Volume-2, Issue-5, May-214 COMPARATIVE HARMONIC ANALYSIS OF VSI FED INDUCTION MOTOR DRIVE 1 NIKHIL D. PATNE, 2 SUSHANT S. ANGRE, 3 MONALISA DASH Student of Electrical Engineering Mumbai University, Student of Electrical Engineering, Mumbai University, Faculty of Electrical Engineering Mumbai University Email: nikhildpatne@gmail.com, sushantangre91@gmail.com, dash.monalisa@gmail.com Abstract This paper mainly focus on harmonics analysis of three phase AC supply available from three phase inverter which is fed to induction motor drive. Ideally the output of inverter should be sinusoidal but due to harmonics the output is having distorted waveforms. Thus, three different types of PWM inverter are used in this paper for comparative study of Harmonic analysis. Harmonic analysis has been done by using the MATLAB Simulink / FFT Keywords Duty Cycle, Pulse Width Modulation, Harmonics I. INTRODUCTION Nonconventional energy resources are very suitable solution for the modern requirement of energy demand. For the future energy demands and gives quality pollution free supply to the growing and today s environment conscious population, the present world attention is to go in the natural, clean and renewable energy sources. Thus solar energy is considered under this paper. Now a days about 6% of total load is motor load & 9% of this is induction motor load because AC motors are preferred on DC motor for industrial application. This is because construction of DC motor is very complicated due to use of commutator and brushes. Also commutation process involves losses due to arcing. Hence induction motor drive gives satisfactory performance as compared to DC motor drive. Energy comes from photovoltaic cell is dc before converting it in ac firstly dc is step up by Boost converter. MOSFET is used as switching devices in boost converter and it has very high switching frequency of range 2 KHz. To get pure DC voltage in output of boost converter a DC π- filter is used. But as induction motor is non linear load it introduce harmonics in supply. These harmonics can be reduce by using different strategies of PWM inverter. Under this paper such different strategies are studied comparatively to find best option to reduce THD. II. PULSE WIDTH MODULATED INVERTER In many industrial applications, PWM inverters are gradually taking over other types of inverter. PWM techniques are characterised by constant amplitude pulses. The width of these pulses is however modulated to obtain inverter output voltage control and to reduce its harmonic content. Different PWM techniques are as under:- (i) Single-pulse modulation. (ii) Multiple-pulse modulation. (iii) Sinusoidal pulse modulation amongst the various techniques of PWM, the sinusoidal PWM is very good and the most popular that gives smooth changeover of V/f, harmonic elimination, four quadrant operation, etc in both open and closed loop applications. Under this paper three different strategies of Sinusoidal-Pulse Width Modulated Inverters are considered and following are the two techniques used in the same. i. Multipulse PWM ii. Multilevel PWM In PWM inverters, forced commutation is essential. The different PWM techniques listed above differ from each in the harmonic content in their respective output voltage. Thus, choice of a particular PWM technique depends upon the harmonic content in the inverter output voltage. In industrial applications, PWM inverter is supplied from a diode bridge rectifier and an LC filters A. Multipulse PWM It is an mosfet inverters having pulse number greater than six. Generally the pulse number is a multiple of 6 (assuming 3-phase system) and 12, 18, 24-pulse and so on circuits are made possible. An 12-pulse inverter, for example, consists of two 6-pulse inverters fed from a DC supply and connected to linear transformer which gives 3-phase output (18- pulse has three 6-pulse circuits and so on). The advantage of using the two 6-pulse circuits like this, rather than just connecting them in parallel (or series), harmonic cancellation takes place and the power quality at the input and output is improved. a. 6-pulse PWM Inverter 17
