Study and analysis of THD and content of Harmonics in Three Phase PWM Inverter with Filters.

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1 Study and analysis of THD and content of Harmonics in Three Phase PWM Inverter with Filters. Abstract- In this paper a traditional three phase inverter drive is constructed with RL load. The inverter and converter system produce harmonics which are induced into the load, in most applications load is a motor. These harmonics can be reduced using many techniques. Filters are generally used to reduce the harmonics. A band pass filter is applied with inductor and capacitor as the passive elements. The filter is tuned to the required frequency and limits the harmonics produced by the inverter at the output. The harmonics induced into the load is reduced drastically, hence the THD of the output is also reduced. The Harmonics and THD of drive with and without filter are compared and presented with the results. Keywords Harmonics, Bandpass filter, THD, three phase Inverter Drive. I. INTRODUCTION The three phase inverter drives are mainly developed to control the speed of the three phase induction motor, the Three phase induction motor has been a work horse of the industries. In most of the induction motor applications the speed of the motor is required to be controlled, this can be done by v/f control method. The PWM inverters are the best choice to control the speed of the induction motor. The major disadvantage of using inverters and converters system produces harmonics, This reduces the life span of the induction motor, hence these harmonics are reduced or suppressed at the output of the PWM Inverter. Many techniques are employed to minimize the harmonics at the PWM inverter are Modulation techniques, different conduction modes, filters are widely used methods to suppress the harmonic content in the output of the inverter. The harmonics are the unwanted currents or voltages that are multiples of the frequencies overlap on the frequency, these Harmonics may cause cables to overheat, damaging their insulation. In industrial applications Induction motors are widely used as the load, the harmonic content at the output of the inverter may heat the stator windings of the motor drastically reduce the life span of the induction motor, and also become noisy and torque oscillations in the rotor can 1 Rajesh B, 2 Manjesh Department of Electronic Science Bangalore University Bangalore, Karnataka, India 1 rajeshb0201@gmail.com, 2 manjesh1972@gmail.com overheat in the most severe cases, the risk of explosion as the dielectric breaks down. Many other faults towards the control or input side of the Inverter, the Electronic displays and lighting may flicker, circuit breakers can trip, computers fail and meters give false readings. In AC motor drives speed can be controlled by using variable frequency supply. Ideally sinusoidal waveforms should be used, generating a variable frequency sinusoidal waveform is very complex and expensive, since a large number of switching elements would be required practically, The out put of the inverter is a symmetrical AC output voltage of desired magnitude and frequency. The output of practical inverters contains harmonics, these harmonics will cause heating of the motor drastically reduces the life time of induction motor and finally breaks down. Harmonic is defined as a sinusoidal component of a periodic wave having a frequency that is an integral multiple of the frequencies. For example, a component, the frequency of which is five times the frequency called a 5 th harmonic. The theoretical maximum amplitude of each harmonic current produced by a converter is equal to that of the component divided by the harmonic order. For example, the 5 th harmonic is equal to 20 the load current and the 7 th harmonic is equal to 14.3% and so on. These values are for an idealized square wave and, in practice, this will be less because of system impedance. The harmonic components are assumed to be in phase with the. The resulting wave shape will depend on the magnitude and the phase relation of each of the harmonic components. The magnitude of voltage distortion of a power system that can be withstand is dependent upon the equipment connected to it and this equipment s susceptibility to non sinusoidal wave shapes. Power utility companies may be more stringent or relaxed in their specifications for the THD, as a rule of thumb, higher the voltage level, the more stringent the harmonic limitations requirements. It is, therefore, necessary to limit the harmonic voltage and current, this may have substantial impact on the drive and filter design. lead to mechanical resonance and vibration. Capacitors 12

