Switchgear selection for secondary stations in MV power distribution

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A cut-away section revealing the interior of the SVS-2 switchgear cabinet.

With cities in the region expanding rapidly and the consequent construction boom that we are all witnessing, the demand for power is also on the rise.

This has in turn resulted in the rapid expansion of distribution networks that must not only provide the required power, but must also be reliable, safe, environmentally friendly and sustainable over the long-term.

Distribution Network

MV secondary stations are the last parts of the distribution network. Their importance is that they deliver power to the transformers located in buildings or just outside buildings in package stations.

Secondary stations number in the thousands and are usually spread-out over the length and breadth of a city, with roughly 250-300 for every primary station.

For example, a city with 50 primary stations would have around 15 000 secondary stations. These stations generally use a type of switchgear called a Ring Main Unit (RMU).

In view of their large number, it becomes impractical, not to mention very expensive, to carry out regular maintenance and therefore a true maintenance-free RMU is the best answer for a company that is interested in lowering labour costs and increasing its productivity levels.

Causes of failure

Problems with the arc interrupter switch and insulation failures are some of the key reasons for switchgear failures.

Traditionally there has been a choice of four technologies for arc interruption in circuit breakers - oil, air, vacuum & SF6 gas. The use of oil & air has been getting lower over the past few years to the extent that it is now virtually unheard of in relation to MV systems.

For MV distribution applications, vacuum switchgear is the preferred technology, although some SF6 switchgear is still used. This is largely because SF6 remains virtually unchallenged in terms of high-voltage transmission.

That said, where vacuum interrupters are used, many manufacturers place these in a SF6 gas-filled enclosure to improve the basic insulation level and to reduce the overall size.

But this solution only partly addresses the problem. It certainly does not render the RMU fully maintenance-free, as SF6 leakage is still a possibility. To combat this, most manufacturers provide a manometer to monitor it.

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