Design of the Zephyros Z72 wind turbine with emphasis on the direct drive PM generator.

Transcription

1 1 Design of the Zephyros Z72 wind turbine with emphasis on the direct drive PM generator. Author: C. J. A. Versteegh, GarradHassan & Partners NL, Sterrelaan 7, 1217 PP, Hilversum NL.? I. ABSTRACT. Index Terms: Zephyros; wind turbine; direct drive; PM generator. The Zephyros Z72 is a gearless variable speed wind turbine with a direct driven PM synchronous generator with a rotor diameter of 70 m. This article describes the design process, testing and prototyping with focus on the generator which till date is the biggest PM generator available on the wind turbine market. The turbine after testing and commissioning has a track record of over 8000 grid connected hours and more than 4700 MWh produced. Tests and operational experience is commented and results are given. Measurements such as the power curve (power versus average wind speed), noise and heat run have been performed and show good results and reassembly with the design calculations. The turbine has been installed in April 2002 and certification design assessment and measurements are completed. II. INTRODUCTION Zephyros b.v. ( for more information) is a small wind turbine manufacturer established in the Netherlands. The company is a spin off of Lagerwey who produces since 1994 a direct driven 750 kw wind turbine. Zephyros has been established in 2000 to make an up scaled design and production of a prototype possible with support of the Dutch government and key suppliers. In order to shorten the prototype phase it was decided to have a full scale factory test done on generator converter system as is learned from experience with the LW750. This has lead to the participation of ABB in the development of generator converter system. As ABB designs and manufactures PM motors the application of PM excitation was considered as proven technology and adopted instead of external rotor excitation. Zephyros now employs 12 persons covering the skills of design, assembly, installation and service and has 30 turbines in the order book. The prototype has a track record of 1.5 year and has produced 4500 MWh during 7000 production hours. Reinforcement of the company by means of a (strategic) investor is strived for and first license contracts are established. III. TURBINE DESIGN REQUIREMENTS The turbine design requirements of initially have been very limited. The market demands bigger turbines hence up scaling of in house technology with use of the market available state of art technology has been the starting point. In order to make the design more attractive and to be able to extend the design life as much as possible both the offshore and onshore markets are considered. This implies transport restrictions but also full enclosure of the generator and high reliability of the turbine. The main turbine specifications followed from the choice of the rotor blades. At the start of the project the first prototype blade sets had become available for a rotor diameter of 70 m. Tip speed restrictions to obtain an acceptable noise level determine the rotational speed and with known transport sizes key parameters for the design could be defined. Maximum outer generator diameter : 4 m Nominal rotational speed : 18 rpm Nominal power : 1500 kw Protection class : IP54 Manuscript submitted May 3, C.J.A. Versteegh is senior engineer at GarradHassan & Partners and based in the Netherlands. He has been responsible for the Zephyros Z72 design. The address of Zephyros is: Arena business park 1, Olympia 1a/1b, 1213 NS Hilversum, Tel: +31 (0) , Fax: +31 (0) , info@.zephyros.com Figure 1: The Z72 wind turbine Z72 Direct drive PMSG page 1 of 7 CJAV

2 2 IV. THE TURBINE DESIGN The Z72/2000 (figure 1) is a wind turbine with three GFRP blades and a steel tubular tower. It has a direct driven multipole synchronous PM generator which is fully integrated in the structural design. Use is made of a single multiple row roller bearing on which the hub is mounted on one side and generator carrier and rotor on the other side. The stator is mounted on the opposite side of the carrier which on its turn is mounted on a compact casted nacelle frame. The advantage of this design is the relative big diameter the load path follows contrary to the traditional designs with a mainshaft hence reduces weight. The turbine is designed according to IEC61400 wind class IB with exception of the tower (presently only class IIB), defined by an annual average wind speed at hub height between 7.5 and 8.5 m/s and a maximum turbulence intensity of 16% at 15 m/s wind speed. The rotor speed during power production is variable. The matching between the available aerodynamic torque and the