This application claims priority to Swedish patent application 0502170-4 filed 29 Sep. 2005 and is the national phase under 35 U.S.C. §371 of PCT/SE2006/050362.
The present invention relates to oil filled power transformer for high voltages with coils comprising a number of stacked winding layers comprising windings of insulated conductors, which winding layers are separated by spacers serving as distance and support members and arranged preferably perpendicular to the conductors, which spacers comprising a central body with upper and lower planes.
The invention further relates to a spacer for separating and supporting stacked winding layers of insulated conductors of a transformer coil at an oil filled transformer, which spacer comprises an elongated central body comprising upper and lower planes.
The main functions of spacers in oil filled transformers are to mechanically separate and support windings. Typically they are also stressed electrically with an AC electrical field and a high impulse electric field in testing, which is often dimensioning for the spacer thickness.
When transformer designs are optimized for maximum compactness the spacer ability to accept a high dielectric stress becomes vital. The allowed voltage between coils in transformers is often limited by the initiation of a breakdown outside the spacer and along the spacer-oil interface.
This effect occurs primarily as a result of the different dielectric constants of typical spacer materials and transformer oil. When a higher dielectric constant material like pressboard and transformer oil meet at a conductor, the electric field in the oil wedge is enhanced by a factor approximately equal to the ratio of dielectric constants, or 4.5/2.2=approximately 2 in the pressboard-oil case. There are several geometric ways that this field enhancement can occur.
Where a rounded spacer is in contact with the conductor, an oil wedge occurs in the contact area of the spacer and the conductor. The electric field in this arrangement increases at the contact area. The field in the contact area is approximately twice the average field away from the conductor. It is also known that the interface along the spacers is a weak point and that electric breakdowns preferable occur in the vicinity of the spacers. The oil volume exposed to this field enhancement depends on the geometry of the spacer, and is normally quite small.
Another critical area is where rounded conductors and spacers comes into contact with spacers which are arranged perpendicular to the conductors.
This oil wedge is present along the conductor on all turns of the transformer and consequently has a quite large volume and consequently a larger probability for triggering a discharge during impulse testing. Such a discharge created between the spacer and the conductor is probably not too dangerous if it happens far from the edges of the spacers, but if it happens close to the spacer edge there should be a substantial risk that the discharge propagates along the spacer-oil interface to the next winding layer, causing a breakdown. The observation in real testing is also that breakdown preferentially does occur at spacers.
Still another critical area is where an axial spacer, conductor corner and a radial spacer meet. At the outermost turn of a disc winding the conductor meets an axial pressboard spacer, which defines the distance to the next barrier. This barrier is followed by a further spacer, a new barrier etc. The result is a similar field enhancement at the axial spacer oil wedge, and a combined axial and radial field enhancement occurs at the outer conductor edge. This is the most vulnerable part of the winding, with the highest failure probability.
The present invention seeks to provide an improved oil filled power transformer and improved spacers getting improved breakdown strength of the transformer.
According to an aspect of the present invention, there is provided an oil filled transformer.
According to another aspect of present the invention, there is provided a spacer.
The insulation system is strengthened by creating barriers to the discharges that occur at the spacer edges, by altering the shape of the spacer. By this the discharge streamers are stopped by the barriers created by the addition of “wings” on the spacers. As these extension wings are thin in relation to the total spacer thickness they do not themselves increase the oil field substantially, as the straight prior art spacer do.
The barriers can be extended around critical corner mentioned above. This is achieved by extending the spacer wing barriers in the longitudinal direction of the spacer and bending it up- and/or downwards around the corner to protect the corner and the radial part of the outer coil edge towards the axial spacer.
The suggested shape of spacers can be applied to a range of possible insulating materials including all cellulose, ceramic as well as polymeric materials. The discharge protection effect would be substantial for all solid materials. The wings extending can be manufactured from the same or different material than the spacer itself.
For spacer materials that have a dielectric constant substantially higher than that of the liquid, and hence causes the largest withstand reduction, the insulation improvement would be particularly high. Further, the suggested shape can be applied for axial and radial types of spacers as well as other similar elements in transformers.