2 II. TRADITIONAL INVERTER DRIVE A. Inverter Drive The Traditional inverter drive or PWM drive has 6 switches. The Fig.1 shows the circuit diagram of the traditional inverter drive. The IGBT s are used as switches, gating signal to each IGBT is provided through a pulse generator. Each phase is120 0 out of phase with each other. The IGBT-2 is turned on with a phase delay of with respect IGBT-1, similarly IGBT-3 is tuned on with a phase delay of 120 with respect IGBT-2. The IGBT s-1,3,5 are the upper switches and IGBT s-2,4,6 are the lower switches. A dead time is provided between upper and lower IGBT s to avoid short circuit, at any instant of time no two switches in the same leg can be ON. The switching pattern of the switches are ON, one from the lower group and two from the upper group or one from the upper group and two from the lower group are ON. This produces a three phase output power used to drive the load. The low pass and the high pass filter can be designed shown in the equation.1. = (1) The cut off frequencies of low pass and high pass filter are known. Either L or C is kept constant, the other element is calculated. The filter designed for 5Hz is with cut off frequencies of 2hz and 7hz. By keeping C constant. The filter designed for 50Hz is with cut off frequencies of 47hz and 53hz. By keeping C constant. Table 1 shows the values of both L and C for different filters. Table:1:Capacitor and inductor values for the filters. Type of Filter Capacitor value Inductor value Low Pass(5Hz) 1000µF 6.327H High Pass(5Hz) 1000µF H Low Pass(50Hz) 1000µF 11.45x10-3 H High Pass(50Hz) 1000µF 9.01x10-3 H. The Filter is inserted between the inverter output and the load, as shown in Fig.3. Fig:3: Filter Inserted between load and Inverter. The calculated L and C values are assigned to the RL block in simulink. The circuit diagram of the inverter drive with filter is as shown in Fig.4. Fig:1: PWM Inverter drive Circuit diagram B. Band pass filter The basic blocks of a Band pass filter are as show in Fig.2. Fig:2: Block Diagram of Band-pass Filter Many applications the filters are major advantage over other techniques, a particular band, or spread, or frequencies need to be filtered from wider range harmonic spikes. Filter circuits can be designed to accomplish this task by combining the properties of lowpass and high-pass into a single filter,is a band-pass filter. Some band pass filters require an external source of power and employ active components such as transistors and integrated circuits, these are known as active band pass filters. Other band pass filters use no external source of power and consist only of passive components such as capacitors and inductors, these are called passive band pass filters. Fig:4: Circuit diagram of Traditional Inverter drive with Band pass Filter. 13

3 L11,C11 constitute the Low pass filter and C12,L12 constitute High pass filter. The 3 phase load constitute the resistive and inductive load. III. RESULTS The Fig.8 shows the percentage of harmonics produced by the inverter drive with filter with 5Hz input frequency. The THD of both experiments with 5Hz input frequency is simulated through a simulator are as shown in Table.2. The simulation work has been done in two stages for two different frequencies for 5Hz and 50Hz to observe the harmonic content and THD of three phase Inverter drive. The traditional inverter drive or 2-level inverter drive is used to drive the 3 phase resistive and inductor load. The harmonics and the THD obtained by simulation using matlab simulator, the harmonic orders and % age of THD are recorded. The output of the inverter current and voltage wave form is as shown in Fig.5. Fig:7: Percentage of harmonics produced by the Inverter Drive with 5Hz input frequency. Fig:5: Line current and Line to neutral voltage waveform with 5Hz input frequency The pulse generator is programmed to 5Hz output. The output of the inverter is also 5Hz. The output of the inverter drive with band-pass filter is as shown in Fig.6. Fig:6: Line current and Line to neutral voltage waveform with band pass filter with 5Hz input frequency. Using band pass filter the output of the three phase Inverter is more sinusoidal than the normal drive is observed. The harmonics produced by the inverter with 5Hz input frequency are plotted as shown in Fig.7. Fig:8: Percentage of harmonics produced by the Inverter Drive with filter with 5Hz input frequency. Table:2:THD with and without filter in the circuit 14 Mode THD Without Filter (Normal drive) 30.7 With Filter The simulation work has been extended to study at the rated frequency of 50Hz, The output of the inverter current and voltage wave form is as shown in Fig.9. The output current and voltage of the inverter drive with band pass filter is as shown in Fig.10.

