Generator circuit-breaker switchgear in combination with steam turbines were installed
Generator circuit-breaker switchgear in combination with steam turbines were installed at a cogeneration combined-cycle power plant in Rayong, Thailand.
Credit: Siemens
Technological developments now enable maximum normal and short-circuit currents to be interrupted using vacuum circuit-breakers, writes Nils Anger

Vacuum circuit-breakers are used in most industrial switchgear – with good reason, since they have their own particular strengths in terms of installation, operation and maintenance.

In the past, it was not possible to use the potentials of the long-proven vacuum technology for generator switching applications. Today, however, the appropriate technical solutions are available, which are tailored to suit the special requirements of power plants and power supply companies.

In power plants, generator switchgear ensures reliable synchronization and maximum operational safety. When installed between a generator and transformer, it synchronizes parameters such as frequency, voltage and phase with the grid.

In the event of a short circuit in the generator or transformer, the integrated circuit-breaker immediately cuts the flow of electricity. This protection averts consequential problems that could compromise the operation of the entire power plant.

Last but not least, the generator can also be switched on and off in a matter of milliseconds in response to the appropriate command, enabling it to be connected to the grid right on target. This function will become more and more important in future as the trend toward renewable energy sources and Smart Grids leads to decentralized power plants with multiple smaller generators.

Generator circuit-breakers are thus assuming a wide range of functions relevant to power plant operation. At the same time, they have to be designed for high currents.

Besides safely breaking short-circuit currents, a key requirement is to immediately extinguish any short-circuit arcing in the circuit-breaker. Until about 15 years ago, there was only one suitable technical process to deal with this: extinguishing the switching arc using the isolating gas sulfur hexafluoride (SF6) in a chamber filled with this gas.

This powerful technology has been successfully used in the power industry until now. More recent technological developments, however, enable maximum normal and short-circuit currents to be interrupted using vacuum circuit-breakers.

The HB3-80 is the first generator switchgear in the
The HB3-80 is the first generator switchgear in the world equipped with a vacuum generator circuit breaker for an IEEE standard C37.013 type-tested switching capacity of 80 kA
Credit: Siemens

This means that all switching duties in a generator switchgear can be assumed in their entirety by vacuum circuit-breakers. Unlike gas-insulated circuit-breakers, vacuum circuit-breakers interrupt the switching arc in a high-vacuum interrupter. This is therefore an especially environmentally friendly and reliable procedure for generator switching applications that is now available as an alternative.

The characteristic benefits of vacuum switching technology are:

High reliability – The use of vacuum circuit-breakers guarantees a high level of personal and operational safety. One reason is the small number of moving parts in the arcing chamber. One of the circuit-breaker contacts is firmly attached to the housing while the other is movable. With this setup, the metal bellows enables the switching stroke and creates the vacuum-tight connection to the interrupter housing. The hermetically sealed vacuum interrupters are also independent of environmental influences. Not the least of the advantages is that no oxidation occurs in the vacuum, which means the metal surfaces remain clean, ensuring a constantly low contact resistance.

Easy to install – Vacuum generator switchgear is a factory-assembled solution that is tested before it leaves the factory. It can be immediately integrated into the power plant technology on site. With SF6-insulated circuit-breakers, on the other hand, the gas is extracted for transportation immediately after manufacture, and the circuit-breakers are then refilled with gas at the time of installation, using a special tool. This means not only increased expenses but also a need for further testing.

Cost-efficient – Vacuum switchgear scores in terms of overall results. The installation and operating benefits described above keep costs comparatively low during the entire operating time (i.e. total cost of ownership) in the case of switchgear with vacuum circuit-breakers.

Nils Anger is Director of Generator Breaker Switchgear at Siemens


HB3-80 – A WORLD FIRST

The latest addition to Siemens’ product range is its HB3-80 generator switchgear.

This enclosed single-phase unit is suitable for power plant blocks of up to 160 MW or 250 MW, depending on the type of power plant and the operating voltage.

HB3-80 is the world’s first generator switchgear with generator vacuum circuit-breakers for currents of up to 10,000 A with natural cooling and a type-tested switching capacity of up to 80 kA in accordance with IEEE C37.013.

The switchgear is also type-tested in accordance with IEC 62721-200 and the Draft Dual-Code Standard IEEE/IEC 62271-37-13. It offers maximum operational safety and a high level of personal safety, since the single-phase enclosure rules out the risk of short circuits between the phases.

The HB3-80 is constructed to be compatible with other products established on the market, from the perspective of retrofitting in particular. Rather like a plug-and-play solution, it can be incorporated or retrofitted into the generator connections with no additional engineering costs. The flexible selection of pole-center distances and diameters means that existing round conductors (isolated-phase busbars, IPBs) can still be used.

Further switchgear types round out the portfolio to provide optimized customer solutions for the demands of a constantly developing market: The compact HB1, with horizontal busbars, is designed for power plants of up to 120 MW or 170 MW.

Its flexible connection concept, comprising bus ducts, cables and solid-insulated bars, offers a broad range of applications. The HB1 is very adaptable to customer-specific requirements. The modular VB1 offers even more flexibility for personalized solutions. This property makes it especially interesting for power plants operated with multiple generators or feeders for auxiliary supply, excitation, or brake disconnectors. It is suitable for the range of ratings up to 140 MW and – considering the high requirements in terms of switching capacity, space and accessibility – for use in hydropower plants and retrofit projects. The centerpiece of all of these systems is formed by the 3AH37 and 3AH38 vacuum circuit-breaker for generator switching applications: The 3AH38 high current and generator circuit-breaker is already the standard for switching normal currents of up to 4,000 A in many power supply companies worldwide.

As one of the first circuit-breakers for short-circuit currents of 63 kA and 72 kA on the world market, it was type-tested in accordance with the criteria of generator circuit-breaker standard C37.013. Its counterpart for higher generator ratings is the 3AH37. A world-leading vacuum circuit-breaker, it can manage a constant normal current of 6,300 A at up to 24 kV with natural cooling. With forced cooling, normal currents as high as 8,000 A are possible. Optimizing the circuit-breakers for joint operation enables them to be used in phase-enclosed switchgear and in retrofitting situations. Maximum normal currents of 12,000 A with a short-circuit breaking capacity of 80 kA and up to 6,300 A at 90 kA can be achieved.

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