|Stator coils can be independently tested before being installed
Head of Operations – UK for Sulzer
An increased focus on controlling greenhouse gas emissions, coupled with escalating demand for electricity and slow progress on nuclear projects, is leading to an expansion of alternative renewable energy sources.
Leading the pack at the moment is hydroelectricity, the basic principles of which were commercialized in the late 1880s and which now accounts for around 16 per cent of global electricity generation capacity.
One of the advantages of hydropower is actually the age of its base-level technology: as a mature generation method, the equipment is tried and tested and although a large proportion of it is due for refurbishment, this means that refinements in materials and manufacturing techniques can be applied to existing installations.
Improved generation efficiency and longevity can now be built in during routine maintenance, delivering 10 per cent increases in power output from the same source, without reinvesting in replacement hardware – surely a massive win/win for the industry.
It should be noted that not all repairers offer the same level of service and expertise, so hydro generator operators need to choose carefully.
The variety of plants is huge, from the smallest Pelton wheel to the largest hydroelectric power station in the world, the 22,500 MW Three Gorges Dam in China.
However, the principle remains the same, using flowing water to turn a turbine and generate electricity. The keys to a successful installation are efficiency and reliability, both of which are benefitting from the latest technological advancements in the sector.
In recent years, many hydro projects have begun operating in ways not envisioned during their original design and new installations have been made possible through improvements in technology. Of course, work is still needed to determine how equipment can be improved through new materials technology as well as identifying actions and tools for predicting and preventing failures.
As hydropower increases in popularity so the demand for new installations grows. Thanks to inherent shortcomings with traditional technology, it may not be suitable to meet these demands in some locations, such as those with large fish populations. Here, development programmes, such as the Alden turbine project, have produced new designs that significantly improve the mortality rate of fish travelling downstream while also improving generating efficiency.
Hydro turbine research has also produced a new fish-friendly design criteria for Kaplan and Francis turbines that can be incorporated either into units during rebuild projects or in new hydroelectric facilities. In the US, the Department of Energy (DOE) has implemented a new hydro programme called The Advanced Hydropower Turbine System Program (AHTS) which aims to develop technology to maximize the use of hydropower while minimizing its environmental effects.
Refurbishment of existing turbine assemblies can now include the latest materials technology such as the high velocity oxygen fuel (HVOF) coating system, which uses tungsten carbide materials. This and other similar coating and wear protection systems can be used to protect water turbines and draft tubes against erosion from water-borne sediment and reduce cavitation effects for Pelton, Kaplan and Francis turbines.
Improving generator efficiency
Advances have been made in the design and construction of the high voltage stator coils used within the generator, which can provide improved efficiency and reliability. The manufacture of high voltage coils is a very precise science; design and construction methods will vary between suppliers, as will the testing procedures, which form the bulk of the proof the client has of the quality of the finished product.
Obviously, when repairs are unplanned, speed is crucial in getting the equipment back up and running, but this cannot be achieved to the detriment of quality, which has much longer-lasting implications. With regards to the testing protocols for these high voltage coils, the hydroelectric industry works to higher standards than some industrial sectors.
For example, Sulzer’s Birmingham Service Centre in the UK, where replacement coils are manufactured for use worldwide, subjects its coils to a series of demanding tests both in-house and by external testing experts to ensure that these standards are met and, in most cases, surpassed in order to deliver a guaranteed product.
|Advances have been made in the design and construction of the high-voltage stator coils used within the generator, which can provide improved efficiency and reliability
Maintaining high standards of design and construction has a number of key benefits, most notably reliability and operational efficiency. Employing a process of continual measurement against specification throughout the manufacturing process will ensure correct initial fit as well as continued reliable service.
In addition to the myriad of small differences in design, construction and materials from one manufacturer to another, there are two distinct insulation methods of coils in generator construction.
It is worth focusing on this aspect of generator repair because it can make a profound difference to the efficiency and performance of a unit, often without having to replace the other major components.
Replacement coils or bars will be manufactured either using the heated and individually pressed Resin Rich insulation system or a vacuum pressure-impregnated (VPI) resin bath method.
While smaller applications might prefer the VPI method, this can become unmanageable for larger installations such as large generators.
The design and construction of the resin-rich coil provides accurate control of the coil dimensions within the slot, which, combined with good winding practice, minimizes any voids between the coil and the slot, resulting in good control of partial discharge (PD) activity within the slot.
PD is one of the main failure modes of the insulation in rotating machines, making it a good indicator of normal service life expectancy.
The resin-rich coil uses a thermosetting epoxy resin-infused mica tape for the slot while the resin for the end-windings contains a flexibilizer which provides a small degree of flexibility within the resin, making it less susceptible to the end-winding cracking often seen in larger VPI coils.
The flexible end-winding tapes allow the coil to be adjusted slightly during installation in the stator, which ensures a more consistent gap between the coil sides in the end-winding.
Epoxy resin is more susceptible to PD activity when compared to mica, and so the resin-rich coils, which contain less resin than the VPI equivalent, provide better resistance to partial discharge activity.
In most cases the use of modern insulating materials will improve both the dielectric and thermal performance when compared to those used by the OEM during original manufacture.
Improvement in the control of stray losses can also be achieved by changing the method of stator coil transposition in order to reduce circulating current losses (a process called Roebel transposition), or by changing winding covers to a non-magnetic material. Improved design software and modern winding equipment also contribute to delivering stator coils with a more efficient and robust construction.
It is not only the stator windings that can influence the efficiency of the generator; the design and construction of the stator core itself also has an influence. As part of a generator refurbishment, it is essential that inspection and tests be carried out on the stator core and rotor. These can highlight any issues with the integrity of the stator core which may have to be replaced before any coils can be installed.