produced electromechanical torque of the generator is determining the rotor speed. The torque-speed curve is programmed in the frequency converter controller and the inverter is adapting the generator stator current in response to the measured generator power frequency. The time constant of this process is in the order of a few milliseconds. The control of the generator torque keeps the rotor running at optimum tip speed ratio for a part of the working range. The power demand is therefore set proportional to the cube of the generator speed. To make the design more attractive for offshore applications an increase of rotational speed has been proposed in order to obtain an installed power of 2 MW without any changing. Offshore the turbine noise (dominated by the rotor and increasing square with the rotor speed) is not a design driver and only the voltage level will raise hence with a high enough insulation value the drive train suits both a 1.5 and a 2 MW design. The AC-DC-AC converter in the tower base allows the generator to operate with a variable speed while the power is fed into the grid with a constant frequency of 50 Hz (or 60 Hz for the countries where this applies). It furthermore assures an average constant power output for wind speeds above rated. The power factor at the grid side is controllable at standstill as well as operating. Above rated wind speed, the blade pitch control maintains a more or less constant rotor speed between admitted boundaries. For blade pitching each blade has its own individual pitch actuator with accompanying pitch angle sensor and a collective control loop maintains equal blade pitch angles at the three blades. When the electric power reaches the nominal value (1.5 or 2 MW), further increase in electric power is avoided by means of a change in the Q-N curve. As a result of this the rotor speed increases due to excess aerodynamic power. Figure 2: Single line diagram This in turn is noticed by the rotor speed controller, which pitches the blades to a larger positive pitch angle (smaller angle of attack), thereby effectively limiting the rotor speed to the set value. The rotor speed controller is programmed in the turbine control system and makes use of advanced routines to avoid overspeed and tower resonance due to pitch movements. A wind estimation routine based upon rotor acceleration monitoring is a further advancement decreasing overshoot and reducing blade pitch activity. The hardware of the Z72/2000 control system is built up in a modular manner. The control and safety functions take place in the same area or space where the needed measurements and control or safety actions are performed. V. THE DESIGN PROCESS The design process has shown a clear design philosophy has to be adopted what not only is technically driven but also has to fit on organizational capabilities and possibilities. Organization structure and available budgets can be a serious obstacle to success if not properly managed. The design process is not only characterized by the concept design, a preliminary design and the detail engineering resulting in shop drawings, specifications and design reports but also so-called RAM (Reliability, Availability, Maintainability) targ ets have been specified what should in principle lead to predictable failure rates per main component or sub-system. Z72 Direct drive PMSG page 2 of 7 CJAV

3 3 In the early concept and design stages of a technical system it can be determined that the system will really achieve the ultimate required availability goal sometime at the end of its specified lifetime. Studies have shown that after the design stage is finished (and just before the manufacturing starts) usually 10% to 20% of the total lifetime expenditures have already been spent. At the same time about 80% of all lifetime costs have been locked-in at that moment as well. It is very clear that the availability can be improved best for the lowest price and with higher returns before the design process is closed. This is also known as a reliability data sources management problem, that in the practice of wind turbine design is hard to solve and also require extensive feed-back from the organization specially the service department. To build up statistics a track record of a significant number of turbines is required. Due to this RAM targets are specified upfront based on actual knowledge and experience but will have to be verified and adapted when statistic data is available. Before ABB participated two designs of a DCSG have been made; water cooled outer rotor type and an air to air cooled inner rotor type. Cooling of the rotor losses of ca 30 kw or stator losses of ca 90 kw adds considerable complexity in case of a fully enclosed