The low voltage winding of SGB-SMIT cast resin transformers are almost always designed as foil winding.

The benefits of this form of winding are self-explanatory:

Cast resin transformers can be equipped with fans for power increase. Through forced cooling a power increase of up to 40% is possible. With optimum interpretation, a greater value can also be reached. This is then determined by means of appropriate cooling in the test room.

The following variants are offered for the forced cooling of cast resin transformers:

• Free standing cast resin transformers
• Installation of the cast resin transformer in a housing, mechanical rating <= IP33

If the solutions shown do not meet your demands, we would be only to pleased to compile customer-specific solutions for you. Please get in touch with us.

Please also look at the following information:

• Fan selection table
• Assembly instruction for additional attachment

Higher acquisition costs due to higher outlay for laminations and winding materials coincide with lower operating costs.

It is relatively straightforward to evaluate idle losses, as these are incurred at a constant level over the entire service life of 8,760 hours/year. Evaluation of the load losses which exhibit quadratic growth or decline according to the load is slightly more difficult.

The use of transformers with reduced no-load losses is also profitable as it helps reduce noise emission.

The place at which the cast resin transformer is installed must be adequately ventilated because with each transformer operation, heat loss occurs which must be discharged from the transformer chamber. In so doing it is worth checking whether the possibility is given for natural airing and ventilation. If dimensions of a chamber or enclosure do not guarantee proper cooling, the air rate must then be guaranteed via forced ventilation. Therefore it is immaterial whether the transformers are operated as self-cooling with the cooling method AN or with attached fans for increased capacity. Therefore the ventilation system must be assessed for maximum heat loss arising which is made by iron core no-load losses and the heat energy caused by the coils. In order to achieve natural air circulation, it is important that the additional air entrance port is always arranged below and with a maximum height up to the beginning of the low voltage coil, in order to further the chimney effect and the circulation of the air flow in the channel between the low and high voltage coil. With regard to the planned openings of additional air – intake and extracted air – outtake, it is important that the lower opening under the transformer is preferably all the way around, whereby the upper opening must normally show a cross section around 10 to 15 % higher in order to take the lower density of extracted air into consideration and consequently prevent heating.

The necessary volume for perfect cooling can be calculated as follows:

Q = Pt / (1.15 x ΔO) [m³ / s ]

Effective area of the lower opening can be calculated as follows:

S = 10,752 x (Pt / ((H x ΔO³)0,5))[m²]

Pt: total losses discharged in kW

ΔO: Temperature gradient in °C between additional air and extracted air

Q: Air change in m³

H: Distance in meters between the axis of the transformer and the axis of the upper opening of the booth

S: Effective area in m² of the lower additional air opening (without bars)

As with dry-type transformers, but not oil-filled transformers with their scoop-proof vessels, a distance of 0.3m can be measured from the vessel, for safety reasons with cast resin transformers, the measurement is made at a distance of 1 m from the reference plane.

LWA and LPA with LS have the set combination in the German Institute for Standardisation 45 635/Part 30:

LWA = LPA + LS

LS = 10 lg S/S0 dB

S = (h + 1) + lm an S0 = 1 m²

lm = 4 MA + (DWA + 2)π

MA = Dual spacing in m

DWA = Coil outer diameter in m

h = Height of core with clamping bar in m (including castors)

When operated, transformers cause electric and magnetic fields. These electromagnetic fields can attain values to whose assessment the following basis applies:

The Federal Immission Control Act

With the coming into effect of the 26th decree on the 1st January 1997 of the implementation of the Federal Immission Control Act (26. BImSchV), low frequency installations with 50-Hz-fields at the place of influence have to abide by the threshold values of 5 kV/m for electrical field strengths and 100 µT for magnetic induction. Place of influence is the place with the greatest exposure at which not only a sojourn from people is to be expected. The electric field outside the cast resin transformer and its connections are hardly effective outside the transformer booth. The protective enclosure of SGB Cast Resin Transformers acts as a Faraday cage. This also applies as far as possible to ceilings and walls in the transformer booths provided that these do not consist of electrical insulating material. The magnetic transformer scatter fields can give rise to disturbances, which in essence consist of three sources.

• Scatter field in no-load

• Scatter field from the lead wires

• Scatter field from current-carrying coils

The no-load field issues mainly at the top and bottom ends of the coil and from the impact points of the iron core. However the no-load field is clearly smaller than the scatter field from the load current-carrying coils. In the literature for instance, factor 10 is mentioned. Separate measurements have resulted in a magnetic leakage flux density at least four times smaller. Therefore you can disregard the no-load scatter field for calculations.

Like each current-carrying conductor, discharges, like connection cables and busbars set up a scatter field. At every point in the chamber this depends on the vectorial sum of the field strengths caused there by the single current-carrying conductors. With conductor set-ups in which the total currents appear as neutral, like outward and return conduction in the AC circuit or three phases with the three-phase current system, the resultant field is all the more smaller the closer these conductors are together. The load current-carrying coils constitute the largest part of the scatter field, which is overlaid by fields of the low voltage-sided discharges.

These scatter fields were recorded by SGB measurements in order to determine the field and with action a reduction occurs in the fields. For instance, the connected low voltage outgoing circuits were set up point-symmetrically at a 1000VA cast resin transformer, so that the field capacity was able to be minimised as much as possible by means of the cable and at the same time, by using a very simple and assembly friendly aluminium shield box, demonstrating that you could reduce the distance for a lower deviation of the flux density in all directions (x - direction, y - direction, z - direction). Equally recommended for the connection of the transformer is field compensation by means of a double low voltage conduction with point-symmetrical phase sequence, were it to be required.

Transformers are regarded as passive elements in accordance with IEC 60076 with regard to electromagnetic interference emission and interference immunity and a CE marking is not permissible.

In accordance with measurements and calculations, the magnetic field around SGB Cast Resin Transformers as well as SGB oil distribution transformers is significantly below the threshold value, which the 26th decree for the implementation of the Federal Immission Control Act (26. BImSchV) of 16.12.96 stipulated. Capacity transformers are basically exempt from this decree as capacity transformers are to be seen as components of a total system or of an installation. Measures are to be sought to reduce the critical field strengths as well as adherence to threshold values in a system or installation which essentially determine the whole field configuration.

SGB had already started with the calculation and measurement of the electromagnetic field well before this date in order to determine the field around a transformer and to try out measures to reduce the field. Under defined measurement conditions the magnetic field of a cast resin transformer is circa 5 μ , for example, 630 kVA/20 kV-0,4 kV in accordance with separate measurements with a distance of 2 m to the cast resin surface of the coil. With an oil transformer of the same capacity it is less than 2 μ with a distance of 1.7m to the geometric centre. It is evident that the partly magnetic fields by means of currents in the coil in this voltage level (0.4 kV) fall significantly below the permitted 100 μ. Disturbances on the display may arise from 1 μ onwards. Further technical disturbance limits are:

Pay attention however to the contribution of the current flow in the low voltage connections, e.g. busbars or feed lines. The magnetic field around the feed lines decreases only with the reciprocal value of the square number of the distance, by contrast, around transformers, with the reciprocal value cubic from the distance. It is simply because the current flow in transformers is restricted in a limited space and in feed lines in one dimension expands at will.Therefore the magnetic field around low voltage connections dominates the level of the entire field e.g. in a supply station. A range of measures to optimise the laying of busbars and feed lines, e.g. the batch of outbound connections,have a great effect in reducing induction. The manufacturers of transformers can already create an "interface" for the connection of such optimum laying in the design.

The 26th decree applies to the "fixed installations for the transformation and transmission of electricity". In its area of application it includes "electrical transformation installations including switch bays with a frequency of 50 Hz and high voltage of 1kV or more". As capacity transformers are to be seen as components of a total system, i.e. an appropriate installation, they are exempt from this decree.

The safeguarding of SGB Cast Resin Transformers is with temperature monitoring.

This is ensured with the installation of resistor temperature sensors (PTC) in the low voltage coils and a temperature monitoring instrument. The functionality of the resistors lies in the fact that its resistance rises sharply in a temperature range with increasing temperature and consequently a certain response temperature is reached to which the temperature monitoring instrument immediately responds. If the coil cools around about three Kelvin under response temperature then the temperature monitoring instrument falls back again. Normally two PTC systems are installed which distinguish themselves by 20K separated rated response temperatures (NAT). A PTC System with rotary current transformers is composed of three resistor temperature sensors in series. A temperature sensor for each phase. Thereby the lower rated response temperature is used as a warning and the higher rated response temperature for shutting off the transformer. The first PTC system signals the temperature being exceeded which forms the basis of the normal service life consumption. In this case provisions must be made, so that the strain does not increase further. The operator is therefore warned and should be prompted to find measures to ease the strain. The second PTC system is geared to the maximum temperatire of the declared temperature class. A third system can, for example, assume control of the fans, if transformers are equipped with fans for an increase in capacity of up to 140 % of the rated power. This control of fans happens with PT 100 and a temperature monitoring instrument like, for example, the TS 02. This instrument recognises and signals faults like a short circuit or broken wire in the measurement line, as well as malfunction of the supply voltage. Furthermore the temperature measured is displayed. In addition remote-reading indicator thermometers can be attached too if the customer so wishes. The cost-efficient alternative of temperature monitoring is the use of resistor sensors without displaying the temperature.

For temperature monitoring there is the following choice:

Temperature sensor: PTC, PT 100
Temperature monitoring instruments: TS – 01, TS – 02, TAA – 12
Remote-reading indicator thermometer: for example JUMO

Inrush current:

When activating the transformer large input currents may flow. The inrush current can for example amount to more than ten times the rated current. In order to be able to correctly put safeguards on the entry side of the transformer, it is advantageous to be familiar with the behaviour of the inrush current of the particular transformer. For that reason SGB provides the opportunity to create a diagram for the customer of the inrush current process.

Inrush current process diagram

However the extent to which the transformer should be safeguarded is at the discretion of the operator. In this way surge diverters against high voltages for example can be attached on the high voltage side. The surge diverters that should be used is to be determined by the operator. However it must also be mentioned that safeguarding is dependent upon the nature of the network and this in turn is the responsibility of the operator.

The cast resin transformer must be connected to the earthing (potential equalisation). For this purpose a connection for the earthing at the chassis is available as standard. Customer requests can naturally be taken into consideration, like for example, earthing at the clamping bar.

Connection of earthing and short circuit equipment:

For this purpose fixed ball points of 20mm or 25mm diameter can be attached to the conductor connection straight or angled. There is also the opportunity to attach earthing switches to the high voltage connection side on the transformer or over the course of the high voltage cable connection on the wall.

The earthing and the potential equalisation on the transformer must be connected and checked correctly. Please bear in mind here the torque of the fixing screws and the minimum cross-sections of the potential equalisation conductors. The cross-section of the potential equalisation conductor must be at least half as big as the largest protective earthing conductor cross-section of the installation, but at least 6 mm² copper for mechanical reasons. For potential equalisation conductors, 25 mm² copper is mentioned for non-electrical installations as a satisfactory upper limit. Other than that the whole protective earthing conductor of 50 mm² is to be chosen as a cross-section for the potential equalisation conductor.

Assignment of the minimum cross-sections of protective earthing conductors and neutral conductors to the cross-section of the outer cable in accordance with the Association for Electrical, Electronic & Information Technologies 0100, part 540:

PEN-conductor >= 10 mm² Cu or >= 16 mm² Al, in accordance with article 8.2.1 of the Association for Electrical, Electronic & Information Technologies 0100, part 540. Unprotected movement of Al conductors is not permitted. Use bare conductors chiefly from a cross-section of the outer cable of >= 95 mm².

If the capacity of a transformer is not sufficient you can then connect several transformers in parallel. In addition the momentary values of the voltages must be equal and the transformers must change their voltages with strain to the same extent. In distribution networks rotary current transformers for example are operated in parallel in order to supply the low voltage network from the high voltage network. With a voltmeter the low voltage side is checked to see whether the parallel connection was correctly made. If a parallel connection is made, before connection you have to check to see if the high and low voltage cables are connected to all transformers at the same phases and that the switch links are at the same position, so that equal voltage is available at all phases and all instruments and at the output the same stress ratio exists. In addition it is necessary to make a conductive connection between the secondary sides of the transformer with a metal rail in order to be able to compare more easily the voltages of the phases. If there is a neutral conductor it is beneficial to employ a metallic connection. With a voltmeter the value of the potential difference must also be checked. If the measured value is zero this means that all requirements are met for a parallel connection and the high voltage switches can be closed.

To connect rotary current transformers in parallel the following requirements are to be met in accordance with the German Institute for Standardisation and the Association for Electrical, Electronic & Information Technologies 0532:

The high and low voltages must be equal so that the transmission of open circuit voltages is equal as much as possible.
The short circuit voltages of transformers should differ by 10% at most from each other. Thereby you can avoid the transformer with the small short-circuit voltage getting too large a current and being overloaded. If the difference is bigger then inductances can be preceded to increase the short-circuit voltage.
The ratio of rated outputs should be less than 3:1. With larger rated output ratios, equalising currents are able to flow and bring about a phase shift between the output voltages.
The coefficient of vector groups must be equal so that phasing of the output voltages corresponds and no short-circuit occurs.

By way of exception rotary current transformers must be connected in parallel and their coefficient differs by 6, for example Dy5 and Dy11. In addition however the connections must be duly alternated. A check of the phasing is then essential.

If the transformers connected in parallel have equal short-circuit voltage, the entire load is then spread out over the transformers in proportion to the rated outputs. If the short-circuit voltage is different then the transformer is loaded more with the smaller short-circuit voltage. The strain is then greater as it corresponds to the ratio of the rated outputs.