Hydro plants demand special requirements from the generator switchgear when compared with other power plants, especially in terms of frequent switching operations, load switching, and easy and flexible retrofitting concepts. Vacuum generator circuit breakers offer a solution to fulfill these requirements.
By Karthik Reddy Venna
Peer Reviewed This article has been evaluated and edited in accordance with reviews conducted by two or more professionals who have relevant expertise. These peer reviewers judge manuscripts for technical accuracy, usefulness, and overall importance within the hydroelectric industry.
In power plants, generator switchgear ensures dependable synchronization and operating reliability. Installed between the generator and the transformer, the generator circuit breaker is used to synchronize the two systems. The important conditions to be fulfilled before synchronization are quantities such as voltage and frequency, and the phase sequences must be in phase with the system to be synchronized. Having a generator circuit breaker ensures the reliable synchronization. In addition, the integrated circuit breaker immediately interrupts the flow of short-circuit current in the event a fault occurs at the generator or the transformer. This protection prevents secondary faults that could impair operation of the entire power plant.
In addition, the generator can be switched on and off within milliseconds with a control command, allowing it to be coupled into the electric transmission grid with high precision. With renewable energy sources and smart grids driving the trend toward decentralized energy centers with several small generators, this function will become increasingly important in the future.
Special requirements in hydropower and pumped-storage units
Due to their technical and physical characteristics, hydropower plants place different demands on generator switchgear than other types of power plants. For instance, hydro generators normally rotate at about 300 to 400 revolutions per minute, which is significantly slower than coal- or gas-fired power plants. This has a direct impact on the short-circuit currents fed by the generator and results in 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 for pumped-storage hydro plants. In these facilities, the circuit breakers must perform multiple load switching or rated current switching operations per day, compared with other power plants where typically no-load switching is performed. This demands the circuit breaker manufacturer ensure the breaker has high reliability as well as a high number of switching operations with load currents to avoid power plant outages due to frequent breaker maintenance.
Further, in both conventional hydropower and pumped-storage plants, back-to-back switching is frequently used to accelerate the generator to its rated speed before it is connected to the system. This means interrupting the fault currents at the frequencies below 50 Hz or 60 Hz is necessary.
Further, Asian countries such as India, Indonesia and Malaysia have ambitious plans to expand their hydropower capacity, whereas in Europe and the USA the focus primarily is on modernizing existing plant equipment, which is nearing the end of its lifetime. Often the complete set of electrical equipment within a plant 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.
Technological advancements
Generator circuit breakers are considered 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. About 15 years ago, the preferred arc extinguishing medium was SF6 for large generators. For smaller machines, vacuum technology has already been the leading technology.
Now recent technical developments enable the switching of high operating and short-circuit currents with vacuum circuit breakers even for large 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 eco-friendly and reliable method with low/no maintenance, making the vacuum generator circuit breakers an efficient alternate solution for generator switching applications.
Advantages of this technology
Following are the three main advantages of using vacuum generator circuit breakers in hydroelectric power facilities.
High reliability
The design guarantees a high level of personal and operational safety and makes them highly reliable because of the fewer moving parts in the arcing chamber. In an extremely unlikely case of vacuum loss, the arc develops and stays inside the interrupter’s envelope until the back-up breaker interrupts the circuit.
In addition, no gas decomposition products exist in the vacuum and the hermetically sealed vacuum interrupter has no influence from environmental effects, which ensure the constant dielectric strength. Further, the contacts cannot get oxidized in the vacuum medium. which ensures very small resistance maintained throughout their life.
Low maintenance
The vacuum circuit breakers manufactured by Siemens 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 are generally maintenance-free as they do not need any auxiliary units such as gas monitoring or monitoring sensors for hydraulic drives.
Simple installation and cost efficiency
With SF6-insulated breakers, the gas is removed from the finished products for transport and then is refilled during assembly. This means additional work and requires testing at the assembly. The vacuum generator circuit breaker is assembled and tested at the factory and can be immediately integrated into the power plant technology on site. This feature makes the technology ideally suited for retrofitting existing projects.
Vacuum circuit breakers are also smaller and more compact in design. Typically, their high degree of flexibility even makes it possible to replace existing circuit breakers. And vacuum circuit breakers are the more cost efficient solution because additional gas monitoring products are not necessary.
Environmental impact
In medium voltage, the vacuum circuit breakers offer the lowest environmental impact over the entire product life cycle. In fact, the Environmental Protection Agency of the U.S. and Denmark have both recommended using alternatives like vacuum instead of gas insulation.
Vacuum switching technology for short-circuit currents up to 100 kA
Siemens offers vacuum circuit breakers in a variety of sizes. The latest addition is generator switchgear HB3-100 for generator ratings up to 450 MVA. The HB3-100 uses vacuum as an interrupting medium for rated currents up to 12,500 A with natural air cooling and short circuit currents up to 100 kA. This air-insulated, single-phase encapsulated switchgear has been type-tested in accordance with International Electrotechnical Commission standards 62271-1, 62271-100, 62271-102, and 62271-200, as well as Institute of Electrical and Electronics Engineers standard C37.013.
The 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. It provides maximum operational reliability as well as a high level of safety for personnel as the inter-phase short-circuits are eliminated, thanks to its single-phase encapsulated design.
In addition, irrespective of isolated phase busbar (IPB) diameters and their pole center distances, HB3 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 equipped with two grounding switches and starting disconnectors. The system can also be optionally equipped with a starting disconnector for starting up the turbine.
Summary
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 those special demands and, with their distinctive advantages, they can offer hydropower plant owners an efficient alternative to conventional gas-insulated circuit breakers.
Dr. Karthik Reddy Venna is a technical expert on vacuum generator circuit breakers in Siemens AG’s Energy Management Division.
Goldisthal case study
The Goldisthal pumped-storage plant in Germany is on the Schwarza River in eastern Thuringia and has a generating capacity of 1,053 MW from four turbine-generator units. The facility was completed in 2004 and is one of the largest and most modern power plants of its kind in Europe.
The plant, owned by Vattenfall, uses surplus electricity generated by lignite power stations to pump water from the lower reservoir, up an elevation difference of 302 meters into the 12 million m3 upper reservoir.
The plant was equipped with SF6 breakers. After having a bad experience with this technology, the plant operators wanted an alternative option that is efficient and reliable. Due to the advantages of vacuum technology – high switching operations, maintenance free and highly reliable – Vattenfall selected Siements vacuum circuit breakers, which are installed in the VB1 generator switchgear. The company is satisfied with the operation of the installed breakers.