design. Both designs are technically feasible but it became clear the use of PM could simplify the design considerably. In case of a PMSG the choice for an inner rotor type without rotor losses and an outer air cooled stator is obvious. An advantage also is no rotor excitation current has to be supplied through a slip ring set but a disadvantage however is it cannot be disconnected either. This means with increase of the rotor speed the voltage increases as well. A contactor has to be used between generator and converter or the insulation level has to be chosen such that over speed situations never lead to over voltage on the system. The choice has been made for the latter because of simplicity and cost. For enclosure a labyrinth with dust seal has been designed. A ventilator is used to pressurize the generator internals. This system has been patented (publication number WO 01/21956) Further design criteria for the PMSG have been: 1. Structural design. As the generator structure is part of the turbine load carrying parts in combination with the single bearing construction, FEM calculations have been made by Zephyros (figure 3) to determine strength and stiffness of structure and bolted connections. With a nominal airgap of 3 mm and an active material length of 1200 mm requirements regarding deformation due to external wind and mass loads and magnetic loads are strict. A maximum deflection of ca 2 mm has been calculated under extreme loads. The stator and rotor dimensions are more determined by the required stiffness to minimize deflection caused by the magnetic pulling forces rather than material stresses. These calculations have been made by the ABB research centre in Mannheim Germany. Figure 3: FEM analyses of the generator structure. 2. Generator mass. The generator mass is important as it has an impact on the turbine installation. The chosen concept, rotor speed and airgap diameter determine the mass. 3. Number of phases. The number of phases is 3 being a common phase number and simplifies the converter design. 4. Generator use. The generator has been optimized for use with a voltage source converter. A back-to-back converter is used providing maximum control. The generator is operated at cos f = 1 hence current is kept in phase with the voltage induced by the airgap field so the torque produced with the combination of stator current and airgap field is maximum. Another advantage of this converter type is the better fault performance at turbine overspeed (voltage control by field weakening through reactive current supply) and loss of electrical load (rated internal e.m.f. in the concept is 10 15% higher than the stator voltage hence DC link voltage would not exceed the rated value). 5. Voltage level. A 7.5 kv insulation level has been chosen. It is believed with the increase of nominal power the voltage should increase and not the current. The choice is made for a medium voltage converter with fewer components than a low voltage converter and a better efficiency. Past years however LV converter have strongly developed and up to 2 MW are still cheaper. Nevertheless overall cost assessment show the MV solution can compete but the converter lacks the cost reduction due to limited production number of MV semi-conductors. Z72 Direct drive PMSG page 3 of 7 CJAV

4 4 In figure 4 for three wind climates with a yearly average of 5, 7 and 10 m/s the relative energy output has been calculated for 4 wind turbine types with the only difference: the conversion system. PMHV is de Zephyros turbine; WRLV (wound rotor low voltage) is a design like the Lagerwey and Enercon designs PMLV is used by the manufacturers WinWind, Vensys, Leitner and MTorres. WRHV is added for comparison but is a non existing design. In an average wind climate (7 m/s) PMHV produces 2% more than WRLV due to avoiding rotor losses and a higher converter efficiency. 7. PM material. For the magnet material Neodymium- Iron-Boron (NdFeB) is used. The cost of high energy product (BH product) magnets has reduced in price with a factor 5 in 10 years time and now cost less than 50 /kg. The magnets are glued on steel modules and then magnetized. These modules are bolted on the rotor and a GRP bandage is wrapped around the rotor before coating. Influence of generator type and voltage level on performance [%] PMHV PMLV WRHV WRLV Windspeed Figure 4: Influence of generator type and voltage level on performance. At low wind sites with more partial load hours the advantage is more obvious than at high wind sites with a higher capacity factor. For locations where a higher noise level can be accepted (off shore) the generator is upgraded to 2 MW by increasing the rotational speed of the turbine. With an equal airgap torque and an increased voltage to 4000 V the nominal power is increased at minor extra cost due to the chosen voltage insulation level in both generator and converter. 