Wind Turbine Asynchronous Generator Control ...

Available online at www.sciencedirect.com

ScienceDirect Procedia Engineering 129 (2015) 927 – 932

International Conference on Industrial Engineering

Wind turbine asynchronous generator control algorithms Smolin V.I., Topolskaya I.G.* South Ural State University, 76, Lenin Avenue, Chelyabinsk, 454080, Russian Federation

Abstract The aspects of mass usage of 7-15 meters diameter wind turbines (WT) on 5-15 kW uninterrupted power supply in the continental region conditions at 3-10 m/s dominant wind speeds and 4-6 m/s average annual wind speed is discussed. A method of generating the desired power characteristics of WT, taking into account the differential chart of wind flow distribution and electric energy consumption structure is prposed. We have studied the control algorithms of frequency-controlled asynchronous generator wind turbine to provide the braking of rotor in the modes of partial loading, stabilizing the nominal power and full stop. The approach is illustrated by the analysis of combined mechanical characteristics of the wind turbine and the generator. We discuss the control system of a wind turbine with gain-frequency control of the generator and automatic distribution of generated power between the consumers of self-contained power grid and the static energy storage. We propose the methods of optimization for the generator operating modes based on the of three phase electromechanical converters generalized energy flow theory. © 2015 The Authors. Published by Elsevier Ltd. © 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license Peer-review under responsibility of the organizing committee of the International Conference on Industrial Engineering (ICIE(http://creativecommons.org/licenses/by-nc-nd/4.0/). 2015). Peer-review under responsibility of the organizing committee of the International Conference on Industrial Engineering (ICIE-2015) Keywords: wind turbine, self-contained power grid; uniterrupted power supply; asynchronous generator; algorithms gain-frequency control; electric energy balance control.

1. Introduction The medium and large power modern wind turbines (WT) are being developed on high nominal wind speed V0N = 11…13 m/s, typical for the coastal regions [1…6]. However the continental part of Russia is limited by 3…10 m/s dominant annual wind flow range in the most regions, and average annual wind speed doesn’t exceed 4…6 m/s [7, 8]. Since the wind energy depends on the third wind speed degree, these WT cannot operate efficiently

* Corresponding author. Tel.: +7-904-978-03-30. E-mail address: [email protected]

1877-7058 © 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license

(http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of the International Conference on Industrial Engineering (ICIE-2015)

doi:10.1016/j.proeng.2015.12.136

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V.I. Smolin and I.G. Topolskaya / Procedia Engineering 129 (2015) 927 – 932

everywhere. The mass application in Russia could be forecasted for autonomous WT with 7…15 meters diameter, generating ɊN = 5…15 kW nominal power [7] on the areas of low wind speeds, providing uninterrupted power supply of consumers [9, 10] together with energy static storage (GB). Actually, the talk is about the back off to the wide usage of power devices with outline dimensions of wind-mills, but developed on principally new base for the small energy demands. Autonomous WTs have as a rule, limited abilities of getting and flattening the impulse energy flow. To support the grid power on nominal level, it’s necessary for WT parameters and control system to meet the requirements of providence the positive energy balance in accordance with the differential characteristics of local area wind flows as well as consumption structure. Let’s discuss the said problem in more details. 2. Theory 2.1. Desired Power Characteristic of Wind Turbine On Fig. 1 we showed a differential characteristic of wind flow, typical for earlier mentioned continental regions.

Fig. 1. Characteristic of WT: a — differential characteristic of wind flow; b — desired power characteristics: Pɧɧ — autonomous grid, Ɋɜɞ — wind turbine, ɊGB — accumulating batteries; V0ɪ, V0ɫ, V0ɤ, V0ɛ — operating, average, root-mean-cube and cut off speeds.

