Implementation of SVPWM Based Three Phase Inverter Using 8 Bit Microcontroller

Transcription

1 International Jornal of Science, Engineering and Technology Research (IJSETR), Volme 4, Isse 6, Jne 015 Implementation of SVPWM Based Three Phase Inverter Using 8 Bit Microcontroller Prof. S. K. Patil 1, Mr. Nikhil D. Wadokar, Miss. Pallavi H. Pikale Associate Professor 1, P.G. Scholor, P.G. Scholor Department of Electrical Engineering, Government College of Engineering, Karad, Maharashtra, India 1 Abstract: This paper describes the digital implementation of Space Vector Plse Width Modlation (SVPWM) techniqe sing 8-bit microcontroller for three-phase inverter. For control of IM nmber of Plse width modlation (PWM) schemes are sed to variable voltage and freqency spply. The most commonly sed PWM schemes for threephase voltage sorce inverters (VSI) are sinsoidal PWM (SPWM) and space vector PWM (SVPWM). There is an increasing trend of sing space vector PWM (SVPWM) becase of it redces harmonic content in voltage, increases fndamental otpt voltage by 15% & smooth control of IM. So, here present the implementation of SVPWM based three phase inverter sing simple microcontroller 89c51. This paper presents simlation and hardware reslts of this techniqe. Keywords Space Vector Plse Width Modlation (SVPWM), Protes, Inverter, VSI. ISSN: I. INTRODUCTION DC to AC converter is known as inverters. The Fnction of an inverter is to change dc inpt voltage to a symmetric ac otpt voltage of desired magnitde and freqency. The otpt voltage cold be fixed or variable at fixed or variable freqency. A variable otpt voltage can be obtained by varying the dc inpt voltage and maintaining the gain of inverter constant. On other hand, if the dc inpt voltage is fixed and it is not controllable, a variable otpt voltage can be obtained by varying the gain of inverter, which is normally accomplished by plsed width modlation (PWM) control within the inverter [4]. Based on operation; Inverters are of two types Voltage Sorce Inverters (VSI) & Crrent Sorce Inverters (CSI). An inverter is called VSI if the inpt voltage remain constant, a CSI if the inpt crrent is maintained constant. Ot of which VSI is widely sed in indstries as it gives independently controlled AC otpt is a voltage [5]. The performance parameters of an inverter sch as switching losses and harmonic redction are principally depended on the modlation strategies sed to control the inverter. In this design the Space vector plse width modlation (SVPWM) techniqe has been sed for controlling the inverter as it can be directly controlled the inverter otpt voltage and otpt freqency according to the space vector.vsi sally operate on Plse Width Modlation (PWM) techniqe. In this is method a fixed dc inpt voltage is given to an inverter and the otpt is a controlled ac voltage. There are different PWM techniqes for control the inverter and its otpt harmonic redction. The most widely sed switching techniqes are the Sinsoidal PWM (SPWM) and the Space Vector PWM (SVPWM). There are nmber of indstry All Rights Reserved 015 IJSETR applications in which indction motors are fed by Space Vector Plse Width Modlated inverter. It provides many benefits to their sers sch as simplicity of circit, redced energy consmption etc. In this paper firstly Space Vector Plse Width Modlation (SVPWM) techniqes for three phase inverter is described. Later the hardware reqirement has discssed. In last part hardware reslt have been explained. II. SVPWM TECHNIQUE Space vector modlation is qite different from PWM methods. With PWMs, inverter can be thoght as three separate psh-pll driver stages, which create each phase waveform independently. SVM, however, treats the inverter as a single nit; specifically inverter can be driven to eight niqe states. Modlation is accomplishes by switching the state of the inverter. The control strategies are implemented in digital systems. SVM is a digital modlating techniqe where the objective is to generate PWM load line voltages that are in average eqal to given load line voltages. This is done in each sampling period by properly selecting the switching states of the inverter and calclation of the appropriate time period for each state. The selection of the states and their time period are accomplished by the space vector (SV) transformation. Space Transformation Any three fnctions of time that satisfy t t t a 0 b Can be represented in two dimensional space. The coordinates are similar to those of three phase voltages sch that the vector [ a 0 0] t is placed along the x-axis, the vector [0 b 0] t is phase shifted by 10 0, and the vector the [0 0 c ] t is phase shifted by 40 0.This is shown in figre 1. SV in complex notation is given by Figre 1: SV Transformation c 1891

