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Fri 26 Dec 2008 04:00 AM

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Switchgear selection for secondary stations in MV power distribution

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.

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.The problem with SF6 gas leakage is that not only will it render the RMU unreliable as well as unsafe, but it could also be harmful to the natural environment.

This is because SF6 is a man-made, highly potent greenhouse gas (one of six) with an estimated atmospheric life of 3200 years and a global warming potential (GWP) 23 000 times that of CO2.

Cost of maintenance

At a rough estimate it could cost as much as 8-9% of the RMU's basic cost in order to perform two gas leakage checks per year.

In addition there are other costs, including disposal, safety and the potential damage to the environment. Consequently, it would be logical for utilities to look into their respective operation and maintenance costs to arrive at the Total Cost of Ownership (TCO) for the commercial evaluation of an RMU, in much the same way that we consider operating losses in the case of a transformer.

Eaton's offering

Seeing the need for a true maintenance-free RMU in power distribution networks, Eaton's Electrical Group has worked to develop and produce a revolutionary GREEN SF6-free RMU under the Holec brand name.

The result is a product that is compact, maintenance-free, reliable and economical. It is called XIRIA and uses air & solid insulation with vacuum interrupters, all housed in a closed metal tank. It has been produced since 2001 and is used by utilities the world over.

Today, XIRIA meets the specifications of most utilities and is tested to fault levels of 20kA/3s. XIRIA also has a number of optional extras that can greatly increase its functionality and value. For example, it can be provided with remote operation capabilities as well as a communication capability and automation.

One of the factors that make it particularly useful in this region is that it is suitable for max 24kV - a very common distribution voltage in man-made island projects.

For more complex secondary stations, where additional feeders are required and an RMU is not adequate, Eaton Electrical offers yet another product, which is SF6-free Cast Resin Insulated extendable switchgear SVS08/12 - which is compact, maintenance-free, reliable and environmentally friendly.

Its Cast Resin Insulation, from bus-bar to bushing per phase, eliminates the possibility of flashovers, inter-phase or phase-earth. This has been in production for more than 18 years and uses our expertise of over 45 years in Epoxy-Resin technology.

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