Pitch systems keep a turbine running and ensure the safety of the turbine in the event of high winds or catastrophic events. The pitch control system, mounted inside the wind turbine’s hub, monitors and adjusts the inclination angle of the rotor blades and thus controls the rotation speed of the blades. Although these systems play an outsized role, they account for less than 3% of a wind farm’s capital expenses.
According to a 2011 ReliaWind research report, pitch system failures account for 23% of all downtime in wind turbines. This is more than any other component or system of the turbine. The ReliaWind report goes on to note that pitch systems tallied the highest percentage of all component failures in wind turbines at more than 21%.
Benchmarking reliability
When it comes to improving reliability in wind turbines, the pitch system is a critical component. One of the challenges faced by the wind industry lies in understanding where efforts to improve component reliability will translate into the greatest return on investment. Most of the research done in the past on this subject has focused on mechanical and electrical system level analysis – providing very little depth in terms of failure analysis at the component level and its impact on cost of energy. Earlier this year, Moog partnered with DNV GL, an international certification body and provider of technical assessments, to quantify the impact of pitch system reliability on turbine failure rate, and the improvement in Levelised Cost of Energy (LCoE) due to advances in pitch system reliability through innovative design.
LCoE measures the net cost to install and operate a wind turbine against expected energy output over the course of the turbine’s lifetime. Moog and DNV GL collected data from 69 projects totalling 5,3 GW of capacity across four million turbine days for wind turbines located in North America, Europe and China. The turbines ranged in size from 1,5 to 3 MW. The Moog-DNV GL benchmarking study confirms that pitch systems (whether electric or hydraulic) have a high rate of failure and significant effect on turbine reliability, downtime, operating expenses and LCoE.
Improving pitch system reliability through innovative design
As part of the study, DNV GL’s research team analysed a number of wind turbine makers’ electromechanical (EM) and electrohydraulic (EH) pitch systems operating in the field as well as a newly designed next-generation pitch system from Moog, known as the Moog Pitch System 3. A typical pitch system used throughout the industry today consists of roughly 3000 to 4000 sub-components and has a system reliability of approximately 5700 hours. Field data collected for the study is a representative sample of these designs.
The Moog Pitch System 3, developed and released in 2016, consists of 66% fewer parts than the pitch systems installed on most turbines today. The benchmarking study shows that turbine reliability can be improved significantly by using Moog’s next-generation pitch control system. This is largely because current industry designs incorporate components manufactured for general-purpose industrial applications with limited customisation for wind turbines. Some of the opportunities for improving on these designs include consolidation of functions and designing electronics specifically for the wind turbine hub environment rather than wiring off the shelf DIN-rail components. Another opportunity for design improvement is through the use of ultra-capacitors instead of batteries, to eliminate backup power failures and minimise periodic maintenance.
The Moog Pitch System 3 capitalises on these opportunities, including pluggable/integrated electronics for the system’s drives, AC servo motor technology and advances in ultra-capacitor design. With permanent magnet synchronous motor technology (i.e. brushless, no fans for cooling), Moog’s engineers improved pitch system motor reliability and reduced periodic maintenance compared to DC motors currently used by the wind turbine OEMs. These improvements have helped the Moog Pitch System 3 set a new benchmark in pitch system reliability. In fact, the new Moog Pitch System 3 improves wind turbine reliability by up to three times over current industry designs.
Improving the reliability of the pitch system directly affects the turbine reliability by mitigating downtime; and reducing maintenance, in turn, lowers LCoE. The DNV GL LCoE model also shows that the Moog Pitch System 3 can save up to R25/ MWh for a typical 3,0 MW turbine. An optimised pitch system with pluggable/integrated electronics for the system’s drives, AC servo motor technology and advances in ultra-capacitor design – along with additional enhancements – has significantly improved pitch system motor reliability and reduced periodic maintenance.
For more information contact Willie Steyn, Moog, +27 (0)12 653 6768, wsteyn@moog.com, www.moog.com
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