Control of Variable-speed PMSG Wind Generation System

Control of Variable-speed Permanent Magnet Synchronous Generators Wind Generation System DENG Qiu-ling1,2;LIU Gou-rong1,2;XIAO Feng1 1.Dept.of Electric and information engineering, Hunan Institute of Engineering, Xiangtan 411101,China; 2. Institute .of Electric and information engineering, Hunan University changsha 410082,China Abstract： This paper describes the optimal control of variable-speed permanent magnet synchronous generators system. In order to maximize generated power, permanent synchronous generator is controlled by a maximum power point tracking(MPPT) control and maximum efficiency control. A sensor-less scheme is proposed for MPPT. Experimental results have been carried out to verify the benefits of the proposed system. I INTRODUCTION Owing to the characteristic of wind power with inexhaustible supply and being free of charge and being a clean energy source, wind power has become a rapidly growing technology for renewable power generation and it could supply 12%of world’s electrical demand by 2020. However wind energy has a drawback of having only 1/800 density and irregularity as compared with that of water energy, it is important that how we can utilize it as a high efficient electric power energy[1]. A maximum power extraction algorithm (MPEA) is an essential requirement for maximizing the aerodynamic efficiency of the wind generation system. In previous work, it is illustrated that the wind turbine operating in variable-speed variable-frequency mode with the MPEA ,for obtaining an optimal tip speed ratio(TSR),could generate additional 9-11% power more than traditional fixed voltage or speed control. One of the problems associated with variable-speed wind power system today is the presence of the gearbox coupling the wind turbine to the generator. This mechanical element suffers from considerable faults and increases maintain expenses. To improve reliability of the wind mill and reduce maintenance expenses the gearbox can be eliminated[2]. Permanent magnets can be used to replace the excitation winding of synchronous machines because of magnet price reduction and magnetic material characteristic improvement. Permanent-magnet excitation allows us to use a smaller pole pitch than do conventional generators, so these machines can be designed to rotate at rated speeds of 20-200r/min, depending on the generator rated power. In addition, permanent magnet synchronous generator has the advantages of simple structure and high efficiency. In the existing controller for the maximum power extraction, most designed controllers use an anemometer to measure wind speed for deriving the shaft speed. Due to stochastic nature of the wind, one anemometer reading could not provide adequate information[3]. As a result, most of the designs are too costly because of much dependence on expensive sensors for measuring the wind speed, wind turbine generator (WTG) rotational speed and torque, microprocessors for achieving elaborate and complicated control strategies. To achieve optical power output, a sensor-less scheme will be proposed for extracting desired output power from the WTG over a wide range of wind speeds. Using the measured dc voltage, dc current and PMSG terminal voltage frequency from the wind energy conversion system(WECS), the proposed MPPT controller allows the conversion system to track the maximum power point very rapidly. This paper describes the operation and control of variable-speed direct driven permanent magnet synchronous wind generators. The generator is connected to the power network by means of a fully controlled frequency converter, which consists of a pulsewidth-modulation(PWM) rectifier, an intermediate dc circuit, and a PWM inverter. The generator is controlled to obtain maximum power from the incident wind with maximum efficiency under different load conditions. Vector control of the grid-side inverter allows power factor regulation of the wind generation system. Experimental results have been carried out to verify the benefits of the proposed system. Ⅱ WIND ENERGY CONVERSION SYSTEM The WECS considered for analysis consist of a permanent magnet synchronous generator driven by a wind turbine, PWM rectifier, an intermediate dc circuit, and a PWM inverter. Fig.1 shows a schematic of the power circuit topology of a variable speed wind turbine system that will be discussed in this paper. The wind turbine converts the kinetic energy presented in the wind into mechanical energy, which drives the permanent Fig.1. Schematic of wind energy conversion system 2454 magnet synchronous generator. Since the wind is the intermitted source of energy, the output voltage and frequency from generator will vary for different wind velocities. The variable output ac power from the generator is first converted into dc using the PWM rectifier. The available dc power is fed to the grid at the required constant voltage and frequency by regulating the modulation index of the inverter. Ⅲ MAXIMUM POWER TRACKING The output mechanical power of the wind turbine is given by eq.(1). The tip speed ratio is given by eq.2. 