Hybrid Five-Level Inverter using Switched Capacitor Unit

IJIRST –International Journal for Innovative Research in Science & Technology| Volume 3 | Issue 04 | September 2016 ISSN (online): 2349-6010

Hybrid Five-Level Inverter using Switched Capacitor Unit Minu M Sageer PG Student Department of Electrical & Electronics Engineering Federal Institute of Science And Technology (FISAT), Mookkannoor P.O., Angamaly 683577, Ernakulam, Kerala

Rabiya Rasheed Assistant Professor Department of Electrical & Electronics Engineering Federal Institute of Science And Technology (FISAT), Mookkannoor P.O., Angamaly 683577, Ernakulam, Kerala

Abstract A multilevel inverter is a power conversion device that produces an output voltage in the desired levels by using DC voltage sources at the input. The number of DC voltages and switches increases with number of output voltage levels for the conventional topologies. This is the main drawback of the conventional topologies. Here a new multilevel inverter is presented, which uses switched capacitor units. The switched capacitor multi-level inverter can produce the desired output voltage and also can boost the input voltage without any bulky transformer. Also, the number of switches and DC voltage sources are reduced in this topology. This reduces the size and cost of the inverter. This topology uses series and parallel combination of the basic unit to produce higher level output voltage. In this paper the operation and simulation results of a hybrid five-level inverter is presented using MATLAB/Simulink software. Multilevel inverters have their many applications in renewable energy systems.

This new multilevel inverter using switched capacitor units can also be used for renewable energy applications.

Keywords: Cascaded H-Bridge, Hybrid Multilevel inverters, Series and Parallel Switches, Staircase Modulation Technique, Switched Capacitor Units _______________________________________________________________________________________________________ I.

INTRODUCTION

The Switched Capacitor Multilevel Inverter can produce the desired sinusoidal voltage waveform and boost the input voltage without any bulky transformer. Because of the inherent voltage unbalancing of capacitors in the switched capacitor multilevel inverters, using complicated capacitor voltage balancing is necessary. Capacitor voltage balancing techniques will be more complex when higher number of voltage levels is produced at the output. In order to mitigate this problem, the hybrid-source switched capacitor topologies can be used. By using this kind of inverter with fewer switching devices and simpler control methods, it is possible to achieve a greater number of voltage levels at the output. The hybrid source switched capacitor topologies can be used in electric vehicle application and PV systems.

The hybrid multilevel inverter using switched capacitor units consist of a combination of the conventional series and parallel switched capacitor units. The hybrid multilevel inverter topology using switched capacitor units reduces number of switches and isolated DC voltage sources, the variety of the DC voltage source values, and the size and cost of the system in comparison with the conventional topologies. In addition, this topology can boost the input voltage without a transformer.

II. HYBRID MULTILEVEL INVERTER USING SWITCHED CAPACITOR UNITS The term “hybrid� means ‘a thing made by combining two different elements, of mixed characteristics, composed of different elements’. Thus the hybrid multilevel inverter is a combination of a switched capacitor topology and an H-Bridge.

Switched Capacitor Unit

Fig. 1: Basic Switched Capacitor Unit

Figure 1 shows the basic unit of a switched capacitor topology. The basic circuit contains one DC power supply, one capacitor, one power diode and two series/parallel unidirectional power switches. The switches P and S connect the capacitor in parallel and series with the DC voltage source, respectively. When the switch P is turned ON, the capacitor is charged to the voltage đ?‘‰đ?‘‘đ?‘? , and when the switch S is turned ON, the capacitor starts to discharge. The switches P and S have complementary operation with each other, which means that, when the switch P is ON, the switch S must be OFF and vice versa. Otherwise, a short circuit

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Hybrid Five-Level Inverter using Switched Capacitor Unit (IJIRST/ Volume 3 / Issue 04/ 014)

occurs across the DC voltage source. When the switch S conducts, the diode D becomes reverse biased and prevents capacitor discharging to the DC voltage source. Thus, in the case of series connection of the capacitor and DC voltage source (S is ON), the capacitor current only flows to the load. Figures 2(a) and 2(b) shows the operating mode of the basic unit. The blocked voltage by each switch in Figure. 2 is đ?‘‰đ?‘‘đ?‘? .

