DEVICE COMPRISING THE SAME

Technical Field The present invention relates to an electrically insulating pressboard comprising polysulfonamide fibrid and polsulfonamide fiber and to an electrical device comprising the same.

Backgr ound

Insulation of oil-filled distribution and power transformers may be made from cellulose and polymer pressboard. The cellulose pressboards are mainly used in transformers with relatively lower thermal stability requirements, and polymer pressboards are mainly for transformers with relatively higher thermal stability requirements. Nomex from Dupont is typical commercially available polymer pressboard. Generally speaking, cellulose pressboards are more extensively used than polymer ones. The major reason is that the cost of cellulose pressboard is much lower than those made of polymer. For certain applications, the combination of cellulose and polymer pressboard are also used for the balance of thermal stability and cost. The cellulose and polymer raw materials are converted by machines to pressboard, which is further converted to different insulation elements used in transformers. These elements include spacers, barriers, press rings and strips. Cellulose pressboards are mostly made from cellulose pulp fiber. The cellulose pulp fiber is prepared and then dried in sheets, by heating and pressing at the same time. Polymer pressboards are mostly made of polymer fiber and polymer fibrid. The polymer fiber for paper and pressboard preparation is short fiber which is generally made of normal continuous fiber with regular diameter, the short polymer fiber could be treated by further beating to develop their sheetmaking properties. The polymer fibrid, a type of fibrous particle used for binding, is with irregular shape and made from polymer solution, the normal process for polymer fibrid preparation is to get the polymer solution in a polar organic solvent first, then get the suspension of polymer fibrid by putting the polymer solution into the high shearing coagulation bath with the mixture of water and polar organic solvent as coagulation bath and finally get the fibrid by washing and filtrating the suspension of polymer fibrid in water several times to remove the polar organic solvent and the water. The polymer fibrid could be treated by further beating to develop their sheetmaking properties.

Although it is possible to prepare polymer pressboards from polymer fibrid directly without any polymer fiber, the polymer fiber is critical for mechanical property of the final product. Thus, the normal polymer pressboard contains both polymer fiber and polymer fibrid. The Nomex pressboard from Dupont, which is made from poly (metaphenylene isophthamide) fiber and poly (metaphenylene isophthamide) fibrid, has been used as key insulation part for transformer and generator successfully where the high thermal stability is necessary. While there are still some problems with Nomex pressboard, one problem is the moisture absorption of Nomex pressboard is very high, at about 4% or even higher, the transformer and generator manufacturers need long pretreatment time to remove the moisture inside the pressboard, if there is pressboard with lower moisture content, the manufacturers can substantially shorten the pretreatment time of pressboard, which will result in the lower fabrication cost obviously; another problem is the dielectric strength of Nomex pressboard in transformer oil is not so good, the dielectric strength of Nomex T994 after vacuum immersion treatment in transformer oil for 48 hour is about 30kV/mm. A pressboard with low moisture absorption, high dielectric strength in oil and high thermal stability is required in industry.

CN 101063280A discloses a synthetic fiber paper, which comprises polyester chopped fiber which is necessary for the disclosed paper, nylon chopped fiber, aramid chopped fiber or polysulfonamide fiber, aramid pulp or polysulfonamide pulp, felt fiber and dispersing auxiliary agent. The introduction of polyester could decrease the paper cost while the thermal stability is also deteriorated, it is well known that the thermal class of NMN laminates which comprise both polyester and aramid is F with using temperature of 155°C, the thermal class of Nomex is H with using temperature of 180°C and even higher. Indeed the introduction of other unstable polymer compared to polysulfonamide also will deteriorate the thermal stability of the final material.

CN 101892611A discloses a process to prepare a polysulfonamide paper, the dielectric strength of polysulfonamide paper with thickness of 0.15mm is only 13.5kV/mm, which is not so good, as the dielectric strength of Nomex T410 paper with thickness of 0.13mm is about 27kV/mm and that of Nomex T410 paper with thickness of 0.18mm is about 33kV/mm.

There is not too much work to improve the performance of polymer pressboard while as we described above, to develop pressboard with low moisture absorption, high dielectric strength in oil and high thermal stability is urgent for electrical industry. Summary

According to the present invention, there is provided an electrically insulating pressboard which comprises fiber and fibrid, wherein the fibrid is polysulfonamide fibrid. According to another aspect of the present invention, wherein the fibrid in the electrically insulating pressboard may have a specific surface area of 3 m 2 /g to 80 m 2 /g, preferably 10 m 2 /g to 40 m ² /g.

