International Journal of Energetica (IJECA) https://www.ijeca.info ISSN: 2543-3717 Volume 4. Issue 1. 2019 Page 37-43 IJECA-ISSN: 2543-3717. June 2019 Page 37 Fuzzy Logic Based Robust DVC Design of PWM Rectifier Connected to a PMSG WECS under wind/load Disturbance Conditions Y. Saidi 1* , A. Mezouar 1 , Y. Miloud 1 , M. A. Benmahdjoub 1 , M. Yahiaoui 2 1 Laboratory of Electro-Technical Engineering, Faculty of Technology Tahar Moulay University of Saïda (20000), ALGERIA 2 University of Mascara, Road of Mamounia, Mascara (29000), ALGERIA Email*: saidi_youcef_20@yahoo.com Abstract – Permanent Magnet Generator has been widely used in Variable-Speed Wind Energy Conversion System (VSWECS). Fuzzy Logic Control (FLC) of the generator side converter has the ability to have a good regulation of the DC-link voltage to meet the requirements necessary to achieve optimal system operation, regardless of the disturbances caused by the characteristics of the drive train or some changes into the DC-load. The main focus of this paper is to present a model for a three phase voltage source space vector pulse width modulation (SVPWM) rectifier which is connected to a PMSG in a wind turbine system, where a direct voltage control (DVC) us- ing FLC based on voltage orientation strategy is used to control the mentioned rectifier. The con- trol algorithm employs fuzzy logic controller to effectively achieve a smooth control of DC-link voltage under wind/load perturbation conditions. Some simulation results, using Matlab/Simulink, are presented to show the effectiveness of the SVPWM rectifier Connected to a PMSG WECS with the proposed control strategy. Keywords: Direct Voltage Control (DVC); Permanent Magnet Synchronous Generator (PMSG); Wind Energy Conversion System (WECS); Fuzzy Logic Controller (FLC). Received: 09/04/2019 – Accepted: 20/05/2019 I. Introduction Recently, the AC-DC converter applications are in- creasing in industry, commerce and house utility. Tradi- tionally, the main parts of converters have been the dio- des and thyristors bridges to rectify the AC power. These rectifiers have the advantages of being simple, robust and having low cost. However, they generate harmonics and reactive power in AC side, which results voltage distor- tion, poor power factor at power supply side and slowly varying rippled DC output at DC side. Therefore, a three- phase PWM rectifier is a more cunning solution for in- dustrial applications, since it has more advantages such as adjustment and stabilization of DC-link voltage, sinu- soidal line current, power factor control and bidirectional power flow [1]. One of the most considerable industrial applications is wind energy. Nowadays, there are two types of generators which are used in large scale wind turbines to transform the wind power into electrical ener- gy, such as: DFIG and PMSG [2]. Because of its ability to operate in all wind speed range and do not require excitation current, PMSG shows good performance in wind farm. As the fast development of wind power tech nology [3], the efficiency of converter device in wind power generation system has become another knotty problem to improve wind power generation system per- formance [2]. The three-phase voltage source PWM rectifier control based on DVC issues are traditionally treated by fixed gain PI controllers [4]. However, the fixed gain control- lers are very sensitive to parameter variations and gener- ally cannot provide good dynamic performance, Such as discussed in [5]. So, the controller parameters have to be continually adapted. This problem can be solved by sev- eral adaptive control techniques such as sliding mode control (SMC) [6]. The design of all of the above con- trollers depends on the exact system mathematical model. For the same purpose around solving these problems, the idea that a linear system is adopted as the consequent part of a fuzzy rule has evolved into the innovative Takagi- Sugeno (TS) model [7], which has become quite popular today. The fuzzy logic have gained great