2 International Jornal of Science, Engineering and Technology Research (IJSETR), Volme 4, Isse 6, Jne 015 t = a + b e j π + c e j π (1) Where / is scaling factor. Eq. (1) can be written in real and imaginary components in x-y domain as t = x + j y () Using Eqs. (1) and (), we can obtain the coordinate transformation from a-b-c axis to x-y axis as given by x y = 1 0 a b c () This can be written as x = v a 0.5 v b + v c (4) y = v b v c (5) The transformation from the x-y axis to the α-β axis, which is rotating with an anglar velocity of ω, can be obtained by α = cos ωt cos π ωt β sin ωt sin π + ωt (6) Using Eq. (1), we can find the inverse transformation as a = R e (7) b = R e e j π (8) c = R e e j π (9) Then, sing Eq. (1), we get the space vector representation as t = V m e jθ = V m e jωt (10) which is a vector of magnitde V m rotating at a constant speed ω in rads per second. Figre : The on and off states of the inverter switches. A. Space vector (SV) The switching states of the inverter can be represented by binary vales q1, q, q, q4, q5, and q6; that is, qk = 1 when a switch is trned on and qk = 0 when a switched is trned off. The pair of q1q4, qq6 and q5q are complimentary. Therefore, q4 = 1 - q1, q6 = 1 - q and q = 1 q5. The switch on and off states are shown in figre. Using three phase to two phase transformation in Eq. () and the line voltage as the reference, the α-β components of the RMS otpt voltage vector can be expressed as a fnction of q1, q, and q5. V Lα V Lβ = 1 V 0 q 1 q q (11) Using the factor for converting the rms voltage to its peak vale, the peak vale of the line voltage is V L peak = V s and that of the phase voltage is V p peak = V s. Using the phase voltage V a as the reference, which sally the case, the line voltage vector V ab leads the phase vector by π 6. The normalized peak vale of the nth line voltage vector can be fond from V n = j n π 6 e = cos n π 6 + jsin n π 6 (1) For n = 0,1,,6 There are six nonzero vectors, V 1 V 6, and two zero vectors, V 0 and V 7, as shown in figre. Let s define a performance vector U as the time integral fnction of V n sch that U = V n dt + U 0 (1) ISSN: All Rights Reserved 015 IJSETR 189

3 International Jornal of Science, Engineering and Technology Research (IJSETR), Volme 4, Isse 6, Jne 015 zero SV (V z = V 0 or V 7 ) shold be sed to obtain the maximm load line voltage and to minimize the switching freqency. As an example, a voltage vector V r in section one can be realized by the V1 and V vectors and one of the two nll vectors V 0 or V 7. In other words, V 1 is active for time T 1, V is active for T, and one of the nll vectors V 0 or V 7 is active for T z. For sfficiently high-switching freqency, the reference vector V r can be assmed constant dring one switching period. Becase the vectors V 1 and V are constant and V z = 0, we can eqate the volt time of the reference vector to the SVs as V r T s = V 1 T 1 + V T + V z T z which is defined as the SVM. This is achieved by sing two adjacent SVs with appropriate dty cycle. The vector diagram is shown in Figre. Expressing the SVs in rectanglar coordinates, above eq. becomes Figre : The space vector representation Where U 0 is the initial condition. According to Eq.(1). U draws a hexagon locs that is determined by the magnitde and the time period of the voltage vectors. If the otpt voltages are prely sinsoidal, then the performance vector U becomes T s M cos π 6 + θ sin π 6 + θ = T 1 (17) cos π 6 sin π 6 + cos π sin π U = Me jθ = Me jωt (14) Where M is the modlation index 0 < M < 1 for controlling the amplitde of the otpt voltage and ω is the otpt freqency in rads per second. U draws a pre circle locs as shown in figre by dotted circle of radis M = 1 and it becomes the reference vector V r. The locs U can be controlled by selecting V n and adjsting the time width of V n to follow the U locs as closely as possible. This is called qasi-circlar locs method. B. Modlating reference vectors Using Eqs. (), (4) and (5), the vectors of three-phase line modlating signals V r abc = V ra V rb V T rc can be represented by the complex vector U = V r = v r αβ = v rα v T rβ as given by v rα = v ra 0.5 v rb + v cr (15) v rβ = v rb v rc (16) If the line modlating signals v r abc are three balanced sinsoidal waveforms with an amplitde of A c = 1 and an anglar freqency ω, the reslting modlating signal in the α β stationary frame V c = v r αβ becomes a vector of fixed amplitde MA c = = M that rotates at freqency ω. C. SV switching The objective of the SV switching is to approximate the sinsoidal line modlating signal Vr with eight space vectors V n, n = 0,1,,.,7. However, if the modlating signal V c is laying between the arbitrary vectors V n and V n +1, then the two nonzero vectors V n and V n +1 and one Figre 4: Determination of state times Eqating the real and imaginary parts on both sides, we get T s Mcos π 6 + θ = T 1 T s Msin π 6 + θ = T 1 Solving for T 1 and T, we get T 1 = T s M cos π 6 +θ cos π 6 θ (18) T = T s M sin θ sin π 6 cos π 6 + T sin π 6 + T cos π sin π = T s Mcos π 6 + θ = T sm sin π = T s M sin θ (19) T 0 = T z = T s T 1 T (0) Where M is the modlation index; θ is the angle between V r and V n ; T s is the switching or sampling period ISSN: All Rights Reserved 015 IJSETR 189

