SGB-SMIT Group is able to deliver Cast Resin Transformers (from production sites in Germany, Malaysia and the USA) with capacities up to 25 MVA and series voltages up to
36 kV. Our offer includes converter transformers, distribution transformers and special transformers. With more than 30 years experience in the construction of cast resin transformers, SGB-SMIT Group has worldwide expertise, and this is expressed in remarkably high quality coefficients such as mean time between failures (MTBF) of over
The Cast Resin Transformer shall be suitable for continuous indoor operation in a tropical climate on a 3 phase distribution system with solidly earthed neutral. It shall be of a short circuit, impulse and moisture prove design.
The cast resin transformer shall be designed, manufactured and tested according to the latest edition of International Electrotechnical Commission (IEC) Standard 60076- 11. It shall be compact and suitable for easy installation on site.
The transformer shall be suitable to operate in a enclosure of the protection class IP20 and IP 23 without derating. For installation of the cast resin transformer in an enclosure of higher protection class the transformer shall be adopted accordingly.
In general the insulation level of the cast resin transformer used in public and in industrial systems shall comply to the requirements of IEC 60076-11 as follows
The relevant figures shall be selected according to the system voltage of the grid. Transformer with an insulation level differing from the above shall be considered separately on request.
The HV and the LV winding shall be of insulation class F in respect of mechanical and electrical strength (Partial Discharge Test).The maximum continuos operation temperature shall be within the limit of the maximum system temperature according to IEC 726, i.e. 155°C. The maximum temperature rise shall be limited to 100 K at 40°C ambient.
The core laminations shall be made of cold rolled grain oriented high quality silicon steel with non hygroscopic insulation material ”Carlite” covering both sides. The mitred interleaved joints of the limbs and yokes shall be executed in the STEP LAP stacking process. The flux density shall be selected in the way that the core material used is efficiently operating.
The clamping structure of the core and coil arrangement shall be of robust construction to withstand the stress to which it may be subjected during transport, installation and service.
The clamping frame shall respect the requirements of high voltage equipment. Tie rods shall be located next to the lamination. Lifting lugs capable to carry the weight of the transformer shall be provided at the core frame.
An adequate painting shall cover the complete core and the clamping structure and shall protect it against corrosion.
The conductor material used at the HV windings shall be of high grade copper or aluminium. Enamel insulated wire and fibre glass insulated wire shall be allowed. The material used for insulation and reinforcement shall be of fibre glass and of pure epoxy resin. Filler shall not be allowed.
The HV winding shall be wound in a multi layer winding technique and shall be in such a way that a linear potential distribution during impulse stresses is achieved.
The resonance frequency of the winding shall be below 20 kHz to avoid resonance with the switching impulse of VCBs and over stress of the insulation.
The winding process must position each winding accurately in order to minimise and control the electric stress on the insulation material.
Cooling ducts formed in the HV winding shall minimise the temperature gradient in the coil and get the cooling most efficient.
The HV coil shall be moulded and cast under vacuum with epoxy resin to ensure complete void-free resin impregnation throughout the entire insulation system. The flexibility of the resin shall be such that the formation of cracks shall be avoided during short circuit stresses and temperature shocks. Penetration of moisture shall be avoided by the vacuum cast process.
The termination of the HV feeder cable shall be made directly at the HV coils.
The HV winding shall be equipped with tappings preferable at -5 -2,5 0 +2,5 +5 % of the rated voltage.
The tappings shall be inter connectable by links which shall be located at the dom of the HV winding and shall form part of the delta connection.
The conductor material used at the LV windings shall be of high grade copper or aluminium formed as a foil. The width of the foil shall form the height of the winding (foil winding).
Epoxy resin pre impregnated solid multi layer type insulation material of Class F shall be used for insulation. After thermal treatment, the winding shall form a solid structure which shall be capable to withstand the stress occurring during a short circuit.
The LV winding shall not be wound on the core to create a cooling duct between core and LV winding Cooling ducts formed in the LV winding shall minimise the temperature gradient in the coil and get the cooling most efficient.
