Air cooled generator state of the art and SMR application perspectives

 

Previous work on this topic

 

The series of large, air-cooled generators has its roots back in the late 1960s, and it has grown over the years to fulfil the needs of the steadily growing gas turbine units. A lot of papers have been written inside A1 study committee, focused on light new designs of ventilation or insulation and therefore increasing power of air-cooled generators.

 

Large, air-cooled generators cover now the turbine range from 150 to 400MVA and match the requirements of the medium and large-size gas turbines. They have also been used successfully with steam turbines. (Fig. 1). Air-cooled turbogenerators are generally characterized by simplicity and ease of operating and maintenance, qualities which are needed for the small modular reactor (SMR) market.

Figure 1: Cut-away view of a large air cooled generator (from CIGRE A1-106-2006)

 

Due to global warming, decarbonation, and the energy transition, the SMR will be a solution to produce CO2 free energy and with more and more flexible operation. A lot of SMR projects are now in development for new greenfield plants or to replace old thermal plants used coal or gas.

Whereas the power output of traditional reactors ranges from 600 to 1,650 MW, SMR range between 20 and 300 or 400 MW per reactor. They are called modular reactors because their design includes factory-built modules that are then transported and assembled on site. This enables a serial production effect, which is essential for the competitiveness of their economic model.

Two criteria are useful for distinguishing SMRs: the type of reactor and the power range.

 

The reactor type allows us to distinguish:

 

• Third-generation SMRs, which use pressurized water or boiling water technologies. As many countries strengthen their decarbonization goals to achieve carbon neutrality by 2050, these SMRs are strong candidates to replace coal-fired power plants.

 

• Fourth-generation SMRs, which are more disruptive models with technologies still in research and development (such as fast neutron reactors, molten salt reactors, etc.). They offer complementary services, particularly for large energy consumers like industrial companies.

 

These types of small nuclear power plants will be equipped with air-cooled turbogenerators with strong requirements of reliability, life duration, modularity, and maintainability.

In addition, these generators shall be designed to operate in compliance with the latest operational and grid code requirements.

 

Summary of SMR designs and technologies across the world's regions

 

 

The contribution of A1-75 on this topic

 

WG A1-75 was created to establish a state of the art for large, air-cooled generators and give information on the possible evolutions with best efficiency in all grid-dependent operation conditions.

 

The working group is investigating and reporting on:

 

• Basic approach to modern air-cooled generator design and state-of-the-art manufacturing processes
• Industry references
• Latest operational and grid code requirements impacting generator design
• Limits
• Degradation modes
• Maintainability
• Standard type tests
• Standard instrumentation and monitoring
• Standardization or modularization
• Future perspectives

 

A questionnaire was sent to CIGRE members and monthly working meetings have been set up.  The table below gives an example of information about instrumentation from the questionnaire that enables provide a view on the international practice in term of instrumentation.

 

 

 

A second example of the content of the A1-75 TB, is the contribution regarding standardization and modularization, the two main streams of potential evolution for an SMR project:

 

 

Standardisation or modularization

 

SMR power stations are planned with the concept of standardizing them as plants. Therefore, the generator for SMR power station will be designed based on its standard specifications, and standardized generators will be provided to match the design specifications of the respective SMR power station types.

 

Currently, there are no proposed specifications for SMR power stations that pose technical challenges, and the design and manufacture of standard generators for SMRs is feasible, supported by prior experience in producing air-cooled generators (with stator coils cooled by air) for capacities up to 400 MVA.

 

When determining the standard specifications for generators, aligning them with the manufacturer's standard designs is an effective way to reduce costs. Among the generator specifications discussed in Chapter 1 of the fiture Technical Brochure, the following items have a particularly significant impact on overall cost.

 

• Output voltage
• Power factor
• Short circuit ratio
• Requirement of peak power output
• Type of excitation system
• Cooling water temperature
• Transport restrictions

 

Deviation from standard specification of generator

 

Depending on the construction site of the SMR power station, factors such as the natural environment, geographical location, and requirements from the connected power system may deviate from the standard specifications for SMR power station.

 

For example, depending on the natural environment and location, the following factors may deviate:

 

• Temperature and availability of cooling water for equipment
• Transportation constraints affecting equipment delivery
• Seismic requirement
• Requirements of the power system to be connected to the SMR power station mean grid code requirements (inertia, voltage variation, frequency variation, etc.).

 

(Chapter 3 of the future Technical Brochure will consider the latest operational and grid code requirements impacting generator design).

 

If the standard generator cannot meet the specifications of the SMR power station, either a new generator must be designed to fulfill the requirements, or the specifications of the associated equipment must be modified. The decision between these options should be made based on a comprehensive evaluation of factors such as cost, delivery schedule, and other relevant considerations.

 

Modularization of generator

 

Modularization is a functional and geographical grouping of equipment allowing optimization of the phases of assembly on site by an optimized arrangement of the components between them (reduction of the networks) and the realization of tests or preliminary control (off site).

 

Turbine generators are manufactured with high precision in factory environments, utilizing advanced production technologies. During the manufacturing process, irreversible assembly techniques—such as welding and shrink fitting—are widely applied. As a result, the number of modules that can be separated after completion is inherently limited.

 

Typical modules of a turbine generator include:

 

• Stator
• Rotor
• Cooler
• End shield
• Bearing
• Brush holder
• Housing for the excitation system

 

In general, turbine generators are shipped from the factory in the most integrated form possible to minimize the complexity of on-site assembly. However, transportation constraints—particularly those related to weight and dimensions—may necessitate alternative handling strategies depending on the specific conditions.

When transportation weight limits make it impractical to ship the generator as a single unit, the stator and rotor are transported separately.

 

Among all modules, the stator typically has the largest weight and dimensions and may therefore exceed transportation limits. In cases where the stator's weight exceeds the allowable weight limit, it is effective to design the stator frame and core as separable module so that they can be transported individually and assembled at the installation site.

 

When the stator's dimensions exceed transportation limits, a commonly adopted method is to design the stator frame so that part of it can be separated for transport. The divided sections are shipped individually and welded together on-site.

 

It has to be noted that the required power is increasing for a few SMR projects to be able to offer cogeneration and electricity power.  Consequently,there may be a need to use hydrogen cooled generators, but depending on the market we could retrieve some new developments of more powerful air cooled generators.

Work on A1-75 is still continuing with a group of high knowledge experts from different sectors of generator technical domain, and the first draft of the Technical Borchure is planned for CIGRE Paris session in 2026.