Vacuum Interrupters and Embedded Poles for Medium Voltage (1)

Vacuum switching technology is nowadays the dominant switching principle in medium voltage. Innovative developments are leading to a continuously increasing market growth, based on the fundamental advantages such as reliability, availability, compactness and, last but not least, the environmental friendliness of the vacuum as a switching medium.

Circuit-breakers have the function in the closed condition of conducting operating and short-circuit currents. They must also be capable of connecting and disconnecting these currents and, in the open condition after interrupting the current, isolating different potentials from each other. Various technologies have been used in recent decades to secure the functionalities described above. Interruption of a current is always associated with the formation of an arc between the switching contacts, irrespective of which quenching medium surrounds those contacts. These media include gases, for instance air or SF6, and liquids such as oil.

While compressed air circuit-breakers and minimum-oil circuit-breakers are practically no longer used in new and retrofitted medium voltage installations and the proportion of SF6 circuit-breakers is constantly dropping, the number of vacuum circuit-breakers is significantly rising. Figure 1 shows the development in shares of the various technologies installed from 1980 to 2010.

A vacuum circuit-breaker is a three phase device, with main components consisting of a mechanical or magnetic operating mechanism and the three pole parts. The current interrupting devices, the vacuum interrupters, are located in the pole parts.

ABB commenced manufacture of the first vacuum interrupters in the early 1980s. By the mid-1990s, annual production had reached around 30,000 units, and that level has risen almost ten times today in response to strong demand from the market. ABB is therefore now the world's largest manufacturer, with over 1.5 million vacuum interrupters installed.

At the same time, development activities (including cooperation with research institutes and universities) have continuously been intensified, new modelling and simulation methods introduced, and innovative manufacturing and testing strategies implemented. One special example is the market launch of embedded poles, in which the vacuum interrupters are directly enclosed in the insulating material, an epoxy resin moulding, in a special manufacturing process. In conjunction with the maintenance-free vacuum interrupters, this solution provides high dielectric insulation strength for service in a wide range of ambient conditions, and has proved successful on the market, above all with the ABB vacuum circuit-breakers of types VD4 and VM1.

Vacuum interrupters and embedded poles as components are a promising solution for manufacturers of medium voltage switchgear. Using these components to construct vacuum switching devices, they can participate in the forward-looking vacuum technology simply and, most particularly, cost-effectively, and position forward-looking competitively on the market.