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1 ABSTRACTPulse Width Modulation variable speed drives are increasingly applied in many newindustrial applications that require superior performance. Recently, developments inpower electronics and semiconductor technology have lead improvements in powerelectronic systems. Hence, different circuit configurations namely multilevel invertershave become popular and considerable interest by researcher are given on them.Variablevoltage and frequency supply to a.c drives is invariably obtained from a three-phasevoltage source inverter. A number of Pulse width modulation (PWM) schemes are usedto obtain variable voltage and frequency supply. The most widely used PWM schemesfor three-phase voltage source inverters are carrier-based sinusoidal PWM and spacevector PWM (SVPWM). There is an increasing trend of using space vector PWM(SVPWM) because of their easier digital realization and better dc bus utilization. 1
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2 INTRODUCTIONThree phase voltage-fed PWM inverters are recently showing growing popularity formulti-megawatt industrial drive applications. The main reasons for this popularity areeasy sharing of large voltage between the series devices and the improvement of theharmonic quality at the output as compared to a two level inverter. In the lower end ofpower, GTO devices are being replaced by IGBTs because of their rapid evolution involtage and current ratings and higher switching frequency. The Space Vector PulseWidth Modulation of a three level inverter provides the additional advantage of superiorharmonic quality and larger under-modulation range that extends the modulation factor to90.7% from the traditional value of 78.5% in Sinusoidal Pulse Width Modulation.An adjustable speed drive (ASD) is a device used to provide continuous range processspeed control (as compared to discrete speed control as in gearboxes or multi-speedmotors). An ASD is capable of adjusting both speed and torque from an induction orsynchronous motor. An electric ASD is an electrical system used to control motor speed.ASDs may be referred to by a variety of names, such as variable speed drives, adjustablefrequency drives or variable frequency inverters. The latter two terms will only be used torefer to certain AC systems, as is often the practice, although some DC drives are alsobased on the principle of adjustable frequency. Figure1.1): Comparison of range process speed control 2
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2.1 Latest Improvements• Microprocessor-based controllers eliminate analog, potentiometer-based adjustments.• Digital control capability.• Built-in Power Factor correction.• Radio Frequency Interference (RFI) filters.• Short Circuit Protection (automatic shutdown).• Advanced circuitry to detect motor rotor position by sampling power at terminals, ASDand motor circuitry combined to keep power waveforms sinusoidal, minimizing powerlosses.• Motor Control Centers (MCC) coupled with the ASD using real-time monitors to tracemotor-drive system performance.• Higher starting torques at low speeds (up to 150% running torque) up to 500 HP, involtage source drives.• Load-commutated Inverters coupled with synchronous motors. (Precise speed control inconstant torque applications.Adjustable speed drives are the most efficient (98% at full load) types of drives. They areused to control the speeds of both AC and DC motors. They include variablefrequency/voltage AC motor controllers for squirrel-cage motors, DC motor controllersfor DC motors, eddy current clutches for AC motors (less efficient), wound-rotor motorcontrollers for wound-rotor AC motors (less efficient) and cycloconverters (lessefficient).Pulse Width Modulation variable speed drives are increasingly applied in many newindustrial applications that require superior performance. Recently, developments inpower electronics and semiconductor technology have lead improvements in powerelectronic systems. Hence, different circuit configurations namely multilevel invertershave become popular and considerable interest by researcher are given on them.Variablevoltage and frequency supply to a.c drives is invariably obtained from a three-phasevoltage source inverter. A number of Pulse width modulation (PWM) schemes are usedto obtain variable voltage and frequency supply. The most widely used PWM schemes 3
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for three-phase voltage source inverters are carrier-based sinusoidal PWM and spacevector PWM (SVPWM). There is an increasing trend of using space vector PWM(SVPWM) because of their easier digital realization and better dc bus utilization. 4
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3 SPACE VECTOR PWM:-3.1 THEORY:-3.1.1 THREE-LEG VOLTAGE SOURCE INVERTERThe topology of a three-leg voltage source inverter is shown in Fig. 2.1. Becauseof the constraint that the input lines must never be shorted and the output current mustalways be continuous a voltage source inverter can assume only eight distinct topologies.These topologies are shown on Fig. 2.2. Six out of these eight topologies produce anonzero output voltage and are known as non-zero switching states and the remainingtwo topologies produce zero output voltage and are known as zero switching states. 5
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3.1.2 VOLTAGE SPACE VECTORSSpace vector modulation (SVM) for three-leg VSI is based on the representationof the three phase quantities as vectors in a two-dimensional (a,b ) plane. Consideringtopology 1 of Fig. 2.2, which is repeated in Fig. 2.3 (a) we see that the line voltages Vab,Vbc, and Vca are given byThis can be represented in the a,b plane as shown in Fig. 2.3(b), where voltages Vab,Vbc, and Vca are three line voltage vectors displaced 120° in space. The effective voltagevector generated by this topology is represented as V1(pnn) in Fig. 2.3(b). Here thenotation „pnn‟ refers to the three legs/phases a,b,c being either connected to the positivedc rail (p) or to the negative dc rail (n). Thus „pnn‟ corresponds to „phase a‟ beingconnected to the positive dc rail and phases b and c being connected to the negative dcrail 7