important, wit- nessed a rapid growth in industrial applications, proved their dexterity of many respects. FLC can achieve satis- factory results in dealing with system, which is difficult Abder Image placée Y. Saidi et al. IJECA-ISSN: 2543-3717. June 2019 Page 38 to de-scribe mathematically or is highly nonlinear beha- viour, as described in [8], [9] which relates to the control of PWM Rectifier whose energy derives from a purely electrical source. In this paper, a detail dynamic model and a simple di- rect voltage control (DVC) strategy using fuzzy logic controller for three-phase voltage source SVPWM rec- tifier connected to a PMSG wind turbine with voltage orientation to improve the system’s robustness and dy- namic response of the dc bus voltage is proposed. In order to improve the dynamic performances of the source current loop, the simulation results show that the FLC can significantly reduce the three-phase rectifier’s vol- tage fluctuation, improve the dynamic response of the dc- bus significantly and assist the system to operate in unit power factor with low harmonic content of current. II. Wind Energy Conversion System The topology of the WECS presented in this study is depicted in Fig. 1. It consists of a wind turbine, a gear- box, a PMSG, Generator side converter and grid side converter. In our strategy studied, the converter on the generator is used to control the DC link voltage whatever the disturbances caused by the characteristics of the wind turbine drive train or the variation in the DC load. Turbine Gearbox Generator Side Converter PMSG Grid Side Converter  Grid Network Transformer PWM Rectifier Figure 1. Wind energy conversion chain based on PMSG The converter on the generator is used to control the DC link-voltage whatever the disturbances caused by the characteristics of the WECS drive train or the variation in the DC load. II.1. Aerodynamic subsystem model The aerodynamic power is dependent on the power coefficient. It is given by [10], [11]:                            1311 643 2 1 32 1035.008.0 --, , 2 1 5       i c i p paer cecc c cC VRCP i (1) Where:  – air density [ 3mKg ], R – blade length [m ], V – wind velocity [ sm ]. The power coefficient pC depends on the ratio  and the pitch angle  is shown in Fig. 2. The aerodynamic torque aerT is calculated by the ra- tio of the aerodynamic power aerP to the shaft speed t : t aer aer P T   (2) C p � 0 5 10 15 -0.2 0 0.2 0.4 0.6 � = �° � = ��° � = ��° � = ��° � = �° ���� ��� Figure 2. Power coefficient variation against tip speed ratio and pitch angle The turbine is usually attached to the generator via a gearbox whose gear ratio G is chosen to adjust the speed of the generator column at the required speed range. The torque and shaft speed of generator are given by:       G GTT tg aerg 1 (3) Where: g – generator shaft speed, gT – torque of the generator. By using the equation (3), the shaft system dynamics can be described as [12]: gvemgg fTTJ   (4) Where: emT – electromagnetic torque, J – equivalent inertia, vf – viscous friction. Fig. 3 illustrates a typical characteristic giving the aerodynamic power of a WT-s which is also used in the simulation section. The extractable power is shown ver- sus the rotor speed for different wind speed values. Each diagram for a constant wind speed has a peak value in which the pair ( opt,g , max,aerP ) are relevant. Y. Saidi et al. IJECA-ISSN: 2543-3717. June 2019 Page 39 P a er [ p u ] g [pu] 0 100 200 300 400 500 0 1 2 3 4 x 10 4m/s 6m/s 8m/s 10m/s 14m/s 12m/s 0.4 0.8 1.2 1.4 1.8 MPPT Pitch control 0 0.5 1 1.5 2 Figure 3. Aerodynamic powers various speed characteristics II.2. Electrical subsystem model The circuit diagram of the three-phase two level voltage source rectifier structure Connected to a PMSG Wind Energy Conversion System is shown in Fig. 4. In order to set up math model, it’s assumed that the filter reactor is linear, IGBT is ideal switch and lossless [13]. and Gearbox VOLTAGE CONTROL dci Ci rr LR , t a b c C Li dcv LR PMSG Turbine Figure 4. Circuit schematic of PWM