# Cast resin transformer

Cast resin transformer

A cast resin transformer is a power transformer for energy technology whose insulation of the high-voltage windings consists of cast resin and in which no transformer oil is used. It is therefore often referred to as a dry- type transformer, which, strictly speaking, includes all transformers that do not contain any liquid insulating materials, such as B. also pure aramid-insulated transformers.

## Differences to the liquid-filled transformer

Cast resin transformer 400 kVA in the transformer station

In the case of liquid-filled transformers, the transformer oil provides electrical insulation and the dissipation of heat loss. In cast resin transformers, the high-voltage winding is cast in epoxy resin , while other solid insulating materials such as prepreg are used in the low-voltage winding . The insulation of the coils from one another and from the core is ensured by sufficiently large air gaps. A vertical air flow along the coil surfaces and in cooling channels in the coils ensures that the heat loss is dissipated. Due to convection , the air flow occurs by itself (cooling type AN - Air Natural ) or it is additionally reinforced with fans (cooling type AF - Air Forced ). With the transformer oil, the cast resin transformer also eliminates the risk of fire and groundwater associated with it .

As a result, cast resin transformers are particularly used where, due to the proximity to people or property, oil-filled transformers are not or only with considerable fire protection measures, such as B. fire walls can be set up. There is also no need for oil collecting pits to protect groundwater. With cast resin transformers there is therefore also the possibility of a simple change of location. Furthermore, they are largely maintenance-free, since z. B. no leaks can occur as with liquid-filled transformers and the problem of hydrolysis of the transformer oil and its possibly necessary processing are eliminated.

On the other hand, the insulating media used in cast resin transformers have a lower dielectric strength than transformer oil . The heat loss can also be dissipated more poorly through air cooling than through liquid cooling. For this reason, cast resin transformers are usually limited to a power range of 50  kVA to 40 MVA and to operating voltages of up to 36  kV . They are only used in the medium-voltage network, primarily as distribution transformers . Furthermore, cast resin transformers must be designed with larger distances between the live parts in order to take into account the lower insulating capacity. The poorer cooling properties must be compensated for by lower losses or a larger coil surface for heat dissipation. This leads to larger dimensions and higher material usage compared to an oil transformer of the same power and operating voltage. In addition, the coil surface of cast resin transformers is not potential-free . In contrast to liquid-filled transformers, which are surrounded by a protective, earthed tank, dry-type transformers are fundamentally more susceptible to moisture and contamination, and therefore not suitable for outdoor installation without a housing and also not safe to touch. Since the insulation is partly guaranteed by the surrounding air, cast resin transformers must be designed for high installation altitudes (according to the standard> 1000 m) with greater distances in order to compensate for the dielectric strength of the air, which decreases with pressure. While insulation faults in liquid insulation media are eliminated to a certain extent by the decomposition products flowing away, this self-healing mechanism is absent in solid insulation. However, discharges in the air gap of a cast resin transformer have no consequences as long as the solid insulation is not damaged.

## construction

### core

As with liquid-filled transformers, the core is designed as a three-legged core made of electrical sheets insulated on both sides . However, the cast resin transformer still needs to be painted to ensure corrosion protection, as the core is not surrounded by oil.

### High voltage windings

Copper , but often also aluminum, is used as the conductor material in the high-voltage windings . The thermal expansion coefficient of aluminum is higher than that of copper and is therefore closer to that of cast resin. For example, an aluminum conductor reduces the internal mechanical stresses in the coil as a result of temperature fluctuations and thus the risk of cracks in the insulation. The winding can either be designed as a wire winding or as a tape winding with plastic film as layer insulation between the individual turns. After winding , the coil is potted with epoxy resin under vacuum . This ensures sufficient electrical and mechanical strength of the winding and protection against dirt and moisture. The encapsulation must not have any cavities or bubbles that would otherwise lead to partial discharges and thus cause voltage breakdown of the insulation in the long term .

The casting resin is primarily used to insulate the electrical conductors from one another within the coil. There is still an air gap between the undervoltage coil and the core. As a result of the different relative permittivities of air ( ) and cast resin ( ), the electric field is “forced” into the air, so only a small part of the voltage in the cast resin is reduced on these insulating sections. Therefore, the coils must not be touched on the surface during operation. ${\ displaystyle \ varepsilon _ {r} = 1}$${\ displaystyle \ varepsilon _ {r} = 4 ... 5}$

The insulation mostly corresponds to insulation class F or H.

### Low voltage windings

Aluminum or copper tape is used as a conductor in the low-voltage winding, as interlayer insulation is usually prepreg used. This not only electrically isolates the windings from one another, but also bonds them to one another, thus ensuring sufficient mechanical strength in the event of a short circuit . The insulation class is usually F or H.

### Connection elements and accessories

In addition to rails and tubes as connecting and connecting elements, other accessories consist of temperature sensors to protect the transformer against overload and, if required, rollers for transport and surge arresters .

## Environment, climate and fire classes

The testing of dry-type transformers according to DIN EN 60076-11 does not fundamentally differ from that of liquid-filled transformers. However, dry-type transformers are also divided into environmental, climatic and fire classes. H. must be verified on a transformer representative of a series.