Rectangular section terminals shall be rigidly fastened to the upper clamping structure and avoid displacement at a short circuit.
Pre set PTC sensors shall be provided and attached to each of the LV winding as close as possible to the hottest spot.
The PTC sensors shall be selected in order to protect both, HV and LV windings.
The transformer shall be made for AN cooling.
On request the transformer shall be equipped with fans in order to achieve 40 % uprating . The fans shall be temperature controlled by means of temperature sensors which shall be installed at the LV winding of the middle leg.
The transformer shall be provided with the following accessories:
under carriage with rollers for bi-directional movement
Each transformer shall be routine tested according to IEC 60076-11. Test report to
Acording SGB Standard.
12 months from delivery ex works
Ventilated enclosures of adequate protection for indoor installation shall be supplied The transformer and the enclosure shall create a compact unit. In order to minimise the space required the HV XLPE cables shall be connected direct to the transformer coils inside the enclosure. Gland plates with cable glands shall be provided at top or bottom. Removable panels shall allow access for cable termination and to the reconnection links.
The LV terminals shall be designed in the way to allow the termination of ample nos. of cables.
In order to minimise the space required the LV terminals shall be placed inside the transformer enclosure.
On request HV and LV cable box shall be provided. Enclosures of IP20, IP21, IP23, IP30, IP31, IP33 degree of protection are available and have to be selected according to the local protection requirements.
SGB Cast Resin Transformers comply with the relevant national, European and international standards of the customer’s order.
DIN VDE 0532 for transformers and reactance coils
DIN VDE 0532 part 6 for dry-type transformers
IEC 60076-11 for dry-type transformers with high voltage for equipment up to and including 36 kV
DIN 42523 (dry-type transformers 50 Hz, 100 – 2500 kVA)
HD 538.1 S1 (General demands for transformers with Um up to 24 kV)
The following requirements are also worth considering when setting up and operating installations.
DIN VDE 0100 (setting up heavy current installations with rated voltage up to 1000 V)
DIN VDE 0101 (setting up heavy current installations with rated voltage above 1 kV)
DIN VDE 0105 (operating heavy current installations)
DIN VDE 0108 (setting up and operating heavy current installations in buildings with crowds of people)
DIN VDE 0141 (Earthing heavy current installations with rated voltage above 1 kV)
Elt Bau VO (Regulation on the construction of operating floors for electrical installations)
Arb. Stätt. VO (Requirements for workplace regulation)
TA-Noise (Instructions for protection against acoustic strain)
Further hints for planning and explanations can be obtained:
AGI-Worksheets J11 or J12 (structural execution, space for transformers) VDI 2078 (Calculating the cooling load in air-conditioned spaces) Worksheets J11 and J12 can be ordered at the following Internet address:
The tender or contract documentation and specifications are decisive in the specific execution of cast resin transformers. The actual measurements such as width, height and weight of the transformer naturally depend upon the relevant customer order. Consequently the types of transformers shown in the selection table with their actual dimensions and weights may differ.
The following constructional principles can be applied in the installation of dry-type transformers in buildings, which are in force in accordance with DIN VDE 0101 as enclosed electrical production sites with an installation height of up to 1000 m. The information relates to dry-type transformers with a rated power of 400 kVA up to 2500 kVA and a rated voltage up to 20 kV.
It is generally said that the installation of the transformer depends upon the type of electrical connections (cable or railing system), position of the cable entry point, position of the switchgear to the transformer chamber (transformer and switchgear can be placed in one chamber), transport conditions, required space and accessibility. When planning transformer chambers, you can possibly consider the option that transformers of a greater capacity can be upgraded later.
The chamber in which a transformer is installed, ought to be free from both ground and high water, turned away from the sun and access for means of transport, operation and fire fighting must be ensured. That is precisely why the chamber should preferably be on the ground floor or platform height. Furthermore any obstruction of free movement of traffic in exits and stairs due to fire and smoke must be avoided. The additional air openings should be aligned towards the north, the area away from the sun. Also avoid the use of piping for fluids, combustible gases and also cables in transformer chambers.
