Volume 120 No. 6 2018, 4079-4097 ISSN: 1314-3395 (on-line version) url: http://www.acadpubl.eu/hub/ http://www.acadpubl.eu/hub/ SENSOR LESS VOLTAGE CONTROL OF CHB MULTILEVEL INVERTER FED THREE PHASE INDUCTION MOTOR WITH ONE DC SOURCE PER EACH PHASE G. V. V. Nagaraju 1, G. Sambasiva Rao 2, CH Rami Reddy 3 1 Department of Electrical & Electronics Engineering, Acharya Nagarjuna University, Guntur, India 2 Department of Electrical & Electronics Engineering, RVR & JC college of Engineering, Guntur, India 3 Department of Electrical & Electronics Engineering, Nalanda Institute of Engineering & Technology, Guntur, India 1 nagaraju006@gmail.com, 2 sambasiva.gudapati@gmail.com, 3 crreddy229@gmail.com. June 20, 2018 Abstract In this paper a single DC source per phase cascaded h bridge (CHB) three phase five level inverter fed induction motor with minimum number of switches and a single capacitor is proposed. Maximum all available switching 1 4079
states are evaluated and a sensor less voltage regulating technique is suggested which controls the second bus voltage as half of the applied single DC voltage source. Voltage levels at the output are zero, the voltage across the capacitor and voltage of DC voltage source. The switching method is mixed with a simple voltage balancing technique which can be possible to implement even with small, simple microcontrollers and Simulation results exhibit the dynamic performance of this method in controlling the second bus capacitor voltage. The low harmonic desired value of five level voltage is regulated by the voltage across the capacitor. Keywords:Three phase CHB inverter; Single DC voltage source; Sensor less voltage regulating technique; Regulating capacitor; Induction motor load. 1 INTRODUCTION The world power demand in the energy market leads to redesign of power converters. The complications with the two level inverter topology are low efficiency and high power losses, which leads to the development of Multi level inverters (MLI). Now a days the use of multilevel inverters is increasing due their advantages and attraction by industries. MLI produces a number of voltage levels at output with the use of many switches with different configurations and DC links, so that the output quasi sine wave has low harmonic distortion [1-3]. The researchers introduce so many types of MLI, among CHB and Neutral point clamped (NPC) inverters are the best ones [4-7]. NPC inverters are finest ones which are attracted by many industries, and provides common DC bus for the application of three phase loads [8]. The CHB inverters have interesting structures and it provides more levels for high power applications, but suffers with many separated DC supplies [9]. Many of the MLI are facing the above mentioned problem [10-20]. Freshly, an attractive structure is designed with the modification of the flying capacitor (FC) inverter, but, suffering with separate voltage ratings and switching frequency [15-17, 21-27]. With the advantages of h bridge inverters a single DC source three phases MLI is designed in Fig.1 2 4080
[28], which has two cells, one cell is connected to DC source and next cell is connected to a charging capacitor. So many studies are implemented for balancing the capacitor voltage for different loads [29-34]. To track the voltage of the source and capacitor it needs a voltage sensor In this paper a new CHB sensor three less inverter fed by induction motor is proposed for generating three phase five level output with single DC source per phase and a single capacitor phase. In each phase when the capacitor is in series to the source and load, it will charge upto half of the source voltage. When it is in series with load the energy is discharged through the load. It has the drawback of reducing voltage levels from seven levels to five levels, but has an advantage of removing sensors at DC source and capacitor. 2 FIVE LEVEL THREE PHASE CHB INVERTER The block diagram of the three phase CHB inverter is presented in Fig.1. It has three single phase CHB inverters, each one is fed by one DC source and one capacitor. Each inverter will act as a single phase CHB inverter, but when connected to induction motor they has a phase difference of 120 degrees. Each one has eight switches and two H- bridge cells in which one cell is connected to supply DC voltage source and other is connected to storage capacitor shown in Fig.2. The voltage across capacitor should be managed as half of the applied voltage source. If the source voltage Vdc is 2E, then the capacitor voltage Vc was E. In the fast published works, the CHB inverter is employed as a seven level inverter with distinct modulation technique, but it is facing voltage balancing problem [32]. But in this paper, we are presenting a sensor less voltage regulating technique which can produce five level output. The switching states of five level voltage waveforms are indexed in the Table. I. Due to no effect on charging and discharging of capacitor voltage some switching positions which produce zero at voltage output are not considered. These switching states are used for reducing the frequency of the inverter. From the switching states listed in Table. I, with paths 3 4081