It could be determined experimentally in the place of estimated WT location with the help of wind speed sensor equipped by electronic clocks. The clock measure and accumulate the timing intervals ǻɌi / Ɍ, during which the wind speed is within the appropriate range V0i + ǻVi within the period of research Ɍ [11]. Differential characteristic helps to estimate objectively the season and annual distribution of wind flows and experimentally determine the operating, average, root-mean-cube and/or cut-off speed [12]. Let’s consider that the integral chart of electric energy consumption of some company during the research period T, contains the constant component Pɧɧ. This part of energy could be selected as a nominal for autonomous local grid of WT. The company should get the missing electric energy from backbone network grid. We predict that the energy generated by WT, would be less expensive and the payback period is acceptable. Power characteristic of WT (Fig. 1b) should meet the statistic distribution of local wind flows. Nominal speed of rotor V0ɧ could be selected from the range V0ɪ … V0ɫ. Reduction of nominal speed relatively the operating one, may lead to significant increase of rotor diameter, and economically failing increase of capital and operating expenditure comparing with the battery price benefits. The tasks of providing the resource and protection of wind rotor (WR) to be solved in power stabilizing mode on Ɋɧ level and in-time cut off (stop of WR). The selection of cut off (storm) speed V0ɛ should be justified by differential characteristic of wind flows. The power ɊGB in calm area is provided by GB discharge. The charge should return back on wind speeds exceeding V0ɧ. Energy balance of autonomous grid on partial loading modes (1-2) is provided by combined operation of WT and GB: Wɧɧ = WGB + WWT. In nominal power stabilizing modes (2-3) the WT operates as a sole energy source: WWT = Wɧɧ + WGB. In the modes (3-4) of short time storm wind speeds the electric supply of autonomous grid is provided by discharge of GB: Wɧɧ = WGB. Accumulating


batteries shouldn’t be progressively discharged during the WT usage. Deficit of charge could be determined daily, weekly, monthly, depending on the type of GB and under the condition that battery state of charge will be recovered. The system of GB recharge should provide the automatic refilling of energy in backbone network grid failure situations. 2.2. Mechanical Characteristics of Wind Rotors WT rotors are being described in most papers by aerodynamic characteristic in the form of dependences of relative rotating torque M and power coefficient ɋɪ on rotor tip speed ratio Z [13, 14]. Electromechanical energy converters are presented as a rule, by mechanical characteristics in the form of dependences of rotating or braking torque on angular velocity [15]. To comprehensively analyse the WT rotor it’s necessary to cast the characteristics of devices to the same base, for instance by re-calculating the aerodynamic characteristics of WR to mechanical characteristics of electric generator. The sample of such re-calculation based on aerodynamic characteristics data of four bladed WR [13], is presented in Fig. 2. Relative angular velocity of non-loaded WR when V0ɧ = const, is taken as one. Under the load the velocity of WR will decrease and in critical mode :ɤ ,M ɤ the action of aerodynamic forces is qualitatively changing. Further braking of the WR causes the reduction of braking torque and WR stops ( M 0 — initial or starting torque). Power characteristic of WR has the demonstrable maximum, which could be a base for nominal power Ɋɧ determination. Relative angular velocity of WR in the mode of normal loading :ɧ 0 ,5...0 , 6 is closed to theoretical, equal 2/3. Let’s get back to the WT characteristics (see Fig. 1). The task of WT operation efficiency increase in mode (1-2) of low wind potential, can be solved by improvement of aerodynamic characteristics of WR and duty optimizing of generator power take off. The extremum on power characteristic of WR, causes the determined preconditions for generator control by criteria of maximum power take off. Characteristics of WR in low speed wind flow range, are shown in Fig. 3.

Fig. 2. Dependence of angular velocity : Z0 Z and power P : M on torque M of four bladed WR on V0ɧ = const: Ȧ0, Ȧ — angular velocities in idling mode and loaded WR.

Fig. 3. Dependence of WT relative power on torque M on small wind speeds.

The re-calculation of these values from nominal characteristic (V0ɧ) on another (V0i) was made in assumption that the power coefficient ɋɪ is constant and the incoming flow is reducing in a cubic dependence on wind speed: P0i | V03i . Extremums of power characteristics allow determining the function of optimal control Pmax f M . Information data base of generator control is enough to determine the electromagnetic torque by algorithmic method [16, 17], based on the control of energy conversions in electric machine. Using this approach we can avoid the application of expensive sensors of braking torque. The task would be even more simple if the control law would be presented in function as a dependence on rotor angular velocity: Pmax f : . The argument can be determined with the help of two sensors: wind speed V0 (V0~Ȧ0) and rotor angular velocity Ȧ. The main problem is stipulated in

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this case by the dynamics of the control process. Instantaneous wind speed has impulse nature comparing with the inertia of rotor. Thus the information channel of the speed : should be filtered. If the wind speed exceeds the nominal, the power of rotor should be limited. The Stall (Stahl) method of control solves this task by profiling the stationary blades of rotor [18]. The exemplary mechanical characteristics with aerodynamic stabilizing, are shown in Fig. 4. When the wind speed increases V0i > V0ɧ, the mechanical characteristics of rotor become more soft. And the coordinates of point Ⱥɧ (ȍɧ, M ɧ ) remain constant in general, in the wide range of wind flows. Extra stabilizing of duty point with the help of control system, allows decreasing the requirements for the accuracy of fabrication of blades.