4 International Jornal of Science, Engineering and Technology Research (IJSETR), Volme 4, Isse 6, Jne 015 The same rle can be applied for calclating the time states of the vectors in sectors or 6. It is assmed that the inverter operates at constant freqency and T s remains constant. D. SV seqence. The SV seqence shold assre that the load line voltages have the qarter-wave symmetry to redce even harmonics in their spectra. To redce the switching freqency, it is also necessary to arrange the switching seqence in sch a way that the transition from one to the next is performed by switching only one inverter leg at a time. Althogh there is not symmetric approach to generate an SV seqence, these conditions are met by the seqence V z, V n, V n +1, V z. If for example, the reference vector falls in section 1, the switching seqence is V 0, V 1, V, V 7, V 7, V, V 1, V 0. The time period T z can be split and distribted at the beginning and at the end of sampling period T s. Figre 5 shows both the seqence and the segments of three phase otpt voltages dring two sampling periods. In general, the time intervals of the nll vectors are eqally distribted, as shown in Figre 5, with T z at the beginning and T z at the end. Figre 6: Flow Chart IV. SIMULATION RESULTS Reslts are obtained sing protes. Actal switching pattern for all six sectors are shown below. Figre 5: Pattern of SVM III. HARDWARE IMPLEMENTATION SVPWM techniqe is implemented sing simple microcontroller 89c51. Otpt pattern of SVPWM is store in the form of lookp table. Time drations are calclated as explained before and stored in lookp table. These times dration sed to provide on time of switches. Timer 0 of 89c51 is sed to provide delay according to calclated time drations. Flow chart shows actal implementation of SVPWM. Sector I Sector II ISSN: All Rights Reserved 015 IJSETR 1894

5 International Jornal of Science, Engineering and Technology Research (IJSETR), Volme 4, Isse 6, Jne 015 Sector III Figre 8: SVPWM Hardware Photo Sector IV Sector V Sector VI Figre 7: SVPWM otpt pattern Actal hardware is as shown below V. CONCLUSION This paper describes the digital implementation of SVPWM techniqe for three-phase inverter. The proposed system ses 8-bit 89c51 microcontroller for generating SVPWM signal needed to trigger the gates of MOSFET bridge of the inverter. The experimental reslts show the ability of the proposed system to generate a three-phase sine wave signal with desired freqency. REFERENCES 1] Sandeep N Panchal, Vishal S Sheth, Akshay A Pandya, Simlation Analysis of SVPWM Inverter Fed Indction Motor Drives International Jornal of Emerging Trends in Electrical and Electronics (IJETEE), Vol., Isse. 4, April-01. ] Mahmod Gaballah, Mohammed El-Bardini, Soliman Sharaf and Mohammed Mabrok, implementation of space vector pwm for Driving two level voltage sorce inverters, Jornal of Engineering Sciences, Assit University, Vol. 9, No. 4, pp , Jly 011. ] Mahesh A. Patel, Ankit R. Patel, Dhaval R. Vyas and Ketl M. Patel, Use of PWM Techniqes for Power Qality Improvement International Jornal of Recent Trends in Engineering, Vol. 1, No. 4, May ] M. H. Rashid, "Power Electronic, Circits, Devices, and Applications", Second Edition, Printice-Hall of India, New Delhi, ] Marwan A.A. Badran, Ahmad M. Tahir and Waleed F. Faris, Digital Implementation of Space Vector Plse Width Modlation Techniqe Using 8-bit Microcontroller, World Appl. Sci. J., 1 (Mathematical Applications in Engineering): 1-8, 01. 6] K. Monika, B. Kiran Bab, Sinsoidal and Space Vector Plse Width Modlation for Inverter, International Jornal of Engineering Trends and Technology (IJETT) Volme 4 Isse 4 - April 01. 7] Mohan, N., First Corse on Power Electronics and Drives, MNPERE, 00. 8] Bimal K. Bose,"Power Electronics and Motor Drives, Advances and Trends,by: Based Expert System Using Object Oriented ProgrammingLangage,006. ISSN: All Rights Reserved 015 IJSETR 1895