rectifier Connected to PMSG WECS Where asi , bsi and csi , are phase currents, C is smoothing capacitor across the DC bus, LR is the load resistance, and Li is load current. The classical electrical equations of the PMSG and converter in the PARK frame are written as follows [14], [15]:                      L dc qsqdsd dc qemqsrsqsrsqs demdsrsdsrsds R v iSiS dt dv C fi dt d LLiRRv fi dt d LLiRRv , , (5) With d,emf and q,emf are the crosses coupling terms be- tween the d-axis and q-axis:          fdsrsqem qsrsdem iLLf iLLf , , (6) Where sR , sL , rR , rL ,are the stator phase resistance and inductance, the rectifier line resistance and induc- tance, respectively and pmsgg p is the electrical speed and pmsgp is the pair pole number, dsi and qsi are the direct and quadrate stator currents, f is magnetic flux. dS , qS are input voltage of rectifier, switch func- tion in synchronous rotating d-q coordinate, respectively and dcv is the dc-bus voltage. The electromagnetic torque is expressed as [16]: qsfpmsgem ipT  2 3 (7) III. Direct Voltage Control (DVC) Strategy In this section, the principle of this control (DVC) based on voltage orientation consists of using a current loop, developed by analogy with the vector control of electrical machines. It consists of orienting the current vector in the same direction as that of the voltage vector, by controlling the current vector in the two revolving d-q axes. Regarding Fig. 5, the current of the d-axis is set to zero while the reference current qsi is set by the DC link voltage regulator. Figure 5. Voltage orientation Once the rectifier is connected to an existing load, the transit of direct and quadrature axis currents must be controlled separately. To obtain a decoupled currents control of rectifier, the method based on voltage orienta- tion can be regarded as the efficient one. There are three control loops in the DVC strategy. The error between the reference dc-bus voltage *dcv and the sampled dc-bus voltage dcv is processed by FLC, which produces the reference active current *qsi . As in the inner loops, d-axis currents loop and q-axis current loop use PI controllers to make the actual currents ( dsi and qsi ) track their reference values ( *dsi and * qsi ). Then, the errors are processed in two conventional PI controllers to produce the output signals of *dsv and * qsv , after coordinates transformation, *sav , *sbv and * scv O  qO  sI  qsi dsi si α si  sV  sv α sv  dO  O  sqs ii  , 0dsi Y. Saidi et al. IJECA-ISSN: 2543-3717. June 2019 Page 40 which can be obtained and used to produce switch- ing signals aS , bS and cS by two-level space vector pulse with modulation (SVPWM). Consequently, the proposed currents control can then be applied, as depicted in Fig. 6, Considering that the direct and quadrature axis currents considered as va- riables to be controlled. With:    srssrs LLTRRA  1;1 sT A s s 1 + * ds v dsi sT A s s 1+ * qsv qsi 1,qsv demf , qemf , + + 1,dsv  dsiController  qsiController demf , ˆ qemf , ˆ - - + + * 1,dsv * 1,qsv dsi - - + + * dsi * qsi Model of rectifier currents Rectifier current control qsi Figure 6. The block diagram of direct and quadrature axis currents control IV. Controller Design IV.1. PI regulator synthesis In order to control the converter used, we must per- form a decoupling by compensation. To make the “d” and “q” axes completely independent. The parameters of the corrector are calculated with a method of imposition of the poles. It is possible to generate reference voltages from given reference quantities. The design of this con- troller is simple. Fig. 7, 8 shows the system scheme regu- lated by a PI corrector. Consequently, the proposed currents control can then sT A s s 1 s K K s,i s,p  * ds i s + - * ds v dsi Figure 7. Direct current regulation loop sT A s s 1s K K s,i s,p  * qsi s + - * qsv qsi Figure 8. Quadrature current regulation loop In fact, the errors ( ds * ds ii  ) and the errors  qs*qs ii  are processed by the PI corrector, in order to design the reference voltages * s,dqv . Using the