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While it is difficult to improve on power plant generators’ already high efficiency, companies are still working on achieving even better performance. But any improvement comes at a cost, making the process an economic balancing act, writes Paul Breeze
Power plant generators, the machines which turn the rotary motion from a prime mover into electricity, rarely hit the headlines. While advances in gas turbine and combined cycle design have meant that efficiencies of over 60 per cent are now achievable, and steam turbine developers are aiming for 50 per cent, the generator that provides the electrical output from each of these plants barely gets a mention. Yet this unsung component can achieve a mechanical to electrical conversion efficiency that is just short of 100 per cent.
This high efficiency is a part of the problem when it comes to headlines. While improvements in thermodynamic cycle design might lead to an improvement of one or two percentage points in overall system efficiency, similar levels of development in generator design — an improved magnetic material, advances in insulating tape or optimization of cooling fluid flows, for example — will struggle to find an additional 0.1-0.2 percentage point improvement. Even a fundamental change in technology such as the introduction of high-temperature superconducting materials into the windings of generators is likely to yield at best an improvement of 0.2-0.4 percentage points. Bearing in mind that this is likely to result in an efficiency of over 99 per cent, perhaps such small yields are unsurprising.
This high efficiency is the result of decades of refinement. The classic generator design in use today is over 140 years old, as Dr Thorsten Krol, manager of Siemens’ generator business development with responsibility for technical marketing of generators in the company’s Power and Gas division, observed, and the efficiency is close to the top achievable. “With current technologies you are at the technological limit,” he said.
When designs have been refined over such a long span of time, finding ways to further improve performance, however slight, is difficult. Even so, it is still possible to win a small improvement here and there. However, any improvement comes at a cost and that cost may be too high to be economically worthwhile. Advanced generator design is thus an economic balancing act.
The dynamo-electric machine
It was Werner von Siemens who, in 1867, originally gave a name to the devices we now commonly call generators. He called them dynamo-electric machines, a name soon shortened to dynamo. The operation of an electrical generator is based on a phenomenon discovered earlier in the nineteenth century by Michael Faraday. It relies on the movement of a conductor through a magnetic field to generate an electric current. Very simple generators have a coil rotating in a stationary magnetic field, but the type of generator used in a large fossil fuel or hydropower power plant will have a rotor that acts as an electro-magnet, providing a moving magnetic field that generates a current in stationary windings (the stator). In order for the rotor to generate a magnetic field, its coils must be fed with an exciting electrical current. This may come from an external source or it may be provided by a second generator, called the exciter, mounted on the same generator shaft. The current in the rotor can be adjusted to control the current generated in the stator coils.
The excitation field draws power which is dissipated within the rotor coils. This generates heat which must be dissipated. While the losses associated with the excitation field may be less than 1 per cent of the total power output of the generator, this can amount to several MW in a large generator. There are also resistive losses within the stator windings and some magnetic losses in the stator core. For state-of-the-art large modern generators, the sum of all these (and some other) losses is generally between 1 and 2 per cent for a machine operating at full load. The losses may increase at part load, but the relative loss will generally be only 1-2 percentage points higher at 25 per cent load than at full load, Dr Krol said. Even at part load, generators remain highly efficient.
The cooling problem
Removing the heat from the rotor and stator coils represents one of the key challenges in generator design. How this is done, and with what, represents one of the important parameters that a generator designer has available. There are three basic media used for cooling generators: air, hydrogen and water. Each has its advantages and disadvantages.
Large generators are designed to meet the IEC standard which specifies a maximum temperature of 155ºC, and machines are built to be able to withstand this temperature during operation. However, it is normal for them to be designed so that they do not exceed a lower temperature, typically 130ºC, targeting some thermal reserves for overload and/or extend the lifetime of the machine.
Air is least effective as a heat removal fluid, as shown in Table 1, but it is the easiest form of cooling to implement and therefore the cheapest. At its simplest, air cooling involves drawing cool ambient air into the casing of the generator and then discharging hot air back to the atmosphere. More complex systems will used a closed air cycle and air-to-water heat exchangers to control temperatures more closely.
