International Journal of Applied Sciences, Engineering and Technology Vol. 03, No. 01, Jan-Dec 2014 Harmonics Analysis of Multilevel Inverter based Single Phase AC-DC-AC Converter KR. DHEIVARAYAN, N. PRIYA Department of Electrical and Electronics Engineering, SASTRA University, Tamil Nadu, India Email: dheivarayankr@gmail.com, priyalakshanya@gmail.com Abstract: This paper introduces a new topology for multilevel inverter based single phase AC-DC-AC converter for different loads. The proposed converter consists of two stages; a controlled rectifier converting AC supply to DC supply and a multilevel inverter converting DC supply to AC supply cascaded to the rectifier. Multilevel inverter is preferred for the proposed system, since it increases the number of output voltage level and hence the Total Harmonic Distortion (THD) is reduced. Thus the different types of loads R, RL and Motor loads are fed with less harmonic AC voltage which will increase the overall efficiency of the system. Closed loop control is done for controlling the output voltage of the inverter using PI Controller. Overall system is simulated using MATLAB/SIMULINK. Keywords: Multilevel inverter, AC-DC-AC converter, Total Harmonic Distortion I. Introduction: Single phase AC-DC-AC converters are employed in large number of applications such as Motor Drives, Yaw Drives Traction and Micro Grids. Conventional AC-DC-AC converters which are widely used for power utility and drive applications are the indirect ac-ac converters. It is necessary to concentrate on adjustable speed drives. The converter connected to the source is a voltage source rectifier and the load side converter is a voltage source PWM inverter. The DC bus is provided between the rectifier and inverter parts of the drive. The ripple content in the output voltage of rectifier must be removed before any transistor switches “on”. If not, this distortion will show up in the output to the load. The inverter part is made up of group of transistor and diode combinations. This converts the DC supply again to AC. The transistor devices of the inverter are switched on and off many times for every half cycle, hence a pseudo sinusoidal current waveform is generated. Figure 1.1 Conventional Single Phase AC-DC-AC Inverter Normally the induction motor drive with performance improved AC-DC-AC converter must have the properties like: In the voltage source Inverter (VSI) side, mainly it should have significant torque and flux operation and maximum output torque for given range of speed operation [1]. In the rectifier fed DC side there should be Bi-Directional power flow, increased Total Harmonic Distortion (THD) in input and Reduction of link capacitor Range. This method of power conversion has disadvantages like, it uses an IGBT PWM inverter for DC-AC conversion, in which the ac line voltage is not pure sinusoidal, and hence there may be harmonics located at high frequencies causing more THD. In case if these inverters are used for Adjustable Speed Drives (ASD), then the voltage ranges used are low, this happens due to factors like: i. The presence of increased dv/dt in the pulse width modulated ac line voltage is not tolerable in the average to maximum voltage ranges ii. Sharing of load power by only four switches of the inverter. There are two methods to approximate near-sinusoidal voltage by utilizing four-switch inverter. i.Current Source Inverter (CSI) connected to a capacitive filter. ii.Voltage Source Inverter (VSI) which includes an inductive (L) or combination of L&C filter at the load terminals. Even though above said topologies have an advantage of producing near sinusoidal voltage waveforms, but they have a disadvantage, that load power is shared only among four power valves for a single phase inverter. So it is difficult when this converter is connected to an adjustable speed drive, since it reduces the motor performance. Hence multilevel inverter topology is used in the proposed system to reduce the dv/dt and to share load power among different switches [4]. Conventional converter also uses a diode bridge rectifier for ac-dc conversion hence control over the DC voltage is lost, but it is converted to firing angle controlled Thyristor Bridge in proposed model. A LC ripple filter is used to remove the ripples in dc output of rectifier. Then this proposed converter is used for a Single phase Induction Motor provided with an auxiliary winding for direct phase angle control [7]. IJASET 030104 Copyright © 2014 BASHA RESEARCH CENTRE. All rights reserved. KR. DHEIVARAYAN, N. PRIYA Direct phase angle control of Single phase induction