More automation and greater dynamism – the demands on the mechanics and control technology are increasing on wind turbines just as they are in traditional engineering. At the same time it is absolutely essential for plant builders and operators to meet the specifications of the Machinery Directive. Guidelines published by Germanischer Lloyd WindEnergie GmbH (GL Wind) also categorise protection against excess rotor speed as a safety-related function. Safe speed monitors meet these specifications and combine fulfilment of safety requirements with economical operation.
The technical development of wind turbines has progressed rapidly over the last few years. Apart from the basic principle of converting the wind’s kinetic energy into electrical energy, the first wind turbines from the 80s have very little in common with today’s megawatt systems. According to the German Federal Wind Energy Association, rated powers have increased tenfold on average in the last decade, while the rotor diameter and hub height have doubled.
As plants increase in size, the demands on safety and control technology also increase. A failure of the control technology can lead to a total failure of the wind turbine. This can be hugely damaging for the plant manufacturer or operator, not only in a material sense but also in terms of image.
Complying with the specifications of the Machinery Directive
When designing and building wind turbines within Europe, manufacturers and operators must adhere to the specifications in the Machinery Directive 2006/42/EC: The Machinery Directive has been incorporated into domestic law in Germany via the Product Safety Act (ProdSG) and is therefore binding for wind turbine manufacturers. This Act calls for a comprehensive risk assessment for all of the plant’s lifecycle phases and operating modes and obliges the manufacturer to issue a declaration of conformity in accordance with Annex IIA of the Machinery Directive.
The 2010 edition of the “Guidelines for the certification of wind turbines” from the leading certifier for wind turbines, Germanischer Lloyd WindEnergie GmbH (GL), provides wind turbine manufacturers with a further set of rules that consider the functional safety aspects of control functions. GL names EN ISO 13849-1, Safety of machinery – Safety-related parts of control systems, Part 1: General principles for design, as an appropriate standard for comprehensive fault consideration. EN ISO 13849-1 describes a procedure for classifying safety-related control functions into a relevant Performance Level (PL) with regard to their functional safety. This can be taken as a measure of the safety function’s reliability.
Standard components are not enough
On wind turbines, the wind’s kinetic energy is transferred to the rapidly rotating generator shaft via the slowly rotating rotor shaft and gear box. According to GL, protection against excess rotor speed represents a safety-related function and is classified as PLd.
If an error occurs, standard components cannot guarantee these safety functions with the PLd calculated in the risk assessment. In the event of a failure, the user may receive no feedback about the actual plant status; in this case, the possibility of a hazardous situation for man and machine cannot be excluded. Personal injuries may result and material damage may also occur, leading in the worst case to a total plant failure.
The implementation of speed monitoring requires components which maintain a defined failure mode even in the event of an error and which also have the necessary approvals; this applies throughout, from the encoder system to monitoring, through to evaluation and shutdown.
Safe reaction to errors
Scalable solutions are used for all wind turbine safety functions in order to meet their specific safety requirements. The automation company Pilz from Ostfildern supplies the safe speed monitor PNOZ s30 to safely monitor the rotor and generator speed, as well as the transmission ratio. The speed monitor can be used as a standalone module for safe monitoring of standstill, speed, position, speed range and direction of rotation in accordance with EN ISO 13849-1 up to PL e and EN IEC 62061 up to SIL CL 3. As the speed monitor is able to safely monitor speed ranges and direction of rotation, the unit is suitable for a wind turbine’s speed-related safety functions. PNOZ s30 also has UL/cUL approval, which is important for international use, and is certified by TÜV.
In order to meet the high technical requirements of safety functions, the speed monitor PNOZ s30 has a dual-channel control architecture throughout. A standard encoder detects the rotor speed as the first channel. The second channel is represented by a standard proximity switch, which measures the generator speed. With this initiator signal as an additional encoder signal, the standard encoder represents a dual-channel redundant and diverse system. The transmission ratio between the rotor and generator is defined through the gear box and represents the third parameter that is to be monitored. In this way the PNOZ s30 monitors the maximum speeds of the rotor and generator shaft as well as their relationship to each other. Overspeeds and infeasibilities in the encoder signals are reliably detected and the appropriate safe reaction is triggered if necessary. In this way, faults or errors can be detected before they result in damage to the plant, or more precisely the gear box.
Save time during commissioning and maintenance
The speed monitor provides a user-friendly basic menu and advanced functions. All the information necessary for operation is entered in the menu. The speed monitor is operated via a rotary knob. Used in conjunction with the monochrome display, time can be saved during commissioning and unit exchange, as no separate software is required. The display shows the set limit values and parameters as well as the current speed. It gives the user constant information about the current speed.
The parameters for an application are stored on the corresponding chip card and so can easily be transferred from one unit to another. This reduces costs and saves time. What’s more, the parameter data is password-protected and so is safe from manipulation or unintended changes.
Application of the new functional safety standards and their implementation presents a challenge to wind turbine designers and operators. The focus is on gear boxes and the associated monitoring of rotor and gear box speeds. Use of the safe speed monitor PNOZ s30 satisfies the GL recommendation for overspeed detection with PLd. Not only does the unit fulfil the safety-related requirements, it can also react quickly to any deviations. In this way, fault or error sources can be identified early, costly downtimes can be minimised and the service life of the plant can be extended.
For more information,
Website: www.pilz.in
