Pak. J. Biotechnol. Vol. 13 (special issue on Innovations in information Embedded and communication Systems) Pp (2016)

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1 COORDINATED CONTROL OF DFIG SYSTEM DURING UNBALANCED GRID VOLTAGE CONDITIONS USING REDUCED ORDER GENERALIZED INTEGRATORS Sudhanandhi, K. 1 and Bharath S 2 Department of EEE, SNS college of Technology, Coimbatore, Tamilnadu, India 1 sudhanandhik@gmail.com; 2 bharathbhadri@gmail.com ABSTRACT This paper presents combined control of rotor-side converter (RSC) and grid-side converter (GSC) of a DFIG system during unbalanced grid voltage conditions. Here RSC is controlled to minimize the torque ripples and GSC is controlled to ensure total balanced currents and constant active and reactive power into grid by reducing ripples which are caused due to unbalance. A ROGI is used to reduce the above problems. Index terms - Doubly fed induction generator, Reduced order generalized integrators, voltage unbalance. I.INTRODUCTION WIND ENERGY is playing a vital role in generation of electrical power due to its availability in plenty, free of cost and pollution free. The various types of generators are used, where doubly fed induction generator is found to be more sensitive due to its variable speed constant frequency operation and reduced power rating of converters, compared with the fixed-speed induction generators or synchronous generators based wind power system [1,3]. Due to wide use of nonlinear loads and asymmetrical faults in a grid, the system gets unbalanced, which affects the overall performance of DFIG s output. Hence PI and ROGI controllers are used to reduce these effects. Under severe and short-time balanced fault conditions, the improved rotor current control schemes are implemented for the rotor side converter [4,5]. In [6,8], the RSC control strategy is designed with two PI current regulators in the positive and negative synchronous reference frame. The sequence decomposing process of both sequence components in the control loop is inevitable, which would introduce the time delay and degrade the dynamic performance. The PIR regulators are used in [9,10] to implement the precise control of both positive and negative sequence currents. The calculations are very complex due to both positive and negative sequence components which are highly dependent on generator parameters [6-10]. The main objective of this paper is to develop a combined control strategy for both DFIG s RSC and GSC under the unbalanced grid voltage, which can achieve the different control targets for the overall DFIG system, i.e., balanced total currents, constant active power or reactive power. In this scheme reduced-order generalized integrators (ROGIs) are employed to eliminate the calculations of the negative sequence current references and the decompositions of the positive and negative sequence voltages and currents. Thus, a simple implementation can be obtained and the dynamic responses of the DFIG system can behave faster and smoother. This paper will be organized as follows. Section III briefly describes DFIG typical performance during network unbalance. In Section IV, a control scheme consisting of PI regulators and ROGIs tuned at twice grid frequency is designed, where three control targets are identified. Sections VI and VII present the simulation results and conclusions. Fig. 1. Block diagram of combined control of DFIG s RSC and GSC II. WIND TURBINE MODEL. Wind turbines produce electricity by using the power of the wind to drive an electrical generator. Wind passes over the blades, generating lift and exerting a turning force. The rotating blades turn a shaft inside the nacelle, which goes into a gearbox. The gearbox increases the rotational speed to that which is appropriate for the generator, which uses magnetic fields to convert the rotational energy into electrical energy. The power output goes to a transformer, which converts the electricity from the generator at around 700V to the appropriate voltage for the power collection system, typically 33 kv. A wind turbine extracts kinetic energy from the swept area of the blades. The power contained in the wind is given by the kinetic energy of the flowing air mass per unit time. That is 3 = ρ A V wind (1) Where P air is the power contained in wind (in watts),ρ is the air density(1.225 kg/m 3 ) at 15 C and normal pressure), A is the swept area in (square meter), and V is the wind velocity without rotor interference, i.e., ideally at infinite distance from the rotor (in meter per second). 552

