Goldisthal pumped storage plant, Germany

Credit: Siemens

Generator circuit breakers are considered the heart of the generator switchgear. Hydropower plants place different demands on the switchgear than other power stations, meaning the generator switchgear has to withstand special requirements to guarantee the plant’s availability and efficiency, writes Dr Karthik Reddy Venna

Hydropower plants place very special demands on the generator switchgear when compared with other types of power station, especially in terms of frequent switching operations, load switching and easy, flexible retrofitting.

Vacuum circuit breakers, which are becoming highly preferred for this kind of application, offer an ideal solution to fulfill these special requirements reliably and efficiently.

In power plants, generator switchgear will ensure dependable synchronization and maximum operating reliability. Installed between the generator and the transformer, the switchgear can synchronize various quantities like frequency, voltage, and phase with the network.

The integrated circuit breaker immediately interrupts the flow of electrical energy in the event of a short circuit in the generator or the transformer. This protection prevents secondary faults which could impair the entire power plant operation.

In addition, the generator can be switched on and off within milliseconds with a corresponding control command allowing it to be coupled into the grid with high precision.

With renewable energy sources and smart grids driving the trend toward decentralized energy centres with several small generators, this function will become increasingly important in the future.

Hydropower requirements

Due to their technical and physical characteristics, hydropower plants place different demands on generator switchgear than other types of power plants.

For instance, the hydro generators normally rotate with a speed around 300-400 revolutions per minute, which is significantly slower when compared with coal- or gas-fired power plants.

This has a direct impact on the short circuit currents fed by the generator and a heavier load on the breaker during the switching process.

In addition, hydropower plants often use a three-winding transformer instead of the typical two-winding transformer to connect the high-voltage network. This results in a high DC component from a system source fault, which in turn necessitates a higher switching capacity.

Another special requirement is that, for pumped storage power plants, the circuit breakers must perform load switching or rated current switching, whereas for other power plants, typically no load switching is performed.

Therefore, generator switchgear installed in hydropower plants must withstand these special requirements and guarantee the maximum plant’s availability.

Added to this, Asian countries like China, India and Malaysia have ambitious plans to expand their hydropower generation capacity, whereas in Europe and the US the development scope is limited and mainly focuses on modernizing existing plants which are nearing the end of their service lifetimes.

HB3-100 generator switchgear

Credit: Siemens

As a result, there is a high requirement for retrofitting, as much as for new switchgear. Often the complete set of electrical equipment is replaced, but sometimes only individual components are exchanged. In this case, the replacement solutions must be easy and quick to install. This also applies to the generator switchgear.

One example of this modernization is the Tinizong hydropower plant in Switzerland, with a rating of 70 MW, which is considered to be the power supplier ewz’s most powerful plant. In 2013/2014, a large portion of the electromechanical plant components were replaced. Siemens supplied the transformers, switchgear equipment including the VB1 generator switchgear, and secondary systems for control and protection as well as high-voltage components. The VB1 generator switchgear is equipped with vacuum generator circuit breakers with a rated current of 3,150 A, and can interrupt short-circuit currents of 50 kA.

Generator circuit breakers are considered to be the heart of the generator switchgear.

For reliable operation, it is necessary to guarantee that the electric arcs in the interrupter tube caused by short circuits are extinguished immediately. Up to about 15 years ago, the most preferred technique was to use SF6 as the arc-extinguishing medium, especially for big generators. For smaller machines vacuum technology is still the leading method.

Eco-friendly method

However, recent technical developments now enable the switching of high operating and short-circuit currents with vacuum circuit breakers, even for big machines – for instance, up to 450 MVA.

Unlike gas-insulated switches, vacuum circuit breakers interrupt the electric arc in a vacuum tube which is hermetically sealed. This provides an exceptionally eco-friendly and reliable method with low/no maintenance which is making the vacuum generator circuit breakers an efficient alternate solution for generator switching applications.

