Testing of Small. Wind Turbines

Transcription

1 Testing of Small Wind Turbines The potential of wind energy in solving the energy crisis is significant and the small wind turbines popularly known as SWTs are poised to go a long way in achieving energy empowerment for rural India. This article seeks to explain the mechanism behind the technology of SWTs. Rajesh Katyal Students visiting the C-WET facility 31

2 traditional mega-power production of electricity is insufficient in today s scenario considering exponential industrial growth along with higher living standards. However mass use of battery stored renewable energy would mitigate and even overcome peak load demands especially in villages. It is in a scenario such as this that micro-generation which includes technologies like small wind turbines/aerogenerators can act as catalysts to bring about cultural changes in consumer attitude. This will in turn facilitate the availability of power to the rural poor in the Indian context. The small wind turbine (SWT) market has been on the upswing globally for the last 2-3 years. SWTs find application in isolated or stand alone systems, mainly rural electrification, commercial applications (telecommunication towers) etc. There can be different types of small wind turbines - upwind, downwind, horizontal axis wind turbines (HAWTs), vertical axis wind turbines (VAWTs) and different variations of HAWTs and VAWTs. Most of the existing systems come under the category of stand alone systems. Hybrid systems constitute a major y shaft z shaft x shaft Fig 1. System of Axes for HAWT Source: IEC y blade y z x z blade x blade Mini wind farms consisting of large number of wind turbines in vast open spaces have become a reality and serve as miniature power plants. In remote places aerogenerators meet the power requirements. share of these isolated systems, as they combine two or more sources of renewable energy to ensure continuity of supply. Mini wind farms consisting of large number of such wind turbines in vast open spaces in cities have also become a reality and can serve as miniature power plants. In remote places where grid connectivity is not available these aerogenerators are handy to meet the basic power requirements. Also they can be employed in localities with weak grids and high winds. Some of the issues that have hampered the growth of SWTs are design, grid integration, policy support from the government and ing and standardisation. This article focuses on the issues and methods, related to ing/ certification of small wind turbines and strategies to accelerate the growth of the small wind turbine sector. Types of Small Wind Turbines Unlike large wind turbines, there are different types of SWTs available in the market which makes the choice of the most suitable SWT difficult. These include: Drag and lift type SWTs HAWT and VAWT Upwind and downwind SWTs Geared and direct-drive SWTs Off-grid, on-grid and hybrid SWTs Hence ing of SWTs has become all the more important to mitigate the customer/investor risk of under performance thereby increasing their confidence. Testing of Small Wind Turbines To cater to the standardisation of the design of these SWTs, the international standard IEC (2006) [3] was formulated for small wind turbines/aerogenerators with a swept area less than 200 m 2 (i.e. rotor diameter less than or equal to 16 m) and generating at a voltage below 1000 V AC or 1500V DC. The standard suggests certain empirical formulae/equations for design documentation. After having dedicated years to the ing and certification of large MW class wind turbines, the Centre for Wind Energy Technology (C-WET) based at Chennai, now has the capability even to the SWTs and issue quality type reports. The methodologies have been standardised and the adherence to procedures is done diligently. Presently the ing programme focuses on engineering integrity, safety philosophy and quality assurance so as to have a fail-safe design for SWTs. IEC Tests for Small Wind Turbines The different classification of s, as per International Electrotechnical Commission (IEC) [3], to be conducted on an SWT, based on which the integrity of the turbine s design is ascertained are given as follows: a. Tests to verify design data, 32

