Going with the flow - Vacuum ensures stability for energy supply

Going with the flow - Vacuum ensures stability for energy supply

Vacuum switches are just small cogs in the complex power grid system. But they play a key role in ensuring that electrical energy is available reliably around the clock.

From the power plant to the consumer
When we switch on the coffee maker or charge our smartphones, we usually don't think about the highly complex infrastructure that provides us with electricity. In Germany, for example, a grid over 1.8 million kilometres in length distributes the energy from power plants to end consumers. High-voltage electricity of 220,000 or 380,000 volts flows through the power lines.

For rough regional distribution, this is usually initially transformed down to 110,000 volts. Then it is brought to a medium voltage between 50,000 and 1000 volts in smaller substations until the electricity finally makes it to municipal and local grids at a low voltage of 230 volts. Due to their high energy requirements, industrial companies, hospitals, larger swimming pools or broadcasting towers often have their own transformer stations, which are usually directly fed from the medium voltage grid. Grid management also ensures a balance between production and consumption. Countless switches are constantly actuated during the transforming process and during ongoing adjustment of current flow.

Sometimes you can see a little spark of light if you press a simple electrical switch in the dark. These flashes are created when the contacts get closer or just after they separate. The electricity ionizes the air between the contacts; the electricity flows through it up to a certain distance further than that between the contacts. The small flash is a tiny electrical arc. While this physical phenomenon is harmless when flipping a light switch at home, it can develop destructive force in high- and maximum-voltage switches. The electrical arc would reach over 5000 degrees Celsius there – hot enough to set fire to most materials in a fraction of a second.

Vacuum prevents electrical arcs

However, contact burning would immediately endanger the functionality of the entire connected power grid. Furthermore, a fire can also spread to other switches or components in high-voltage stations. This is why air is already evacuated from high-voltage switches during the manufacturing process. A vacuum interrupter encloses the switching contacts. In vacuum, there are no air molecules that could potentially be ionized and become a carrier of the electric arc.

But a small electrical arc still occurs because small amounts of metal from the contacts evaporate under high voltage current. Their certain type of shape – cylindrical with slanted slits – and their material ensure that these electrical arcs are only able to develop low heat. Also, in a vacuum they remain isolated and cannot cause damage. Evacuating the switch housing is thus an important prerequisite for grid stability.

Faults in power plants are among the obvious causes of power outages. But when a generator fails, other power plants normally provide more electricity in appropriate amounts within a very short time, thus compensating for the failure. A power outage lasting a few seconds can also be caused if lightening hits a pylon that is part of the medium voltage grid. If a power outage in a large section of the grid lasts for minutes, or even hours, energy experts refer to it as a blackout. In these cases there is complete power failure. Some examples of events that can trigger blackouts are fallen trees that sever an important line, or a short circuit. Extreme weather conditions in winter can also have serious consequences. If melting snow or sudden rain after a long period of cold causes a thick layer of ice to build up on power lines, its weight can cause them to break, thus interrupting energy transmission. Sometimes the affected pylons also tip over and, in the worst case, a domino effect is triggered in which other pylons tip over. But if we take a look at the largest blackouts in history, they were most often caused by the grid itself: by excessive fluctuations or overloads, or due to technical malfunctions of important grid components.

In Europe, the frequency of power outages is indicated with the so-called SAIDI value (System Average Interruption Duration Index). It establishes a ratio between the length of outages with the number of consumers and indicates the total outage in minutes per year. In 2013, Luxembourg had the lowest SAIDI value with 10 minutes, closely followed by Denmark, Switzerland and Germany.

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