sensor less voltage control of chb multilevel inverter … · 20 cycles. from fig.5, when the...

SENSOR LESS VOLTAGE CONTROLOF CHB MULTILEVEL INVERTERFED THREE PHASE INDUCTIONMOTOR WITH ONE DC SOURCE

PER EACH PHASE

G. V. V. Nagaraju1, G. Sambasiva Rao2,CH Rami Reddy3

1Department of Electrical & Electronics Engineering,Acharya Nagarjuna University,

Guntur, India2Department of Electrical & Electronics Engineering,

RVR & JC college of Engineering,Guntur, India

3Department of Electrical & Electronics Engineering,Nalanda Institute of Engineering & Technology,

Guntur, [email protected],

[email protected],[email protected].

June 20, 2018

Abstract

In this paper a single DC source per phase cascaded hbridge (CHB) three phase five level inverter fed inductionmotor with minimum number of switches and a singlecapacitor is proposed. Maximum all available switching

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International Journal of Pure and Applied MathematicsVolume 120 No. 6 2018, 4079-4097ISSN: 1314-3395 (on-line version)url: http://www.acadpubl.eu/hub/Special Issue http://www.acadpubl.eu/hub/

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states are evaluated and a sensor less voltage regulatingtechnique is suggested which controls the second busvoltage as half of the applied single DC voltage source.Voltage levels at the output are zero, the voltage acrossthe capacitor and voltage of DC voltage source. Theswitching method is mixed with a simple voltage balancingtechnique which can be possible to implement even withsmall, simple microcontrollers and Simulation resultsexhibit the dynamic performance of this method incontrolling the second bus capacitor voltage. The lowharmonic desired value of five level voltage is regulated bythe voltage across the capacitor.

Keywords:Three phase CHB inverter; Single DCvoltage source; Sensor less voltage regulating technique;Regulating capacitor; Induction motor load.

1 INTRODUCTION

The world power demand in the energy market leads to redesignof power converters. The complications with the two level invertertopology are low efficiency and high power losses, which leads tothe development of Multi level inverters (MLI). Now a days theuse of multilevel inverters is increasing due their advantages andattraction by industries. MLI produces a number of voltage levelsat output with the use of many switches with differentconfigurations and DC links, so that the output quasi sine wavehas low harmonic distortion [1-3]. The researchers introduce somany types of MLI, among CHB and Neutral point clamped(NPC) inverters are the best ones [4-7]. NPC inverters are finestones which are attracted by many industries, and providescommon DC bus for the application of three phase loads [8]. TheCHB inverters have interesting structures and it provides morelevels for high power applications, but suffers with manyseparated DC supplies [9]. Many of the MLI are facing the abovementioned problem [10-20]. Freshly, an attractive structure isdesigned with the modification of the flying capacitor (FC)inverter, but, suffering with separate voltage ratings and switchingfrequency [15-17, 21- 27]. With the advantages of h bridgeinverters a single DC source three phases MLI is designed in Fig.1

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[28], which has two cells, one cell is connected to DC source andnext cell is connected to a charging capacitor. So many studiesare implemented for balancing the capacitor voltage for differentloads [29-34]. To track the voltage of the source and capacitor itneeds a voltage sensor

In this paper a new CHB sensor three less inverter fed byinduction motor is proposed for generating three phase five leveloutput with single DC source per phase and a single capacitorphase. In each phase when the capacitor is in series to the sourceand load, it will charge upto half of the source voltage. When it isin series with load the energy is discharged through the load. Ithas the drawback of reducing voltage levels from seven levels tofive levels, but has an advantage of removing sensors at DC sourceand capacitor.

2 FIVE LEVEL THREE PHASE CHB

INVERTER

The block diagram of the three phase CHB inverter is presentedin Fig.1. It has three single phase CHB inverters, each one is fedby one DC source and one capacitor. Each inverter will act as asingle phase CHB inverter, but when connected to inductionmotor they has a phase difference of 120 degrees. Each one haseight switches and two H- bridge cells in which one cell isconnected to supply DC voltage source and other is connected tostorage capacitor shown in Fig.2. The voltage across capacitorshould be managed as half of the applied voltage source. If thesource voltage Vdc is 2E, then the capacitor voltage Vc was E. Inthe fast published works, the CHB inverter is employed as a sevenlevel inverter with distinct modulation technique, but it is facingvoltage balancing problem [32]. But in this paper, we arepresenting a sensor less voltage regulating technique which canproduce five level output. The switching states of five levelvoltage waveforms are indexed in the Table. I. Due to no effect oncharging and discharging of capacitor voltage some switchingpositions which produce zero at voltage output are not considered.These switching states are used for reducing the frequency of theinverter. From the switching states listed in Table. I, with paths

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2, 3, 5 & 6 we can analyze whether the capacitor is charging ordischarging. With paths 2 & 6, the capacitor is in series with theDC voltage source and load, hence the capacitor would charge upto E and delivers power to a load. In the sequence 3 & 5 thecapacitor is only connected to load so it will discharges power tothe load. By introducing the voltage balancing technique intoswitching techniques, the controller structure gets simple whichcan be easy to implement by using cheap microcontrollers. Thecharging and discharging effects of a capacitor after introducingvoltage balancing techniques into switching techniques are listedin Table. I

Figure 1: Block diagram of the three phase CHB five level inverterfed Induction motor