Knowing the measurements of the transformer to be used, i.e. length, height and width, is vital for the dimensions of the chamber. The chamber length and width should amount to the transformer length or width plus twice 80 cm (ambulatory measurement). The ambulatory measurement of 80 cm is recommended as the width for the all-round ambulatory around the transformer for installation or maintenance work, measured from the outer edge of the transformer. With noise-insulated installations the chamber measurements are to be extended accordingly.
The chamber height complies with the maximum height of the transformer, the voltage and protection distances of electrical connections, the type of ventilation of the transformer chamber and the height of the used air opening. The height of the chamber should be at least 50 cm higher than the maximum transformer height.
With a transverse arrangement the door opening should be twice 10 cm transport safety distance plus transformer length and with arrangement lengthways, the door opening should be twice 10 cm transport safety distance plus transformer width. For the height of the door, transformer height plus 20 cm transport safety distance applies.
Windows should be avoided in transformer chambers. If this is not possible, then any access must be made difficult. Doors must open outwards, on the door outside a high voltage danger sign must be attached and the door locks must be used in such a way that any unauthorized entry is prevented at all times. However without a key it must be possible to open the door from inside with a handle or something similar, even if it is locked from the outside. Ventilator openings for the transformer chamber are provided, for example, with grates. These are used as it is difficult for foreign matter to infiltrate, people will not be put in danger and snow and rain will not be able to get in. Protective grates must be secured so that they are not moved or taken off.
An opening of 80 x 20 cm is recommended for the cable gland with transformers up to 630 kVA and with 800 kVA to 2500 kVA transformers, an opening of 140 x 20 cm. The loading assumptions for the cable supporting frames are to be considered and also ensure that there is adequate fastening.
Installations for lighting, ventilation etc in the transformer chamber should be arranged in such a way that they can be operated safely when the transformer is being used and there is as little risk as possible due to any arcing fault or mechanical effects.
Again, always refer to the worksheets AGI J11 and AGI J12, which give helpful constructional planning principles.
The requirements of SGB cast resin transformers concerning conditions of installation at site are marginal. This is due to the regulations mentioned for ground water protection, fire prevention, functional preservation according to DIN VDE 0101, DIN VDE 0108 and ELT Bau VO. With SGB Cast Resin Transformers, no protective measures for the prevention of water pollution are necessary. However with the transformer being executed with insulating fluids like, for example, mineral oil or silicone oil, drip pans and collecting pits would be essential, however, discharge of fluid from the collecting pit must also be prevented. Equally the Water Management Act and German Land regulations must be observed.
With protective measures for fire prevention and functional preservation in accordance with DIN VDE 0101, the following conditions must be met:
Chambers are fire-resistant separated according to F90A.
Doors are fire-retardant designed according to T30.
Doors which lead into the open air must be of low flammability.
These three conditions mentioned do not apply when fast-acting protection is available.
When installing in an open-air installation, no measures are necessary for fire prevention and functional preservation with SGB Cast Resin Transformers.
Should the cast resin transformer however be equipped with rated voltage above 1 kV for installations for crowds of people in accordance with DIN VDE 0108 and Elt Bau VO , the following additional demands arise:
The cast resin transformer must be installed in an isolated electrical commercial unit.
Transformer chamber walls must be fire-resistant.
Doors must be fire-retardant and from non-flammable building materials.
With doors into the open air, a design from non-flammable building materials is sufficient.
Automatic protection against the effects of overloads as well as from internal and external faults.
SGB Cast Resin Transformers comply with the most defined classes E2 and C2.
Air moisture, condensation, pollution and ambient temperature are environmental conditions for dry-type transformers. These factors are of importance not only during operation but also during storage before installing the transformer.
With regard to air moisture, condensation and pollution, three different environmental classes are defined.
Condensation does not appear on the transformer, pollution is negligible. This condition is normally met with installation in a dust-free and dry interior.
