Common problems in medium voltage switchgear

Electrical switchgear: how it’s got its name?

Many years ago power stations used simple disconnectors, open knife switches, to isolate or de-energize equipment. With the continuous increase in power levels, it became no longer safe to open such disconnectors manually and geared switches were a need which is then combined to be “Switchgear”.

Switches didn’t only be geared, but also it was developed to include all the regulating, controlling, metering and switching devices in one unit.

Electrical switchgear: what is it?

As shown in figure.1 it’s a device in a metal-enclosed structure used to isolate and de-energize electrical equipment. It contains combination of disconnectors, fuses or circuit breakers and protection relays for the aim of isolation and protection, control panel for the aim of control and current and potential transformers for the aim of measurement.

It is also filled with oil and replaced now with air, insulating gas like SF₆ or vacuum in high ratings.

After knowing what the switchgear is, it’s obvious that we don’t want to lose it. So knowing the problems that usually switchgears have is very important to be able to avoid them.

Problems a medium voltage usually faces

In this article we take a deeper look into the common two types of faults and failures reported in switchgears so in future we will be able to avoid them and ensure the availability of the equipment for the longest time ever.

Mechanical Failures

As it’s seen in figure.2 the mechanical failures represents the highest ratio with nearly 30 % which is increase to 38 % when the switchgear is run in a faulty manner. So working on detecting the problems because of mechanical failure and trying to avoid it will potentially effective in protecting the equipment.

In the surveys done by IEEE, which are mentioned in IEEE Standard 493-1997, IEEE Recommended Practice for Design of Reliable Industrial and Commercial Power Systems. It was reported that the exposure to moisture contributes with 30% in the failure of the bus of the switchgear.

Whenever it’s exposed to high humidity conditions switchgear will absorb moisture, and hydrophobic insulation surfaces will be attacked by voltage stresses which were designed to inhibit moisture absorption. Water due to any reason can lead to short circuits, insulation damage, and long term metallic component corrosion, among other complications.

Also the exposure to dust contributes with 19 % in failure of the bus of switchgear.

Partial discharge

As it’s seen in figure.2 the partial discharge failures comes second in the highest ratio with nearly 26 %. Partial discharge is the conduction of two electrodes and a discharge of electricity passes through them. At any location within the insulation, wherever the strength of the electric field is more than that of the insulating material breakdown, partial discharge may happen.

The faulty connections in switchgear cause an increase of resistance which leads to an increase in heat. That continuous increase in heat will cause thermal failure of the connection resulting in a fault. Failure of insulation usually occurs at jumper cables, bus and cable terminals.

The partial discharge creates signals which usually used as a warning before complete failure of insulation. So perform regular infrared inspections of all switchgear can be used to inspect for surface partial discharge activity.

To conclude, I’d say that billions are spent each year on equipment maintenance. So detecting failures in equipment and trying to avoid it is mandatory to ensure continuity of service, save your money and save your time too.

Thank you for reading,

References (pictures and table):

• schneider-electric.com
• Failure Contributors of MV Electrical Equipment and Condition Assessment Program Development
• Gabe Paoletti, P.E.- IEEE Senior Member, Martin Baier, P.E. – IEEE Member, Eaton’s Cutler-Hammer Performance Power Solutions
• maintenanceonline.org