4 with input frequency of 50Hz are plotted as shown in Fig.12. Fig:9: Line current and Line to neutral voltage waveform with 50Hz input frequency Fig:11: Percentage of harmonics produced by the Inverter Drive with 50Hz input frequency. The THD of both experiments with 50Hz input frequency is simulated through a simulator are as shown in Table.3. Table:3:THD with and without filter in the circuit Fig:10: Line current and Line to neutral voltage waveform using band pass filter with 50Hz input frequency Fig:11: Percentage of harmonics produced by the Inverter Drive with 50Hz input frequency. The harmonics produced by the inverter with input frequency of 50Hz are plotted as shown in Fig.11. The harmonics produced by the inverter with band pass filter Circuit THD Without Filter With Filter 1.25 Table:4: Comparison of Harmonics with and without Band Pass filter for 5Hz input Frequency. Harmonic order without filter with filter The Table.4 shows 5 th, 7 th,11 th and 13 th harmonics magnitudes has been reduced drastically. It can be 15

5 concluded that the band-pass filter suppresses the harmonic content in the output of the inverter drive. Table:5: Comparison of Harmonics with and without Band Pass filter for 50Hz input Frequency. Harmonic order without filter with filter The Table.5 shows 5 th, 7 th and 11 th harmonics magnitudes has been reduced drastically. It can be concluded that the band-pass filter suppresses the harmonic content in the output of the inverter drive in both the cases. IV. CONCLUSION The study of Harmonics and its THD of three phase PWM inverter has been simulated.the percentage of Harmonics present at the output of the inverter drive is obtained. A Band-pass filter is designed to eliminate the dominant 5 th harmonic at the output of the inverter and studied its THD at the output of the three phase PWM inverter and the result obtained with and without band pass filter. The study is carried out at two different frequencies 5Hz and 50Hz. The results have been compared and it is found that the THD using band pass filter used at the output of the three phase PWM inverter is found to be less as shown in the results. REFERENCES [1] T.Prathiba*, P.Renuga, A comparative study of Total Harmonic Distortion in Multi level inverter topologies. Journal of Information Engineering and Applications,ISSN (print) ISSN (online) Vol 2, No.3, 2012.P: [2] Mahrous Asmed, Saad Mekhilef, A Three Phase Three-level Voltage Source Inverter with a Three-phase Two-Level Inverter as a Main Circuit. Pg [3] M. J. Meco-Gutiérrez (1), A. Ruiz Gonzalez (1), F. Vargas-Merino (1), J.R. Heredia-Larrubia (2), Reduction in induction motor heating fed by a new PWM technique: results obtainedin laboratory experiments. [4] M. Shuja Khan*, I. Intesar, M. S. Raheel, M. Babar Ali, U. Asad, M. Farid, U. Ahmed, P. Ali Ayub, Implementation of a Passive Tune Filter to Reduce Harmonics in Single Phase Induction Motor with Varying Load. International Journal of Engineering & Technology IJET-IJENS Vol: 11 No: 03.pg [5] Ying-Tung Hsiao, Design of Filters for Reducing Harmonic Distortion and Correcting Power Factor in Industrial Distribution Systems. Tamkang Journal of Science and Engineering, Vol. 4, No. 3, pp (2001),pg: [6] Prof. Dr. Suhail. A. Qureshi, Ahmed Hassan, Azeem Talib, Design and Simulation of Harmonic Filters Using MATLAB Software. pg:1-6 [7] Seema P. Diwan, Dr. H. P. Inamdar, and Dr. A. P. Vaidya, Simulation Studies of Shunt Passive Harmonic Filters: Six Pulse Rectifier Load Power Factor Improvement and Harmonic Control. ACEEE Int. J. on Electrical and Power Engineering, Vol. 02, No. 01, Feb 2011,pg:1-6 [8] Kuldeep Kumar Srivaztava, Saquib Shakil, Anand Vardhan Pandey, Harmonics & Its Mitigation Technique by Passive Shunt Filter. International Journal of Soft Computing and Engineering (IJSCE) ISSN: , Volume- 3, Issue-2, May 2013,pg: [9] M. A. Latif1, M. J. Alam2, M. A. Rashid*3, A. Karim3, N. H. Ramly3, I. Daut3, Microcontroller based PWM Inverter for Speed Control of a Three Phase Induction Motor. M. A. Latif et al. / International Journal of Engineering and Technology (IJET), Vol 5 No 2 Apr-May 2013.psg: