| JP3665229 | MAGNETIC SHIELD STRUCTURE FOR CURRENT CONVERTER FOR MEASURING INSTRUMENT |
| WO/2002/019350 | A METHOD OF MANUFACTURING AN INDUCTOR |
| JP52109140 | TRANSFORMER |
CLAIMS
1. EMC protection for a distribution transformer to eliminate the health-endangering effects of magnetic fields in the vicinity of a dry-type transformer, cha racter i z ed in that a uniform and substantially seamless metal jacket (4) which damps the magnetic fields is disposed over a distribution transformer (1) and all of its four sides, substan- tially along their entire height, enclosing the distribution transformer from a small distance; closed air circulation is provided between the metal jacket and the transformer; and that a heat exchanger (6) is provided in connection with the metal jacket to cool the air circulation.
2. The EMC protection for a distribution transformer according to claim 1, chara ct e r i z ed in that the metal jacket extends over the entire height of the transformer. 3. The EMC protection for a distribution transformer according to claim 1 or 2, charact eri z ed in that the metal jacket extends under the transformer, so that it encloses the transformer form all sides. 4. The EMC protection for a distribution transformer according to claim 1, chara ct e r i z ed in that the small distance is 12cm for transformers of 1OkV and 22cm for transformers of 2OkV, according to the electric safety regulations. 5. The EMC protection for a distribution transformer according to any one of claims 1 to 4, charact e r i z e d in that a temperature sensor is provided in connection with the transformer to control the operation of the heat exchanger. 6. The EMC protection for a distribution transformer according to claim 4, chara ct e ri z ed in that the temperature sensor is mounted to the transformer. |
EMC PROTECTION FOR A DISTRIBUTION TRANSFORMER
FIELD OF THE INVENTION
The invention relates to EMC protection for a distri- bution transformer as defined in the preamble of claim 1.
BACKGROUND OF THE INVENTION
In addition to safety, reliability and maintainabil- ity, an electric device is required to function faultlessly with other devices in the designated operating environment. Undisturbed functioning is ensured by electromagnetic compatibility (EMC) of devices intended for the same operating environment.
Different interference levels have been determined for residential and industrial environments in order for the devices to be able to function normally with each other. All devices in a given operating environment can be made compatible with each other when two factors are taken into account:
1) none of the devices generates higher interference than the agreed level,
2) all of the devices stand interference at the agreed level.
Presently, in both residential and industrial buildings, distribution transformers are normally located in the basement or the ground floor of the building, nevertheless being often close to residential or office facilities. Currents of up to thousands of amperes circulate in the transformers and three-phase bus bars departing from the transformers, generating in the immediate vicinity of the transformer strong
magnetic fields which, according to the new regulations, are not allowable.
The presently used solution to eliminate the problems provided by the magnetic fields is to line substantially the entire transformer room for example with an aluminum plate which prevents access of the magnetic field in the surrounding space. Installation of the protection as such is simple, but doors, windows, ca- ble and ventilation inlets make it more difficult to obtain good protection. Moreover, the protection is extremely expensive because the rooms to be protected are relatively large, and consequently large amounts of expensive protective metal sheets are needed.
OBJECTIVE OF THE INVENTION
The objective of the invention is to eliminate the drawbacks of the prior art referred to above. In particular, the objective of the invention is to disclose a novel EMC protection for a distribution transformer, which is efficient, does not require much space, is easy to build for already existing transformers as well and is inexpensive in terms of building costs.
SUMMARY OF THE INVENTION
For the features characteristic of the invention, reference is made to the claims.
The invention relates to EMC protection for a distri- bution transformer to eliminate the health-endangering effect of magnetic fields in the vicinity of a dry- type transformer. The invention relates specifically to dry-type transformers, i.e. air-cooled transformers. According to the invention, a uniform and sub- stantially seamless metal jacket which damps the mag-
netic fields is disposed from the top over a distribution transformer and all of its four sides substantially along their entire height, i.e. over most of their height, enclosing the distribution transformer from a small distance. Further according to the invention, closed air circulation is provided between the metal jacket and the transformer and, additionally, a heat exchanger is provided in connection with the metal jacket to cool the air circulation, i.e. to eliminate the heat losses from the transformer and thereby prevent excessive heating of the transformer. In this context, the closed air circulation is essential because it efficiently prevents access of impurities, such as dust, to the transformer, and also steadier running temperature is provided for the transformer .
Since strong currents generating dangerous magnetic fields normally occur at the upper end of the trans- former, in the bus bars located therein, and since the facilities to be protected from the magnetic fields are often located above and beside the transformer, the rotating magnetic fields can be cut and their harmful effects thereby prevented merely by suitably protecting the upper portion and the sides of the transformer. Of course, the best protection is provided when the metal jacket extends under the transformer as well, in which case the metal jacket encloses the transformer from all sides.
In this context, a small distance refers to the smallest distance determined by the electrical safety regulations, which is for in situ built untested structures 12cm for transformers of 1OkV and 22cm for transformers of 2OkV.
The efficient cooling provides clear advantages for the use of the transformer. When the temperature of the transformer is steady and sufficiently low in all conditions, it can be temporarily overloaded for up to 40% without problems. In addition, due to steady service conditions, the service life of the transformer increases for about 20%. Preferably, a temperature sensor is used in connection with the transformer and is most suitably mounted to the transformer for con- trolling the cooling. In this manner, the cooling operates in real time so that the transformer will not heat up and always delivers maximum power.
The magnetic field damping structures known per se, such as aluminum sheets, μ-metal sheets, various multilayer metal sheets and combinations thereof, may be used for the EMC protection of the transformer. The invention provides considerable advantages as compared to the prior art. The invention protects effi- ciently the surroundings of a distribution transformer, including the transformer chamber, from health risks induced by magnetic fields. It does not require much space and is easy and quick to build in the already existing facilities and transformers. The mate- rial requirements are small as compared with the known protections. Furthermore, the invention permits even heavier temporary overloading of the transformers and significant increase to their service life.
LIST OF FIGURES
In the following section, the invention will be described in detail with reference to the accompanying drawings, in which
Fig. 1 presents the known, protection technique and Fig. 2 presents the protection technique according to the invention.
DETAILED DESCRIPTION OF THE INVENTION
According to Fig. 1, in the prior art, a distribution transformer 1 is disposed in a transformer chamber 2. As the transformer generates around it a strong magnetic field, the impact of which extends also outside the transformer chamber, the inner surfaces of the transformer chamber are covered for example by a 5mm thick aluminum sheet 3. Various two- and three-layer structures have been used as well. The protection is efficient and prevents the effects of the magnetic field outside the chamber, but inside the transformer chamber, which people have to enter at least from time to time, the effect of the magnetic field is strong. Furthermore, the metal sheets suitable for the protection are relatively expensive and are easily consumed in tens of square meters even for smaller transformer chambers .
In the protection according to the invention presented in Fig. 2, a metal jacket 4 enclosing the transformer 1 is disposed right around it so that only a small air gap 5 is left between the transformer and the protection. This air gap 5 is provided with closed air cir- culation using a heat exchanger 6. In this manner, air is circulated around the transformer by the heat exchanger, and the heat is exhausted inside the transformer chamber 2. By the heat exchanger 6, the temperature of the air in the gap 5 and consequently of the transformer 1 is maintained steady and suitably low. It is, of course, possible that the heat be transferred outside the entire transformer chamber 2 by the heat exchanger, either to be utilized in other manners or into the outside air.
In addition, Fig. 2 illustrates a temperature sensor 7 mounted to the transformer 1 for controlling the heat exchanger β in real time. In this manner, no delay is provided in the cooling, which would be the case if only the temperature of the circulating air was measured.
In the schematic presentation of Fig. 2, the air gap 5 is relatively large, but in reality it may be fabri- cated quite small to the level required by the electric safety regulations, i.e. 22cm for transformers of 2OkV.
The EMC protection according to Fig. 2 prevents effi- ciently health risks induced by magnetic fields resulting from the strong currents of the transformer, both outside the transformer chamber 2 and in the transformer chamber itself. In this manner, the maintenance operations and other tasks conducted in the transformer chamber may be performed in compliance with the safety required by the new EU directives.
The invention is not limited merely to the examples referred to above; instead, many variations are possi- ble within the scope of the inventive idea defined by the claims.