Fig. 4. Mechanical characteristics of rotor with aerodynamic stabilizing.

To stop the rotor, it’s necessary to apply braking torque M 0i to the rotor (see Fig. 4), not exceeding the nominal value. The task of rotor stop can be also solved by turning the electric machine into the motor reversible mode. Braking by generator allows the decreasing of requirements for the design of electromagnetic brake. 3. Results and discussion

3.1. Joint Analysis of Duties of Rotor and Generator Let’s discuss the opportunity of usage of the braking modes of WR with the help of asynchronous generator (AA) with amplitude-frequency control. Let’s combine the mechanical characteristic of WR with analog characteristics of asynchronous electric machine of corresponding power, operating in braking mode. We would combine the characteristics by transferring the WR characteristic from the first quarter of coordinate map into the second one, which characterizes the operation of electric machines in generator mode (Fig. 5). The said analysis is qualitative, without analysing any specific parameters of WR or AA.

Fig. 5. Combined characteristics of Wind Turbine: A1 – mode of partial loading, A2 – mode of stabilizing of nominal power, A3 – mode of stop.


In the modes of partial loading (A1) and stabilizing of nominal power (A2) the control of AA should be handled by software in accordance with the law of optimal power takeoff obtained experimentally or by modeling [19]. AA control system should have a corresponding technical means. To turn the rotor into the braking mode, it’s necessary to shortly increase the braking torque up to critical M ɤ and further smoothly decreasing the torque in accordance with the rotor angular velocity reduction [16]. The full stop with the help of electric machine in generator mode is impossible. It’s necessary to have an external source of braking torque generated by turning the AA into the generator mode (A3) [20]. The AA control system should contain the reversible converter of frequency with amplitude-frequency control in the low speed area.

3.2. WT Control System The desired power characteristics of WT can be realized by control system (Fig. 6), containing the earlier marked basic components.

Fig. 6. Functional chart of WT control system: 1 – wind turbine, 2 – asynchronous generator, 3 – controller, 4 – wind speed sensor, 5 – wind rotor sensor, 6 – reduction unit, 7 – measuring converters of phase currents and voltages, 8 – reversible frequency converter, 9 – converter, 10 – batteries (GB), 11 – external charging device (compensator of charge for GB), 12 – voltage autonomous inverter, I – self-contained power grid, II – central power grid.

Controller keeps in memory the main parameters of power characteristics and function of control Pmax f M or Pmax f : , obtained as the result of testing or modeling of wind rotor. Using the information about current wind speed V0 and angular velocity Z of wind rotor, the controller controls the power of AA, adjusting it in accordance with the tabular Pmax. Reversible frequency converter based on the autonomous voltage inverter, should solve two tasks simultaneously during the control process: generate a synchronous frequency and amplitude of AA three-phase voltage [21]. Among the main tasks to be handled by controller, there is a function of support of positive energy balance of autonomous grid of WT. Converter is functionally connected with the controller, and should provide the automatic distribution of AA power between consumers of autonomous grid and accumulating battery module. Static energy storage compensates deficit of generator power in low speed modes. The recovery of energy balance of WT autonomous grid should be realized with the help of external charging device controlled by controller. Control of three phase electric machine in generator mode can be implemented by the known methods [22…24] of theory of alternating current drive. New opportunities in implementation of required AA braking modes would give out the control algorithms based on the theory of generalized energy flow of three phase electromechanical converters, in particular amplitude control of torque and energy efficient AA control under criteria of constant overloading ability. 4. Conclusion

We discussed the aspects of mass usage of 7-15 meters diameter wind turbines (WT) on 5-15 kW uninterrupted power supply in conditions of continental regions with 3-5 m/s dominant wind flow range and 4-6 m/s average annual wind speed. We proposed the method of generating the desired WT power characteristic taking into account