3 b. Mechanical load ing, c. Mechanical component ing, d. Environmental ing, and, e. Tests on electrical sub-systems. Tests to Verify Design Data: The design of the SWT is quantified ultimately by the dimensions and the specification of the operation of the individual components and hence the s which can be performed in-house by the manufacturer to verify: Designed power at rated wind speed, Designed rpm, Designed shaft torque at rated wind speed, Maximum rpm, and, Maximum yaw rate. These verifications result in the appropriate technical specification details finalised for the designed model. Mechanical Load Testing: Since the material used in the construction of the SWT, by virtue of its properties, has to be capable enough to withstand the load flow occurring during its operations; one critical that has to be done is to determine the loads induced at critical locations in the structure along with the meteorological parameters and SWT operational data (rotor speed, electrical power, yaw position, turbine status). The loads may include blade root bending moments, shaft loads and loads acting on the support structure. These s needs to be performed as per IEC [5]. Mechanical Component Testing: This procedure includes s carried on the load carrying components such as the blade, hub, nacelle frame, yaw mechanism and gearbox. These s can be performed in-house by the manufacturer. The resulting data from these s provide the concise picture of the components of capabilities and whether they live up to the design requirements envisioned on the design stage; in effect ensuring the reliability of components. Environmental Testing: If the SWT is designed for external conditions such as extreme cold, hot, desert/ dusty conditions outside the normal external conditions, the turbine shall be subjected to s simulating those conditions, thus ensuring that the design adequacy in these spheres are verified. Tests on Electrical Sub-Systems: Tests shall be carried on all safety critical electrical sub-systems of the turbine such as the electrical generators/alternators, electronic sub-systems, etc. in compliance with the relevant IEC and national standards. Type Tests The present type ing programme focuses on the following essential s of the SWTs. Power performance, Every small wind turbine has to perform its operations, yield the utmost benefits promised during the commercial transaction and yet safeguard itself for an extended life. Safety function, and, Duration s (replacement of load measurement as in the case of large wind turbines). The three elements of type ing for the conformity statement of a SWT/ aerogenerator are as shown in Fig 2. Power Performance Test: The productive yield of a turbine is always quantified by the annual generation of electrical power harnessed by the turbine. The power performance provides verified data for the purpose and is to be carried in accordance with the IEC standard, (Wind Turbine Generator Systems, Part 12: Power Performance Measurement Techniques, IEC [2]). Small wind turbines can be used as battery charging devices, grid connected systems or can directly run electrical loads like motors or resistive load. For a battery charging turbine, the power performance measurement must take into account the effect of battery state of charge (i.e. voltage variations in the load including float voltage levels) on power output. The turbine s power performance characteristics are determined by the measured power curve and the estimated annual energy production (AEP). The measured power curve is determined by collecting simultaneous measurements of wind speed and power output for a period that is long enough to establish a statistically significant database over a range of wind speeds and under varying wind conditions. The AEP is calculated by applying the measured power curve to reference wind speed frequency distributions, assuming 100 per cent availability of the turbine.as per IEC , the anemometer, to measure wind speeds, must be located between 2-4 times the rotor diameter from the turbine. Another anemometer will be located at 1.5m below the primary anemometer as a check on the primary anemometer. Measurement Setup for Power Performance Testing: The measurement system involves acquisition of signals from meteorological sensors, status signals and DC power signals at the output of the charge controller. The power curve of the wind turbine is plotted as a function of wind speed at standard air conditions (air density kg/m 3 ). A typical power curve of a SWT normalised to the peak power at standard wind speed conditions is depicted in Fig 3. 33

4 Fig 2. Type s of SWTs Type conformity statement Power performance Source: IEC Safety function Other environmental (optional) Duration Safety and Function Test: Every SWT has to perform its operations, yield the utmost benefits promised during the commercial transaction and yet safeguard itself for an extended life. This sole purpose is catered to through the safety and function ing which verifies whether the SWT under displays the behaviour predicted in the design and that provisions relating to personnel safety are properly implemented and get deployed during the appropriate time. The purpose of the safety and function is to ensure a fail-safe operation of the turbine under all conditions. It is undertaken in accordance with provisions listed in IEC Unlike large wind turbines, the control and protection systems for SWTs are very simple and often passive. The safety and function consists of the following: Emergency Shutdown Operation: Brakes will be applied to check if the SWT shuts down at normal and high wind speeds. Power and Speed Control: This is to ensure that the power and speed of the SWT remain within design limits. The power output and the rpm of the SWT will be measured. The maximum rpm shall be determined by interpolation or extrapolation to Vref, corresponding to the SWT class. Yaw Control: This shall be verified by visual inspection. Loss of Load: The condition shall be simulated by an open circuit at the SWT electrical generator terminals and the braking mechanism of the SWT shall be verified. Over-Speed Protection: This verifies how the over speed protection mechanism of the SWT works under fault The purpose of the safety and function is to ensure a failsafe operation of the turbine under all conditions. It is undertaken in accordance with provisions listed in IEC conditions or above design cut-out wind speeds. Start-Up and Shutdown above the Rated Wind Speed: The start up and shutdown mechanism above rated wind speed for the SWT are monitored. In addition to the above mentioned critical functions, the following may also be verified if applicable. Excessive Vibration Protection: The verifies the vibration protection mechanism of the SWT. Battery Over- and Under-voltage Protection: The verifies the over and under voltage protection mechanism of the SWT at battery voltages outside set points, for battery charging systems. The battery voltage, dump load status and load status are monitored and correlated. Cable Twist: This is to verify whether the SWT cable untwist influences the yawing mechanism. Anti-islanding (for grid connections): This verifies whether any electrical system (capacitors) that by itself can operate the SWT shall be automatically disconnected from the network and remain safely disconnected in the event of loss of network power. Duration Testing This is performed at the end of all the s and is based mostly on the visual observations at that point of time. The Fig 3. Power Curve of a SWT Electrical Power (P. U.) Source: SWT ing at C-WET Wind Speed m/s purpose of the duration is to investigate the following: Structural integrity and material degradation (corrosion, cracks, deformations) Quality of environmental protection of the SWT Dynamic behaviour of the SWT. The load measurement and blade fatigue s of large wind turbines are replaced with the duration [3],[4]. As per IEC , the duration requires that the SWT has a reliable operation during the period including at least 6 months of operation, at least 2500 hours of power production in winds of any velocity, at least 250 hours of power production in winds of 1.2 Vave and above or at least 34

5 25 hours of power production in winds of 1.8 Vave and above, where Vave is the annual average wind speed at hub height. The Vave value is determined by the SWT class under which the turbine falls. Reliable operation means operational time fraction of at least 90 per cent without the following events: No major failure of the turbine or components in the turbine system, Significant wear, corrosion or damage to turbine components, and, Significant degradation of produced power at comparable wind speeds. Measurements that will be taken during the duration are power production, turbine operational time fraction, and wind speed consisting of 10-minute average, turbulence intensity and wind direction. Power is measured by a power transducer at the point of connection to the electrical load. To check any hidden degradation in power performance, the power levels shall be binned by wind speed every month. The binned power levels shall be plotted as a function of time for each wind speed and any visible trends shall be investigated. At the completion of the duration, a detailed component wear and durability check for the entire turbine will be conducted. It will include a visual assessment of the structural integrity and material degradation (corrosion, cracks, and deformations). Selection of Test Site The guidelines followed for the selection of site for undertaking s on SWT are as per standard IEC The basic requirements for selection of sites will depend on the terrain conditions and minimum wind speed requirements. Presently, the s on SWTs are conducted at the Wind Turbine Testing Station (WTTS), Kayathar, Tamil Nadu. The wind speed statistics for Kayathar are as follows: a) Maximum average wind speed : 5-6 m/s b) Peak wind speed : 25 m/s Conclusion The main drivers to the growth of SWTs are the demand supply gap in energy, increasing fossil fuel prices, improved small wind turbine technology and the diverse application to which it can be used both for grid-tied and stand alone systems. The market for SWT technology is encouraging in India also, and may require market drivers like favourable policies, encouragement to the net metering concept, adoption of micro generation technologies, and reduced costs etc., to reach a significant level. b The author is Scientist and Unit Chief, Wind Turbine Research Station (WTRS) Chennai. katyal@cwet.res.in Inviting articles for Akshay Urja The need to have a sustainable supply necessitates the exploitation of available energy sources, and among these, renewable resources are at the forefront. It is now an established fact that RE (renewable energy) can be an integral part of sustainable development because of its inexhaustible nature and environmentfriendly features. RE can play an important role in resolving the energy crisis in urban areas to a great extent. Today RE is an established sector with a variety of systems and devices available for meeting the energy demand of urban inhabitants, but there is a need to create mass awareness about their adoption. Akshay Urja is an attempt to fulfil this need through the dissemination of 20,000 copies in India and abroad. The magazine publishes news, articles, research papers, case studies, success stories, and writeups on RE. Readers are invited to send material with original photographs and statistical data. The photographs should be provided in high resolution files on a CD or through . Akshay Urja will pay a suitable honorarium to the authors for each published article of 1500 words and above. The publication material in two copies, along with a soft copy on CD/ DVD/ may be sent to Editor, Akshay Urja Ministry of New and Renewable Energy Block 14, CGO Complex, Lodhi Road, New Delhi Tel , Fax aktripathi@nic.in 35