Over the years, the rating of generators has increased – particularly as the output of gas turbines has increased. Recently, the world’s largest air-cooled generator began commercial operation at the Al Hidd power station in Bahrain.
Oliver Drubel, Reinhard Joho, Alstom, Switzerland;
Yves Sabater, Alstom, France
Since the first turbogenerators appeared almost a century ago, their history has been marked by a steady rise in unit rating. During the course of time, turbogenerator designers turned from air to hydrogen and even water for direct conductor cooling, as these media proved to be more efficient coolants. Then with the introduction in the 1960s of gas turbines for power generation, the development of air-cooled turbogenerators re-emerged.
Today, air-cooled generators represent an important part of the installed 300 MVA generator fleet for the 250 MW class gas turbines and steam turbines. Now some ten years after their introduction to the market for this power output level, this technology is fully proven and 300 MVA air-cooled generators are now a standard in the power generation industry.
Figure 3. The 400 MVA TOPAIR 25 at Al Hidd II
As gas turbines developed, combined cycle plants of larger and larger sizes were built. Subsequently, operators accustomed to air-cooled generators for the gas turbine units started to also consider this technology for the steam tail generator. This meant air-cooled technology was also increasingly being considered for conventional fossil fired steam power plants. Thus, air-cooled generators far above 300 MVA will be expected for all types of applications.
To this end, Alstom has worked towards the development of larger and larger generators. Indeed, Alstom successfully tested a 500 MVA class air-cooled generator at the end of the 1990s. But perhaps more worthy of note is the recent start of commercial operation of the world’s largest air-cooled generator at Al Hidd.
Al Hidd II project
The Al Hidd II combined cycle project (KA13E2-3) in Bahrain is capable of producing 696 MW electrical power while simultaneously meeting the steam demand for a desalination plant of 370 kg/s.
The Ministry of Electricity & Water (MEW) of the State of Bahrain awarded the contract to Alstom for the turnkey construction of the CCGT power plant in September 2001. This order followed the Al Hidd I simple cycle desalination plant, also located at the same site.
The turnkey contract included the supply of three GT13E2 gas turbines and a steam turbine that will be used for a combination of desalination and power supply. Alstom was also responsible for the engineering, procurement and construction of the plant. Constructing on schedule the plant during the period of a booming gas turbine market in 2001 and 2002 was a real challenge.
However, due to the highly standardized plant concept it was possible to reach first ignition in less than 9.5 months. From the notice to proceed in September 2001, ignition of the first gas turbine took place in June 2002.
The Provisional Acceptance Certificate (PAC) was received on July 15, 2004 following the successful completion of the 30-day plant reliability run on the evening July 14, 2004. All start up / load control tests and performance tests were successfully carried out to the satisfaction of the client (MEW) and client’s engineer, and the plant has now commenced summer operation to support the peak electricity demand in Bahrain.
The four turbogenerators at the plant are of the highly standardized Alstom TOPAIR family which has been installed on some 1000 steam and gas turbines all over the world. It has been type tested up to 500 MVA and has been coupled and offered together with all major turbines available on the market.
The Al Hidd II CCGT project uses three 165 MVA TOPAIR 21 generators for the gas turbines, and opted for the 400 MVA TOPAIR 25 air-cooled technology for the steam turbine.
The use of fully air-cooled technology allowed simplifications in plant design, instrumentation and control. This was important to MEW which wanted to minimize maintenance efforts. Since no hydrogen system or CO2 system is needed, all purging and the special attention which is needed for hydrogen has been avoided.
MEW also maximizes the reliability of the generator especially for the site operation between 5°C and 50°C ambient temperature. High temperature variation within short times may cause additional loss of hydrogen due to its expansion. Special measures would have had to be taken in the balance of plant system to avoid this. The ambient temperature of 50°C also calls for special requirements on the auxiliary systems. All these factors are avoided with air as the cooling medium. The wide temperature range determined the rated output to be 400 MVA.
Adequate cooling is a major consideration when increasing output. Existing cooling techniques have to be adapted for an air-cooled generator of the 400 MVA class, in order to keep the material temperatures within the specification.
The basis for the great leap forward in air-cooled technology is the reverse cooling concept in which the axial cooling of the rotor and the axial segmentation of the radial cooled stator, both proven ventilation principles, are maintained. A refined multi-chamber cooling system allows the generation of a quite homogeneous axial temperature distribution in the indirectly cooled stator winding, leading to an equalized thermal utilization of the insulation. This is a basic condition for reaching 400 MVA with air-cooling.
The reverse cooling concept has the fans arranged upstream of the coolers. This eliminates a 17 K-preheating of the cold air by the fans. As with the standard design, there are two symmetric, closed-loop cooling circuits, each responsible for one half of the machine. The losses of the entire system are removed by six water/air coolers embedded within the foundation. This is known as totally enclosed water air cooling (TEWAC).
The refined multi-chamber system, the well balanced cold air flow through the end windings combined with a perfect Roebeling of the winding bars, produces a uniform temperature distribution along the stator conductors. The operation is according to the usual IEC class 130 (B). On one side this is secured by the limits given for the embedded temperature detectors. On the other side, and according to long-standing practice, all hot spots are kept within 130°C.
The stator bars are insulated with the insulation system known as ‘Micadur’. Developed over more than 40 years, Micadur is a system for class 155 (F) and beyond. A thermal reserve of at least a full class remains as a safety margin. This might, for example, be needed in gas turbine peaking operation whereby the hot spots remain in the class 155 limits.
Also, mean temperature in the rotor the winding is kept within the limits according to IEC class 130 operation. It is longstanding Alstom practice to have rated rotor conductor hot spot temperatures at 130°C. It remains a full class margin to the expected class 155 system level.
Advantages of air-cooling
Air-cooled generators have a number of advantages over conventional hydrogen- cooled generators .
The benefits are initial costs savings through:
- Simplified generator design: no pressurized frame and no shaft seals.
- No auxiliaries: H2 and CO2v storage (or production) and distribution system, or seal oil system.
- Reduced piping work
- Simplified interfaces (C&I)
- Simplified civil work and less space occupied
- Reduction of engineering works and project management
- Reduction of equipment delivery time (by 3 months)
- Simpler and shorter erection and commissioning.
These can represent a minimum 40 per cent saving of about $1.5 million to $2 million on the generator cost for 300 MVA.
Figure 3. The 400 MVA TOPAIR 25 in Al Hidd II
There are also operation and maintenance costs savings:
- No H2 or CO2 consumption
- Reduced auxiliaries electric consumption
- Spare parts stock reduced
- Reduced supervision and maintenance
- Avoid H2 safety concerns. Staff training reduced
- Improved reliability & availability : Less systems.
These will provide an estimated total annual saving for a 300 MVA generator of $200 000 to $400 000 i.e. up to $4 million over 10 years.
Using an air-cooled generator makes sense, if two conditions are fulfilled: the reliability of the generator should be of the highest level ; and the efficiency should be comparable to the best hydrogen-cooled generators.
The most advanced designs of TOPAIR series generators meet these two conditions. This will secure the attractiveness of air-cooled technology for larger and larger power outputs and forms a unique base for the future development of comprehensive and consistent families of air-cooled generators up to 500 MVA and beyond.