How extreme weather causes premature aging in medium voltage equipment

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If you own or manage any large building, plant, or public infrastructure, you depend on reliable electrical distribution services. You also need equipment to be as sustainable as possible in terms of maximum lifespan and minimum cost of ownership. Most switchgear manufacturers and design engineers have recognized that new approaches are needed to protect equipment from increasingly extreme environmental conditions. Medium voltage (MV) equipment is getting particular emphasis due to its critical role in keeping operations running, as well as it typically being more exposed to the elements.

In this two-post series, we’ll discuss some of the conditions that expose MV equipment to special conditions that may influence its ratings and lifespan. We’ll also look at some of the advances in testing and technology that are helping switchgear survive those conditions, lasting longer with higher levels of reliability.

Understanding service condition changes in IEC 62271-1

Electrical switchgear contains a variety of materials – e.g. coated and uncoated steel, copper, aluminum, and insulating plastics and epoxies – that undergo changes during their lifecycle. Most equipment is designed for normal service conditions; however, performance and reliability can be negatively affected by corrosion of metals and aging of insulating materials. And both of these can be accelerated by extreme weather.

The IEC 62271-1 standard defines ‘normal’ service conditions in terms of daily temperature and humidity, and expecting an atmosphere that is not significantly polluted. The more often that the weather exceeds those conditions, the faster we can expect materials to deteriorate and associated failures to occur. The environmental conditions that switchgear is exposed can be complex. Let’s take a closer look at some of these.

Excessive heat

Switchgear standards define maximum operating temperature to be 35 °C average over 24 hours, while any switchgear is expected to operate within an average ambient temperature range during an expected lifespan. Exceeding these defined temperatures can, of course, have an effect on reliability and aging. Certain types of installations can be prone to this, especially when switchgear is installed inside a building – even more so if the building is non-insulated, exposed to sun radiation, and not ventilated well enough. And the presence of an indoor, oil-immersed power transformer can increase ambient temperature further.

High humidity

According to the IEC 62271-1 standard, the normal levels of humidity and vapor pressure for indoor switchgear are 95% and 2.2 kPa, respectively average over 24 hours (90% and 1.8 kPa average over a month). Obviously, one can expect that high humidity will be more of an issue in tropical climates than in temperate or arid ones. In these zones, vapour pressure can easily exceed 3 kPa over a month, while humidity does not typically exceed 80%. With humidity and changes in temperature can come condensation. Excessive condensation indoors can be addressed with suitable ventilation, heating, tight cable penetration or dehumidification.

Airborne pollutants

The two main factors in accelerated equipment degradation are condensation combined with airborne pollutants. Pollutants can include dust, smoke, corrosive or flammable gasses, or salt. This is why it’s important to assess the conditions at a planned installation site to specify any special service conditions. The IEC 60815-1 standard provides some guidance for measuring pollution deposits at a site to determine the potential impacts on insulating materials, while the ISO 9225 standard defines measurement requirements for environmental and atmospheric parameters. For example, a substation having a protection index for its housing won’t protect the switchgear with the same level if it’s installed in a coastal area (due to the presence of salt), an industrial area (due to presence of SO², NOx, etc.), or a rural or mining area (due to dirt and mineral deposits).

When considering levels of pollution, it’s important to take into account the salinity of the atmosphere. This can be caused by proximity to a coastal zone or anywhere de-icing is taking place, such as mountain roads. Also, the speed of salt deposits may typically be lower if switchgear is in a sheltered and ventilated area such as a substation; however, speeds of corrosion may be higher compared to outdoor equipment that is naturally washed by rain.

In fact, designers should be aware that though ventilation systems help to reduce heat and humidity, they can also act to bring salt and pollutants inside a building where switchgear is installed. This risk can be reduced by regular cleaning of equipment, as well as designing ventilation systems that use baffles and filters, and deliver the minimum flow required to evacuate heat.

In my next post we’ll have a look at some planning steps and MV innovations that can be used to further avoid the corrosive effects of extreme weather. To learn more about this subject, refer to the paper ‘Extreme Weather Has Become The Norm- Is Your MV Switchgear Ready For It?’. Also, to learn how Schneider Electric can help improve your design and specification expertise, please visit the Consulting Engineer page.

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