motor involves the Magnitude and Phase angle control of Auxiliary winding supply of the motor in order to control the motor torque, which in turn controls the speed of the motor. This is done by generating the reference voltage for auxiliary winding based on the feedback from motor’s speed. This paper proposes a control of single phase thyristor bridge rectifier along with a new hybrid Multilevel Inverter. For the single phase loads, the output voltage of the multilevel inverter is fed back to the controller and compared with the reference voltage. Based on the load voltage requirements the gating signals are generated for the thyristors. Figure 1.2 Overall Block Diagram II. Proposed Hybrid Multilevel Inverter: The New Hybrid cascaded seven level inverter shown in Fig. 2.1. is an alternative idea for conventional cascaded multilevel inverter. The name hybrid indicates the combination of one H-bridge and two auxiliary switches with dc voltage source [2] [5]. The seven level inverter generates a output voltage such as: 0, +2Vdc/3, +Vdc/3, +Vdc, -Vdc/3, -Vdc/3,-2Vdc.The circuit configuration of the proposed Inverter consists of six power switches, one bidirectional switch and three capacitors to balance the voltage from dc supply which act as voltage divider. Table 2.1: Switching States Output voltage Conducting switches Zero Vdc/3 2Vdc/3 Vdc -Vdc/3 -2Vdc/3 -Vdc S3-S4(S1-S2) S4-S6 S4-S5 S1-S4 S2-S5 S2-S6 S2-S3 A. Multiple Reference PWM Technique The modulation strategy given for the proposed 7level hybrid inverter is Multiple Reference Modulation Technique. In this technique, three reference signals Vref1, Vref2,Vref3 are obtained by taking absolute of sine wave operating at fundamental frequency, and the reference signals below zero level is offset with a value and in phase with each other. All the three reference signals with carrier frequency (fs) have same amplitude of Am. It uses one carrier wave which is a triangular wave with amplitude of Ac operating at switching frequency (fsw). Fig.10 shows the switching pattern for the switch 1 to switch 6. Figure 2.2 Multiple Reference Modulation Technique for 1kHz The gate pulses are generated by means of comparing the 3 reference wave with a one carrier wave as shown in fig. 2.2. When Vref1 crosses the peak amplitude of Vcarrier, Vref2 will be compared with Vcarrier till ref2 crosses the amplitude of carrier signal. From that, Vref3 will take the responsibility till its amplitude crosses zero if Vref3 reaches zero, Vref2 will take the responsibility till its amplitude crosses zero then Vref1will be compared with carrier signal. The modulation index for a Multiple Reference modulation technique is given by: Figure 2.1 Proposed Hybrid Multilevel Inverter Table 2.1 describes the operation of new hybrid multilevel inverter with 7levels and also discussed the conduction of switches to produce the different levels of load voltage. Where, Am- Amplitude of reference wave Ac- Amplitude of carrier wave III. Design of LC Filter: This filter consists of inductor L in series with load and capacitor C across the load. The dominant harmonics in the output dc voltage of rectifier is blocked by the inductor. The capacitor which is connected in parallel to the inductor provides an easy International Journal of Applied Sciences, Engineering and Technology Vol. 03, No. 01, Jan-Dec 2014, pp 14-18 Harmonics Analysis of Multilevel Inverter based Single Phase AC-DC-AC Converter path to the nth harmonic ripple currents. The general formula for effective filtering is given as [8], For Voltage Ripple Factor, VRF=0.5, f=50Hz, R L=50 , LL=10mh, L&C values are calculated as, 10 C= =50 F, 2 2 R (2LL ) 2 connected in cascade to Thyristor bridge rectifier. A 230V 1 phase AC input is given to Thyristor Bridge Rectifier. By controlling firing angle of Thysistor under closed loop using PI controller, the input voltage to the load is controlled significantly. Fig. 5.1 shows the simulation of AC-DC-AC converter with R, RL and Motor load and closed loop control of output voltage. Here RMS value of the output voltage is compared with a reference voltage to generate an error. This error is synthesized using a PI controller and then given to the firing angle generator. This firing angle generator (shown in fig. 5.2.) generates the required delay angle to thyristors. =0.5 1 VRF= 2 2 3 (2 ) LC 1 IV. SIMULATION RESULTS The given AC-DC-AC converter is simulated using MATLAB/SIMULINK. Proposed 7-Level Inverter is simulated by generating the gating signals using Multi-Reference PWM technique. Then inverter is Discrete, Ts = 1e-005 s. RM S Vline powergui