15 Mar 2023
By Gregory Wolfe
This article provides an update to the article initially published in this magazine in August of 2020. In the original article, specific detail was provided on the benefits of monitoring harmonic distortion contained within the wind turbine generator’s electrical output signal. This information was key to understanding both the mechanical and electrical stresses within the system.
In case you missed it the first time, or are new to the industry, following is a recap of the original article.
You’ve secured the land, obtained all necessary permits, negotiated utility contracts, secured capital funds, established the operations team, your maintenance contract is in place and the turbines have been erected. Now it’s time to throw the switch!
How do you best manage your investment, nurture it, eliminate risk, maximize availability, and help ensure your wind turbines exceed their forecasted life?
More and more wind farm operations are realizing the key to long term profitability is managing their harmonic levels. By doing so, they are able to minimize undue stress on drive-train components — this not only can improve production availability, but also can significantly increase the operational life of the wind turbines.
What is harmonic distortion?
Based on physics, the output signal of any rotating electric generator (including a wind turbine) has a sinusoidal shape, alternating, up-and-down, at a frequency of 50 or 60 times per second (depending on the country in which it is designed to operate). In a perfect world, this signal would be very smooth in shape, with all sub-components working together in perfect harmony, each contributing to a perfectly and uniformly rotating system. In the real world, however, the output of a typical wind turbine generator is not smooth at all, but distorted in shape; in some cases, severely distorted. The magnitude of this distortion is known as the system’s harmonic distortion.
Ideal generator output (void of distortion)
Distorted generator output (indicative of degrading internal components – reducing the life of the wind turbine asset)
What causes harmonic distortion in a generator?
Within a rotating generator, there are numerous sub-components, each playing a specific role in helping the generator rotate in a uniform fashion, and each having its own specific operating frequency. Interestingly, most system sub-components have operating frequencies which are very different than the overall system operating frequency. For example, while the system operating frequency of the generator may be 60 hertz (rotating at a multiple of 60 times per second), the operating frequency of a critical sub-component (such as a gear tooth or winding flux interface) could be 432 Hertz. Even in newly commissioned wind turbines, all sub-components have imperfections; these imperfections each distort the output signal of the generator, the cumulative effect of which comprises the total harmonic distortion. Over time, as components degrade, these imperfections get worse, and the total harmonic distortion of the system increases accordingly.
How bad is bad when it comes to harmonic distortion?
Harmonic distortion can be thought of as the measure of stress on a system. For a wind turbine generator, the electrical output signal is the result of thousands of pounds of magnetic force applied to high-speed rotating equipment (upwards of 3600 RPM in some cases); any distortion in the output signal is actually an opposing force, not only against the rotating magnetic field, but against every sub-component working to keep the system in perfect rotation. Therefore, as the harmonic distortion of the system increases, the stress on each sub-component increases as well. This stress increases not only on the sub-components within the wind generator, but also on components downstream (pad mount transformers as well as grid components outside the wind farm).
Excessive harmonic distortion creates undue stress on all system components, leading to poor reliability and a reduction in the operational life of the equipment and components involved. It is for this reason that IEEE-519 was recently revised to redefine harmonic distortion acceptability limits and how harmonic levels should be calculated. These new requirements are making their way into an increasing number of Agreements between wind farm operators and grid operators in the territories in which the wind farms reside. Wind farm operators face stiff penalties for non-compliance.
Turning it into a positive
As a sub-component degrades in health, its contribution to the overall distortion of the output signal increases proportionately. These contributions can be isolated and tracked over time for each specific sub-component, so that preventative action can be taken. Monitoring harmonic distortion levels between the contributions of the various sub-components can be used as a source of actionable insight for proactive maintenance programs; this can help avoid costly unplanned outage, prevent undue stress on system components, improve availability, and increase the operational life of the wind assets.
Continuous monitoring for proactive maintenance, which now provides early warning indicators of both controller and transformer issues
A specific example
A common source of failure of a wind turbine generator system is a degrading wye-ring joint. Such a condition can lead to catastrophic failure at worst. At the very least, it can lead to an unplanned outage, loss in availability, and an overall reduction in the life of the wind asset. This degradation is typically caused by a 6th harmonic pulsating torque within the generator, which causes isolated stress on the wye-ring joints. Interestingly, a 6th harmonic pulsating torque is typically caused by excessive 5th and 7th harmonic signals (these, resulting from a mis-match in capacitor compensation). Thus, an early warning sign of a pending wye-ring failure is excessive 5th and 7th harmonic content. In advanced stages of wye-ring degradation, harmonic distortion around the system operating frequency is observed, equally spaced above/below the fundamental frequency, separated by sidebands located at twice the slip-frequency of the system.
Visualize degradation and validate repairs
In traditional monitoring systems, this type of wye-ring degradation would typically go undetected because those systems primarily focus on mechanical vibrations. Harmonic distortions are electrical in nature, and provide insight into both mechanical and electrical health conditions, including those which do not show on a vibration spectrum. Comprehensive monitoring of individual components helps wind operators make more accurate decisions to keep their turbines running longer and more efficiently.
In monitoring the electrical output signal of the generator, it was discovered that, with only minor modifications to the input sensors, both the WTG controller and step-up transformer could be included in the monitoring regimen. This opens up the availability of gaining pre-fault monitoring and health insight into the performance of controller and transformer sub-systems within the overall WTG system.
This new capability is only possible with CMS monitoring systems, which include high-resolution monitoring of the electrical output signal of the WTG.
Documented case-studies have identified early indications of various controller problems, as well as transformer insulation, winding, and load balancing issues. Since the original publication of this article, image 3 has been updated to include early warning indicators of both controller and transformer issues.
This valuable tool provides a holistic monitoring approach, helping owners and operators gain further insights into the health and performance of their WTG systems, helping improve profitability and extending the operational life of their critical assets.
Gregory Wolfe is CEO/CTO and Co-Founder of Fischer Block, Inc. His roots are in designing missile guidance systems for the United States Air Force. He later obtained certification as a black-belt in Six-Sigma statistical techniques while holding leadership positions within the electrical power industry, running operations in both the United States and in Europe before co-founding Fischer Block, Inc.
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