Fig 1 Circuit Diagram of 6-Pulse Inverter The inverter is fed by a dc voltage and has three phase-legs each consisting of two MOSFETs and two Diodes. A common inverter control method covered in sine-triangle pulse width modulation (SPWM) control. With SPWM control, the switches of the inverter are controlled by comparison of a sinusoidal control signal and a triangular switching signal. The sinusoidal control waveform establishes the desired fundamental frequency of the inverter output, while the triangular waveform establishes the switching frequency of the inverter. The ratio between the frequencies of the triangle wave and the sinusoid is referred to as the modulation frequency ratio. The switches of the phase legs are controlled based on the following comparison: Volume-2, Issue-5, May-214 When looking at 12- pulse inverter schematically, it becomes readily apparent that the higher pulse numbers are achieved by combining 6 pulse inverters so that their outputs are skewed from each other. By skewing multiple 6-pulse inverters, relative to one another, the 6- pulses become intertwined. This is most commonly done using phase shifting transformers. Two transformers, identical in all other regards, yet with delta windings or wye windings, exhibit a 3 phase shift. By means of even more winding configurations, almost all phase shift can be achieved. The input and the output characteristics can be improved by increasing pulse numbers, but makes circuit more complex. As number of components increases. Any statistical analysis of system reliability dictates that increased component count translates to lowers overall reliability. B. Multilevel PWM Numerous industrial applications have started to acquire higher power apparatus in recent years. Some medium voltage motor drives and utility applications require medium voltage and megawatt power level. For a medium voltage grid, it is troublesome to connect only one power semiconductor switch directly. As a result, a multilevel power converter structure has been introduced as an alternative in high power and medium voltage situations. A multilevel converter not only achieves high power ratings, but also enables the use of renewable energy sources. Renewable energy sources such as photovoltaic, wind, and fuel cells can be easily interfaced to a multilevel converter system for a high power application Fig 2 6-Pulse Sinusoidal PWM b. 12-Pulse PWM Inverter I. 3-Level PWM Inverter In order to create a signal which is closer to a true sine wave, a 3 level PWM signal can be generated with high, low, and zero voltage levels. For the resulting 3-level PWM signal to correspond to a sine wave, the signal comparison stage must also be 3-level. A triangle wave is used as it is in the 2-level PWM comparison, but it half the amplitude and summed with a square wave to compare one half of the sine reference signal at a time. The resulting PWM signal is used to control one half of an H-bridge, which controls how long the bus voltage is allowed through to the load. The other half of the H-bridge controls the polarity of the voltage across the load, and is controlled by a simple square wave of the same frequency and in phase with the sine signal. Generally, this square wave can simply be created in a stage of the sine wave generation circuit. One Sample output of 3-Level PWM Inverter is shown in Fig 3 Circuit Diagram of 12-Pulse Inverter 18