Embodiments of the present invention are schematically illustrated, by way of example only, in the drawings where
a and b show spacers provided with bent shields according to an embodiment of the invention,
When transformer designs are optimized for maximum compactness the spacer ability to accept a high dielectric stress becomes vital. The allowed voltage between coils in transformers is often limited by the initiation of a breakdown outside the spacer and along the spacer-oil interface. There are several geometric ways that this field enhancement can occur as we be illustrated in the following
Oil wedges 10 between conductors 3 and at the surface of a spacer 6 are shown in
At the outermost turn of a disc winding 5 the conductor 3 meets an axial pressboard spacer 12a, which defines the distance to a next barrier 13. This barrier 13 is followed by a further spacer 12b, a new barrier etc. as illustrated in
In
In
In
In order to take full advantage of the new spacer shape it could also be extended around critical corners. This can be achieved by extending the discharge barriers in the longitudinal direction of the spacer and bending it up- and/or downwards around the corner forming bent shields to protect the corner and the radial part of the outer coil edge towards the axial spacer. An example of such a design is shown in
The suggested shape of spacers can be applied to a range of possible insulating materials including all cellulose, ceramic as well as polymeric materials. The discharge protection effect would be substantial for all solid materials. The discharge barrier and bent shields can be manufactured from the same or different material than the spacer itself.
For spacer materials that have a dielectric constant substantially higher than that of the liquid, and hence causes the largest withstand reduction, the insulation improvement would be particularly high. Further, the suggested shape can be applied for axial and radial types of spacers as well as other similar elements in transformers.
Oil filled transformer according to the invention is designed for high voltage, suitably in excess of 10 kV, in particular in excess of 36 kV, and preferably more than 72 kV and up to very high transmission voltages, such as 400 kV to 800 kV or higher. Further, the oil filled transformer preferably is designed for a power range in excess of 0.5 MVA, in particular in excess of 20 MVA, and preferably more than 100 MVA up to very high power as 1000 MVA and above.
The core of such transformers has a diameter of more than 300 mm and the corresponding coil can have a diameter up to 4000 mm and the conductors cross section has the dimension height×width from 4×1.2 mm up to 18×6 mm.
Any range or device value given herein may be extended or altered without losing the effect sought, as will be apparent to the skilled person for an understanding of the teachings herein.
Disclosures in Swedish patent application No. 0502170-4 of Sep. 29, 2005 from which applications this application claims priority, are incorporated herein by reference.
Preferred embodiments of an oil filled transformer and spacers according to embodiments of the invention have been described. A person skilled in the art realizes that these could also be varied within the scope of the appended claims. Although the transformer has been described as oil filled, it will be appreciated that the transformer also can be filled with other liquids than oil/transformer oil, provided that the liquid having suitable properties as regards dielectric strengths, electric insulation and cooling.
Number | Date | Country | Kind |
---|---|---|---|
0502170 | Sep 2005 | SE | national |
Filing Document | Filing Date | Country | Kind | 371c Date |
---|---|---|---|---|
PCT/SE2006/050362 | 9/29/2006 | WO | 00 | 5/4/2010 |
Publishing Document | Publishing Date | Country | Kind |
---|---|---|---|
WO2007/037756 | 4/5/2007 | WO | A |
Number | Name | Date | Kind |
---|---|---|---|
3748616 | Weber et al. | Jul 1973 | A |
3775719 | Gainer et al. | Nov 1973 | A |
3902146 | Muralidharan | Aug 1975 | A |
4145679 | Mitchell, Jr. | Mar 1979 | A |
4219791 | Moore et al. | Aug 1980 | A |
5296829 | Kothmann et al. | Mar 1994 | A |
6870374 | Perkins et al. | Mar 2005 | B2 |
Number | Date | Country |
---|---|---|
10337153 | Mar 2005 | DE |
53085332 | Jul 1978 | JP |
6021513 | Feb 1985 | JP |
9092549 | Apr 1997 | JP |
2001345228 | Dec 2001 | JP |
Number | Date | Country | |
---|---|---|---|
20110037551 A1 | Feb 2011 | US |