6. Stator winding. Pre-formed or flat wire has been used what is inherent to the chosen insulation level. No mass produced round wire can be used what on its self is cheaper but the insulation quality is less. The slot fill factor of 0.7 is better than 0.45 for round wire what saves weight on active material. The disadvantage of flat wire is the bigger number of connections and the necessity to use magnetic slot wedges. The stator is vacuum impregnated. Figure 5: Generator efficiency as a function of power and rotational speed. The blades are commercially available but had had to be verified for the Z72 load spectrum. The loads are calculated with a computer code with following input: 1. Model of the wind spectrum 2. Model of the pitch and generator control 3. Aerodynamic model of the blades 4. Dimensions and mass and stiffness distribution. The loads are calculated in the time domain and are rain flowed and presented in Markov matrices containing mean values, amplitudes and number of cycles for different locations of the turbine in three directions and/or resulting loads). Critical bolt connections of blade-bearing-hub and hubbearing-generator/nacelle as well as the hub, generator and nacelle structures are designed with use of FEM calculations. Z72 Direct drive PMSG page 4 of 7 CJAV

5 5 Table 1: Generator specifications. Rated shaft power 1670 kw Temperature rise class Rated electrical power 1562 kw Insulation class F (H) Rated air gap torque 862 knm Standards IEC34 Rated voltage 3000 V Protection by IP54 enclosure Rated current 327 A Cooling type IC40 Power factor 0.92 Rotor inertia kgm 2 Frequency Hz (rated) Total weight kg Rotational speed rpm (rated) Stator weight kg Pole number 60 Rotor weight kg Pole angle 33.5 deg. Bearing support cone Torque harmonics 100% fundamental (862 knm) < 1% 6 th harmonic (55.5 Hz) < 1% 12 th harmonic (111 Hz) < 1% 24 th harmonic (222 Hz) Bearing weight F PT 100 stator 6 PT generator air PT 100 bearing 2 Short circuit current 569 A (sustained) Airgap distance sensors Ambient temperature 40 C Bearing greasing unit Radial pull 98 kn/mm between Maximum stator and rotor due magnetic force to excentricity 5000 kg 4000 kg kn magnetic pulling force of one pole. 4. Firing through. This test is to check the mechanical strength of the DC link and the braking capacity of the main circuit braker. 5. Continuous load test. This test is to proof if the drive train meets the specifications in steady state conditions. The power will be ramped up to the maximum. Following parameters have been mo nitored: temperatures of generator, converter and main transformer, speed, load angle, frequency of the generator, generator power, power of auxiliaries, grid power output, total losses of test bench and interactions generator converter. 6. Optimization of the grounding concept and the flange filter to minimize the influence (dv/dt, common mode and differential mode voltage) to the generator. 7. Fast variation of torque to optimize the closed loop control. VI. TESTING The generator has been manufactured and tested in the ABB factory in Helsinki. In the factory 2 systems are mounted back to back as drive equipment with low rpm and such high torque are not available. Two synchronous generators are mechanically coupled. The two generators have been cooled with external fans and a speed and a position signal of the motor generator shaft had to be provided for overspeed tests. The power converter Nr 1 on the left side of figure 6 is the device under test, the other one gives the load for the generator. Due to the losses in the converters, motor and generator, the output power of the converter on the generator side is reduced by ca. 15 %. This resulted in a power of ca. 1.5 MW of the converter of the generator side hence test at maximum current could be executed. Following tests have been exe cuted (in this overview limited to the generator): 1. Light load test. Each drive train is tested on its own with no load. This is to check the normal function. Protection. Protecting levels and functions are checked by forcing different faults to the converters. 2. Overspeed. This test is to check the external overspeed protection. 3. Force generator short circuit. This test is to determine the short circuit current and peak value of the torque. Figure 6: Generator test bench schematics. Figure 7: The test bench. Z72 Direct drive PMSG page 5 of 7 CJAV