3 SENSOR-LESS VOLTAGE

REGULATING TECHNIQUE

From Table. I, it is well known that the capacitor may be chargedor discharged in any one half cycle, But to maintain the capacitorvoltage fixed, the switching technique of the capacitor is designedin such a way that it should be charged during the positive halfcycle and discharged in the negative half cycle. Due to switchingtechnique of capacitor and output waveform frequency the

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Figure 2: Single DC source multilevel inverter R Phase

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capacitor charge is increases to half of Vdc supply. The capacitorcharge increases when it is connected in series with the load andVdc source, the charging states of capacitor are 2 and 6, and theload voltage is E. These charging and discharging states aremathematically represented in the equation (1)

If the primary source voltage Vdc is 2E, to produce the desiredload voltage the charging capacitor voltage Vc must be E. Thecharging time and discharging time of the capacitor will matainthe capacitor voltageVc to E. Hence, to have equivalent chargingtime and discharging times, in the charging state 2 the capacitoris connected in series with the voltage source in the positive halfcycle and from switching state 5 the capacitor is discharged in thenegative half cycle by connecting in series with the load. It shouldbe known that the capacitor charging and discharging depends onthe type of load only, but not on the output frequency orswitching frequency. The type of load connected will directlyaffect the size of the capacitor. The self regulating voltageprocedure is mathematically proved with energy storage relationsof the capacitor. The output voltage and current waveforms of afive level CHB inverter is shown in Fig.2. Mathematically theoutput voltage and current waveforms can be written as anequation (2) and (3)

Where Vm, Im and are the maximum value of voltage, currentand phase angle between voltage and current. The load currentflowing through capacitor can be written as

Where I, V, q and U are the current flowing through thecapacitor, the voltage across the capacitor, the charge oncapacitor plates and energy stored in the capacitor respectively.From equations (3) and (4) the charging energy of the capacitorcan be written as

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In the same way the discharging energy of the capacitor can bewritten as

From equation (5) and (6), we can observe that the outputvoltage is symmetric about positive and negative half. Hence wecan assume an equation (7) as

The energy stored in positive half cycle and negative half cyclesare same but has opposite in polarity

From equation (8) the energy stored or discharged by thecapacitor is balanced and constant and also it keeps the capacitoroutput voltage constant irrespective of all conditions. Forpreparing the hardware setup the sensor less voltage regulatingtechnique is integrated with modulation technique. The Multicarrier switching technique is used as modulation technique [17].For a five level inverter PWM scheme is implemented with fourcarrier waveforms (Cr1, Cr2, Cr3, and Cr4) and reference sinewave are shown in Fig. 3.

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Figure 3: Five-level PWM scheme using four level shifted carrierwaves

Figure 4: Proposed sensor-less voltage regulating approachintegrated into switching technique

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The four carrier waves are chipped vertically for modulatingthe reference sine wave. The firing pulses related to Table.I areproduced after comparing the carrier waves with the referencewaveform. The algorithms for producing the firing pulses arepresented in Fig.4. This algorithm produces the five level outputafter seting the capacitor voltage at a currect value without anyfeedback sensor. This technique does not depend on the type ofthe system model (e.g. average modelling), modulation index,feedback sensors, grid frequency and switching frequency . It canoperate the system voltage to any arbitrary value and also atvarying 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 modewith induction motor as a load. We can use the standaloneinverters as power supply units for motor drives. The simulationparameters of the test system are listed in Table. II. To evaluatethe behavior of the proposed method induction motor load isconnected to the inverter. When the capacitor is connected withthe source and induction motor capacitor voltage starts rising andit reaches the desired value which is half of the Vdc value within20 cycles. From Fig.5, when the source voltage Vdc is 200 V, thecapacitor voltageVc starts increasing and tracks the desired valuewhich is half of the source voltage is 100 V. To observe thechanges in the voltage and currents, in Fig.5, the correspondingwaveforms are captured.

The skyrocket of the three phase multilevel output voltage of theproposed converter is shown in Fig.6. And its zoom is representedin Fig.7. These results, which show that when the frequency ofswitching is low, then switching pulses are visualized. The loadcurrent and its harmonic spectrum without filters are shown in theFig.8. Due to high starting torque of induction motor, initiallyit draws more current and after some time it will come to steadystate. The symmetrical five level balanced voltage will regulate thevoltage of the second bus. The speed and torque waveforms are

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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 proposedCHB three phase inverter

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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 startingtorque is 12 times the rated torque starting.

Figure 9: Speed and torque waveforms of induction motor withproposed converter

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5 CONCLUSION

In this paper a new sensor less voltage controlling technique issuggested for the multilevel inverter fed induction motor withsingle DC source and a capacitor for each phase. The capacitor ischarged in the second bus up to half of source voltage, when it isconnected to the DC source and an induction motor. Withouthaving any feedback from DC links and loads it will provide fivelevel output voltage. By integrating it with the switchingtechnique industrial products are implemented with a very lessnumber of switches and one DC source and capacitor per phase.The demerits of diode clamped and flying capacitor inverters likecapacitor voltage balancing, isolated DC sources are eliminated bythis converter. This method is simulated in Matlab, the resultsshows the good dynamic performance of this method for inductionmotor load. The power quality is improved.

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sensor less voltage control of chb multilevel inverter … · 20 cycles. from fig.5, when the...

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