2 Although Equation (1) gives the power available in the wind, the power transferred to the wind turbine rotor is reduced by the power coefficient, C p C p = (2) P wind turbine = C p *P air (3) = ρa A maximum value of C p is defined by the Betz limit, which states that a turbine can never extract more than 59.3% of the power from an air stream. In reality, wind turbine rotors have maximum C p values in the range 25-45%. It is also conventional to define a tip speed ratio as (4) Where is rotational speed of rotor (in rpm), R is the radius of the swept area (in meter).the tip speed ratio λ and the power coefficient C P are the dimensionless and so can be used to describe the performance of any size of wind turbine rotor. III.DOUBLY FED INDUCTION GENERATOR Wind turbines use a doubly-fed induction generator (DFIG) consisting of a wound rotor induction generator and an AC/DC/AC IGBT-based PWM converter. The stator winding is connected directly to the 50 Hz grid while the rotor is fed at variable frequency through the AC/DC/AC converter. The DFIG technology allows extracting maximum energy from the wind for low wind speeds by optimizing the turbine speed, while minimizing mechanical stresses on the turbine during gusts of wind. The optimum turbine speed producing maximum mechanical energy for a given wind speed is proportional to the wind speed. Another advantage of the DFIG technology is the ability for power electronic converters to generate or absorb reactive power, thus eliminating the need for installing capacitor banks as in the case of squirrel-cage induction generator where Vr is the rotor voltage and Vgc is grid side voltage. The AC/DC/AC converter is basically a PWM converter which uses sinusoidal PWM technique to reduce the harmonics present in the wind turbine driven DFIG system. Here Crotor is rotor side converter and Cgrid is grid side converter. To control the speed of wind turbine gear boxes or electronic control can be used. The stator is directly connected to the AC mains, whilst the wound rotor is fed from the Power Electronics Converter via slip rings to allow DIFG to operate at a variety of speeds in response to changing wind speed. Indeed, the basic concept is to interpose a frequency converter between the variable frequency induction generator and fixed frequency grid. The DC capacitor linking stator- and rotor-side converters allows the storage of power from induction generator for further generation. To achieve full control of grid current, the DC-link voltage must be boosted to a level higher than the amplitude of grid line-to-line voltage. The slip power can flow in both directions, i.e. to the rotor from the supply and from supply to the rotor and hence the speed of the machine can be controlled from either rotor- or stator-side converter in both super and sub-synchronous speed ranges. As a result, the machine can be controlled as a generator or a motor in both super and subsynchronous operating modes realizing four operating modes. Below the synchronous speed in the motoring mode and above the synchronous speed in the generating mode, rotor-side converter operates as a rectifier and stator-side converter as an inverter, where slip power is returned to the stator. Below the synchronous speed in the generating mode and above the synchronous speed in the motoring mode, rotor-side converter operates as an inverter and stator side converter as a rectifier, where slip power is supplied to the rotor. At the synchronous speed, slip power is taken from supply to excite the rotor windings and in this case machine behaves as a synchronous machine. A.BEHAVIOUR OF RSC DURING NETWORK UNBALANCE The DFIG model in this study is based on the stator voltage orientation (SVO). All the voltages and currents contain both positive- and negative-sequence components of the unbalanced voltage. Fig. 2. DFIG equivalent circuit on the reference frame. The stator/rotor voltage and flux is given by (7) (8) Where Rs and Rr are stator and rotor resistances, is rotor angular speed, and is slip angular speed, (9) (10) and are the self-inductance of stator and rotor windings,, and are stator and rotor leakage inductances and mutual inductance, respectively. According to equations 5, 6, 7 and 8 the rotor voltage in the frame can be represented as Where (5) (6) (11) (12) σ is the leakage factor and is the equivalent rotor back The electromagnetic force and given as (13) The electromagnetic torque is given by 553

3 (14) (15) B.BEHAVIOUR OF GSC DURING NETWORK UNBALANCE The GSC behaves as a GC-VSC under unbalanced supply. The voltages and currents consist of both positive- and negative sequence components. Thus, the GSC under unbalanced supply can be represented in the frame as Where GSC. (16) refers to the control voltage produced by the (17) IV COMBINED CONTROL OF BOTH RSC AND GSC The combined control can provide the overall DFIG system with enhanced performance and high output power quality to meet the system requirements, including balanced total current, constant total active or reactive power. This control scheme is implemented in the positive synchronous reference frame. The positive and negative sequence fundamental components are converted