In addition, Siemens vacuum generator circuit breakers are tested as per the North American ANSI standard IEEE C37.013 (1997). This is currently the only standard in the world that takes into account the higher requirements to which the switching components are subjected during generator switching applications. This makes it possible to fully utilize the advantages of vacuum technology in a power plant’s generator switchgear for almost all operating modes and energy sources.

Siemens continues to use the vacuum technology even for the high-current and generator switching applications up to the 450 MVA range. The following are the distinctive advantages of vacuum generator circuit breakers over other technologies:

High reliability. The VGCBs guarantee a high level of personal and operational safety and make them highly reliable. This is mainly because there are fewer moving parts in the arcing chamber and, in the extremely unlikely case of loss of vacuum, the arc develops and stays inside the interrupter’s envelope until the backup breaker interrupts the circuit.

In addition, there are no gas decomposition products existing in the vacuum and the hermetically sealed vacuum interrupter has no influences from environmental effects which ensure the constant dielectric strength. Further, the contacts cannot get oxidized in the vacuum medium, which ensures that their very small resistance is maintained throughout their lifetime.

Low maintenance requirement. Siemens vacuum circuit breakers are designed for a service life of more than 20 years. They are considered maintenance-free up to 10,000 operating cycles or 30 short-circuit switching operations. The vacuum interrupters themselves are generally maintenance-free as they do not need any auxiliary units such as gas monitoring or monitoring sensors for hydraulic drives, which mark them as no-/low-maintenance equipment.

Simple installation and cost-efficiency. With SF6-insulated breakers, the gas is removed from the finished products for transport and then refilled during assembly using special tools. This not only means more work, it also requires testing at the assembly. The vacuum generator circuit breaker, on the other hand, is a factory-assembled solution that has already been tested at the factory. It can be immediately integrated into the power plant technology on site. This feature makes it ideally suited for retrofitting projects.

Vacuum circuit breakers are also smaller and more compact in design. Typically, their high degree of flexibility even makes it possible to replace the existing circuit breakers. With vacuum circuit breakers it is just plug-and-play. And, for eliminating the need for additional gas monitoring products as well as due to their low maintenance, vacuum circuit breakers are considered to be a cost-efficient solution.

The latest addition to Siemens’ product range is the generator switchgear HB3-100 for generator ratings up to 450 MVA. The HB3-100 is the only generator switchgear in the world using vacuum as an interrupting medium for the rated currents up to 12,500 A with natural air cooling and short circuit currents up to 100 kA. This air-insulated, phase-segregated switchgear has been type-tested in accordance with IEC standards 62271-1, -100, -102, -200, and IEEE C37.013.

Flexible design

The series HB3 generator switchgear, designed for indoor and outdoor installation, are suitable for power plants up to 400 MW, depending on the plant type and operating voltage. HB3 switchgear is considered an ideal solution not only in hydropower plants, but also in pumped storage, combined cycle, coal-fired, geothermal and solar thermal power plants, as well as in the oil and gas and paper and pulp industries.

In addition, irrespective of IPB diameters and their pole center distances, HB3 generator switchgear can be implemented due to its flexible design concepts. The switchgear can be operated with the overpressure required for the IPB system. The integrated generator circuit-breaker module 3AH36 for up to 100 kA consists of generator circuit breaker and disconnector, and can be optionally equipped with two grounding switches and starting disconnectors. The system can also be optionally equipped with a starting disconnector for starting up the turbine.

Generator circuit breakers in hydropower plants need to fulfill special requirements, especially with respect to frequent switching operations, load switching and ease of retrofitting. Generator switchgear with vacuum switching technology can fulfill these special demands and, with their distinctive advantages, they can offer power plant owners an efficient alternative to conventional SF6 circuit breakers.

Dr Karthik Reddy Venna┬áis Siemens’ expert for vacuum generator circuit breakers in the company’s Energy Management Division