Occasional condensation can appear on the transformer (for example if the transformer is turned off). Pollution in a confined area is possible.
Frequent condensation or considerable pollution or a combination of both factors.
The climate class takes the lowest ambient temperature into consideration.
It is distinguished between two climate classes:
The transformer is suitable for operation with ambient temperatures not below –5°C , but can be exposed during transport and storage to ambient temperatures of up to –25°C.
The transformer is suitable for operation, transport and storage with ambient temperatures up to –25°C.
The necessary classes regarding environment and climate must be defined by the operator.
The necessary fire class must be defined by the operator. However SGB Cast Resin Transformers comply strictly with the most defined class F1.
Two fire classes are defined:
No specific fire risk is to be taken into account. With the exception of the features available as a result of the design of the transformer, no special measures have to be provided in limiting the fire hazard. Nevertheless the emission of toxic materials and smoke which impedes sight must be cut to an absolute minimum.
By this we mean transformers, which take into consideration the dangers starting from fires. It is necessary to limit the fire hazard. The emission of toxic materials and smoke which impedes sight must be cut to an absolute minimum.
SGB Cast Resin Transformers are therefore safe to operate both with damp condensation and being exposed to dirt. However maintenance intervals should be adapted to environmental conditions. That is to say with considerable pollution we recommend the coil surfaces be cleaned.
Equally they are suitable for open-air installation for temperatures up to -25°C in an IP 23 protective enclosure with a special coating and comply with a large part of fire prevention.
As SGB Cast Resin Transformers only have cast resin parts in low and high voltage coils, the fire load is defined for the most part as these components. It should be indicated using the example of a 1000 kVA –execution and likened to liquid-cooled transformers. In the latter case, for the sake of simplicity, the fire load of the coil and supporting insulation is disregarded and only the insulation fluid is taken into consideration.
Type of transformer
Insulating amount [kg]
Heating value [kWH/kg]
Fire load [kWH]
In the laboratories of ALLIANZ AG the cast resin moulding material from a SGB transformer coil as well as samples from all insulants were carbonised and the fire gases analysed. There were no halogens, such as fluorine, bromine and chlorine in the moulding material. Dioxin formation can therefore be ruled out. Sulphur dioxin does also not occur, as P-moulding materials contain no sulphur. Apart from trace contents of other hydrocarbons, the smoke gas is composed practically of carbon monoxide, carbon dioxide, steam and carbon black. That means it is a question of the same waste products which arise with each combustion, like for example, wood and paper. Corrosivity of the combustible products was analysed by the Allianz Centre for Technology in Munich in accordance with the VDE-directive 0472, Part 813. The result of the analysis proves that the cast resin moulding material used by us is technically free from sulphur and halogen and consequently does not lead to corrosion in this respect.
For many years cast resin transformers have been used as a safe means of production and with the correct specification and installation, they are the equal of oil transformers in every way in their durability. To this day however cast resin transformers are not suitable for any exposed open-air installation. For this they require a protective enclosure. With the vents in the enclosures, moisture, salt content, aggressive gases and dust can reach the transformer. If the cool air is polluted considerably it possibly must be filtered. Climatic conditions such as extreme cold in winter or strong sunshine in summer must also be taken into consideration. Equally in air-conditioned chambers make sure that the cooled air from the air conditioning is not blown directly onto the hot transformer coils. This extreme difference in temperature can lead to cracks forming in the moulded coils, which, at worst, may lead to malfunction of the cast resin transformer.
Cast resin transformers are designed in accordance with the relevant standards for the following cool air values:
The cool air temperature does not exceed
40 °C at any time;
30 °C in the monthly mean of the hottest month;
Annual mean 20 °C;
and does not exceed
-25°C with open-air transformers;
-5°C with interior transformers.
The monthly and annual mean values are defined in chapter 3.12 of IEC 60076-11.
The average monthly temperature is half the total of the mean values of the daily maxima and the mean values of the daily minima during a certain month, calculated over many years. The average yearly temperature is one twelfth of the total of the average monthly temperature.
