DE1079167B - Resistance block for surge arrester - Google Patents

Description

GERMAN

Resistors for surge arresters consist of resistance material with voltage-dependent Resistance, d. i.e., the material has a very high resistance value at normal stress while the resistance decreases very quickly in the event of overvoltages, so that large currents can be diverted. When the voltage returns to normal, the resistance increases in turn, which causes the current is reduced to a very low residual current.

The resistance blocks for surge arresters are generally made of granular silicon carbide produced, to which a suitable binder and a mineral filler is added. Under application By pressure the resistor blocks are pressed into a suitable shape and then baked to to remove the excess moisture. A currently widely used filler material is Volume. The filler material and the binder act as an insulating agent with which the silicon carbide grains are in electrical contact and a breakdown of the block is prevented. The mineral Filler material also has a "smear" effect during the molding process, so that less pressure is applied is required to shape the block while at the same time achieving good closeness of the block will. It is essential that the block be made compact so that the adjacent Grains of silicon carbide have good electrical contact with each other. The resistance owns namely its unique dissipation properties due to the properties of the contact between the Grains of silicon carbide and the specific resistance of silicon carbide grains. However, it is difficult to get pure clay with uniform dielectric properties and lubricity, and therefore difficult to obtain a uniform structure of the block.

The known resistance blocks are also easily broken off when handling and changes in shape and bulges are unusable during the drying process. Such dents and deformations can interrupt the contact between the silicon carbide grains. In addition, there is the pressing due to the great pressure required with the binders and fillers currently used the possibility of the electrical properties of the block being adversely affected. One Mixture that could be molded at lower pressures is therefore desirable.

Sodium silicate, commonly used as a binder, is a hygroscopic material and avidly absorbs humidity. This fact is very disadvantageous because of the absorption of resistance block
for surge arresters

Applicant:

Westinghouse Electric Corporation,
East Pittsburgh, Pa. (V. St. A.)

Representative: Dr.-Ing. P. Ohrt, patent attorney,
Erlangen, Werner-von-Siemens-Str. 50

Claimed priority:
V. St. v. America. August 30, 1957

Norman K. Osmundsen; Wilkins Township, Pa.

(V. Si Α.),
has been named as the inventor "

Water the resistance block changes its resistance properties. It loses its valve properties and is unable to maintain its resistance value after a drainage process, if it has absorbed a certain amount of moisture. That's why then does a strong residual current flow, which leads to the destruction of the block? One has in the past tried to use non-hygroscopic binders, but with little or none Success, mainly because it does not have the excellent binding properties of sodium silicate own.

The aim of the invention is to provide a resistor block with a voltage-dependent resistance create that is mechanically strong, uniform in structure and resistant to water absorption and which the 'binder gives a high resistance after the discharge process.

In the case of a resistor block made of resistor material with a voltage-dependent resistor, in particular Made of silicon carbide, for surge arresters with an additive containing calcium silicate The object is achieved in that, according to the invention, the block is wollastonite as a filler (Calcium metasilicate) contains.

This creates an improved material for a resistor block for surge arresters.

90Ϊ 769/407

using a mineral filler material that has a needle-like structure. Through this the result is a networked, well-connected structure as well as a large dielectric Strength as well as good resistance to destructive flashovers. The invention is on Hand of the following, detailed description in connection with the drawings explained in which

Fig. 1 is a plan view of a resistor block for surge arresters and

FIG. 2 shows a section along the line H-II in FIG. 1 shows, the thickness of the coating of the resistor block in the interests of clarity was considerably exaggerated.

The illustrated resistance block 1 for surge arresters consists of resistance material with voltage-dependent resistance. It is roughly cylindrical. However, the invention is not limited to any certain size or shape of the block is limited. The resistance material preferably consists of granular silicon carbide mixed with a filler and a binder. To the electric To facilitate connection of the block, a zinc or copper plating 2 is placed on the opposite one Applied end faces of the block. In particular, zinc can be sprayed directly onto the block. A hard, solid, high-resistance sheath 3 is applied to the side surfaces of the block to prevent flashovers along the outside of the block and to avoid certain types of destruction of the block. The casing is made up of a suitable binding agent and a filler material. That The filler material is preferably wollastonite. However, it can also be any other suitable powder-form heat-resistant mixture, such as. B. talc, can be used. As a binder is generally Sodium silicate used. However, another water-soluble silicate can also be used. That Mixing ratio of filler material and the remainder sodium silicate or another water-soluble Silicate can vary between 35 and 65 percent by weight. The filler material and the binder are mixed until they form a homogeneous high viscosity mixture, adding water as needed is added. The jacketed block is then baked to remove the moisture from the jacket 3 to drive out and to harden and mechanically strengthen the coating. The sheath has one very high electrical resistance compared to the resistance of the block material 1. The one described Sheathing has already been suggested elsewhere.