### Environment classes E0 - E3

The environmental class provides information about the degree to which the transformer can be used even in adverse environmental conditions such as high humidity , condensation and pollution. It applies to:

• E0: Condensation must not occur on the transformer, pollution is negligible. I.e. the transformer must be set up in a dry and dust-free interior. No test certificate is required for this class
• E1: Occasional condensation and limited pollution is allowed. This class can be carried out by means of a moisture precipitation test. The transformer is exposed to a humidity of more than 93% in a chamber for 6 hours, which is achieved by atomizing water. The water must have an electrical conductivity between 0.1 S / m and 0.3 S / m, so that condensation creates a conductive layer of water on the transformer. The transformer is then operated for 15 minutes at 1.1 times the nominal voltage . No flashover or dangerous tracking must occur.
• E2: Frequent condensation and / or heavy pollution is possible. For verification, the test is carried out as for E1, but with more conductive water in the range from 0.5 S / m to 1.5 S / m. In addition, a moisture penetration test must be carried out. The transformer is stored for 144 hours at 50 ° C and 90% humidity and then subjected to standardized tests with applied and induced alternating voltage, but with voltage values ​​reduced to 80%. Here, too, no flashover or dangerous tracking must occur. This also proves that no damage is caused by moisture gradually penetrating into the transformer.
• E3: DIN EN 60076-16 ( transformers for wind turbine applications ) also describes an even higher class E3. It differs from E2 in that the conductivity of the water used in the moisture precipitation test is increased to 3.6 S / m to 4 S / m and the humidity is increased to 95% for the penetrant test.

### Climate classes C1 and C2

This climate class defines the minimum temperatures at which the transformer can be transported, stored and operated. Due to temperature changes (switched off transformer: ambient temperature, transformer in nominal operation: usually> 100 ° C) the cast resin insulation can develop cracks due to the different expansion coefficients of the conductor material and the cast resin if incorrectly dimensioned. It then applies to

• C1: The transformer must not be operated below −5 ° C ambient temperature, but stored and transported down to −25 ° C. This climate class must be verified by cooling the transformer to −25 ° C, heating it to −5 ° C in 4 hours and then quickly heating it up in a shock with twice the nominal current . The final temperature is determined by the insulation class (e.g. 140 ° C for F). After the test, the insulation must not have any cracks or slits, the transformer must pass the standardized voltage tests with voltage values ​​reduced to 80%, as well as partial discharge tests.
• C2: The transformer can be transported, stored and operated down to an ambient temperature of −25 ° C. The verification is carried out as with C1, however, the heating with twice the nominal current starts already at −25 ° C, the slow warming up to −5 ° C is not necessary.

### Fire classes F0 and F1

The fire class provides information about the fire load in the event of a fire in the vicinity of the transformer and the development of toxic smoke gases that obstruct the view.

• F0: There is no specific fire risk to be considered. The emission of toxic substances and smoke that obstructs the view must nevertheless be reduced to a minimum.
• F1: It is necessary to limit the risk of fire. The emission of toxic substances and smoke that obstructs the view must be reduced to a minimum. To prove this, “one third” of the transformer, i.e. an upper and a lower voltage coil with core leg, is exposed to flame from ignited alcohol and radiation from a radiator in a fire chamber . Certain maximum temperatures in the chimney of the chamber must not be exceeded. The light transmittance in the smoke must not fall below certain values.

## Areas of application

The classic areas of application of the cast resin transformer are where there is a large, spatially concentrated power requirement in the vicinity of people or high material assets. Due to their low fire load, the cast resin transformers can be installed there close to the consumer, so that the low-loss medium voltage can be brought close to the consumer and the low- voltage lines with higher losses can be shorter. This is the case in building complexes such as department stores, office buildings, hospitals and airports, as well as in industrial plants, underground and suburban trains. They are used as converter transformers on drilling rigs, ships, cranes, in rolling mills and paper mills and in mining.

With the spread of wind energy , a new area of ​​application has opened up: In offshore and onshore wind turbines , a cast resin transformer is often installed in the nacelle in order to transform the generator voltage up to the mains voltage.

## Individual evidence

1. ^ A b Germar Müller, Bernd Ponick: Fundamentals of electrical machines . 9th edition. John Wiley & Sons, 2012, ISBN 978-3-527-66097-1 , chap. 1.7.4, p. 180 f . ( limited preview in Google Book search).
2. ^ A b Georg Flegel, Karl Birnstiel, Wolfgang Nerreter: Electrical engineering for mechanical engineering and mechatronics . 9th edition. Carl Hanser Verlag, Munich 2009, ISBN 978-3-446-41906-3 , chap. 12.1.8, p. 154 ( limited preview in Google Book search).
3. Siemens builds the most powerful cast resin transformer in the world. (No longer available online.) In: TGA - Technical building equipment. WEKA-Verlag Gesellschaft mbH, November 13, 2007, archived from the original on January 6, 2014 ; accessed on January 6, 2014 . Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice.

## literature

• German standard DIN EN 60076-11: Power transformers - Part 11 dry-type transformers (IEC 60076-11 2004)
• Günter Springer: Expertise in electrical engineering . 18th edition, Verlag - Europa - Lehrmittel, Wuppertal, 1989, ISBN 3-8085-3018-9