Volume-2, Issue-5, May-214 Fig(4) III. SIMULATION RESULTS Fig 4 3-Level Pulse Width Modulation II. 5-Level PWM Inverter In order to create a PWM signal which more closely follows the desired sine wave output, the design described for the 3-level PWM technique can be expanded to 5-, 7- and 9+ level PWM. Each additional 2 levels added on top of the 3-level design adds an H-bridge (added in series), a comparator, and a summer. The added accuracy to the signal due to increase in the level results additional components, space, cooling arrangements, and power requirement. One of the advantages of higher level PWM generation is that there is less of a voltage swing from the minimum and maximum of each step, which results in less power loss due to the slope up and down for each step (known as dv/dt losses). This reduced power loss results in higher efficiency for the inverter. This increased efficiency must be considered in balance with the addition of components and frequency effects which must be filtered out. Sample output of 5-Level PWM Inverter is shown in Fig(5) The simulink model consists of supply, pi filter, PWM inverter, LC filter (Lowpass) and Induction motor as load. The voltages and current at different point of simulink model can be obtained from V-I measurement block from there filtered value of inverter line voltage is obtained. The simulink block diagram is designed to obtain the different waveforms. For the satisfactory performance of a drive output should be ripple free so that the input taken from the inverter after filter (stator current) is analyzed for harmonics. Firstly, the harmonics are reduced by pulse width modulation (PWM) technique. Then by the use of filters ripples are reduced at the output. For the comparative study the Simulink model, output of simulations and FFT window is shown for three different models as below :- a. 6-Pulse 3-Level PWM Inverter b. 6-Pulse 5-Level PWM Inverter c. 12-Pulse 3-Level PWM Inverter A. 6-Pulse, 3-Level PWM Inverter Fig 6 Simulink Block Diagram of VSI Fed Induction Motor Drive with 6-Pulse PWM Inverter The average output voltage of boost converter is found approximately same to the rated value of output voltage of boost converter. The output of boost converter is not pure. 8 7 6 Output Voltage without Filter 5 4 3 2 1 Fig 5 5-Level Pulse Width Modulation.1.2.3.4.5.6.7.8.9.1 Fig 7 Boost Converter Output without Filter 19
Volume-2, Issue-5, May-214 For getting a smooth dc an pie (π) filter is used at the output side of boost converter. Due to this filter output voltage of boost converter becomes approximately smooth and very less ripples are present at the output voltage. 8 As the Induction Motor itself introduces harmonics, harmonics are present at the supply of motor. Thus THD of Fig 9 is 91.64%, THD of Fig 1 is.29% and THD of Fig 11 is 1.81%. This THD can be improve by using 6-Pulse 5-Level PWM Inverter. 7 6 B. 6-Pulse 5-Level PWM Inverter Output Voltage with Filter 5 4 3 2 1.1.2.3.4.5.6.7.8.9 1 Fig 8 Boost Converter Output with Filter Then pure dc is fed to 6-pulse PWM inverter so that it is converted to ac, which is required for induction motor. 1 Vab inverter 8 6 4 2-2 -4-6 -8-1.2.4.6.8.1.12.14.16.18.2 Fig 9 6-Pulse 3-Level PWM Inverter Output without Filter Further LC filter is connected to filter the harmonics as it is lowpass filter and sinewave is generated which is near to pure. 6 Fig 12 Simulink Block Diagram of VSI Fed Induction Motor Drive with 6-Pulse 5-Level PWM Inverter This model is similar to previous one the only change is that two carrier signals are summed to perform the 5-Level PWM technique. Thus the improvement at the output of PWM inverter, LC filter and stator current are as follows. 4 Vaa inverter Inverter output voltage with Filter 2-2 -4-6.2.4.6.8.1.12.14.16.18.2 Fig 1 Inverter Output voltage with Filter Inverter output voltage without filter 6 4 2-2 -4 Fig 11 shows the waveform for the supply current of induction motor. -6.1.2.3.4.5.6.7.8.9.1 Fig 13 6-Pulse 3-Level PWM Inverter Output without Filter 5 <Stator current is_a (A)> 6 4 3 4 2 Supply Current of Induction Motor 1-1 -2 Inverter output voltage with filter 2-2 -3-4 -4-5.1.2.3.4.5.6.7.8.9 1 Fig 11 Induction Motor Supply Current (Stator Current) -6.2.4.6.8.1.12.14.16.18.2 Fig 14 Inverter Output voltage with Filter 2
