North offset: an often forgotten issue
During the life cycle of a wind farm, multiple operational issues can appear with significant impact on the energy yield, performance and lifetime. A good example of this is the North deviation or North offset that corresponds to the difference between the True North and the SCADA turbine North.
Illustration of a North offset: the difference between the True North and the SCADA turbine North that creates an offset on the SCADA nacelle direction measurement.
Each wind turbine nacelle direction is continuously measured as part of the standard SCADA system: this information is often measured by an incremental sensor located between the tower and the nacelle. It operates by detecting each change of yaw direction (detection of each bearing passing in front of the sensor) and adds the angle delta to the previous direction value.
After the initial calibration (during commissioning, for example), it is expected for the sensor to operate 100% of the time without any issues: even a single measurement error, bearing passing missed or the sensor not being temporarily powered will create a north offset. In addition, software upgrades and reboots might reset any offset you have applied in the past to compensate for the north deviation.
And contrary to common knowledge, north offsets are not automatically checked/corrected in the wide majority of turbine models nor are they part of the standard O&M schedule. This could explain why it is one of the most frequent and recurrent issues found on operating wind farms.
The reason why North offsets and nacelle direction measurements have not been a priority in the industry is because they don’t impact the normal operation of a turbine, as the normal operation is not wind sector dependent. But, in reality, multiple constraints and power limitations have been increasingly imposed on wind farms: Wind Sector Management plans (to protect turbines from particularly rough wind conditions), acoustic curtailments (to avoid excessive noise emission on close by housings), shadow flicker curtailments, … and the majority of those are actually wind sector dependent !
Example of a time series of the North offset on an operating wind farm extracted from ExpertWind’s online dashboard: besides the significant north offsets detected (up to 150°), multiple changes over time were also visible.
When these limitation plans are imposed on a wind turbine, the controller will determine if the turbine should be curtailed by looking at the nacelle direction measurements coming from the SCADA sensor. But if a north offset is present, the nacelle direction information is not correct and therefore the turbine is going to be curtailed on the wrong wind sectors. This has a double negative impact: turbines are not curtailed when they should and they are curtailed (losing unnecessary energy yield) when they shouldn’t.
Curtailment plans can reduce the energy yield of a wind farm up to 10% of the expected AEP, making them one of the most critical parameters to be monitored on a wind farm.
Furthermore, as we explained in one of our previous articles (Turbine Performance), wind turbine performance is strongly influenced by external factors including wind direction. So if we want to perform a reliable performance monitoring, north offsets should be accounted for in order to, for example, assess impacts of wind sectors on turbine performance.
With so many challenges and so much to consider, how can we effectively manage it all?
Stay tuned for our upcoming article where we’ll be sharing how we at ExpertWind handle and correct North offsets and present real use-cases of issues detected and their impact on operating wind farms.
In the meantime, if you have any questions or if you want to learn more about our services and how your wind farms can benefit from them, feel free to Contact Us.