firing angle frequency pulse width Van 50 frequency rms signal PID Controller alpha_deg pulse PI(s) ref Step 4 Pulse Generator FIRING ANGLE GENERATOR + v - 5 pulse width g 7-level INVERTER L1 + + A - B A B + -i - Scope1 + v - C Thyristor Converter Figure 5.1 Simulation of AC-DC-AC Converter (Thyristor Bridge)-Closed Loop RM S1 Trigonometric Function1 Divide1 Gain2 1 alpha_deg -K- acos Fcn 2 ref (2*1.4144/pi)*u(1) rms signal Figure 5.2 Firing Angle Generator Vab Amplitude (V) 400 200 0 -200 -400 0.1 0.105 0.11 0.115 0.12 0.125 0.13 0.135 0.14 0.145 0.15 0.13 0.135 0.14 0.145 0.15 Iab Amplitude (A) 4 2 0 -2 -4 0.1 0.105 0.11 0.115 0.12 0.125 Time Figure 5.3 Output of Multilevel Inverter International Journal of Applied Sciences, Engineering and Technology Vol. 03, No. 01, Jan-Dec 2014, pp 14-18 1 Vline KR. DHEIVARAYAN, N. PRIYA For R load, R=100ohm, L=10mH Vab Amplitude (V) 500 0 -500 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 0.6 0.7 0.8 0.9 1 Iab Amplitude (A) 5 0 -5 0 0.1 0.2 0.3 0.4 0.5 Time FFT Analysis of output current of Converter: Figure 5.4 Simulation Output of AC-DC-AC Converter for R-Load (Thyristor Bridge)-Closed Loop For RL load, R=100ohm, L=10mH Vab Amplitude (V) 500 0 -500 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 0.6 0.7 0.8 0.9 1 Iab Amplitude (A) 5 0 -5 0 0.1 0.2 0.3 0.4 0.5 Time FT Analysis of output current of Converter Figure 5.4 Simulation Output of AC-DC-AC Converter for RL-Load (Thyristor Bridge)-Closed Loop For Single Phase Induction Motor load Vab Amplitude (V) 400 200 0 -200 -400 1 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2 1.6 1.7 1.8 1.9 2 Iab 20 10 0 -10 -20 1 1.1 1.2 1.3 1.4 1.5 Time FFT Analysis of output current of Converter: Figure 5.4 Simulation Output of AC-DC-AC Converter for Motor-Load (Thyristor Bridge)-Closed Loop International Journal of Applied Sciences, Engineering and Technology Vol. 03, No. 01, Jan-Dec 2014, pp 14-18 Harmonics Analysis of Multilevel Inverter based Single Phase AC-DC-AC Converter Figure 5.3 shows that for a change in reference voltage from 200V to 250V, there is change in output voltage of the converter. FFT analysis shows that the THD of the output current of the converter is given in table 5.1. Table 5.1 THD Comparison LOAD THD in % R 16.32 RL 3.79 Motor 8.32 V. Conclusion: Thus, a new configuration of AC-DC-AC converter is designed. The proposed converter requires less number of power switches compared to conventional model, it also achieves less total harmonic distortion and the obtained output voltage is approximately sinusoidal wave. RL and Single phase motor type of loads are discussed. Calculations of LC filter to remove ripples in output of rectifier are done. FFT analysis shows that THD for output current of R, RL and motor load is shown. Also a closed loop control to maintain required output voltage at the load is provided. This shows that the converter proposed in the paper reduces the harmonics in output voltage effectively and reduce THD in AC-DC-AC converter by utilizing Multilevel Inverter and responses are shown for different types of Load. So the proposed converter is well suited for single phase adjustable speed drives, like Yaw drives employing single phase motors and controls the axle of wind blade according to wind direction also for Traction Drives and UPS Systems. References: [1] Euzeli Cipriano dos Santos, Jr., Member, IEEE, October 2011, Cursino Brandão Jacobina, Senior Member, IEEE “Component Minimized AC–DC–AC Single-Phase to Three-Phase FourWire Converters”, IEEE Transactions on Industrial Electronics, Vol. 58, No. 10. [2] Rodriguez, J. Lai and FZ. Peng, Aug. 2002, “Multilevel converters: a survey of topologies, controls, and applications”, IEEE TransIndustrial Application, vol. 49(4), pp.724– 738. [3] B. Wu, P. G Song, 2004, "Comprehensive Analysis of Multi-Megawatt Variable Frequency Drives", Transaction of China Electromechanical Society, Vol.19, No. 8, pp.42-50. [4] B.-R.Lin and H.-H.Lu, July 1999, “Multilevel AC/DC/AC converter for AC drives”, IEE Proc.-Electr. Power Appl., Vol. 146, No. 4. [5] Gerardo Ceglia, Victor Guzman, Carlos Sanchez, Fernando Ibanez, Julio Walter, and Maria I. Gimenez Sep. 2006, “A New Simplified Multilevel Inverter Topology for DC-AC Conversion,” IEEE Trans. Power Electron., vol. 21,no. 5, pp.1311-1319,. [6] W. J. Morrill April 1929, “the Revolving field theory of the capacitor motor,” Trans. AIEE, vol. 48, pp.614-629,. [7] P.S. Bimbhra, ‘Power Electronics’, fourth edition Khanna Publishers. International Journal of Applied Sciences, Engineering and Technology Vol. 03, No. 01, Jan-Dec 2014, pp 14-18