Switching gears

Many ring main units and packaged substations employed by utilities and businesses for medium voltage (1-40kV) power distribution use SF6 gas-filled enclosures.

However some medium voltage switchgear uses vacuum circuit-breakers in conjunction with solid dielectric insulation to achieve a truly ‘green’ solution.

SF6 and the environment

The electrical industry accounts for 75-80% of SF6 use. It serves two functions: as an arc-interrupting medium in mv and hv switchgear; and as an insulating material in switchgear, gas-insulated transformers, substations and transmission lines.

SF6 is a man-made gas and one of the six greenhouse gases for which the Kyoto Protocol set emission limits to be achieved by 2012.

Although its concentration in the atmosphere is much lower than that of other greenhouse gases such as CO2 and methane it has a global warming potential 24,000 times that of CO2. This means that one tonne of SF6 has the same effect as 24,000 tonnes of CO2.

Furthermore, SF6 has an estimated atmospheric life of up to 3,200 years compared to 50-200 years for CO2. It is important to prevent any release of SF6 into the atmosphere and to ensure its safe disposal at the end of equipment life.

Switchgear technology

While low voltage (up to 1,000V) switches and circuit-breakers normally operate in air, higher voltage equipment generally needs special measures to help extinguish the arc drawn by the opening contacts.

Traditionally there has been a choice of four technologies for arc interruption in circuit-breakers – oil, air, vacuum and SF6 gas.

The use of oil and air circuit-breakers is now virtually non-existent for mv and hv applications. For mv (power distribution) applications, vacuum switchgear is the preferred technology, although some SF6 switchgear is still used. SF6 remains unchallenged for hv transmission.

However, even where vacuum interrupters are used, many manufacturers place these in a gas-filled enclosure to improve the basic insulation level. SF6 is generally used in this gas-insulated switchgear (GIS).

Because it is so important to control greenhouse gas emissions, any plant containing SF6 should be equipped with monitoring systems to warn of leaks. Regular inspection is necessary, typically twice a year, which inevitably adds to the total operating costs of the switchgear.

Vacuum technology

Although SF6 is non-toxic and non-flammable on its own, when used as a circuit-interrupting medium it can produce a number of dangerous and toxic decomposition products. This calls for special precautions when working on any SF6-filled equipment.

Even more serious is the issue of disposal at the end of equipment life. This has been compared with the problems experienced with the disposal of polychlorinated biphenyls (PCBs) in transformers. With an anticipated life of 20 years or more for switchgear, it is difficult to ensure that it is dismantled under controlled conditions. The alternative, vacuum interruption, is a mature technology with proven reliability.

Vacuum circuit-breakers were first used more than 40 years ago. Continuous development has seen the size of a 15kV vacuum interrupter bottle reduce from 180mm to 50mm diameter between 1967 and 2007.

Meanwhile, modern sealing techniques ensure that units retain their vacuum for more than 30 years; this exceeds the mechanical life of the circuit-breakers of which they form a part. Any leaks are, of course, completely harmless to the environment.

Vacuum circuit-breakers are suitable for a wide range of mv switching applications including transformer secondary protection, capacitor switching and motor switching, as well as use as ring main units.

They are suitable for operation at voltages from 1kV to more than 40kV; current ratings from 100A to more than 4,000A; and fault levels from 6kA to 63kA.

Aside from their compact size, vacuum circuit-breakers offer excellent electrical performance. They will normally cope with a rated ac power frequency withstand voltage (an overvoltage due to power system switching operations) of two to four times the normal operating voltage.

Rated lightning impulse withstand is 4-12 times normal operating voltage. However, in normal service the breaker contacts are closed, so lightning overvoltages are mostly seen by the line-to-earth or line-to-line insulation.

In the rare event of a lightning impulse appearing across the open contacts of the vacuum interrupter, the current will be quickly broken. Under similar conditions an SF6 puffer-type, air or minimum oil circuit-breaker would probably explode.

An interesting characteristic of the vacuum circuit-breaker is the self-conditioning of the contacts. Rough spots that can occur on the contact surfaces are smoothed out by the discharge when the contacts are opened on load.

Vacuum interrupters offer exceptional performance under load switching conditions, far exceeding the mechanical life of any circuit-breakers and reclosers of which they form a part.

They are suited to motor switching duties in excess of one million operations, arc furnace switching and capacitor switching.

Contact resistance remains low throughout their life because contact erosion only occurs at the cathode and eroded material is deposited uniformly on the anode; the contacts act randomly as cathode and anode so the effect is evened out.

In SF6 circuit-breakers, contact resistance increases progressively during life.

Vacuum interrupters are built from materials that can be recovered and recycled at the end of life. They do not contain greenhouse gases, nor do they present potential health hazards due to the products of decomposition.

No special precautions are needed to protect the environment from the results of leaks or during disposal.

Insulation issues

The compact size of modern vacuum insulator bottles means that special measures are needed to improve insulation levels.

A 150mm ceramic length will only have a basic insulation level (BIL) of 125kV in air. For this reason, insulators may be immersed in a dielectric medium such as oil or SF6 gas to raise the BIL to 170kV.

Oil is being phased out due to fire risks, so many manufacturers favour SF6 insulation, which brings back all the problems of leakage monitoring and end of life disposal.

An alternative approach is to enclose the vacuum interrupter in a potting compound such as polyurethane or epoxy. In some cases an epoxy insulator with a contoured profile, similar to the sheds used on overhead line insulators, is used to increase creepage distances.

This is especially valuable when the equipment is used in industrial environments involving heavy atmospheric pollution or condensation.

In some cases the entire interrupter and associated busbar are enclosed in solid insulation. Solid insulation means that the circuit-breaker can operate in a normal enclosure with no special sealing or gas filling and there is no need for costly monitoring equipment.

Maintenance is negligible and the life can be 30 years or more.

Cost of ownership

While the unit cost for GIS is lower than solid insulated switchgear, its total cost of ownership is much higher for GIS equipment. The specialist nature of the pressure checks that are needed by GIS equipment means that trained personnel with specialist equipment must carry out the work.

One estimate has put the annual cost of this maintenance as 9% of the equipment value – that is 45% over the first five years. This does not include any other safety and insurance costs involved.

At transmission voltages, the relatively low number of circuit-breakers means that a small specialist team can carry out maintenance on SF6 equipment. However, a completely different situation exists at distribution voltages.

A city with fifty 132/11kV primary substations could have as many as 125,000 ring main units. The advantage of these being maintenance-free is obvious.

Disposal costs for GIS equipment at end of life are difficult to quantify. Recycling parts and by-products is not practicable and dismantling, transport and disposal costs will be high.

In contrast, the solid-insulated equipment is fully compliant with ISO14001, covering environmental management systems and standards and all parts can be recycled.

The US Green Building Council’s Leadership in Energy and Environmental Design (LEED) system is being adopted in many projects as a way to quantify and compare sustainability.

Vacuum switchgear with solid dielectric insulation can help to achieve the objectives of the LEED and similar standards, while ensuring the best possible total cost of ownership.

Praveen Kashyap is Middle East Regional specialist, MV Systems with Eaton.