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.
The DoE’s Renewable Energy Independent Power Producer Procurement Programme (REIPPPP) has resulted in the construction of several wind farms over the last few years that collectively house in the region of 1 400 wind turbines.
In October of 2021 the Department of Mineral Resources and Energy (DMRE) announced the preferred bidders for the Fifth Bid Window (BW5) under this programme which will see a further 1 600MW of onshore wind energy being developed.
The adequate functioning of most utility-scale conventional wind turbines depends to a large extent on the performance of the gearbox. 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.
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 same study also looked at data representing about 27 000 turbines ranging from 500kW to 5MW 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.
Therefore, reliability is essential to the success of wind energy systems, and this requires appropriate condition monitoring of sub-systems like the gearbox. It 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.
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.
The test slate offered by WearCheck as part of their wind turbine oil analysis programme is designed to achieve these functions.
To illustrate the potential savings that could be realised with a proactive maintenance strategy like oil analysis the following cost benefit analysis was performed.
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 R5.1 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 R561 000 per turbine.
Applying this savings to wind farm with 50 – 3 MW turbines equates to a savings of more than R28 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.
