Short circuits in high voltage components, a CHP plant that would not start, and a single transformer fire may sound severe. They also led to power outages in a large number of areas in North Jutland; among others Aalborg, Silkeborg, Kjellerup, Struer, Brande, Engesvang, Tjørring near Herning, and Bedsted in Thy.
But the real reason for these events was Storm Malik and the way it unfolded over the weekend, explains Johannes Guldbæk, network operations department manager at N1:
“It was a relatively long and very dry storm. This means that salt from the sea scattered across Jutland and settled on the high voltage components. Normally, such a storm would be at its peak for a few hours and it would be accompanied by rain, which quickly washes away the salt,” says Johannes Guldbæk, who only remembers one more case in which salt from a storm had been the cause of a power outage during the last 25 years.
He emphasizes that isolators are designed to be able to cope with dirt and filth settling on them, which are frequently washed away when it rains. But salt poses a particular challenge because even though water itself is not electrically conductive, it is a completely different matter when it is mixed with salt. And this is exactly what happens when it gets foggy and humid:
“What happens is a corona discharge with sparks around isolators. Adding the salt to the equation contributes to a substantial outage,” Johannes Guldbæk says.
When a corona discharge occurs, a short circuit can appear from that phase to the ground:
“And since we are using a so-called ‘arc suppression coil grounded’ network, the ground potential rise moves to the faulted phase. This means that the voltage of the other two phases is increased by a square root of 3 in relation to the ground potential rise. This challenges the already stressed isolators, which means that the component then disconnects,” Johannes Guldbæk explains.
The distribution network operator can tell that salt has settled everywhere by looking at the number of outages and the fact that they have happened on all voltage levels. That is, ranging from 10, 15, 20, and 60 kV, which are operated by N1, and up to 150 and 400 kV, which are operated by Energinet.
“We have also experienced problems at the transformer stations, where salt has settled on the ‘horns’ of the transformers. In one place, we have had a fire in a mast-mounted 10 kV distribution transformer,” Johannes Guldbæk says.
The solution to the problem has been to lower the voltage level by 10–20 percent to reduce the voltage difference across the isolators. At first glance, this is not something that causes significant problems for ordinary consumers. But companies may have experienced their circuit breakers activating, and in Hjørring, the local CHP plant could not connect:
“When we lowered the voltage, a synchronization error occurred at the power plant. There we had some relays that couldn’t connect the synchronous generator because the voltage difference between the mains network and the generator was too large,” he explains.
The numerous power outages in North Jutland are definitely unfortunate and a great inconvenience to consumers, but Johannes Guldbæk emphasizes that all design requirements for high voltage components are of course complied with and that special consideration is given to particularly exposed areas:
“But we often install slightly longer isolators on high voltage lines if we are close to saltwater, because we know that problems can occur there,” he says, hoping that there will soon be substantial rain that can wash away the salt from the components.
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