Electrical breakdown of insulation media
Like all matter, insulation media appear in solid, liquid and gaseous form. There is a great variety of materials that may be used for insulating purposes. For instance wood, glass, porcelain, rubber and cotton have been used and are still in use as solid insulators, next to more modern materials like plastics, epoxy and cross linked polyethylene (XLPE). Mineral oil is for well over a century in use as insulating and cooling medium for high voltage transformers and is used in oil insulated cables, be it that this type of cable has mostly been replaced by XLPE cables. The wide use of air as gaseous insulating medium is well shown by overhead transmission lines and outdoor substations, but atmospheric air is also used in metal enclosed medium voltage switchgear as principal insulator. For metal clad switchgear at higher voltages, air under pressure was used as insulating medium and extinction medium in circuit breakers. From the mid-seventies of the last century, inorganic Sulphur hexafluoride (SF6) gas is widely used while it has outstanding characteristics to withstand high electric field strengths and superior arc extinguishing capabilities, which is gratefully used in all high voltage circuit breakers. The downside of this insulator is its extreme global warming potential as most potent greenhouse gas. It is therefore that tightness of equipment containing this gas is of paramount importance in order to reduce leakage to an absolute minimum. In the following paragraphs, we discuss the breakdown mechanisms of gaseous, liquid and solid insulating materials. But we start our survey discussing an insulator showing no matter at all. This vacuum was widely used in electron tubes in electronic equipment and Cathode Ray Tubes (CRT’s) in television sets. These applications have been taken over by solid state devices. Nowadays vacuum is still used in X-ray tubes and since the 1970’s, the vacuum circuit breaker is increasingly applied in medium voltage power systems.
Vacuum is space where matter is totally absent and one would expect that if no (charged) particles are present in the gap between electrodes, the withstand voltage of such an empty system must be extremely high, because where nothing exists, nothing may break down. In reality complete emptiness does not exist, so breakdown will eventually occur as we will see, be it that technical vacuum shows high breakdown voltages. In practice (high) vacuum is well attained when the resulting gas pressure is below 10-5 Torr (1 Torr = 133.3 Pa = 1.333 x 10-3 bar) when the number of particles left in the area is less than 1.5x1011per cm3. The length of the mean free path λ, that is the distance any particle may travel without colliding with another particle, is in that case well over 7 metres, which outmeasures normal dimensions of vacuum insulated constructions.While for all media holds that breakdown occurs when ionizing processes lead to conductance of the insulating medium, a large mean free path prevents ionization processes in the vacuum gap between electrodes. So, if electrons are not available in the space between electrodes, the electrodes must be the supplier of charged particles and electrons must be ‘freed’ from the electrodes to arrive at an electrical current between positive electrode (anode) and negative electrode (cathode). If this current becomes large enough, breakdown manifests itself by the occurrence of an electric arc between anode and cathode, accompanied by a steep voltage drop across the electrode-gap and thus low resistance of the insulation medium.