The wind power and petroleum industries may be seen as rivals, yet oil is essential throughout a wind farm to ensure smooth operation. As such, it is necessary to select the right accumulators for both hydraulic and lubrication applications.
Wind turbine manufacturers typically speak of having availability levels in the range of 97% to 98%, but a 2008 study by British wind consultants Garrad Hassan found actual availability levels to be lower. In North America, turbines were averaging an availability of 94% to 95%.
The reasons for downtime vary widely. Sometimes a fault condition is the result of voltage or current being too high or too low. Grid stability issues can cause downtime, as can rotor overspeed conditions, vibration protection alarms and problems with the pitch rate.
Some of these issues are normal in any new equipment design experiences, and more mature turbine designs usually have a higher mean time between fault (MTBF) than newer models. Downtime caused by overheating or premature failure of bearings, gearboxes, or other components due to inadequate or uneven oil pressure is unacceptable.
This issue is not limited to the wind industry. In 2001, for example, the Nuclear Regulatory Commission (NRC) found that “inadequate adherence to maintenance instructions” resulted in “a loss of lubrication and subsequent bearing failure” on an auxiliary feed water pump turbine at Calvert Cliffs Nuclear Power Plant in Lusby, Md. This failure resulted in one of only seven “yellow findings,” the second highest level in terms of potential safety hazard, that the NRC issued from 2001 to 2005. As NRC Regional Administrator Hubert J. Miller wrote, “The finding has substantial importance to safety due to the equipment’s intended function of removing decay heat if called upon, as well as the length of time this condition existed.”
With wind turbines, we are not concerned with radioactive safety, but even short-term loss of flow can lead to excessive wear and premature replacement. More severely, it can lead to failure of the braking pitch or yaw controls, which in high-wind conditions can result in the loss of the entire turbine. This can be more complicated from a maintenance standpoint, because while a single steam turbine generator can consistently produce hundreds of megawatts, it takes hundreds of wind turbines to produce the same output, vastly magnifying failures of a lubrication system. Furthermore, wind turbine controls must operate continually to compensate for ever-shifting wind speeds and direction.
Given the multitude of hydraulic and lubrication systems in each turbine, and the dozens or hundreds of turbines at each site, it is essential to select a high quality hydraulic system, whether as part of the original equipment or as an upgrade to boost reliability. To prevent unnecessary damage or catastrophic failure in that system, properly sized and maintained accumulators are needed to provide a temporary supply of lubricating oil when the oil pump fails. In hydraulic systems, accumulators provide rapid response and minimize pressure fluctuations.
Function of accumulators
Hydraulic accumulators are energy storage devices that smooth out the pulsation of oil pumps and provide short-term oil pressure during switchover between oil pumps. They help maintain a constant oil pressure during temporary changes in demand. In hydraulic systems, accumulators provide storage for fluid at a constant pressure allowing quick and precise movement of actuators. The number of accumulators in a wind turbine varies by manufacturer, but they typically have several with sizes ranging from about 10 in.3 up to 25 gallon.
Lube oil systems consist of three elements: a pump, a reservoir and an accumulator. Lube oil system accumulators (LOSA) prevent bearing damage and increase bearing life by supplying oil to the bearings when a power failure shuts down the pump, or when changing between the primary and backup oil pump.
Hydraulic and lubrication oil systems, with their accumulators, are placed throughout the hub, nacelle and ground equipment, serving a wide range of functions. These include the hydraulics used for pitch control and braking, and the lubrication of the gearbox, turbine, drive train and other components. When specifying an accumulator to mount within the hub, one has to be selected that would withstand continuous shaking and vibration.