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a differential chart of wind flow distribution and electric energy consumption. We formulated the task of research of control algorithms for frequency controlled asynchronous generators WT, to provide the braking of wind rotor in modes of partial loading, nominal power stabilizing and full stop. The solution of the problem was illustrated by the analysis of combined mechanical characteristics of wind rotor and generator. We showed that the application of Stall method of aerodynamic stabilizing of rotor nominal power provides the ability of using the braking mode without increase of generator dimensions. We discussed the system of WT control with gain-frequency control of generator and automatic distribution of generated power between the consumers of local grid and the static energy storage. We proposed methods of optimizing the generator duties on the base of theory of generalized energy flow of three phase electromechanical converters. References [1] Information on http://www.gwec.net/wp-content/uploads/2013/02/ GWEC-PRstats-2012_english.pdf. [2] V.P. Kharitonov, Self-contained wind turbine, GNU VIESKh Publ., Moscow, 2006. [3] Yu.N. Petrenko, S.Ⱥ. Stankevich, Development of a Control Algorithm that Provides the Highest Possible Power output of Wind Turbine with Energy Storage, Electronics, Automation and Measuring Technology. (2011) 47–52. [4] V.S. Krivtsov, A.M. Oleynikov, A.I. Yakovlev, Inexhaustible energy. Bk. 1. Wind turbine, KhAI Publ., Kharkiv, 2003. [5] Information on http://cdn.intechweb.org/pdfs/16255.pdf. [6] S.A. Sinkevich, Yu.N. Petrenko, The Use of Fuzzy Logic Controller to Control the Wind Turbine, Information Technology and Systems 2012: Proc. of the Int. Scientific Conference. (2012) 66–67. [7] E.A. Muravleva, S.P. Rudobashta, Efficiency Electricity Farmhouse Based on a Small Wind Turbine Power, Vestnik FGOU VPO MGAU. 1 (2013) 24–27. [8] V.V. Yelistratov, Monitoring the Development of Renewable Energy in the World and Russia, Akademiya energetiki. 2 (2008) 22–44. [9] E.V. Solomin, I.M. Kirpichnikova, A.S. Martianov, The Iterative Approach to the Design and Optimization of Vertical Axis Wind Turbines, Proc. of the VII Int. Scientific Conference of Young Scientists "Electrical. Electrotechnology. Energy". (2015) 92–95. [10] S.V. Kozlov, A.N. Kudryashov, E.V. Solomin, An Analysis of the Effectiveness of Energy Storage Systems, International Scientific Journal "Alternative Energy and Ecology". 6(170) (2015) 10–23. [11] V.E. Belyakov, System of Automated Simulation of Wind Turbines, Generation of the Future: Looking for young scientists. (2014) 391–395. [12] S.N. Udalov, Renewable energy, Publ. NGTU, Novosibirsk, 2009. [13] P.P. Bezrukikh, Wind power, Publ. «ENERGY», Ɇoscow, 2010. [14] V.P. Kharitonov, Fundamentals of wind Energy, Publ. GNU VIESKH, Moscow, 2010. [15] A.N. Kindryashov, A.S. Martyanov, E.V. Solomin, Electric cars wind turbines with a vertical axis of rotation, International Scientific Journal "Alternative Energy and Ecology". 1/2(118) (2013) 59–62. [16] V.I. Smolin, D.V. Topolsky, N.N. Gudaev, Measurement of a Torque of the Electrical Machines According to Their Electrical Parameters, EPE Conference’97, Proceedings. 3 (1997). [17] I.G. Topolskaya, D.V. Topolskiy, V.I. Smolin, Non-contact Inspection Method Shock in Wind Turbines, International Scientific Journal "Alternative Energy and Ecology". 10(78) (2009) 69–73. [18] I. Munteanu, A.I. Bratcu, N.-A. Cutululis, E. Ceanga, Optimal Control of Wind Energy Systems, Advances in Industrial Control, SpringerVerlag London, 2008. [19] I.M. Kirpichnikova, A.S. Martyanov, E.V. Solomin, Simulation of Wind Turbine Generator, Electrical engineering. 10 (2013) 46–50. [20] G.I. Volovich, E.V. Solomin, D.V. Topolskiy, I.G. Topolskaya, On the Development of Automation in the Energy from Renewable Energy Sources, International Scientific Journal "Alternative Energy and Ecology". 9(131) (2013) 59–64. [21] V.I. Smolin, I.G. Topolskaya, The Theory of Generalized Energy Flow Three-phase Electromechanical Transducers in Control Electric, Proc. of the VIII Int. (XI All-Russian) Conference on Automated Electric AEP 2014. (2014) 111–115. [22] F. Blasche, The Principle of Field-orientation as Applied to the New Transvektor Closed-loop Control System for Rotating-field Machines, Siemens Review. 34 (1972) 217–220. [23] M. Depenbrock, Direct Self-control of Inverter-fed Induction Machine, IEEE Transactions on Power Electronics. 3 (1988) 420–429. [24] R.E. Betz, B.J. Cook, Instantaneous Power Control of Induction Machines, Journal of Electrical & Electronics Engineering. 1 (2001) 57–63.