Laplace transforma- tion, the closed-loop transfer function is given as follows:   s si s sps sisp s s T KA T KA ss KsK T A CLTF ,,2 ,, 1           (8) The calculated terms are in these tables: TABLE 1. The calculated PI gains spK , siK , PI controller   ss AT /1-2 0 ss AT /20 Value 150 10 4.2. Fuzzy Logic Controller (FLC) To regulate the DC voltage, FLC is used because of the nonlinearity of the system. The basic formation of a FLC is consisted of four parts: Fuzzification block de- termining inputs membership values. The Fuzzy Infe- rence System FIS evaluates at each time which control rules are appropriate, using the fuzzy knowledge based block. The deffuzification block calculates the crisp output of the rules leading to the optimal plant con- trol [17, 18]. Fig. 9 shows the block diagram of the fuzzy control. D ef u zz if ic at io n F u zz if ic a ti o n Inference system Rules Base u uKeK deK dt d e de *y y - + Figure 9. Fuzzy logic controller structure The input and output linguistic variables of the fuzzy controller have been quantized in the following five fuzzy subsets. Where the error e and its rate of change de are the input variables; eK , deK and uK are inputs and outputs scaling gains. For the proposed FLC of DC link voltage, we use diagram scheme of Fig. 10. TABLE 2. Fuzzy rule-base for the controller ε dε NL NS ZE PS PL NL NL NL NL NS ZE NS NL NS NS ZE PS ZE NL NS ZE PS PL PS NS ZE PS PS PL PL ZE PS PL PL PL There are two input signals to the FLC; the first input is the error between the reference and the measured value of the DC voltage, the second one represents the varia- Y. Saidi et al. IJECA-ISSN: 2543-3717. June 2019 Page 41 tion of this error. These two signals are expressed by:      )1()()( )()()( * * nvnvne nvnvne dcdcdc dcdcdc (9) V. Results and discussion The fuzzy sets have been determined as: NL, Negative Large, NS, Negative Small and ZE, Zero, PS, Positive Small, PM positive medium, PL, Positive Large, respec- tively. The input/output variables used in this paper are fuzzified by seven symmetrical and triangular member- ship functions (MFs) (Fig. 10(a), (b) and (c)) normalized in the universe of discourse between -1 and +1. The FLC surface is depicted in Fig. 10(d). Then, the outputs of the DC-link voltage fuzzy controller are *qsi . Finally, the overall simulation scheme of a three phase PWM Rectifier under DVC strategy connected to a PMSG WECS is given in Fig. 11. -1.00 -0.35 0 0.35 0.70 1.00 NL ZE PL 1.0 NS PS -0.70 1.0 e e -1.00 -0.35 0 0.35 0.70 1.00 NL ZE PL 1.0 NS PS -0.70 1.0 de de (a) (b) -1.00 -0.35 0 0.35 0.70 1.00 -0.70 NL ZE PL 1.0 NS PS 1.0 du du (c) (d) Figure 10. The memberships of the: (a) – Error, (b) - Error variation, (c)- Command variation, (d) - Control surface Turbine g PMSG t asi bsi csi dq abc dsi qsi qsrs iLL )(  fdsrs iLL  )(- demf , qemf , S V P W M dq αβ demf , - PI PI * qsv * sv - qsi dsi * ds i * qsi * dc v dcv + dcv G S C  dt d Encodeur  * sv qemf , * dsv + + + + - - LR Gearbox Figure 11. The block diagram of the proposed DVC approach Fig. 12a shows the input phase current of SVPWM- rectifier. As can be seen in this figure, the current wave has a sinusoidal shape with very low harmonic distortion. The FFT analysis is applied to the line current of phase A of the rectifier. This analysis gives 0.62 [pu] as an effec- tive value for the fundamental component of line current (THD about 2.25%) that is shown in Fig. 12b, is im- proved when compared to conventional voltage orienta- tion technique (THD=16.06%) [5]. So, It is found that most of the harmonics are low ranks. S ig n a l [p u ] 0.445 0.45 0.455 0.46 0.465 -1 0 1 (a) Time [s] M ag ( % o f F u n d am en ta l) 0 5 10 15 20 0 50 100 Fundamental (50Hz) = 0.9417 , THD= 2.25% 0 10 20 0 1 2 (b) Harmonic order Figure 12. Harmonic spectra of line current of phase A of the rectifier All the simulation results were elaborated with a fixed-step size of 0.1 [ms] with a view to digital imple- mentation in future works. Three tests are under study to prove the robustness dynamic