The resistor material itself comprises a mixture of granular silicon carbide and, in one Embodiment, a binder made of sodium silicate and, according to the invention, wollastonite as filler material. A mixture with different proportions of silicon carbide, sodium silicate and Wollastonite can be used. The preferred value of the mixing ratios is as follows:

65

70

weight
at least percent
at most Silicon carbide 84.25
14.62
1.13 84.25
10.75
5.00 Sodium silicate 100.00 100.00 Wollastonite (CaSi O ₃ )

The above substances are carefully mixed until you get a homogeneous mass without lumps receive. The mixture is granulated and then brought into molds, where it is pressed into a appropriate shape is pressed, preferably as a cylindrical disc. The block is then out of the mold taken and baked to drive off the water. This results in a solid mass of resistor material with voltage-dependent resistance value, which is very suitable to serve as an overvoltage protection device. After the drying process the block is surrounded with the previously described sheathing and, as described above, dried.

As a result of the fibrous structure of wollastonite, there is an improved mechanical strength of the Blocks. It has a greater resistance to deformation and bulging during the first drying process. Deformation and bulging is a serious problem with the known resistor blocks and contributes significantly to scrap, which results in a correspondingly more expensive block.

Another major advantage of the new WoIlastonite filling material is its superior electrical insulation property and its uniformity. the The clay filling materials used up to now fluctuate between values of high insulation strength and those with low insulation strength. It is already known as a binding agent to use Portland cement for voltage-dependent resistances, which contains calcium silicate, among other things. However, the composition of Portland cement varies depending on the type of production and the raw materials within wide limits. Pure wollastonite, on the other hand, is constant in its properties, uniform in its dielectric values and has a higher insulation strength than the best so far used sound. Due to the handling during the manufacturing process before the final coating So far the side edges of the block can be knocked off. The new filler material results in a greater mechanical strength, as the whole block has a more uniform structure. It was further found that the lubricating properties of wollastonite are better than the lubricating properties of Volume. As a result, less pressure is required for molding, which reduces the adverse effect of the high pressure on the electrical properties, while at the same time a good electrical Contact between adjacent silicon carbide grains is achieved. The wollastonite makes one Even and very pure filling in the joints between adjacent silicon carbide grains. Through this reduces the possibility that the resistor block as a result of a breakdown of the Binder is destroyed.

Although only one embodiment of the resistor block is shown in the drawing, should yet another arrangement will be described. Since the section of FIG. 2 for both forms of is the same, no further representation is required.

It has been found that sodium silicate shrinks on drying to become microscopic Cracks on the surface. Such a surface has a strong tendency to moisture take up, whereby the material swells a little. Swelling can cause contact between the grains to be interrupted. In another embodiment of the invention, potassium silicate is used in place of

Sodium silicate used, with the weight proportions of silicon carbide, potassium silicate and wollastonite are identical to the proportions by weight of silicon carbide, sodium silicate and wollastonite, as they are in the first embodiment. The potassium silicate shows no tendency when drying to tear at the surface. Because of its dense surface, it absorbs water very little. Therefore, in the second embodiment, there are additional advantages to the use of wollastonite as a filler material, improved resistance to absorption of water and the disadvantages resulting therefrom, which are caused by replacing the sodium silicate is achieved by potassium silicate.

The invention results in a reduction in the number of failures in the resistor blocks for Surge arrester without affecting the threshold voltage or the closing voltage of the blocks will. The threshold voltage is the voltage at which the block becomes conductive and without further ado derives when the voltage rises. The closing voltage is the voltage at which the block absorbs the leakage current

interrupts in the decaying part of the surge wave.

Claims (3)

PATENT CLAIMS: 1. Resistance block made of resistance material with voltage-dependent resistance, in particular Resistance block made of silicon carbide, for surge arresters with a Calcium silicate-containing additive, characterized in that the block is wollastonite as a filler (Calcium metasilicate) contains. 2. Arrangement according to claim 1, characterized in that the block to 84.25 ° / »of his Weight of silicon carbide, 10.75% to 14.62% of a water-soluble silicate, in particular Sodium silicate, and essentially all of the remainder of wollastonite. 3. Arrangement according to claim 1, characterized in that the water-soluble silicate Potassium silicate is used. Considered publications:
German patent specification No. 645 870. 1 sheet of drawings ® 905 769/407 3. 60-