According to one embodiment of the present invention, the fiber in the electrically insulating pressboard is polysulfonamide fiber. According to another embodiment of the present invention, the fibers may have a length of 1 mm to 20 mm and preferably 2 mm to 7 mm. According to one embodiment of the present invention, the polysulfonamide fiber is present in the electrically insulating pressboard in an amount of from 10 to 90 wt , preferably 30 wt to 70 wt , the polysulfonamide fibrid is present in the electrically insulating pressboard in an amount of from 10 wt to 90 wt , preferably 30 wt to 70 wt , based on the total weight of the electrically insulating pressboard. According to one embodiment of the present invention, the electrically insulating pressboard may further comprise a nano filler and the said nano filler may be selected from nano silica, nano alumina or their mixtures.

Another aspect of the present invention relates to an electrical device comprising the above electrically insulating pressboard, such as an electrical transformer or an electrical motor. Yet another aspect of the present invention relates to the use of polysulfonamide fibrid and polysulfonamide fiber for preparing an electrically insulating pressboard. Such pressboard in turn can be used to prepare electrical devices, such as a transformer or an electrical motor.

The inventors have found that the pressboard made from polysulfonamide fibrid and polysulfonamide fiber showed excellent dielectric strength in oil and low moisture absorption. Specifically, the pressboard with 60 wt polysulfonamide fibrid and 40 wt polysulfonamide fiber displays excellent performance compared with Nomex pressboard, the dielectric strength in oil is more than 50kV/mm and the moisture absorption is lower than 1%.

Brief Description of the Drawings Embodiments will be described, by way of example, with reference to the accompanying drawings, in which:

Fig 1 is a schematic flow chart of an embodiment of a method according to the present invention.

Detailed Description Embodiments will now be described more fully hereinafter with reference to the accompanying drawings, in which certain embodiments are shown. However, other embodiments in many different forms are possible within the scope of the present disclosure. Rather, the following embodiments are provided by way of example so that this disclosure will be thorough and complete, and will fully convey the scope of the disclosure to those skilled in the art. According to some embodiments of the present invention, there is provided an electrically insulating pressboard comprising fiber and fibrid, wherein the fibrid is polysulfonamide fibrid. Preferably, the fibrid used in the electrically insulating pressboard may have a specific surface area of 3 m 2 /g to 80 m 2 /g. According to some embodiments of the present invention, the more preferred fibrid may have a specific surface area of 10 m7g to 40 m g.

According to some embodiments of the present invention, the fiber is polysulfonamide fiber. Preferably, the fiber used in the electrically insulating pressboard may have a length of 1 mm to 20 mm, preferably 2 mm to 7 mm. According to some embodiments of the present invention, the electrically insulation pressboard essentially comprises polysulfonamide fiber and polysulfonamide fibrid. In particular, the polysulfonamide fiber is present in the electrically insulation pressboard in an amount of from 10 to 90 percent by weight, preferably 30 to 70 wt%. The polysulfonamide fibrid can be used in an amount of 10 wt% to 90 wt%, based on the total weight of the electrically insulation pressboard. Preferably, the polysulfonamide fibrid is used in an amount of 30 wt% to 70 wt%, based on the total weight of the electrically insulation pressboard.

According to some embodiments of the present invention, the electrically insulating pressboard may further comprise fillers. Preferably, the fillers are nano fillers. In particular to the current invention, the amount of the nano fillers is preferably from 0 wt% to 10 wt%, based on the total weight of the electrically insulation pressboard. There is no limitation to the specific types of filler, provided that better dielectric strength and electrical creepage resistance can be obtained. Commonly used nano fillers in the field are nano silica, nano alumina and/or their mixture.

As can be seen from the below Example 1 in which the pressboard comprises 60% polysulfonamide fibrid and 40% polysulfonamide fiber based on the total weight of the pressboard, by the combined use of polysulfonamide fibrid and polysulfonamide fiber, the pressboard showed excellent dielectric strength in oil, and lower moisture content. The dielectric strength in oil can be improved to more than 50kV/mm, which is much higher than that of Nomex T994 pressboard, which is about 30kV/mm. the pressboard had a moisture content of about 0.9 wt , which is much lower than Nomex T994 pressboard, which is more than 4%. The lower moisture content can substantially shorten the pretreatment time of pressboard for transformer application, which results in the lower fabrication cost. With the improved dielectric strength in oil, the transformer could have better performance stability.