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Hydrogen is much more efficient than air at removing heat from a generator. Depending on the pressure, it will be three to four times more effective than air. However, the use of hydrogen presents several problems that must be overcome. In order to operate under pressure, the generator must be contained within an enclosure capable of withstanding 2-3 bar internally. The gas is explosive so leaks must be carefully monitored, and the amount of air that is allowed to mix with the hydrogen must be maintained below the explosive limit. In addition, a gas cycling system as well as a system to provide inert gas for hydrogenization and dehydrogenization of the generator is required. This all adds to the complexity, and therefore the cost, of the generator.
For the highest cooling efficiency, water is used. This is 50 times more effective than air and more than ten times as efficient as hydrogen, but it is even more complex to implement than hydrogen cooling. It is therefore generally only used in the largest generators. Where water cooling is implemented, it is used to cool the stator of the generator while hydrogen cooling will be applied to the rotor.
The type of cooling used in a generator depends upon both its size and type. While the cross-over points vary from manufacturer to manufacturer, for typical fossil fuel generators air cooling can be used in sizes up to around 300 – 350MW, hydrogen cooling up to 550MW and a mixture of water and hydrogen cooling for units larger than 500MW.
Large hydropower generators, which rotate much more slowly and, as a result, usually have many more poles, use water and air cooling, but generally not hydrogen. According to Thomas Hildinger, vice-president for generator technology at Voith Hydro Engineering Center, it is useful to talk in terms of the output per pole. Up to 30 MVA/pole air cooling is typical, while above that the generator will probably be water cooled. With many hydropower generators mounted vertically, rather than horizontally as in fossil fuel plants, the implementation of hydrogen cooling is much more difficult, he added.
Optimization of the cooling flow, particularly in air-cooled generators, is one significant method of reducing losses and increasing efficiency. In the past generators were seen as ‘heavy build’ machines but today, as Dr Krol explained, they are viewed from a fluid-dynamics perspective. In an air-cooled machine, cooling circuit losses represent one of the main losses so optimization here can bring significant benefits. As with all generator design, however, this depends on the cost and whether the gain in efficiency offers a cost-effective benefit. That may be down to the cost of electricity in the market into which the unit will be delivering its power.
One potential area for generator development is to extend the maximum capacity range for air-cooled generators. This is an avenue being explored by Brush Electrical Machines, as Sarah Allen, the company’s generator development manager, explained. The company has identified a demand for larger air-cooled generators, particularly for gas turbine applications.
Brush currently manufactures air-cooled generators up to around 200 MVA but is planning to extend this. “We believe the market wants air-cooled generators in the 300-400 MVA range,” Allen said. This is seen as more viable than hydrogen cooling for some applications.
Nigel Daft, senior electrical design engineer at Brush Electrical Machines, elaborated that the company is deploying a computational fluid dynamics package to compare different ventilation schemes. The object is “to fully optimize the existing ventilation system” on the current range of large machines. This will allow the power handling to be increased, making larger machines feasible.Ansaldo is also pushing the boundaries of its air-cooled machines and is in the process of building a 400 MVA air-cooled generator, a jump of 50 MVA from its previously largest air-cooled machine. The Italian company is applying a similar approach to its hydrogen-cooled generators and is planning to extend its hydrogen cooled range to 650 MVA.
As with Brush, this is being achieved at Ansaldo by optimizing cooling as well as refining the design of all major components. One area the company has highlighted is advances in the design of the tape used to insulate the stator windings from the core. Conventional tapes tend to have a relatively high thermal resistance, reducing heat flow and hence cooling. Tapes are being developed with higher thermal conductivity to help improve cooling. In addition, tapes that can withstand a higher voltage are also being developed. These allow the coils to operate at a higher voltage and hence lower current, reducing the resistive heat generation.
While cooling efficiency will allow larger machines to run at a lower temperature, reducing other losses will have a similar effect. The less energy is wasted as heat, the less generator heating occurs. So, for example, reducing the mechanical vibrations generated when the machine rotates will also affect the temperature — as well as reducing a source of mechanical fatigue.