into the dc signals and twice-order harmonic signals of 2ω1, respectively. As in [15], PI controllers can only regulate the dc components of the feedbacks (positive-sequence fundamental components) to track the dc references due to the lower amplitude responses at high frequencies. Thus, the average active and reactive powers can be still regulated by PI controllers. A generalized integrator tuned at the twice grid frequency can be selected to suppress the 2ω1 components. Fig. 4 shows the collaborative control scheme for DFIG s RSC and GSC under unbalanced voltage conditions. The proposed scheme consists of two regulators in either RSC or GSC: 1) PI current controller; and 2) ROGI tuned at twice grid frequency, providing infinite gain only to -2ω1 ac signals. The PI controllers and 2ω1 ROGIs transfer functions are given as (18) (19) Where, and are the proportion, integral and resonant gain in the continuous-time system, respectively, is a cutoff frequency for determining the width of the frequency 2,which is generally set around 5 15 rad/s. In RSC, since the generator torque ripples may deteriorate the lifetime of the drive shaft and mechanical units, it is necessary to eliminate the twice frequency torque ripples when DFIG works on the unbalanced voltage Different from the solutions in [11 14] to control the negative-sequence rotor currents tracking the commanded values, it is noted that the torque is directly regulated by a ROGI tuned at - in the proposed control strategy. The negative-sequence rotor voltage reference is generated directly by controlling the torque instead of controlling the rotor currents; thus, the calculations of the negative-sequence current references based on the positive- and negative-sequence voltages can be eliminated. The feedback references of this ROGI can be given as (20) This ROGI can only provide the high gain for the selected 2ω1 ac signals, while the ac signals of other frequencies and the dc signals can be blocked. In other words, the ROGI only regulates the 2ω1 components of the torque and has no control capability of other frequency components. Thus, the torque can be used to replace the twice frequency ripples as the feedback references of the ROGI. Since he ROGI can almost achieve zero steady-state errors at the twice frequency ripples can be eliminated if the commanded reference of the ROGI in the RSC can be set as zero, i.e.. During the balanced grid conditions, the torque is constant without 100 Hz ripples and the ROGI s output is zero. Thus, ROGIs have no impacts on the rotor current control loop of the active and reactive power regulation. However, during network unbalance, since the torque contains 100 Hz ripple, as analyzed previously, the ROGI s output is a vector rotating in the negative direction. The d- and q-axes components of this vector are added to the d- and q-axes rotor voltage references produced by PI controllers in the rotor current control loop. Then, the required rotor voltages are generated, which can be used to control the DFIG. As for the GSC, PI controllers in the current control loop are used to maintain a common dc-link voltage regardless of the magnitude and direction of rotor power flow. In the view of the grid requirements, the GSC can be controlled to achieve one of the following targets [14]. Target II: To ensure no double-frequency oscillations in the total active power. Target III: To ensure no double-frequency oscillations in the total reactive power. For these control targets, by using the proposed control strategy in Fig. 3, different feedback references can be designed as follows. For Target I, to ensure no negative-sequence currents into the grid, the feedback of ROGI can be expressed as (21) For Target II, to remove the oscillations in the total active power, the feedback can be obtained as (22) For Target III, to remove the oscillations in the total reactive power, the feedback can be calculated as (23) Similar to the RSC, ROGIs are employed in the GSC control system. Since the ROGI can provide adequate gain only at and attenuate other frequency compo-nents, the electromagnetic quantities, including the current, the total active and reactive power, instead of the respective pulsations can be directly controlled by the ROGI in this scheme. The twice frequency pulsations can be eliminated if the 554