6 6 The converter system and generator have fulfilled its requirements in normal and extraordinary conditions. The measured values of the drive train lie within its limits. The system has proven for a given active power reference the generator is able to track it very closely with fast dynamic response. The reactive power to the generator is controllable in such a manner that the terminal voltage has not exceeded the rated value. The test had some limitations: 1. The dynamic tests could not be carried out with nominal load. The reason for this is the very low inertia which is ca 40 times smaller than with the turbine rotor mounted. 2. The cooling is not according real circumstances as no wind is present. The influence of switching on some external cooling fans could clearly be measured and gave comfort. Cooling in practice only can be tested on site. Other tested components have been: 1. Blade. The behavior under the fatigue and extreme loads. 2. Pitch drive. The durability on endurance loads and thermal behavior due to extreme loads. 3. Control cubicle. Vibration test, salt spray test and thermal test to meet specifications regarding corrosion, vibration and temperature range. 4. Control software. System and response test in the workshop to compare with the response of the computer model. maximum generator temperature measured past summer was 86 C at an ambient temperature of 17 C. For cooling ventilation is applied for bearing and electronics in hub and nacelle. Most of the operational problems were in the components that have been adopted from the existing Lagerwey design but should have been paid more attention in the up scaled design although also QA aspects contributed to it. It concerns wear and malfunctioning of the service brake and a lack of control on the yaw brake passive torque to avoid overload on the yaw drives. The design has been adapted on these points Power [kw] Wind speed [m/s] Calculated Measured Figure 8: Measured power curve and calculated power curve VII. OPERATIONAL EXPERIENCE The turbine has been installed in April 2002 and became fully operational in November The first year of operation the overall availability has been 84% and to date 4700 MWh have been produced. For the site (Maasvlakte near Rotterdam, The Netherlands) this means a capacity factor of 27%. If this is corrected for an expected availability of 97% it would lead to 31% what is excellent for a site with an average wind speed at hub height of 7.5 m/s. The measured power curve (as a function of the wind speed and measured acc to IEC ) given in fig. 8shows good comparison with the calculated curve and supports the excellent performance of the high voltage generator-converter system. An initial problem has been the noise generated by resonance of tower shell sections due the switching frequency (480 Hz) of the semiconductors. This has been solved by changing the frequency to 800 Hz. A heat run has been performed being a period of 24 hours continuously at full load. At an ambient temperature of 0 C the maximum stator temperature does not exceed 75 C. The Z72 Direct drive PMSG page 6 of 7 CJAV

7 7 VIII. BENEFITS AND COST X. CONCLUSION Although the Z72 can compete with turbines in the same power range, margins still can improve by taking advantage of series production. Common catalogue prices start at 1500 k. The benefits are in the cost of operation: 1. Reduced maintenance cost due to limited number of components and systems. 2. Higher energy output (2%) 3. Few moving and wearing parts hence eventually lower insurance cost. 4. Due to the full power 4q converter good grid connectivity; universal Hz design, electric braking and positioning of turbine rotor, and capable to operate under line dips. The design has proven to work and the decision to do a full scale conversion system test has considerably shortened the prototyping. The integration of the generator in the structural design leads to a very compact design and saves weight. Although first sales are realized (30 pcs), even for this number of turbines a price reduction already is realized. The volume however should increase to improve the margins. distribution of costing rotor : 15% 2%0% 29% drivetrain: hydraulic: nacelle: cover : yaw mechanism: 25% tower: generator: 10% E-system/converter: 12% 1% 0% 3% 3% transformer : auxiliary equipment: Figure 9: Typical cost distribution of a DD wind turbine. Figure 10: The Z72 IX. FUTURE DEVELOPMENT The short term development is an upscale of the turbine rotor in order with the same rated power and generator design to increase the energy capture thus improving the price performance ratio of at least 10%. The long term development is a recent started government supported concept study to develop a 4 5 MW turbine with similar concept. The first phase has to result in a bid book and a preliminary design before the end of Z72 Direct drive PMSG page 7 of 7 CJAV