An efficient, responsive, and reliable blade-pitch control on a wind turbine is made from many valves, pumps, hoses, reservoirs, and brakes. They must all work together to meet the demanding requirements placed on the pitch control in a turbine hub.
An effective hydraulic blade-pitch control (BPC) requires knowing the performance characteristics of each component and their likely interactions with other components. The task also calls for many tests that go beyond those applied to less demanding applications. In addition, the design work requires experience with the peculiarities of wind turbines’ operating environment.
Valve selection provides a good example of the complexity of the designer’s task. Selecting the right hydraulic proportional valves for BPC systems is a critical step.
From a functional viewpoint, valves in a BPC system are not required to do anything particularly difficult or unusual. They simply control the flow of fluid to cylinders in response to sensor signals in a fairly straightforward closed-loop servo control circuit.
What is unusual is that the valves are housed in a nacelle on a several-hundred-foot tower. What’s more, the tower is likely in an inhospitable environment, such as offshore or on a mountaintop.
The valves are subject to temperature extremes, high and variable rotational and vibration loads, and long periods between maintenance. Servicing such valves requires the attention of highly specialized personnel who can climb and don’t mind working at heights while lugging up equally specialized safety equipment in addition to their normal tools.
These factors combine to increase the premium placed on BPC valve reliability. Standard testing protocols will not sufficiently stress a valve to prove its ability to perform under the extreme conditions.
In standard life testing, the operating parameters of an application are known and the test sample is subjected to a test within them. Simple product survival is considered a success.
Based on its experience, my company has adopted a more rigorous protocol called Highly Accelerated Life Testing (HALT) to help engineers better understand the performance of hydraulic valves operating under the harsh conditions found in wind-turbine nacelles. HALT uses parameters well outside those encountered in the typical service life of the application and then tests products to failure. HALT results let a team of hydraulic designers focus on making, for example, a valve more reliable in the areas where a failure is most likely. More important, HALT gives the company an objective, documented, and repeatable way to qualify its KB proportional valves, the series the company recommends for wind turbines.
In response to the wind industry’s need for more precise control, KB Series proportional valves are also being equipped with an improved control interface featuring CANbus communication using the CANopen protocol. These integrated electronic controls are packaged to industry IP67 environmental standards to meet the needs of the next generation of wind turbines.
Beyond the valve
A lot can go wrong in a nacelle. A valve may function well but a broken hose or leaking fitting could let hydraulic fluid onto the floor. Or, if the electronic controls have insufficient bandwidth to handle the signal load, the best outcome to hope for is an automatic shutdown that takes the turbine out of service.
The latest generation of CANbus compatible valve controllers operating under the CANopen protocol have the bandwidth to accommodate almost any practical load of control signals plus inputs from sensors that monitor valve and actuator performance. By identifying small degradations in performance, sensors and software in today’s systems can proactively schedule preventive maintenance and even component replacement during scheduled downtime.
Hoses and fittings are probably not high on most lists of what some designers call critical components, but they should be. In addition to their role in hydraulic circuits, hoses and fittings are key components in gearbox lubrication, typically an active system with constantly circulating fluids.
Hoses with class-zero leakage do not weep in extreme temperature variations or on cool down, so selecting them eliminates a potential source of fluid loss. Such hoses also promote a safer working environment inside the nacelle by keeping its floor drier and delivering extended service lives.
Premium hoses made with Teflon PTFE are resistant to bulging under pressure, making them a first choice for critical braking circuits. They are also chemically inert which is important in gearbox lubrication circuits.
Color coded fittings, such as those on Match Mate series from Eaton, can help prevent assembly errors on a shop floor, and potentially catastrophic failures that might result when the system is put into service.
Even parts as simple as a reservoir is a potential point of failure requiring features to function well in a wind turbine. Most hydraulic power units are not simultaneously subjected to vibration and rotation. But the combination is common in wind turbines.
Ordinary tank designs will not keep hydraulic fluid from escaping through reservoir breathers and covers because they are not designed to withstand pressure at their tops. Most wind turbine OEMs now require that tanks withstand at least the equivalent pressure of 12-in. column of fluid above the tank top without leaks.
Not every component in a wind turbine BPC system must be specially engineered. Most pumps used, such as the Vickers PVM piston series, are often selected without modifications. Hydraulic component manufacturers have been building similar pumps for use on off-highway equipment and in nuclear power plants for more than 40 years.
Clutches and brakes are additional components often omitted from critical lists. Wind turbines could not function without yaw brakes that hold nacelles into the wind. Drive-shaft brakes lock rotating equipment to avoid damage under extreme conditions and provide a safe environment for maintenance. Clutches connect the blades to the gearbox and generator.
Increasing system complexity
As system complexity increases, how components interact plays a great a role in system reliability. In systems as complex as today’s BPCs, properly matching operational characteristics of individual components can have a tremendous impact on overall system efficiency, performance, and reliability.
This idea suggests a single-supplier design philosophy as opposed to a mix and match approach. Regardless of which manufacturer’s products are chosen, experience shows that the system is more likely to be successful if all of the major component parts come from that manufacturer.
The single-supplier approach can also impact ongoing reliability and operating costs of a system if the supplier maintains a parts and service footprint matches the global distribution of wind turbine installations. This is an important issue when one considers the geographic locations in which wind turbines are likely to be installed. They can be almost anywhere on the planet.
If spare parts and service are not locally available, a turbine needing a replacement valve, pump, hose, clutch, or anything else is likely to be out of service for an extended period until the parts arrive. On the other hand, if the component manufacturer has a global distribution system, parts and services are more likely to be locally available when needed.
Factory-level repair and remanufacturing are also important concerns given the relatively high cost of the more specialized components used in wind turbine BPC systems. Such OEM repaired valves also carry original factory warranties, something non-OEM facilities cannot provide
Experience shows that minimizing the number of individual manufacturers involved in each step of the process, and taking a big picture approach is one of the best ways to maximize the probability of a successful hydraulic wind turbine outcome.
Discuss these considerations for designing turbine hydraulics at www.EngineeringExchange.com
Global Sales & Market Development
Eden Prairie, Minn.
Filed Under: Featured, Hydraulics