response of the proposed DVC approach, firstly the variation on the wind speed (9 to 7 [m/s]) at t=0.5[s] as shown in Fig. 13a, then the variation on the reference voltage of DC link (0.8 to 1 [pu]) at t=1[s] is shown in Fig. 13c and DC load resistance variation (500 to 5*500[Ω]) at t=1.5[s] is seen in Fig. 13b. Fig. 13c shows the output DC voltage. As can be seen in this figure, the DC-Link voltage, after a short transient time, is correctly regulated at its reference voltage (0.8 then 1 [pu]) with soft regulation without over-hoots. The mechanical speed of the PMSG shaft is given in Fig. 13d, it is clear that it takes the same shape as the wind speed. Y. Saidi et al. IJECA-ISSN: 2543-3717. June 2019 Page 42 The electromagnetic torque is shown in Fig. 13e, with a negative value, which proves that the machine used func- tions as a generator. Fig. 13f shows the setting of direct and quadrature axis currents. The d-axis current is main- tained at its zero reference value, while the q-axis current regulation is done by reference DC-voltage control. However, the effect of the coupling between the two control axes (d and q) is observed, since the variation on the reference DC voltage at time t=1[s] induces low os- cillation on the d-q axis currents. V [ m / s ] Ω g [ P u ] 0 0.5 1 1.5 2 0 5 7 9 10 (a) Time [s] 0 0.5 1 1.5 2 0 0.5 1 1.2 0.5 1 1.5 2 0.7 0.72 0.74 (d) Time [s] R L [ Ω ] T e m [ p u ] 0 0.5 1 1.5 2 500 1 500 2 500 0 3000 (b) Time [s] 0 0.5 1 1.5 2 -2 -1.5 -1 -0.5 0 0.5 (e) Time [s] V d c [ p u ] I d q , s [ p u ] 0 0.5 1 1.5 2 0 0.5 0.8 1 1.2 Vdc* Vdc 1.45 1.5 1.55 1.6 0.995 1 1.005 1.01 1.015 (c) Time [s] 0 0.5 1 1.5 2 -1.5 -1 -0.5 0 0.5 1 iqs ids (f) Time [s] Figure 13. Simulation results: (a) Wind speed profile, (b) Load resis- tance variation, (c) DC-Link voltage, (d) Generator speed, (e) Electro- magnetic torque, (f) d-q axis current Appendix In this part, simulations are investigated with a 1.5 MW generator wind turbine [19]. The parameters of the turbine are presented below: Turbine PMSG Rectifier 3/22.1 mKg MWPsn 5.1 mHRr 37 mR 25.35 80pmsgp  3,0rL deg0  mRs 17.3 FC  1100 30G mHLs 07.3 Wbf 7.0172 VI. Conclusion A cascaded control algorithm was properly designed to ensure the optimal operation of the whole system, based on fuzzy logic controller (FLC) with voltage orien- tation technique. Furthermore output DC link voltage is smooth despite a wind/load fluctuation. The control sys- tem based on DVC includes two PI controllers which are used to regulate the AC current and an outer DC voltage loop is composed by FLC strategy. The simulation results shows a good performance and a robust control of DVC proposed method at start- up and during wind/load variations, providing a good regulation of output DC voltage, sinusoidal AC current and low total harmonic distortion. It can be concluded from the simulation results, which demonstrate the inhe- rent ability of the DVC fuzzy logic controller to deal with this kind of noise operation under wind/load disturbance conditions. REFERENCES [1] Y. Saidi, A. Mezouar, Y. Miloud, MA. Benmahdjoub. “A Robust Control Strategy for Three Phase Voltage Source PWM Rectifier Connected to a PMSG Wind Energy Conversion System”, Presented at the 3rd IEEE International Conference on Electrical Sciences and Technologies in Maghreb, Algiers, Algeria, 546-551, 2018. [2] A. Ben Amar, “Direct Torque Control of a Doubly Fed Induction Generator”, International Journal of Energeti- ca, Vol. 2, pp.11-14, 2017. [3] Y. Saidi, A. Mezouar, Y. Miloud, MA. 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Fathi, “A review of low-voltage ride-through enhancement methods for permanent magnet synchronous generator based wind turbines, ” Renewable and Sustainable Energy Reviews, vol. 47, pp. 399-415, 2015. I. Introduction II. Wind Energy Conversion System II.1. Aerodynamic subsystem model II.2. Electrical subsystem model III. Direct Voltage Control (DVC) Strategy IV. Controller Design IV.1. PI regulator synthesis V. Results and discussion Appendix VI. Conclusion