In standard operations the normal service life consumption of the transformer is attained in the process. For service life consumption, the average yearly temperature and strain are crucial. If the ambient temperatures differ from standard operations in the annual mean, then the power rating of the transformer will naturally change in form so that with a reduction of the ambient temperature in the annual mean, a greater power rating of the transformer is possible and with an increase of the ambient temperature in the annual mean, the power rating of the transformer would decrease. This correlation can be seen in the following table.
Ambient temperature (Annual mean)
On certain conditions each cast resin transformer can be overloaded, just like liquid-filled types.
The temperature monitoring system which brings about the shutting down of the transformer with a fixed rated response temperature limits the overload capacity with cast resin transformers.
Our choice of rated response temperature comes from the heating permitted in the low voltage coil. Including the hotspot rate at the measuring point depending on the heating of the coil it is 120° - 150° C, based on the required maximum coolant temperature of 40°C in accordance with VDE 0532, Part 6.
If in a practical operation, the strain under the rated power and the coolant temperature are below 40 °C, then coil temperatures below the threshold values permitted arise as a result. This extent can be exploited for overloads until the fixed response temperature of the thermistor is obtained. The height and duration of the overload are determined through the preceding preload, the actual ambient temperature and the coil time constant. The diagram stated below for a 630 kVA cast resin transformer shows this correlation, based on a coolant temperature of 20 °C and different permanent preloads.
From this it follows that the transformer can be operated with a 100% permanent preload, still about 27 minutes with 130% of its rated power until the temperature monitoring system responds. As the "coil time constant" parameter depends very much on the required technical data and constructive execution, universal overload diagrams are not indicated. On request we can issue them as such whereby all interpretation-specific parameters are taken into consideration.
We have deliberately only shown the overload capacity for the temperature produced with temperature monitoring. As a result we do not exploit a more comprehensive extent* above the threshold temperature as is proposed in IEC/VDE in guidlines on strain for dry-type transformers currently under discussion. Hereby prevent uncontrolled overloads from leading to such thermal strains as it can lead to an abnormally short service life.
To calculate the service life of cast resin transformers you start with the thermal ageing of the cast resin compound insulation in the coils. The thermal effect basically changes the chemical constitution and reduces significant electrical and mechanical features such as partial discharge use, disruptive discharge voltage, tensile strength, elongation etc. These ageing properties can be deduced from the thermal resistance diagram of the relevant insulation system in accordance with DIN / IEC 216 or VDE0304 guidelines.
Example of a thermal resistance diagram for temperature index or limiting temperature 155° C and half life temperature interval 10 K.
Subsequently, in accordance with standardised definitions, for lower limiting values of the relevant test features from the regressing laws on Arrhenius, you obtain temperature index or the limiting temperature and the half life temperature interval. The resultant limiting temperature is based on an underlain service life of 20,000 hours.
The half life temperature interval in Kelvin lets you know the temperature level from which a halving or doubling of the service life arises, based on the limiting temperature calculated (Montsinger-Rule). For cast resin compound mould material a value of 8 – 10 K is expected.
Provided that a cast resin transformer is constantly operated with an aforementioned temperature, in theory you can expect a service life of at least 2¼ years (which is equivalent to about 20,000 hours).
In practice however long-term usage with limiting temperature, i.e. nominal load and 40°C coolant temperature, is unusual. For an estimation of the service life you must take into consideration the daily and annual fluctuations of the coolant temperature with an average of 20K.
Analogously you can also calculate an equivalent permanent load for the customer induced load fluctuation. If these are accepted with a relatively high value of, for example, 88 %, a reduction in heat of 20K will come about, as heating measurements have shown.
Together with the average coolant temperature all in all this means a lower temperature load around 40K compared to the limiting temperature of the insulant.
With an assumed half life temperature interval of 10K you can already calculate a service life of 36 years, based on 2¼ years with limiting temperature, in these cases without partially interpreted safety margins being taken into consideration.