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Proceeding on similar lines the six non-zero voltage vectors (V1 - V6) can beshown to assume the positions shown in Fig.2.4. The tips of these vectors form a regularhexagon (dotted line in Fig. 2.4). We define the area enclosed by two adjacent vectors,within the hexagon, as a sector. Thus there are six sectors numbered 1 - 6 in Fig. 2.4. 8
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Considering the last two topologies of Fig. 2.2 which are repeated in Fig. 2.5(a)for the sake of convenience we see that the output line voltages generated by thistopology are given byThese are represented as vectors which have zero magnitude and hence arereferred to as zero-switching state vectors or zero voltage vectors. They assume theposition at origin in the a,b plane as shown in Fig. 2.5(b). The vectors V1-V8 are calledthe switching state vectors (SSVs). 9
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3.1.3 SPACE VECTOR MODULATIONThe desired three phase voltages at the output of the inverter could be representedby an equivalent vector V rotating in the counter clock wise direction as shown in Fig.2.6(a). The magnitude of this vector is related to the magnitude of the output voltage (Fig.2.6(b)) and the time this vector takes to complete one revolution is the same as thefundamental time period of the output voltage.Let us consider the situation when the desired line-to-line output voltage vector Vis in sector 1 as shown in Fig. 2.7. This vector could be synthesized by the pulse-widthmodulation(PWM) of the two adjacent SSV‟s V1(pnn) and V2 (ppn), the duty cycle of 11
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each being d1 and d2, respectively, and the zero vector ( V7(nnn) / V8(ppp) ) of dutycycle d0 :where, 0 m 0.866, is the modulation index. This would correspond to a maximumline-to-line voltage of 1.0Vg, which is 15% more than conventional sinusoidal PWM asshown ]All SVM schemes and most of the other PWM algorithms [1,4], use above equationsfor the output voltage synthesis. The modulation algorithms that use non-adjacent SSV‟shave been shown to produce higher THD and/or switching losses and are not analyzedhere, although some of them, e.g. hysteresis, can be very simple to implement and canprovide faster transient response. The duty cycles d1, d2, and d0, are uniquely determinedfrom Fig. 2.7, and above equations, the only difference between PWM schemes that useadjacent vectors is the choice of the zero vector(s) and the sequence in which the vectorsare applied within the switching cycle. 12
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The degrees of freedom we have in the choice of a given modulation algorithmare:1) The choice of the zero vector - whether we would like to use V7(ppp) or V8(nnn) or both,2) Sequencing of the vectors3) Splitting of the duty cycles of the vectors without introducing additionalcommutations.Four such SVM algorithms are considered in the next section, namely:1) The right aligned sequence ( SVM1)2) The symmetric sequence (SVM2)3) The alternating zero vector sequence ( SVM3)4) The highest current not switched sequence (SVM4) . 13
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4 SIMULATION of SPWM Simulation tools used – 1. SCILAB 2. PSIM 3. MATLAB Simulink 14
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5 ADVATAGES:- 1. Average value proportional to duty cycle,D 2. Low power used in transistors used to switch the signal fast switching possible due to MOSFETS and power transistors at speeds in excess of 100 kHz 3. Digital signal is resistant to noise 4. Less heat dissipated versus using resistors for intermediate voltage values 16
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6 CONCLUSIONSpace vector modulation and the control of multilevel power electronicsconverters is really the result of generalizations and insights gained from the theoreticalstudy of a practical problem, such as the design of a power conditioning system forsuperconductive magnetic energy storage. The engineering challenge in designing thepower conditioning system (or more precisely, in the design of the digital controller forthe power conditioning system) was to enable the power conditioning hardware to controlthe bi-directional power flow between the magnet and the utility line. In fact, thechallenge was to control the power conditioning system in order to provides stable,controllable and high-quality power on both the magnet and utility sides.The research in the application of the new fast SVM algorithm still has potentially veryinteresting research opportunities. Perhaps the most immediate research topic can befound in the attempt to generalize the algorithm to converters with more than three phasesby exploiting the hidden symmetries in the higher-dimensional spaces of the more theseconverters. Another interesting opportunity can be found in the challenge to generalizeand organize the conversion of the switching vectors back into the switching states. 17
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7 REFERENCES 1. A. Nabae, I. Takahashi, H. Akagi. “A neutral-point clamped PWM inverter”, IEEE Trans. on I.A., Vol.IA-17, No. 5, 1981, pp. 518-523. 2. Cataliotti A, Genduso F, Ricco Galluzzo G. A New SVM Algorithm for VSI based electrical drive fed by Photovoltaic arrays” EERR (Electrical Engineering Research Report) Naples, April 2002; 3. J. Holtz, “Pulse width modulation for electronic power conversion,” Proc. IEEE, vol. 82, pp. 1194–1214, Aug. 1994. 4. Power Electronics by Dr. P.S. Bimbhra. Khanna Publishers, New Delhi, 2003. 3rd Edition. 5. Modern Power Electronics and AC Drives, by Bimal K. Bose. Prentice Hall Publishers, 2001 6. A Power Electronics Handbook by M.H. Rashid. Academic Press 2001. 7. ww.mathworks.com. 18