Volume-2, Issue-5, May-214 <Stator current is_a (A)> 6 6 4 Supply c urrent of motor(s tator current) 2-2 -4-6 I n v e r t e r o u t p u t v o l t a g e w i t h f i l t e r 4 2-2.1.2.3.4.5.6.7.8.9 1 Fig 15 Induction Motor Supply Current(Stator Current) -4 Thus THD of Fig 13 is 58.47%, THD of Fig 14 is.2% and THD of Fig 15 is 1.43%. This THD can be further improved by using 12-Pulse 3-Level PWM Inverter. C. 12-Pulse 3-Level PWM Inverter -6.2.4.6.8.1.12.14.16.18.2 Fig 18 Inverter Output voltage with Filter <Stator current is_a (A)> 5 4 3 S u p p ly c urre nt o f m o tor(s ta to r c u rre n t ) 2 1-1 -2-3 -4-5.1.2.3.4.5.6.7.8.9 1 Fig 19 Induction Motor Supply Current(Stator Current) Thus THD of Fig 17 is 7.79%, THD of Fig 18 is.2% and THD of Fig 19 is.28%. RESULT Fig 16 Simulink Block Diagram of VSI Fed Induction Motor Drive with 12-Pulse 3-Level PWM Inverter In this Simulink model two universal Bridges are use to perform 12-Pulse operation to reduce the THD and thus output changes are as follows. I n v e r te r o u t p u t v o l t a g e w i t h o u t fi l te r 6 4 2-2 -4 Vaa inverter -6.1.2.3.4.5.6.7.8.9.1 Fig 17 12-Pulse 3-Level PWM Inverter Output without Filter From the above simulation results we can find out amplitudes of different harmonics and THD for three different strategies of PWM inverter and perform the comparative study of Harmonic contents for the same. Comparative result with percentage amplitude of fundamental is shown in following table Harmoni c Number 6-Pulse 3-Level PWM inverter Table I 6-Pulse 5-Level PWM inverter 12-Pulse 3-Level PWM inverter 2 1.75 1.3.2 3.2.3.1 4.25.35.7 5.1.3.5 7.15.2.4 9.1.1.2 11.1.1.2 13.1.1.2 15.1.1.2 THD 1.81 % 1.45 %.28 % 21
CONCLUSION In this paper the model circuit has been simulated and various waveforms of output voltage and current have been analyzed. The practical value of voltage, current, speed and torque is same to the rated value of induction motor drive. The waveform of inverter output voltage is not pure sinusoidal because of presence of harmonics. The THD is analyzed from the FFT analysis for three different strategies of PWM inverter, as mentioned in the result. These harmonics are generated in two manners. The power semiconductor device which are used as switching devices in the inverter and the nonlinear load (induction motor drive) are the sources of harmonics. Due to nonlinear load inter harmonics also come in inverter output voltage. Thus from THD values which are mentioned in the result, it is known that, if we increase the level of PWM inverter from three level to five level, THD is improved from 1.81% to 1.45%. But if 12-Pulse inverter is used instead of 6-Pulse inverter then THD is improved from 1.45% to.28%. Thus for improvement of THD it is better to improve pulse of inverter rather than using level of inverter. But as 12-Pulse inverter requires two, 6-Pulse inverter it increases the cost of equipment. Volume-2, Issue-5, May-214 Thus from the above analysis it is understood that, 12-Pulse 3-Level inverter works better on THD improvement with increased cost, whereas 6-Pulse 5-Level inverter provides optimal results with regard to both THD improvement as well as cost and size of equipment. Thus for THD improvement it is better to increase number of pulses rather than increasing number of levels. REFERENCES [1] Manjari Mehrotra, Dr. A. K. Pandey Harmonics Analysis of VSI Fed Induction Motor Drive IJEIT 212 [2] B. Biswas and P. Purkait Current Harmonic analysis of inverter fed induction motor drive system under fault condition IMECS29. [3] Rasagna Golla HARMONIC ANALYSIS OF PWM INVERTER AND PWM INVERTER FED INDUCTION MOTOR S.R.M. ENGINEERING COLLEGE May29 [4] Ian F. Crowley, Ho Fong Leung PWM Techniques: A Pure Sine Wave Inverter 21-211 Worcester Polytechnic Institute Major Qualifying Project [5] M. D. Singh, K.B. Khanchandani Power Electronics Textbook, McGraw-Hill publication. [6] Ken Michaels, Fundamental of Harmonics Electrical construction and maintenance, June 1, 1999. [7] Narain G. Hingorani, Laszlo Gyugyi Understanding FACTS Textbook, Wiley-India Edition. 22