Blade pitch control systems offer new challenges to designers and manufacturers

Colin Cooper, Eaton Corporation

The explosive global growth of the wind turbine industry has created several unique challenges for suppliers of the hydraulic components and systems used to control blade pitch. These challenges include an often hostile operating environment, difficult access for maintenance, and combinations of force, vibration, and motion seldom, if ever, experienced in more traditional applications.

Designing an effective hydraulic blade-pitch control (BPC) system requires knowledge of the performance characteristics of each valve, pump, hose, reservoir, and brake used, as well as 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 environments.

Valve selection

Selecting the right hydraulic proportional valves for BPC systems is a critical step, so looking at valve selection process provides a useful insight into the complexity of the designer's task. 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 several hundred metres above the ground on top of a tower. Moreover, the “ground” is very likely situated in an inhospitable environment such as offshore, or an exposed high ground position.

Inside the nacelle, the valves are subject to temperature extremes, together with high and variable rotational and vibration loads. Maintenance intervals also tend to be long because servicing such valves requires the attention of highly specialised personnel who can climb and don't mind working at heights while having to carry specialised safety equipment in addition to their normal tools. These factors combine to place a high premium on BPC valve reliability.

A new testing protocol

As one of the early suppliers to the wind turbine industry, Eaton learned that standard testing protocols do not sufficiently stress the valves to prove their 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.

But based on experience with a variety of wind turbine manufacturers, Eaton has adopted a more rigorous protocol called Highly Accelerated Life Testing (HALT). HALT helps engineers better understand the performance of hydraulic valves operating under the harsh conditions found in wind-turbine nacelles. The HALT protocol uses parameters well outside those encountered in the typical service life of the application and then tests products to failure.

HALT results let hydraulic designers focus on making a valve, for example, 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.

Expanded control capabilities

A lot can go wrong in a wind turbine nacelle. Even if the valves function properly, a broken hose or leaking fitting can cover the floor with hydraulic fluid very quickly. Or, if the electronic controls lack sufficient bandwidth to handle the signal load, the best possible outcome is an automatic shutdown that takes the turbine out of service.

In response to the need for more robust and precise control, KB series proportional valves are now being equipped with an improved control interface featuring CANbus communication using the CANopen protocol. These electronic controls are completely integrated into the valves, and packaged to industry IP67 environmental standards to meet the needs of the next generation of wind turbines.

This latest generation of CANbus compatible valve controllers operating under the CANopen protocol has sufficient bandwidth to accommodate virtually any practical load of control signals and inputs from sensors monitoring valve and actuator performance. Sensors and software in today's systems can proactively schedule preventive maintenance and even component replacement during scheduled downtime by identifying small degradations in performance.

Every component is a critical component

Hoses and fittings usually aren't high on lists of critical components, but they should be. Beyond their essential role in hydraulic circuits, hoses and fittings are key components in gearbox lubrication which on a wind turbine is typically an active system with constantly circulating fluids.

Selecting hoses with class-zero leakage eliminates a potential source of fluid loss because they do not weep in extreme temperature variations or on cool down. This not only adds to system reliability by extending hose service live, but also promotes a safer working environment inside the nacelle by keeping its floor drier.

In critical braking circuits, Premium hoses made with Teflon PTFE are the first choice because they resist bulging under pressure. They are also chemically inert which is important in gearbox lubrication circuits.

Colour coded fittings, such as those on Eaton's Match Mate series, can help prevent assembly errors on a shop floor, and potentially catastrophic failures that might result when the system is put into service.

Even something as commonplace as a reservoir requires special features to function in a wind turbine. This is because hydraulic power units typically aren't subjected to simultaneous vibration and rotation which is common in wind turbines.

Ordinary tanks are not designed to withstand pressure at their tops and will not keep hydraulic fluid from escaping through reservoir breathers and covers. Today, most wind turbine manufacturers specify tanks built to withstand at least the equivalent pressure of 0.025 bar on the tank without leaks.

Not every BPC system component must be specially engineered, though. Most of the pumps, such as the Vickers PVM piston series, are used without modifications because similar pumps have been proven on off-highway equipment and in nuclear power plants for more than 40 years.

Clutches and brakes are also commonly omitted from critical lists. But wind turbines could not function without yaw brakes that hold nacelles into the wind; drive-shaft brakes to lock rotating equipment to avoid damage under extreme conditions and provide a safe environment for maintenance; and clutches to connect the blades to the gearbox and generator.

Complex systems work best with simple supply chains

Properly matching operational characteristics of individual components can have a tremendous impact on overall system efficiency, performance and reliability. In today's increasingly complex BPC systems, component interaction is quickly becoming as important a consideration as component performance.

This situation amplifies the advantages of a single-supplier design philosophy as opposed to a mix and match approach. No matter which manufacturer is chosen, experience shows that the system is more likely to be successful if all of the major component parts of the system come from a single manufacturer. A single-supplier approach can also impact ongoing reliability and operating costs of a system, particularly when the supplier's parts and service footprint matches the global distribution of the turbine manufacturer's installed base. Since wind turbines are being installed today literally everywhere on the planet, this is a very important issue.

A turbine needing a replacement valve, pump, hose, clutch, or anything else is likely to be out of service for an extended period if spare parts and service are not locally available. The more global the component supplier's distribution system, the more likely parts and services are to be locally available when needed. Given the sophistication and relatively high cost of the more specialised components used in wind turbine BPC systems, convenient availability of factory-level repair and remanufacturing are also important concerns. OEM-repaired valves also carry original factory warranties, something non-OEM facilities cannot provide.

All things considered, experience shows that minimising the number of individual manufacturers involved and taking a big picture approach to their selection, are among the best ways to maximise the probability of a successful hydraulic wind turbine BPC system design.

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