Monday, 10 October 2022 14:27


Traditionally, South Africa has relied on non-renewable coal resources for the production of electricity. In fact, more than 80% of the country’s power is generated from this fossil fuel. However, as the demand for electricity has surged – and at times, outstripped supply – it’s become critical to consider more viable energy resources. One such resource is wind – an abundant, renewable energy resource that has been used to generate power for thousands of years. Wind is a clean, free and infinite resource that presents viable opportunities for energy production in South Africa.

South Africa’s Integrated Resource Plan (IRP), under the leadership of the Department of Energy (DoE), foresees renewable energy contributing to 42% or 17.8GW of the country’s new generation capacity by 2030. One of the ways they plan to achieve this is with 8.4GW of wind generated power.

While wind energy in South Africa is still, relatively speaking, in its infancy, when compared to more developed nations, the benefits of developing this alternate energy resource have become abundantly clear. Over time, wind energy could help to alleviate the country’s dependence on expensive, harmful and diminishing fossil fuels, while providing a sustainable and secure energy resource for a greater number of people. The message is clear, the transition to clean renewable energy is about making an investment in the future of our civilization and the planet.

According to a new analysis published by the Global Wind Energy Council (GWEC), South Africa added 515 MW of new wind capacity in 2020 cementing its wind leadership on the African continent.

By the end of last year, South Africa had 2 500 MW of cumulative wind capacity installed, representing about 34% of the 7 300 MW of capacity installed across Africa and the Middle East.

However, these two regions represent only a fraction of the total market, with global additions of about 70 GW in 2020 and with the cumulative installed base expected to grow to about 800 GW in 2021.

The DoE’s Renewable Energy Independent Power Producer Procurement Programme (REIPPP) has already overseen the completion of four successful bidding windows with a fifth window launched in March of 2021.

The approved wind farm bids from all four bidding rounds has resulted in the construction of several wind farms over the last few years that collectively house more than 1200 wind turbines with many more installations on the horizon.

The much anticipated fifth round of this procurement program (REIPPP) has been issued with the Ministry of Mineral Resources and Energy (MMRE) which seeks to procure a further 1.6 GW of wind energy.

It is self-evident from the above that in the current environment of renewable energies, wind energy has played an important role as a driver of change in the generation of clean alternative energy. However, this rapid increase has also given rise to several operational and performance issues related mainly to equipment design and maintenance practices.

As the growth in wind energy has continued, the average size and capacity of wind turbine generators has also increased. With this increase in size comes an increase in the cost of operation, and specifically the cost of repairs, downtime, and unscheduled maintenance.

Added to this the estimated life span of wind turbines is about 20 years, compared to conventional steam turbine generator units that have averaged 40 years. The failure rate of wind turbines is about 3 times higher than that of conventional generators. Therefore, reliability is essential to the success of wind energy systems and this requires appropriate condition monitoring. 

As a result of external variants, wind turbines undergo constantly changing loads, unlike conventional power plants. Due to these highly mutable operational conditions the mechanical stress placed on wind turbines is unmatched in any other form of power generation and they consequently require a high degree of maintenance to provide cost effective and reliable power output throughout their expected 20-year life cycle.

Ensuring long-term asset reliability and achieving low operation and maintenance costs are therefore key drivers to the economic and technical viability of wind turbines becoming a primary renewable energy source in South Africa.

The adequate functioning of most utility-scale conventional wind turbines depends to a large extent on the performance of the gearbox. These gearboxes transform slow speed, high torque wind turbine rotor rotations to the higher speed required by the generator, which converts the mechanical power to electricity. They are typically configured to have planetary gearsets and bearings that require special attention due to their extreme operating conditions and high lifetime expectations.

The wind turbine gearbox is the most critical component in terms of high failure rates and down time. These premature gearbox failures are a leading maintenance cost driver that can substantially lower the profit margin of a wind turbine operation as they typically result in component replacement.

According to a study published by the National Renewable Energy Laboratory (NREL) the mean downtime for gearbox failure was in the range of 6-15 days for a study performed on European on-shore wind turbines over a 13-year time period. The mean downtime could be even longer if a spare gearbox is not available and crane availability is an issue. The same study also looked at data representing about 27,000 turbines ranging from 500kW to 5 MW over a six-year period and found that the cumulative downtime caused by gearbox failures was more than one hundred and fifty thousand hours.

Further analysis found that 76% of gearbox failures were bearing related and mostly due to High Speed Shaft (HSS) & Intermediate Speed (IMS) bearing axial cracks.

Despite significant advancements in gearbox design, they remain an operation and maintenance cost driver due to the very high associated repair costs coupled with a high likelihood of failure through much of the wind turbine’s life cycle.

Therefore, reliability is essential to the success of wind energy systems and this requires appropriate condition monitoring of sub-systems like the gearbox. Its is for this reason that condition monitoring techniques like oil analysis are considered far more effective predictive/proactive tools for achieving optimum gearbox performance as, in the case of oil analysis, the technique can be used to detect the onset of early damage as well as tracking the severity of the damage.

Routine oil analysis is one of the most widely used predictive / proactive maintenance strategies for wind turbines and utilises a test slate that evaluates the condition of the in-service lubricant and helps evaluate the condition of internal mechanical components. In short, Routine oil analysis is used as a frontline defence against premature gearbox failures.

The three main objectives of oil analysis are to monitor the health of the gearbox, monitor the health of the oil and monitor contaminants. Active monitoring of the above provides early warning of abnormal operating conditions that can lead to catastrophic failures if not corrected.

These three objectives can be expanded to six main functions of an oil analysis programme which are to: detect abnormal wear, detect oil degradation, detect contamination, optimise service intervals, avoid loss of production and ultimately, to save money.

The test slate offered by WearCheck as part of their wind turbine oil analysis programme is designed to achieve these six main functions.

To illustrate the potential savings that could be relalised with a proactive maintenance strategy like oil analysis the below cost benefit analysis was performed.

The cost involved in installing a commercial-scale wind turbine can vary significantly depending on the manufacturer of the turbine, the number of turbines, finance and legal costs, construction contracts, the location of the project, infrastructure requirements as well as numerous other factors outside the scope of this document. For the purpose of this analysis the assumption has been made that a utility scale wind turbine costs about ZAR 17 million per MW of capacity.

With replacement of the main gearbox being approximately 10% of the overall wind turbine cost, replacing a 3MW wind turbine gearbox could easily exceed ZAR 5.1 million with transportation, crane rental etc. Over the 20-year expected life span of the wind turbine the main gearbox is expected to be replaced 2.2 times due to failure.

If the gearbox life could be extended by even one year, the replacement costs associated with the wind turbine life cycle will be reduced by ZAR 561,000 per turbine.

Applying this savings to wind farm with 50 – 3 MW turbines equates to a savings of more than  ZAR 28 million over the life cycle of the wind farm. The saving does not take into consideration the down time costs associated with loss of electricity production which in itself would be validation for a proactive maintenance programme of this nature.

Oil analysis provides a solid foundation on which to build an effective condition monitoring programme in many applications. In the case of wind turbine gearboxes, oil analysis has the potential to reduce unscheduled maintenance, improve reliability and extend service life. The oil analysis tests performed by WearCheck can help wind farm operators get maximum value from their oil sampling program. When these tests are performed on a routine basis and the results analysed by our experienced technicians, oil analysis can facilitate the maintenance of wind turbine gearboxes and ultimately, support this promising form of power generation on the African continent and beyond. Let WearCheck help you protect the assets that protect our planet.

By Steven Lumley

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