3 2 1 wpmec vAcP ρ= (1) w w V R ωλ = (2) Where ρ is the air density (kg/m3);A is area swept by wind turbine rotor (m2); wV is wind speed（m/s）； pC is power coefficient of wind turbine; R is radius of the rotor(m); wω is mechanical angular velocity of the generator(r/sec). If the rotor speed is adjusted according to the wind speed variation, then the tip-speed can be maintained at the optimum points, which yield maximum power output from the system. maxpc is the maximum power coefficient developed by wind turbine at the optimum tip-speed ratio optλ .The relationship between the Cp and tip speed ratio λ is usually provided by the turbine manufacturer in the form of a set of non-dimensional curves as shown in Fig.2. The rate of the rotor speed is proportional to the inverse of the inertia and difference between wind turbine mechanical torque mT and the generator electrical torque eT . )(1 em w TT Jdt d −= ω (3) The wind turbine output mechanical torque is affected by Fig.2. Power coefficient vs. Tip speed ratio with β =0 ( β :blade pitch angle) the Cp. In order to maximize the aerodynamic efficiency, the eT of the PMSG is controlled to match with the wind turbine mT to have maximum possible maxpc . With a power converter, adjusting the electrical power from the PMSG controls the eT ; therefore , the rotor speed can be controlled . For the system to operate at maximum power at all wind speeds, the electrical output power from the power converter controller must be continuously changed so that under varying wind speed conditions the system is matched always on the maximum power locus. From the power curve of the wind turbine, it is possible to operate the wind turbine at two speeds for the same power output. In practice, the operating range at region 1 is unstable as the rotor speed of the WTG belongs to the stall region. Therefore, the controller has to be designed to keep the operating point inside the desired region. IV CONTROL OF SYSTEM A Decoupling of the generator axes The voltage equations of generator in dq-components can be given as[4]: dsqqSdSSd iLiRu • +−= ψω (4) qfsddSqSSq iLiRu • +++= ψωψω (5） Here sdu and squ are the terminal voltage of the generator, sdi and sqi are the stator current, SR is the stator winding resistance, ω is the stator angular frequency, dL and qL are the stator direct and quadrature inductance, and fψ is the excitation flux linkage. The electric torque eT of the three-phase generator can be calculated as follows: ])[( 2 3 sqsdsqqde iiiLLpT ψ−−= (6) Here p is the number of pole-pairs. For a non-salient-pole machine the stator inductances dL and qL are approximately equal. This means that the direct-axis current sdi does not contribute to the electrical torque.The electrical torque of the machine can be simplified as: Sqe ipT ψ2 3 = (7) Our concept is to keep sdi to zero in order to obtain maximal torque with minimum current. In order to design independent controllers for the two coordinates, the influence of the q-axis on the d-axis component and vice versa must be eliminated. This can be done by a decoupling device. Fig.3 shows the block diagram for calculating the decoupling voltages ddecu and qdecu ,which are added to the current controller outputs(see Fig.4.), resulting in the control signals for the PWM-rectifier.. 2455 Fig.3. Decoupling of the generator axes B Control scheme of the permanent magnet generator–side converter Fig.4 is the control scheme of the permanent magnet generator-side converter. In order to combine a fast response of the controlled variable to a change of the setpoint with zero steady-state deviation, proportional plus integral(PI) current controllers are chosen. The required d-q components of the rectifier voltage vector are derived from two PI current controllers: one of them controlling the d-axis component of the current and the other one the q-axis component. According to eq.4 and 5 and the decoupling structure of Fig.3 the linear transfer function of sdi and sqi respectively are: sLRsU sI sF dSSd Sd + == 1 )( )( )( '1 (8) sLRsU sI sF qSSq Sq + == 1 )( )( )( '2 (9) Where '' , SqSd UU are the voltage components in the d-q axes that control the corresponding current components, and S is the Laplace operator. Decoupling terms as compensation are added to improve the dynamic response. The control requires the measurement of the stator currents, dc voltages, and rotor position. Space-vector modulation(SVM) is used to generate the switching signals for the power converter semiconductors. An optimal speed reference is applied in order to obtain maximum power from the accident wind. Generally speaking, the maximum-power-extraction algorithm(MPEA) can be classified to three methodologies including tip speed ratio(TSR) control, power signal feedback(PSF) control, and hill-climb searching(HCS) control[5]. In order to avoid measuring wind speed using expensive anemometer PSF control is adopted. Fig.4. Control scheme of the permanent magnet generator-side converter Speed reference is obtained from measured power. In the PSF control, the maximum-power curve (i.e., the generated power versus generator speed) has to be established by examining the wind generation system in the possible occurrence of wind speeds, and this curve then provides a maximum-power speed reference according to the measured generator speed to obtain a constant tip speed ratio. Q-axis current reference ∗sqi is controlled by generator torque as this current is proportional to generator torque. Direct current component sdi is obtained through a calculator )(Ωf in order to minimize power losses. Control technology aiming to obtain the loss minimization can be roughly summarized into two main categories: loss model control and search control algorithm. The former requires knowledge of both an accurate system model and a reliable identification of its parameters, as well as the variation of the parameters with temperature, current, etc. By expressing the losses as a function of the control variables of the machine, it is possible to impose an operating condition to obtain the maximal efficiency. Search control algorithm mainly consists on changing step by step the value of a control variable, then measuring for each operating point the active power flowing into the machine. Finally , by comparing the measured result with previous one at fixed operating conditions, the maximum power consumption of the machine is obtained. In the loss model control algorithm, the power losses caused by the fundamental harmonic of the current in