The other main areas in which gains can be made are reducing the electrical losses in the generator coils and the magnetic losses in the stator core. Reducing electrical and mechanical losses is a matter of materials. It is possible to reduce the electrical, resistive losses in the coil windings by reducing the coil resistance. This can be achieved by increasing the amount of copper in the coils. Meanwhile, the magnetic losses in the stator core can be reduced by using a higher-grade silicon steel. Steel is graded according to the loss under a standard test using a 50 Hz and 1.5 Tesla magnetic field strength. For common grades the loss is between 2 W and 5.3 W for each kg. The higher the grade of steel (and the more expensive), the lower the loss. Losses can also be reduced in the stator core by using thinner steel laminations. Grain-oriented steels can also be used to advantage, but they must be used carefully or they can have the opposite of the desired effect.
The problem with all of these techniques is that they raise the cost of the generator – so, as Hildinger pointed out, accurate evaluation of the cost of each measure needs to be carried out to ensure it is cost-effective. It is possible to calculate the increase in output from a large generator resulting from the use of an improved steel or more copper and the cost of that change, resulting in a cost/kW for each improvement. If this is lower than the price that the generator owner will receive for the sale of each kW of power, then the improvements are worthwhile; otherwise they are not.
This will depend on the market. In markets where the cost of electricity is high, using higher specification materials can be cost-effective, but elsewhere they often are not.
A more radical material change to generator design is the application of high-temperature superconductors. Dr Krol suggested that the use of these could become a reality within ten years. However, he pointed out that the materials, while offering low coil resistance, are not necessarily well-suited to use in generators where the output changes often. This type of operation is becoming an increasingly important duty cycle for all large generators. Gas turbine plants, steam plants and hydropower plants are all being asked to operate in much more flexible ways than were previously required.
The new renewables question
This new paradigm is the result of the introduction into grids of large volumes of power from variable-output wind and solar power plants. Conventional fossil and hydro plants, once primarily used for base load generation, must now support the new renewable input. This change is leading to changes in generator design.
Generator cycling has an impact on all areas of the design of the machines. For example, the winding of the generator will undergo thermal cycling resulting from frequent changes in output. This will accelerate the aging of the insulation and will also lead to mechanical aging, such as wear and fatigue, of moving components. Therefore generators used under these conditions have to be adapted to resist the new stresses they undergo.
Another trend identified by Hildinger is the greater use of hydropower plants as peaking plants. Again this is down to new renewable capacity and the need to support it, with hydropower offering one of the cheapest means of doing so. This has led hydropower generator designers to start looking at variable speed machines, particularly for pumped storage applications.
Variable speed generators are more expensive than conventional generators and so are not normally economically viable, but in markets such as Japan and Europe where the cost of electricity is high and where grid support services can attract an income, their use is increasingly cost-effective. With a variable speed generator the actual rotation of the turbine-generator is decoupled from the grid frequency through the use of power electronics. This allows turbine speed to be optimized to hydraulic conditions, increasing the overall efficiency of the turbine-generator combination and giving greater flexibility for grid support.
Aside from the variability, one of the most significant consequences of the introduction of large amounts of solar and wind power into a grid is a reduction in the overall system inertia. In a grid populated by conventional power plants, the rotating machinery has a large rotational inertia which can maintain grid stability in the event of a major change to grid conditions while the grid operator acts to correct the change. For a conventional grid there is typically a four- to five-second inertia-supported period during which correction can be made.
New renewable plants have much less inertia, and this means the grid is likely to become unstable more quickly after a major event. To counter this, it may prove necessary to increase the inertia of those remaining large rotating machines on the grid.
One way of creating higher inertia is by adapting the design of generators. There are two principal ways of achieving this. The first is to increase the diameter of the generator rotor. However, the extent of this is limited, particularly in hydro machines, by the maximum rotational force the materials can withstand. The second option is to increase the mass of the rotor, which means higher cost. Such modifications are not yet a part of generator design, but the fact that they are under discussion shows how generator advances still have a crucial role to play.
Paul Breeze is a freelance journalist focusing on energy matters.
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