4 commanded value of the ROGI in the GSC can be set as zero, i.e.. Fig. 3. Schematic diagram of the proposed collaborative control strategy for DFIG s RSC and GSC. During network unbalance, similar to the RSC, the d- and q-axes components of the ROGI s output are added to the d- and q-axes voltage references produced by PI controllers in the current control loop, respectively. Then, the commanded voltage of the GSC is produced to control the GSC. The overall simulink diagram of combined control of DFIG s both RSC and GSC is shown in fig 6. Fig. 5.Overall simulink diagram of combined control of DFIG s both RSC and GSC. Fig. 4. Overall block diagram of the proposed control strategy: (a) RSC; and (b) GSC. The proposed control strategy has the advantages: 1) simple implementations due to the elimination of the calculations of the negative-sequence current reference; and 2) fast dynamic responses due to the avoidance of the sequential decompositions of the positive and negative-sequence components. V SIMULATION MODEL OF PROPOSED SYSTEM TABLE I: PARAMETERS OF THE TESTED DFIG Rated power 2.0 MW Rated voltage 690 V p.u DC voltage 1150 V p.u p.u p.u p.u Stator/rotor turns ratio 0.33 Pole number 0.33 VI SIMULATION RESULTS Fig.6(a).Electromagnetic torque without controller Fig.6(b).Real and Reactive power without controller 555

5 Pak. J. Biotechnol. Vol. 13 (special issue on Innovations in information Embedded and communication Systems) Pp. Fig.6(c).Electromagnetic torque with controller Fig.6(d).Real and Reactive power with controller The simulation results for combined control of both DFIG s RSC and GSC are shown in Fig.6. During network unbalance conditions the ripples in electromagnetic torque and real and reactive power are more without controller which is shown in Fig.6(a) and (b). By using ROGI controller, the ripples are found to be reduced which is shown in Fig.6(c) and (d). VII CONCLUSION This paper proposes a collaborative control for DFIG s RSC and GSC under unbalanced grid voltage conditions. The RSC is controlled to reduce torque ripples, while either the current unbalance, or the oscillations in the total active or reactive power is suppressed as the different control targets for the GSC. The ROGIs in the positive synchronous reference frame were employed to restrain the pulsating components. Furthermore, the proposed control scheme using ROGIs can provide: 1) simple implementation without the calculations of the negative sequence current reference; and 2) fast dynamic responses due to the avoidance of the sequential decompositions. Finally, simulation results during network unbalance are given to validate the feasibility and availability of the proposed collaborate control scheme. REFERENCES [1] F.Blaabjerg, M.Liserre and K. Ma, Power electronics converters for wind turbine systems. IEEE Trans. Ind. Appl. 48 (2): (2012). [2] Z. Chen, J. M. Guerrero, and F. Blaabjerg, A review of the state of the art of power electronics for wind turbines. IEEE Trans. Power Electron 24(8): (2009). [3] J. M. Carrasco, L. G. Franquelo, J. T. Bialasiewicz, E. Galvan, R. C.P. Guisado, A. M. Prats, J. I. Leonand N. Moreno-Alfonso, Power electronic systems for the grid integration of renewable energy sources: A survey. IEEE Trans. Ind. Electron 53(4): (2006). [4] F. K. A. Lima, A. Luna, S. Member, P. Rodriguez, E. H. Watanabe, S. Member and F.Blaabjerg, Rotor voltage dynamics in the doubly fed induction generator during grid faults. IEEE Trans. Power Electron 25(1): (2010). [5] S. Xiao, G. Yang, H. L. Zhou and H. Geng, An LVRT control strategy based on flux linkage tracking for DFIG-based WECS. IEEE Trans. Ind.Electron 60(7): (2013). [6] L. Xu and Y.Wang, Dynamic modeling and control of DFIGbased wind turbines under unbalanced network conditions. IEEE Trans. Power Syst. 22(1): (2007). [7] Y. Zhou, P. Bauer, J. A. Ferreira and J. Pierik, Operation of grid connected DFIG under unbalanced grid voltage condition. IEEE Trans.Energy Convers 24(1): (2009). [8] M. I. Martinez, G. Tapia, A. Susperregui and H. Camblong, DFIG power generation capability and feasibility regions under unbalanced grid voltage conditions. IEEE Trans. Energy Convers 26(4): (2011). [9] J. B. Hu, Y. K. He, L. Xu and B.W.Williams, Improved control of DFIG systems during network unbalance using PI-R current regulators. IEEE Trans. Ind. Electron 56(2): (2009). [10] P. Van Tung and H.H. Lee, Performance enhancement of standalone DFIG systems with control of rotor and load side converters using resonant controllers. IEEE Trans. Ind. Appl. 48(1): (2012). [11] H. Geng, C. Liu, and G. Yang, LVRT capability of DFIG-based WECS under asymmetrical grid fault condition. IEEE Trans. Ind. Electron 60(6): (2013). [12] L. Xu, Coordinated control of DFIG s rotor and grid side converters during network unbalance. IEEE Trans. Power Electron 23(3): (2008). [13] J. B. Hu, H. L. Xu, and Y. K. He, Coordinated control of DFIG s RSC and GSC under generalized unbalanced and distorted grid voltage conditions, IEEE Trans. Ind. Electron 60(7): (2013). [14] J. B. Hu and Y. K. He, Reinforced control and operation of DFIG-based wind-power-generation system under unbalanced grid voltage conditions, IEEE Trans. Energy Convers 24(4): (2009). [15] C. J. Liu, F. Blaabjerg,W. J. Chen, and D. H. Xu, Stator current harmonic control with resonant controller for doubly fed induction generator. IEEE Trans. Power Electron 27(7): (2012). 556