Voltage breakdown

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If a voltage that is higher than the dielectric strength or breakdown voltage is applied to an insulator (which can be solid, liquid or gaseous), an electrical breakdown occurs , also known as a voltage breakdown . For a certain time, a channel is formed in which an electrically conductive plasma is created from the material of the insulator through heat and ionization . The ultraviolet radiation emanating from the plasma knocks further electrons out of the material of the insulator, which further increases the conductivity in the channel. Depending on the nature of the power source, the breakdown can quickly extinguish as a spark or continue to burn as an arc . In the event of a voltage breakdown, the insulating material along the route taken by the spark is often irreversibly changed or even destroyed. Plastics can partially carbonize due to the heat of the spark and are then useless as insulators. Insulating oils develop gases through thermal decomposition, which z. B. in the Buchholz protection of transformers to detect an insulation fault are registered.

Vacuum breakdown

Even in a vacuum , electrical breakdown can occur between two adjacent metallic conductors with a high potential difference. Since there is no insulation material between the conductors in a vacuum that could be ionized, the breakdown of electrons is initiated, which overcome the potential barrier (work function) from the metal due to the high electric field strength between the conductors ( field emission ). With copper, these energies are around 4.5 eV - this corresponds to electrical field strengths of around 1 MV / mm. This is an upper limit and only the case with ideally smooth surfaces of the electrical conductor. In practice, small bumps in the metal surface result in locally much higher field strengths, while the mean field strength is in the range of only 10 kV / mm. This can lead to electron emission even at such low mean field strengths (application, for example, in the field emission microscope ). The vacuum flow generated in this way is ideally constant.

If the current is sufficient, however, a breakthrough can occur according to two mechanisms:

  • Cathode-initiated breakthrough: Fine unevenness of the surface that is practically always present evaporate due to the high current densities of the field emission current and release the metal vapor necessary for the subsequent ionization process. Local current densities of over 100 MA / cm 2 occur in the metal tips . The vaporized metal forms an ionized gas cloud over the metallic conductor. The metal ions are now used for transport, as the current increases like an avalanche due to the ever lower resistance.
  • Anode-initiated breakdown: The few electrons emerging from the negatively charged conductor are strongly accelerated towards the positive conductor by the electrical field and strike it. This heats up the anode strongly - parts of the metal surface evaporate and form an ionized, conductive gas. When the electrons collide, X-rays are also produced , which in turn can contribute to ionization.

In practice, combinations of the two breakdown processes usually occur with vacuum breakdown.

literature

  • Andreas Küchler: high voltage technology, basics - technology - applications . 4th edition. Springer-Vieweg, 2017, ISBN 978-3-662-54699-4 .