2, 3, 5 & 6 we can analyze whether the capacitor is charging or discharging. With paths 2 & 6, the capacitor is in series with the DC voltage source and load, hence the capacitor would charge up to E and delivers power to a load. In the sequence 3 & 5 the capacitor is only connected to load so it will discharges power to the load. By introducing the voltage balancing technique into switching techniques, the controller structure gets simple which can be easy to implement by using cheap microcontrollers. The charging and discharging effects of a capacitor after introducing voltage balancing techniques into switching techniques are listed in Table. I Figure 1: Block diagram of the three phase CHB five level inverter fed Induction motor 3 SENSOR-LESS VOLTAGE REGULATING TECHNIQUE From Table. I, it is well known that the capacitor may be charged or discharged in any one half cycle, But to maintain the capacitor voltage fixed, the switching technique of the capacitor is designed in such a way that it should be charged during the positive half cycle and discharged in the negative half cycle. Due to switching technique of capacitor and output waveform frequency the 4 4082
Figure 2: Single DC source multilevel inverter R Phase 5 4083
capacitor charge is increases to half of Vdc supply. The capacitor charge increases when it is connected in series with the load and Vdc source, the charging states of capacitor are 2 and 6, and the load voltage is E. These charging and discharging states are mathematically represented in the equation (1) If the primary source voltage Vdc is 2E, to produce the desired load voltage the charging capacitor voltage Vc must be E. The charging time and discharging time of the capacitor will matain the capacitor voltagevc to E. Hence, to have equivalent charging time and discharging times, in the charging state 2 the capacitor is connected in series with the voltage source in the positive half cycle and from switching state 5 the capacitor is discharged in the negative half cycle by connecting in series with the load. It should be known that the capacitor charging and discharging depends on the type of load only, but not on the output frequency or switching frequency. The type of load connected will directly affect the size of the capacitor. The self regulating voltage procedure is mathematically proved with energy storage relations of the capacitor. The output voltage and current waveforms of a five level CHB inverter is shown in Fig.2. Mathematically the output voltage and current waveforms can be written as an equation (2) and (3) Where Vm, Im and are the maximum value of voltage, current and phase angle between voltage and current. The load current flowing through capacitor can be written as Where I, V, q and U are the current flowing through the capacitor, the voltage across the capacitor, the charge on capacitor plates and energy stored in the capacitor respectively. From equations (3) and (4) the charging energy of the capacitor can be written as 6 4084
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In the same way the discharging energy of the capacitor can be written as From equation (5) and (6), we can observe that the output voltage is symmetric about positive and negative half. Hence we can assume an equation (7) as The energy stored in positive half cycle and negative half cycles are same but has opposite in polarity From equation (8) the energy stored or discharged by the capacitor is balanced and constant and also it keeps the capacitor output voltage constant irrespective of all conditions. For preparing the hardware setup the sensor less voltage regulating technique is integrated with modulation technique. The Multi carrier switching technique is used as modulation technique [17]. For a five level inverter PWM scheme is implemented with four carrier waveforms (Cr1, Cr2, Cr3, and Cr4) and reference sine wave are shown in Fig. 3. 9 4087
