Keeping the lights on

Published:  05 June, 2014

As the global population increases so the demand for energy rises too, but while in developed countries the rise in both is very similar in percentage terms, for less developed countries the demand for energy is rising much faster. In order for this increased demand to be met, new facilities need to be installed but the existing generating equipment also needs to be more efficient and reliable.

For the majority of power stations a fuel source is used to produce steam, which is passed through a gas turbine, which turns a generator. In fact, for 2008, over 89% of the world's energy was produced using this combination and a great deal of that generating capacity was installed over 20 years ago. Since then, technological advances in turbine and generator design have made great improvements in efficiency which could now be retro-fitted in a proactive maintenance programme.

A power station needs to optimise output to meet demand, but inevitably there comes a time when maintenance repairs, either electrical or mechanical, will be required. While this may not affect the ability of the power station to meet current demand, it will place increased pressure on the remaining capacity, which in-turn needs to be robust enough to cope. Therefore, a programme of periodic, preventative maintenance can be combined with condition monitoring techniques to improve the reliability of the equipment.

Condition monitoring

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 that is used to identify potential issues with bearings, poor electrical connections or the imbalance of phase loadings, which 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. The appropriate use of condition monitoring provides an excellent maintenance tool and can ensure efficient generator output.

Once a turbine and generator set is taken offline for maintenance, it is essential that the required repairs and upgrades are completed as quickly as possible, so planning and proper coordination of the various engineering teams is essential. The skills and equipment required for removing, repairing and commissioning a gas turbine are very different from those involved in the refurbishment of a generator coil and stator.

Turbine repairs

Once a turbine has been removed, it is essential to complete the inspection and assessment process as soon as possible in order to prepare the reconditioning proposal. The ability to carry out weld inspection down to a microscopic level and provide accurate analysis of the balance of the turbine is important in determining potential areas for improvement. Checking that the turbine is in balance is crucial to ensuring that the plant operates free from vibration.

Delivering high levels of maintenance is essential, meaning any potential issues must be repaired to precise standards using the appropriate techniques according to the specification of the turbine. Damaged turbine blades must be replaced, new bearings sourced and installed and the completed assembly carefully balanced to ensure smooth running once it is reinstated.

Generator repairs

In some cases the full extent of a repair does not become apparent until the generator is removed and examined closely. While the stator coils will usually be replaced, there may also be an issue with the laminated core and the rotor assembly. Modern insulation technology allows for thinner layers which can withstand greater dielectric stress and higher temperatures, which also allows more space for copper within the same slot area. This reduces the resistance of the stator winding, which runs cooler, allowing a small increase in output.

By using the latest CAD software, the bars or coils can be precisely formed to ensure an exact fit in the stator slot, making the installation process more efficient. By using comprehensive testing techniques, the reliability of the new coils can be guaranteed, which is especially important for coils being used in hydro generators, which can be required to pass a 400 hour voltage endurance test to provide evidence of the build quality of the coils.

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