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Acoustic Screens, Acoustic Enclosures and the danger of Impressed Voltages

One of the dangers of working close to the high voltages found within an electricity sub-station is the risk of impressed voltage. It’s not something that anyone would normally have to deal with or have even ever heard of.

The technical definition of an Impressed Voltage doesn’t make it that much clearer to most of us either. National Grid, who own the electricity and gas transmission system in England and Wales with day-to-day responsibility for balancing supply and demand, define it as “Voltages that occur on conductive elements that are not locally earthed.”

What this means in layman’s terms is that the parts of the installed transmission system present hidden dangers that are not always obvious. The hazard zone for impressed voltages can reach well outside of the footprint of the apparatus and means that direct contact with the installation is not necessary in order for people and property to be at risk. All that is required is the presence of three elements – an energy source (usually the installed system), a coupling mechanism, and a receptor. With the presence of these three elements it is possible for energy to be transferred indirectly by magnetic fields (induction), by electrical fields, by stored energy, and by transferred potentials.

As you can imagine, this presents huge challenges for the team when they are designing and building acoustic enclosures, acoustic screens, acoustic fences or other structures to reduce nuisance noise from a variety of substation equipment such as supergrid transformers, shunt reactors, or static VAR compensators etc.

Back in 2014 the Kimpton Acoustic team completed a series of acoustic screens around a number of items of equipment within the Beauly Substation near Inverness. The project itself was already high profile with some press coverage around the previous ‘humming’ noise of the substation that was built as part of the Beauly to Denny power line upgrade

The Kimpton Acoustics team on site as the Acoustic Barriers are dismantled with the decommissioned transformer to the side working around impressed voltage

Recently we were back on site to assist with essential maintenance works during a planned electrical outage. This required two of our acoustic screens to be partially removed to enable access to withdraw the equipment inside and then re-erect it again once it has been skidded back into place. With the demand on the power transmission system being so great, outages are a headache for the network and downtime has to be kept to a minimum. With this in mind Kimpton were called upon to plan and execute their work in a series of brief phases to tie in with the maintenance team’s programme to deliver the required work in the shortest possible timescale.

The space has been made to slide the new transformer into place by removing the acoustic barriers

This site is also home to one of our very innovative products. Part of the original design brief required a non-conductive acoustic screen due to its proximity to equipment that presented an impressed voltage risk. To have used a steel acoustic screen in this location around a static VAR compensator could have been catastrophic and not only presented risks to people but also to the structure itself. Kimpton’s design team solved the problem by deploying their polypropylene acoustic screen for this application. The plastic acoustic screen, although a more expensive option, is not conductive and so solved the problem at a stroke and meant the screen could be erected exactly where it needed to be.

With the new transformer in place, the new acoustic barriers are installed to enclose and reduce the transformer noise impressed voltage

You can see from the following image the scale of the enclosures and the huge equipment they contain. All of the Kimpton Acoustic team are fully conversant with the requirements for working in controlled and complex industrial environments, and the contracts team have all undertaken specialist training to work on UK substation sites.


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