Figure 3: Five-level PWM scheme using four level shifted carrier waves Figure 4: Proposed sensor-less voltage regulating approach integrated into switching technique 10 4088
The four carrier waves are chipped vertically for modulating the reference sine wave. The firing pulses related to Table.I are produced after comparing the carrier waves with the reference waveform. The algorithms for producing the firing pulses are presented in Fig.4. This algorithm produces the five level output after seting the capacitor voltage at a currect value without any feedback sensor. This technique does not depend on the type of the system model (e.g. average modelling), modulation index, feedback sensors, grid frequency and switching frequency. It can operate the system voltage to any arbitrary value and also at varying DC source conditions. 4 SIMULATION RESULTS AND DISCUSSION The CHB inverter depicted in Fig.1 is simulated with Matlab/ Simulink, the results shows its performance in standalone mode with induction motor as a load. We can use the standalone inverters as power supply units for motor drives. The simulation parameters of the test system are listed in Table. II. To evaluate the behavior of the proposed method induction motor load is connected to the inverter. When the capacitor is connected with the source and induction motor capacitor voltage starts rising and it reaches the desired value which is half of the Vdc value within 20 cycles. From Fig.5, when the source voltage Vdc is 200 V, the capacitor voltagevc starts increasing and tracks the desired value which is half of the source voltage is 100 V. To observe the changes in the voltage and currents, in Fig.5, the corresponding waveforms are captured. The skyrocket of the three phase multilevel output voltage of the proposed converter is shown in Fig.6. And its zoom is represented in Fig.7. These results, which show that when the frequency of switching is low, then switching pulses are visualized. The load current and its harmonic spectrum without filters are shown in the Fig.8. Due to high starting torque of induction motor, initially it draws more current and after some time it will come to steady state. The symmetrical five level balanced voltage will regulate the voltage of the second bus. The speed and torque waveforms are 11 4089
Figure 5: Voltage across the capacitor voltage Figure 6: Output voltage of the proposed CHB three phase inverter Figure 7: Zoomed waveform of the output voltage of the proposed CHB three phase inverter 12 4090
Figure 8: (a) Load current of inverter for induction motor load (b) THD spectrum of load current in R phase shown in Fig.9. Which shows that the induction motor starting torque is 12 times the rated torque starting. Figure 9: Speed and torque waveforms of induction motor with proposed converter 13 4091
5 CONCLUSION In this paper a new sensor less voltage controlling technique is suggested for the multilevel inverter fed induction motor with single DC source and a capacitor for each phase. The capacitor is charged in the second bus up to half of source voltage, when it is connected to the DC source and an induction motor. Without having any feedback from DC links and loads it will provide five level output voltage. By integrating it with the switching technique industrial products are implemented with a very less number of switches and one DC source and capacitor per phase. The demerits of diode clamped and flying capacitor inverters like capacitor voltage balancing, isolated DC sources are eliminated by this converter. This method is simulated in Matlab, the results shows the good dynamic performance of this method for induction motor load. The power quality is improved. References [1] L. G. Franquelo, J. Rodriguez, J. I. Leon, S. Kouro, R. Portillo, and M. A. M. Prats, The age of multilevel converters arrives, IEEE Ind. Electron. Mag., vol. 2, pp. 28-39, 2008. [2] H. Abu-Rub, M. Malinowski, and K. Al-Haddad, Power electronics for renewable energy systems, transportation and industrial applications: John Wiley & Sons, 2014. [3] B. Singh, A. Chandra, and K. Al-Haddad, Power Quality: Problems and Mitigation Techniques: John Wiley & Sons, 2014. [4] H. Abu-Rub, J. Holtz, J. Rodriguez, and G. Baoming, Medium voltage multilevel convertersstate of the art, challenges, and requirements in industrial applications, IEEE Trans. Ind. Electron., vol. 57, pp. 2581-2596, 2010. [5] M. Sharifzade, H. Vahedi, A. Sheikholeslami, H. Ghoreyshi, and K. Al-Haddad, Modified selective harmonic elimination employed in four-leg NPC inverters, in IECON 2014-40th 14 4092
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