DE1808810A1 - Contact material for high performance vacuum switches - Google Patents

Description

Contact material for high-performance vacuum switches "The invention refers to a contact material for high performance vacuum switches, vacuum switches for medium and high voltages have become increasingly important in recent years attained. They stand out as compared to switches with gases or liquids Isolation and extinguishing media due to a smaller volume, due to a simple mechanical Construction, a very short recovery time after a short circuit shutdown and a long service life.

However, the contact material is essential for a vacuum switch to function properly decisive. Higher demands are placed on him than on contact materials, of switches that work in air or in 41, To the usual demands after low, constant contact resistance, after smaller Burn rate in the arc and low tendency to weld with current flow occurs with contact materials for vacuum switches as a further condition that the contact piece surfaces in the case of local melting, no gases are allowed to emit, because this reduces the working vacuum would be deteriorated and the switch would no longer be able to handle short-circuit currents to interrupt. In switches for higher currents, it is still important that the contact pieces acting as electrodes supply enough metal vapor so that the arc burns to zero current.

Depending on the switching capacity, these requirements have different weight and determine the contact material.

Solid tungsten, for example, is suitable up to a few thousand amperes; in In its very pure form, it can switch off short-circuit currents of up to 15,000 A.

At higher currents, a specific one for the vacuum arc sets in Appearance - the breaking of the electric arc - ("current chopping") of use pure tungsten limits.

As a result of the low level of evaporation of this refractory metal the discharge path becomes impoverished before the alternating current arc crosses zero so far on charge carriers that the arc suddenly breaks at approx. 10 A. In inductive Load circuits of medium voltage networks arise thereby voltage peaks up to 100,000 V, which can cause damage to the systems.

Therefore, other contact materials are used for higher cut-off currents (around 20,000 A) such as gas-free copper, which is a metal used to prevent the arc from tearing off is added with high vapor pressure and high atomic weight.

The alloy evaporates due to its reduced thermal conductivity lighter and the heavy metal vapor atoms slowly diffuse out of the arc column into the vacuum.

It is preferable to add a metal to the copper, which at low and mean currents, the arc voltage and thus the energy consumption at the same time lowers in the arch column.

These properties of the low-melting Eontaktwerkstoffes, the are desirable at currents around 20,000 A, but limit its usability Currents over 40,000 A.

The loss of material from the contact pieces becomes too great and the metal vapor concentration becomes too great in the contact gap is so high that a rapid re-solidification of the switching path can no longer be reached.

When a circuit current of 40,000 A is switched off, Transient processes briefly occur with current peaks of 100,000 A. The energy expenditure in the vacuum arc this achieves values in the order of magnitude of 10 megawatts.

The major part of the arc energy is applied around the switching gap concentrically arranged sheet metal transferred on which the metal vapor should condense.

The energy transfer takes place largely through metal vapor and Melt droplets thrown off the contact piece surfaces. The condensation sheet heats up to a temperature of several hundred degrees; its coefficient of accommodation decreases so much that it is practically ineffective. The metal vapor returns in the plasma column and prevents reconsolidation through its concentration in the contact gap. The vacuum switch loses its advantage in this way fast reconsolidation and long service life.

If the distance between the condensation shield and the plasma column is increased, then the resulting energy density and thus the temperature will decrease, but it is the effect is limited and is bought at the expense of a larger switch volume. Alloyed Contact materials, e.g. copper-based, are evident at cut-off currents of around A 50,000 have reached their limit of employment.

Tests with currents up to 150,000 A have now surprisingly shown that the loss of substance from contact pieces made of pure metals and desires a short-term arc load with very high power densities normal evaporation takes place, but mainly by throwing off very small melting pots.

The ratio of droplet weight to vapor weight is less the switching conditions between 9: 1 and 2: 1. The heating of the condensation plate can therefore be significantly reduced if one considers the energy transport through melt droplets prevents.

According to the invention, this is now done in that the boiling metal melt is held in a framework of a refractory metal. The loss of substance of the electrodes sinks on average to 25 percent of the burn rate of the pure Metal. Tungsten is suitable as a framework metal, but other high-melting points are also suitable Metals such as molybdenum, tantalum, hafnium or niobium can be used.

This measure not only increases the service life of the contact pieces extended to a multiple, but also the effectiveness of the condensation screen Significantly improved and the reconsolidation of the discharge path after extinction of the arc accelerates.

A prerequisite for this is, however, that the structure of the high-melting point The material itself emits as few thermoelectrons as possible, and thereby re-solidification delayed. Not only must the metal be gas-free, but it must also not contain any impurities contain, which favor electron emission. It has to be in particular be free of thorium, rare earths, alkali and alkaline earth metals and carbon and boron.

The following example shows the manufacture of a contact material described to switch off short-circuit currents of more than 50,000 A in vacuum switches Capable of: Highly pure, zone-melted tungsten is made smaller as a powder or in the form of a smaller product Balls (# 0.1 mm) with analytically pure copper or silver powder, in particular may not contain any of the mentioned emission-promoting elements, to blanks with a space filling between 40 and 70 percent. The proportion of tungsten should be between 70 and 98 percent by weight. This body is in a high vacuum (p = 1016 Torr) inductively to a temperature above the copper or silver melting point heated, whereby the liquid additive material wets the grains of the tungsten framework and releases absorbed gases during pumping. Then the body is through indirect or direct explosion deformation or through magnetodynamic deformation in a vacuum to a disk (d 10 cm, h 3 cm) with a degree of space filling of more compressed as 99%, Highly pure can also be used in the same way Contact materials based on Mo, Ta and Nb are produced. Zr and Ti are suitable to a lesser extent because of their high affinity for gases.

Claims (3)

Patent claims: 1. Contact material for high performance vacuum switches with condensation shield characterized by a composite material in which a framework consists of a high-melting point Metals such as tungsten, molybdenum, hafnium, niobium or tantalum, which are free from gas, are rare Earths, alkali and alkaline earth metals, thorium, carbon and boron is one for high voltage switchgear contact material known per se, such as gas-free silver, copper or nickel, or an alloy of these metals is embedded. 2. Material according to claim 1, characterized in that a part of the metal or metal alloy stored in the device by a metal high vapor pressure, such as antimony, thallium, lead or bismuth is replaced. 3. Material according to claim 1, characterized in that the material of the stored metal fills 2-30%, preferably 3-15% of the volume.