the windings, and the power losses caused by he fundamental harmonic of the air-gap flux linkage in the iron stack, can be expressed as a function of the direct current sdi and electric angular speed Ω and generator torque eT . ( ) ( )Ω+Ω=Ω ,,,),,( esdFeesdcuesdC TiWTiWTiW (10) The value of sdi that minimizes the electric losses can be analytically calculated by differentiating express (10), with respect to the sdi variable, and sdi is function of the speed. The value of sdi obtained is tabulated as a function of generator speed, which is expressed as the function ( )Ωf in fig.4. ( )Ωf can be obtained off-line. C . Control of the line-side converter Fig.5 is the control scheme of the grid-side converter. The dynamic model of the grid connection when selecting a reference frame rotating synchronously with the grid voltage space vector is: q d didd Lidt di LRiuu ω+−−= (11) q d didd Lidt diLRiuu ω+−−= (12) Where L and R are the grid inductance and resistance, respectively, and idu and iqu are the inverter dq-axes voltage components respectively. The active and reactive power may be expressed as dd iuP 2 3 = (13) 2456 qd iuQ 2 3 = (14) Fig.5 control scheme of the grid-side converter. Active and reactive power control can be achieved by controlling direct and quadrature current components, respectively. The control of this converter is quite similar to that of the generator. Two control loops are used to control the active and reactive power , respectively. An outer dc voltage loop is used to set the d-axis current reference for active power control, this assures that all the power coming from the rectifier is instantaneously transferred to the grid by the inverter. The second loop controls the reactive power by setting q-axis current reference to a current loop similar to the previous one. The current controllers will provide a voltage reference for the inverter that is compensated by adding rotational EMF compensation terms. D. control strategy of the system Operating modes of wind power generation system include maximum power output under rated wind velocity and constant power output above rated wind velocity. Once generator rated power is reached at rated wind velocity ( rv ), output power must be limited. In some wind turbines, when working with the maximum power coefficient, rated speed is obtained at a wind velocity lower than that of generator rated power. When wind turbine speed is reached at a wind velocity ( 1wv ),control must be changed so that a higher wind velocity no longer increases turbine speed but increases generated power until generator rated power, increases in rotor speed of about 10%are allowed during transients because of the slow pitch control response. Taking into account these stages, the control strategy is the following, depending on the wind velocity. 1) when wind velocity is under cut-in wind speed, the wind turbine is shut down because the power is so low that it is hardly worth working with. 2) When wind velocity is between cut-in and 1wv : the blade pitch angle is set at an optimal value that allows the turbine to extract maximum energy from incident wind. An optimal speed reference is applied to obtain maximum power. Speed control loop bandwidth must be as low as possible (about 2 rad/s) in order to obtain a smooth power output. To achieve efficient control by minimizing the power losses, not only active but reactive generator current component is imposed by the power converter. 3) When wind velocity is greater than 1wv : generator speed is held at its rated value by limiting speed reference. The speed control loop will act on the quadrature-axis current component to achieve this objective. Rated torque is obtained at a rated wind velocity. 4) When wind velocity is higher than rated speed: a mechanical actuator is usually employed to change the pitch angle of the blades in order to reduce power coefficient and maintain the power at its rated value. V EXPERIMENT RESULTS The experiment setup consists of a three-phase,16 pole PMSG, a frequency converter with two back-to-back insulated gate bipolar transistors(IGBTs: 600V,15A) bridges, an inductive filter designed to limit harmonic current injection into the grid and a transformer(400/230V)used for grid connection to allowed the operation of the inverter with leading power factor. The PMSG is rated for 3kW,220V(wye connection), 375r/min. The results obtained from the experimental investigation is illustrated as Fig.6. Fig.6(a) shows the wind velocity, Fig.6(b) shows the generator speed; Fig.6(c) shows the generator output power ; Fig.6(d) shows the reference and actual values of quadrature-axis and direct-axis current. It can be observed that the system tracks the maximum power point until rated generator speed is achieved. VI CONCLUSION An efficient generator control means not only extracting maximum power from accident wind, but also minimizing the power losses. To achieve this objective, a stator q-axis current component is used to develop generator torque aiming at adjusting the wind velocity to capture maximum output power. The optimum generator d-axis current component is imposed by the power converter,i.e,, the current that leads to the minimum losses. Experimental results indicate the system is able to track maximum power using generator power as input when exciting the system with a real wind profile. 2457 Fig.6 Experimental results.(a)Wind turbine velocity(m/s). (b)Generator speed(rpm). (c)Generator power(w). (d)Reference and actual dq-axes current components (A) REFERENCES [1] Tomonobu Senjyu,Satoshi Tamaki,Naomitsu Urasaki,Katsumi Uezato.Wind Velocity and Rotor Position Sensorless Maximum Power Point Tracking Control for Wind Generation System[J].IEEE Power Electronics Specialists Conference.2004(35):2023-2028. 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