Preferably, the pressboard according to the present invention has a thickness of higher than 0.9 mm. More preferably, the thickness of the pressboard is 1-12 mm, and most preferably 1-8 mm. The pressboard is electrically insulating and is suitable for use as insulation material in an electrical device. The pressboard may be, for example, used as electrical insulation in an electrical device, such as in a power transformer, whereby the pressboard may be a high voltage insulation material.

As mentioned above, the electrically insulating pressboard may have especially beneficial electrically insulating properties in an oily environment. Thus, the electrically insulating pressboard may be at least partly soaked in oil.

The present invention further provides an electrical device comprising the electrically insulating pressboard according to the present invention. The electrical device may be any electrical device which comprises electrical insulation, e.g. an electrical transformer or an electrical motor, which may especially benefit from the pressboard, such as with lower moisture absorption, less pretreatment time is needed for the removal of moisture. Especially, the electrical device according to the present invention is an electrical transformer.

The electrically insulating pressboard may be in the form of a spacer, barrier, strip or press ring for insulation of an electrical device, such as an electrical transformer. The electrically insulating pressboard has electrically insulating properties which may be useful in any electrical device but the electrically insulating pressboard may be especially advantageous in an oily environment, such as in an electrical transformer. Specifically, the electrically insulating pressboard may be used for making electrically insulating spacers in a transformer winding.

An improved electrical device is obtained by using the electrically insulating pressboard in accordance with the present invention. In particular, the electrically insulation pressboard displays lower moisture absorption and improved dielectric strength in oil.

Fig 1 is a schematic flow chart of an embodiment of a method 1 according to the present invention for producing polysulfonamide based electrical insulation pressboard, e.g. for an electrical transformer. Fibrids are provided, see 2, and fibers are also provided, see 3. The fibrids and fibers are then mixed with each other, see 4. A pressboard press, multi-daylight hot press of the like, is then used for pressing the mixture to provide a pressboard see 5. The pressing also comprises heating, see 6, and drying the mixture, see 7, as well as pressing the mixture to the pressboard, see 8. The pressboard was then cooled, see 9. The cooled pressboard may then be cut into desired insulation parts, for example, for use in a transformer or any other electrical device. For instance, a spacer, barrier, strip or press ring for insulation of an electrical transformer, can be produced from the pressboard from this invention.

Examples

An electrically insulation pressboard according to the present invention, which is comprising polysulfonamide fibrid and polysulfonamide fiber, was produced and tested for its properties under the IEC (International Electrotechnical Commission) standard 60641-2.

Example 1

A pressboard was made according to the process in Figure 1. The solid materials used in the making of this pressboard were 60 weight percent of polysulfonamide fibrid (Shanghai Tanlon Fiber Co., Ltd) and 40 weight percent of polysulfonamide fiber (Shanghai Tanlon Fiber Co., Ltd). This pressboard had a basic weight of 1160 g/m , a thickness of 1 mm and a density of 1.16 g/cm ³ .

The weight reduction of the final pressboard is 0.93%, 0.94%, 0.94% and 0.94% after drying for 24h, 48h, 168h and 288h respectively according to IEC 60641-2, item 13, and the weight reduction of Nomex T994 is 3.08%, 3.31%, 3.97% and 4.1% after drying for 24h, 48h, 168h and 288h respectively. It is clear that the moisture content of invented pressaboard is much lower than that of the commercialized Nomex pressboard T994, the transformer manufacturers can substantially shorten the pretreatment time of pressboard, which will result in lower fabrication cost obviously.

Following the dielectric strength testing method in IEC 60641-2, item 20, the dielectric strength of the invented pressboard in oil is 53kV/mm, while the dielectric strength of Nomex T994 is about 30kV/mm which is much lower than that of the invented pressboard.

The 5% decomposition temperature of the invented pressboard determined by thermogravimetry analyzer (TGA) in air with heating rate of lOoC/min is about 432°C and that of the Nomex T994 pressboard is about 421 °C.