Acting on the Costs: Position and motion sensors for wind power plants

Kubler 8/1/2015

Acting on the CostsIn order to increase the profitability of wind power plants, the components used in them must become more cost-efficient - this also applies to the sensors. Nevertheless, quality shall in no case suffer from this. The right strategy allows acting successfully on the costs: costs can be saved with the encoders, the safety modules and with smart comprehensive solutions.
Cost pressure has significantly increased in the area of wind power plants construction. Therefore, in order to further increase the profitability of wind power plants, the components must become less expensive. This also applies to sensors. The Fritz Kubler company, who develops and manufactures encoders for all speed and positioning tasks in wind power plants, shows that the cost-efficiency of wind power plants must not be in contradiction with the very high demands on the components used in them. 
Encoders provide the right drive

Encoders used for rotary speed control can be found at several places in the drive train of wind power plants. Bearingless magnetic systems can measure the speed directly on the hub. Rotor speed and position measurement is taken in charge by encoders in the slip ring. Hollow shaft encoders with isolated shaft are used on the high-speed generator shaft. How can costs be saved on the development, manufacture and implementation of these products and yet comply in the same time with the high requirements of wind power plant manufacturers? 
Solution approaches for high-quality and cost-oriented sensors

Kubler's approach for cost-efficient position and motion sensors in wind power plants is based on three solution approaches: 
1. Optimize costs and material usage thanks to Lean Design

Kubler includes here all actions that purge the range of functions of the multiturn encoder Sendix 58 with optical gear. This encoder family is used in pitch systems, that is to say in speed-controlled wind turbines in full-load operation - directly on the motor or as a standalone blade encoder including the measuring pinion and the mounting bracket.

In this case, lean means individualization: first, the various functions of the encoder are analyzed to determine whether they are really necessary for wind power plants. Unnecessary functions are then removed. This results in individually customized absolute multiturn encoders: the compact Sendix F36, a fully optical encoder with electronic multiturn in a 36mm housing, the efficient Sendix F58, also a fully optical encoder with electronic multiturn in a 58mm housing, and the versatile Sendix 58, a fully optical multiturn encoder with gears. 
2. Cost-oriented design thanks to Design-to-Cost

Within the framework of Design-to-Cost, Kubler takes into consideration, besides the technical features and the deadlines, costs as a main project parameter for the development of encoders. In line with this strategy, Kubler extended its hollow shaft incremental encoders range with an encoder tailor-made for the generator speed requirements in wind power plants.

Kubler counts specification among the unique features of its products: the Sendix 5020 can be used for applications with limited space, shaft isolation being ensured with isolating inserts. The A02H with large hollow shaft is a slim encoder with large ball bearings for higher bearing load and large shaft diameter. Shaft isolation is ensured by an isolating sleeve with steel insert. The third of the trio is the Sendix Heavy Duty H120 for applications with high bearing loads and strong bearing compensation currents. 
3. Intelligent solutions for directives (Machinery Directive 2006/42/EC)

The legislation requires a comprehensive technical safety analysis and, if necessary, the upgrade of wind power plants. Kubler offers for this safe incremental and absolute encoders ranging from SIL2/PLe to SIL3/PLd. These encoders are mainly used for the safe monitoring of rotor speed and rotor position.

Besides the safe encoders, the company offers new safety modules, which are used as small safety controllers for the monitoring of speed and position as well as for further safety-relevant measured values of a wind power plant. They consist of basic and expansion modules that can be integrated, along with the suitable encoders, in a safety solution for wind power plants. Functional Safety is completed with adapted service packages. These range from the risk assessment, through the safety concept, up to the installation and commissioning of a complete plant. This way, besides saving time and costs, the customers also minimize their risks.

A tower oscillation sensor also used for plant monitoring has been added recently to the portfolio. Before Machinery Directive 2006/42/EC made the technical safety analysis of wind power plants a binding obligation, the rotary speed of the hub was considered as the main safety parameter. Consequently, speed was measured, outside of the operation controller, by a separate speed module and by proximity switches scanning a perforated disc or screw heads on the hub.

Further parameters such as cable twisting, tower oscillation, wind direction, emergency stop buttons or rotor blocking were processed partly redundantly by the operation controller, leading to the shutdown of the wind power plant. 
Encoder signals are used for safety monitoring

Today's safety controllers monitor the input signals coming from limit switches or oscillation sensors. But they generally have no encoder interface and cannot process high-frequency speed signals. Therefore, the manufacturers had to add to low-frequency sensors redundant with respect to the operation controller in order to be able to monitor rotor speed safely - or to go on working with a separate speed module.

Implementing Kubler's SIL encoders allows using the encoder signals both for the operation controller and for safety monitoring. Moreover, the signals can be split. The safety controller synchronizes with the SSI clock between the encoder and the operation controller and monitors safely both speed and position. This applies to the encoders mounted on the hub, on the generator and on the azimuth and pitch adjustment.

Equipped with safe digital and analog inputs, the Safety-M modules can monitor all other safety-relevant measured parameters of a wind power plant such as cable twisting, tower oscillation, emergency stop switches, wind direction or blocking. Cost pressure on development, manufacture and maintenance of wind power plants will go on increasing in the future - as the safety requirements will. Many new solutions will be necessary in many places, also in the sensors area, to meet these requirements. Cost reduction by saving unnecessary functionality is a practicable solution, whereas cost reduction at the expense of quality would be fatal.

Safety technology is unjustly denounced as a price driver. Used properly, it can save redundant sensors and redundant processing and therefore reduce the costs. Two prerequisites are necessary to reach the many ambitious goals: active branch management and a methodical approach.