Engineers always want to look into how something works, understand it, and tinker with it. Human nature, however, tends to lull us into complacency when we feel we've solved a problem. When something works, we tend not to revisit the issue. But innovation often requires retracing our steps. In the case of wind turbines, this is no different. 

When compared to the size of a wind turbine, a pitch control system appears small. 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 monitors and adjusts the inclination angle of the rotor blades to control the rotation speed of the blades. Although these systems play an outsized role, they account for less than three percent of a wind farm's capital expenses (CapEx).

According to a 2011 ReliaWind research report, pitch system failures account for 23 percent 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 percent. 

Benchmarking reliability
When it comes to improving reliability in wind turbines, the pitch system is critical. One of the challenges faced by the wind industry involves identifying which improvements to component reliability bring the greatest return on investment. Most of the research done in the past focused on mechanical and electrical system level analysis, but provided very little depth in terms of failure analysis at the component level and its impact on cost of energy. Earlier this year, DNV GL, an international certification body and provider of technical assessments, quantified:

• the impact of pitch system reliability on turbine failure rate; and
• the improvement in Levelized 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 (incentives excluded). DNV GL collected data from 69 projects, totaling 5.3GW of capacity across four million turbine days for wind turbines located in North America, Europe, and China. The turbines ranged in size from 1.5MW to 3MW.

The 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 analyzed electromechanical (EM) and electrohydraulic (EH) pitch systems from several wind turbine makers, operating in the field. 

A typical pitch system used throughout the industry today consists of roughly 3,000 to 4,000 sub-components and has a system reliability of approximately 5,700 hours. Field data collected for the study is a representative sample of these designs. However, the newer pitch systems are lighter, smaller and more than three times more reliable than those more commonly used. Some of the latest designs have as few as 1,200 sub-components, which greatly reduces complexity and improves maintenance. 

The benchmarking study shows that turbine reliability can be improved significantly by using smaller pitch control systems with fewer sub-components. Current industry designs incorporate components manufactured for general-purpose industrial applications, with limited customizations for wind turbines. Opportunities for improvement include optimizing the drive electronics by using pluggable PCB modules, instead of wiring off-the-shelf DIN-rail components, and using ultra-capacitors, instead of batteries, to eliminate backup power failures and periodic maintenance.  

With AC synchronous motor technology (i.e., brushless, no fans for cooling), engineers are improving pitch system motor reliability and reducing periodic maintenance, compared with the AC Induction or DC motors currently used by the wind turbine OEMs. These improvements are helping the new pitch systems increase reliability over existing industry designs, by a remarkable 223 percent. 

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 lighter, smaller pitch systems can save up to $1.70/MWh for a typical 3.0MW turbine. 

One of the wind industry's biggest opportunities is improving reliability. Operators of wind farms and makers of wind turbines should not overlook pitch systems as a key to improving uptime, purely because the systems are such a small expense relative to the cost of the turbine.

Mr. Prasad Padman is an instrumentation and control engineer with a master's degree in finance and marketing, and has been with Moog for eight years in various roles. He is currently responsible for developing needs for next-generation motion control solutions.

Moog, Inc. | www.moog.com