Rebuilding the core using new lamination segments which are manufactured from a specific lower-loss grade of magnetic steel can result in a 10 per cent reduction in losses when compared to the material used in the original construction.
In addition, the rotor field coils can be stripped and re-insulated with improved Class F insulating materials.
Sulzer can also redesign, manufacture and fit new AC excitation systems to replace DC systems. The main benefit of this change is to remove the possibility of deposits from carbon brushes contaminating the stator and rotor components; an added benefit is the reduction in brush gear maintenance time giving additional generator uptime.
The use of modern materials for both the stator core and the insulation thereby helps to ensure the continued reliable operation of a facility after the generator has been refurbished, but it can also increase the maximum output of the newly repaired generator by over 10 per cent.
Stator coils can be independently tested before being installed, giving the repair centre and the client peace of mind that every coil has passed the numerous tests available.
The electrical testing of the completed coil includes the Tanδ, which is a measure of the integrity of the slot wall insulation, where a lower figure indicates a better quality coil.
At Sulzer, the figure achieved is always less than half the international standard, with an aim to realize less than one third of the standard.
Further testing can be carried out on sample hydro-generation coils by independent laboratories, including the thermal endurance test, which is carried out at 30 kV for 500 hours, a simple enough task for a high-quality coil. However, a more arduous test is done at 35 kV for 250 hours; all the coils provided by Sulzer have passed this standard as well.
In the majority of cases, the client will specify the test parameters for the coils, which are normally specified to operate at 11 kV. As a baseline assessment, coils are subjected to 23.9 kV for 400 hours while being heated to normal operating temperature, around 120°C. This test is in accordance with the demanding IEEE standard 1553, which relates specifically to hydro generator coils.
|The balancing pit in Birmingham incorporates an HV motor and advanced electronics and diagnostics to provide state-of-the-art trouble-shooting capabilities
Additional coils can be initially subjected to the withstand test, which evaluates the ability of the coils to operate in overvoltage conditions which may be expected during their life. Coils are expected to pass this test which is conducted at 28 kV, as well as a further test at 60 kV and an end-winding test at 22 kV, with no detectable issues with the insulation.
While the IEEE standards set out the framework for these tests, the exact test voltages and durations are specified by the client to ensure that the coils meet the requirements of the individual application.
High-speed rotor balancing
As the stator is completed, so attention falls to the rotor, which can also benefit from the improved insulation materials technology. Rotor field coils will usually be reinsulated as part of a refurbishment project along with the replacement of any damaged components. Clearly, the rotor will need to be re-balanced before it is returned to service.
Dynamic balancing of rotating elements is an important aspect of the manufacture and repair of any turbo machinery. A rotating element that is out of balance can cause major operational difficulties, which may prevent the timely startup of a facility if it has to be re-balanced.
Furthermore, the unbalanced element can cause internal damage that will rob a machine of its design efficiency, reduce machine reliability, and increase the costs of operation and maintenance.
For any applications that require high-speed balancing, this process requires a specialized balancing bunker, but very few of these are available to the independent repair market.
Globally, Sulzer has invested in a number of overspeed balancing pits which can be used as part of each repair project for its clients – though, due to their rarity, Sulzer has also made these available to OEMs and other customers. Each pit is equipped with advanced electronics and diagnostics to provide state-of-the-art troubleshooting capabilities.
Pumped storage systems
Typically used as fill-in generation at times of peak demand, although green in terms of generation, pumped storage systems obviously consume power when pumping water back up to the high-level storage.
Efficiency in the motors, pump sets and generators is crucial to an installation’s viability as they are normally run on very low percentage margins. An improvement in generating or pumping efficiency, especially during routine maintenance work, could therefore make the difference between a site being viable or not.
Some pumped storage installations use separate turbines and pumps while others employ reversible turbine/generators which can act as both pump and turbine. One cost-effective alternative approach is to use a reverse-running centrifugal pump, which can improve the efficiency of the pumping cycle and thus reduce operating costs.
Maintaining these pumps and the associated generators requires expertise and experience as well as access to the latest in pump design innovations.
A power station needs to optimize output in order to meet demand as efficiently as possible, but inevitably there comes a time when maintenance repairs, either electrical or mechanical, will be required. Therefore, a programme of periodic preventive maintenance can be combined with condition monitoring techniques to improve the reliability and longevity of the equipment.
Employing a range of condition monitoring equipment, combined with suitable analysis techniques, can provide an accurate assessment of the status of turbines and generators, allowing operators to ensure continued production.
One of the most useful tools is vibration analysis, which can indicate potential problems with bearings and large rotating parts. Combined with thermal imaging, it can produce an accurate indication of the overall status of the plant.
Additional testing of the electrical windings, especially partial discharge (PD) analysis, can also provide very useful information on the overall condition of the generator. PD can cause insulation degradation which, if not remedied, can lead to reduced output of the generator and premature failure of the insulation system causing stator failures. The appropriate use of condition monitoring provides an excellent maintenance tool and can ensure efficient generator output.
The use of infra-red cameras for predictive and preventive maintenance is becoming more common, especially when assessing high-value equipment or where an unexpected failure could cause considerable disruption. These cameras can be used to analyze the running temperatures of bearings, motors, electrical connections and other equipment and, as such, can indicate potential areas for concern without having to stop any of the machinery.
By applying infra-red technology, engineers are only required to carry out maintenance procedures when they are required, rather than on a fixed basis, which improves efficiency and reduces maintenance costs. Sulzer believes that more clients can benefit from this technology and is offering free thermographic surveys to both new and existing customers.
Graeme Robertson is Head of Operations – UK for Sulzer.
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