of the coil or filled with additional conductor copper. In the first case the winding gets more compact. Consequently, the diameter of outside windings gets reduced, too, and entire active part dimensions get smaller. This leads to savings in materials used and reduces losses. In case where increase of conductor area is chosen, the saving on materials is not observed but losses can be reduced.
An example of transformer design where semi-helical winding is implemented is presented in table 8. The load losses got reduced by 2.5%. It must be emphasized that this loss reduction did not result in any increase of transformer cost. Cost of more expensive insulation materials was compensated by savings on components like core steel or transformer oil.
Table 8: Comparison of helical and semi-helical LV winding design in 30 MVA transformer helix semi-helix
Avg wdg rise [K]
Conductor insulation
Winding height [mm]
Copper strand width
[mm]
Radial build [mm]
Core loss [W]
Load loss [W]
65 cellulose
1 676
5.44
51.3
18 858
130 900
75 aramid fiber
1 676
6.38
45.4
18 766 (-0,5%)
127 585 (-2.5%)
Core weight [kg]
Copper weight [kg]
Oil weight [kg]
Total weight [kg]
Material cost [$]
10 954
6 817
13 928
40 005
132 485
10 900 (-0.5%)
6 827 (+0.1%)
13 619 (-2,2%)
39 554 (-1,1%)
132 423 (0.0%)
It must be remembered, that efficiency of using semi-helical winding may vary from design to design. It can be more suitable for windings relatively cold before re-engineering or for windings supported by wide radial spacers, where eliminating of cooling ducts affects cooling surface to less extent.
Semi-helical winding can be used in low voltage disc type windings of power transformers. Similar approach can be found also in design of distribution transformers, where axial cooling ducts can be eliminated. This increases temperatures of windings to limits which can be withstand by high performance insulation materials. Eliminating the axial ducts from windings not only saves space but also increases mechanical strength of windings, as the structure is more solid. This could even eliminate necessity of epoxy bonding of winding layers.
Windings with reduced number of axial cooling ducts or with ducts completely eliminated have smaller diameters, which reduces size of entire active part and the tank, too. Less raw materials is needed in such a case and the transformer is relatively smaller and lighter.
While generally accepted for the winding insulation, the use of high temperature insulation has been recently considered

for another transformer component which are the lead cables connecting windings to bushings or tap changers.
Currently the kraft paper is commonly used for that application. The interest in applying high temperature materials is based on the same principles as in the windings.
The potential benefits include allowing higher operating temperature for peak load demands and reducing the amount of copper for cost reduction and making the cable more flexible and easier to install.
Thermal and electrical insulation properties of cellulose based paper and aramid crepe paper Nomex® 7T411 have been characterized in publication [7] . Because these results show that the aramid insulation has better thermal performance, a concept of lead cable design was developed.
The aramid insulated cables could be sized to run at higher temperature without any reduction in reliability or impact on the thermal aging of the cable insulation.
Table 9: Example of cable selection depending on insulation material and temperature gradient allowed across the insulation
Example 1
20 MVA, 6 kV
1925 A
Example 2
50 MVA, 20 kV
1443 A
Kraft
20 K
Number of cables
Size of cable
[mm
2
]
Current per cable [A]
Total current allowed [A]
Number of cables
Aramid
50 K
Size of cable
[mm
2
]
Current per cable [A]
Total current allowed [A]
2
253
1009
2018
2
152
999
1998
2
177
785
1570
1
253
1441
1441
Copper savings vs. kraft
Number of cables
Size of cable
[mm
2
]
40%
2
127
28%
1
253
Aramid
60 K
Current per cable [A]
Total current allowed [A]
Copper savings vs. kraft
963
1926
50%
1562
1562
28%
Two examples of insulation substitution are presented in table 9. The kraft insulated cables sized for 20 K temperature gradient across the insulation thickness have been replaced with aramid insulated cables sized for 50 K or
60 K gradient. Example 1 shows reduction in cross section area of cables used (253 mm
2
down to 152 mm
2 or
127 mm
2
) while example 2 shows reduction in the number of cables (2 down to 1). The degree of material cost saving will depend on the specific substitution. For example, the aramid insulated cable of 253 mm
2
with 3.2 mm insulation build could be loaded with 55% more current at 60 K gradient comparing to kraft at 20 K gradient.
The total cost of insulated cable required is lower for the aramid insulated cable. The cost of the 253 mm
2
, 3.2 mm build, aramid insulated cable is approximately 2% higher than the comparable kraft insulated cable but number of cables could be reduced due to the additional capacity per cable gained. Additional cost savings can be achieved due to reduced space in the transformer required for the cables, reduced complexity of supporting structure and finally reduced labor associated with installation of smaller cables or fewer cables.
Operating of transformers at the temperatures higher than conventional gives a chance to make designs with reduced weight and size. This can be a valuable approach in some applications even though the losses generated by such designs may be higher. There are also techniques which allow to reduce materials use without increasing the losses, which not only results in saving the space or weight of the equipment but also gives an economical advantage.
Bio-SLIM® is registered trademark of Pauwels International
N.V., Belgium
Nomex® is registered trademark of E.I. du Pont de Nemours
& Co., USA
REFERENCES
[1] IEC/TS 60076-14:2004, Design and application of liquid-immersed power transformers using hightemperature insulation materials
[2] R.P. Marek, 2006, “Discussions on a New IEC
Document for Liquid-Filled Transformers ”, 10 th
Insucon
International Conference, Birmingham, 75-81
[3] Pauwels International N.V., leaflet on mobile transformers
[4] Pauwels International N.V., leaflet on BIO-Slim transformers
[5] R. Malewski, A. Rojek, J.C. Duart, 2001,
“Zastosowanie izolacji Nomex®-celuloza do transformatorów prostownikowych zasilających sieć trakcyjną” , VIII Sympozjum: Problemy Eksploatacji
Układów Izolacyjnych Wysokiego Napięcia EUI’01,
Zakopane, 305-313
[6] A. Zbudniewek, R. Malewski, 2003, ”Zastąpienie wyeksploatowanych autotransformatorów 160 MVA,
220/115 kV przeciążalnymi jednostkami o zmniejszonej mocy znamionowej” , Elektroenergetyka Nr 3/2003 (46),
16-21
[7] R. Szewczyk, J.C. Duart, L.C. Bates, 2007, “High
Temperature Operating Lead Cables for Power
Transformers” , CWIEME “Inductica” conference, Berlin