Fig. 2(a): Operating Mode of Basic unit when Capacitor Charging (V L = VC = Vdc)

Fig. 2(b): Operating Mode of Basic unit when Capacitor Discharging (iL = iC = is)

H-Bridge Unit An H-Bridge Unit produce positive, negative and zero voltages at the output. Figure 3 shows an H-Bridge unit.

Fig. 3: H-Bridge Unit

An H-Bridge unit consists of a DC power supply and four switches T 1, T2, T3 and T4. When T1 and T2, conducts a positive output voltage is produced. And when T 3 and T4, conducts a negative voltage is produced. If two switches of the same leg (i.e. if T1 and T4 or if T2 and T3) are ON or if no switches are turned ON, then the output voltage will be zero. Figures 4(a) and 4(b) shows the operation of an H-Bridge Unit.

Fig. 4(a): Operation of an H-Bridge unit to Produce Positive Output Voltage (Vo=Vdc)

Fig. 4(b): Operation of an H-Bridge unit to Produce Negative Output Voltage (Vo= - Vdc)

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Hybrid Five-Level Inverter using Switched Capacitor Unit (IJIRST/ Volume 3 / Issue 04/ 014)

General Topology of a Hybrid Multilevel Inverter using Switched Capacitor Units Figure 5 shows the generalized topology of a Hybrid Multilevel Inverter using Switched Capacitor Units.

Fig. 5: Generalized Topology of a Hybrid Multilevel Inverter using Switched Capacitor Units

The generalized topology is derived from the series combination of several basic units. The switches Si (i = 1, 2, . . . n) connect the capacitors in series, and the switches P i connect the capacitors in parallel with the DC voltage sources. To produce zero and negative voltage levels, an H-bridge has been used at the output. When T1 and T2, conducts a positive output voltage is produced and when T3 and T4, conducts a negative voltage is produced. If two switches of the same leg of the H-bridge conduct or if all the switches of the H-bridge are open, then the output voltage will be zero. The blocked voltage by each switch in Figure 5 is Vdc. Calculation of Different Parameters of the Hybrid Multilevel Inverter Using Switched Capacitor Units  Number of output voltage level, Nstep=2n+3  Number of switches, NIGBT=2n+4  Number of diodes, Ndiode=n  Maximum output voltage, V(o,max)=(n+1)Vdc Where n is the number of capacitors.

III. HYBRID FIVE-LEVEL INVERTER USING SWITCHED CAPACITOR UNIT A Hybrid Five-Level Inverter Using Switched Capacitor Unit is formed by combining a basic switched capacitor unit with an HBridge unit.

Figure 6 shows a Hybrid Five-Level Inverter using Switched Capacitor Unit.

Fig. 6: Hybrid Five-Level Inverter using Switched Capacitor Unit

The switches S and P are the series and parallel switches respectively, which connect the capacitor in series and parallel with the source. The switches T1, T2, T3 and T4 form the H-Bridge. The switches T1 and T2 are operated to produce a positive voltage, while the switches T3 and T4 are operated to produce the negative level voltages.

Operation of a Hybrid Five-Level Inverter using Switched Capacitor Units Figures 7(a), 7(b), 7(c) and 7(d) show the operation of a Hybrid Five-Level Inverter using Switched Capacitor Unit.

Level 1 (Vo =Vdc)

Fig. 7(a): Operating Mode of a Hybrid Five-Level Inverter using Switched Capacitor Unit to get Vo=Vdc

If the switches P, T1 and T2 are turned ON, then the capacitor C charges to Vdc and the output voltage across the load will be Vdc. Thus, Voltage across the capacitor C, Vc = Vdc Voltage across the load / output voltage Vo =Vdc

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Level 2 (Vo =2Vdc) Now the parallel switch P is turned OFF and the series switch S is turned ON. The diode D becomes reverse biased and the Capacitor C discharges. The Switches T 1 and T2 are turned ON, so that the output voltage across the load will be 2V dc.

Fig. 7(b): Operating Mode of a Hybrid Five-Level Inverter using Switched Capacitor Unit to get Vo=2Vdc

Thus, Voltage across the load / output voltage Vo = 2Vdc Level 3(Vo= - Vdc)

Fig. 7(c): Operating Mode of a Hybrid Five-Level Inverter using Switched Capacitor Unit to get Vo= - Vdc

In this mode, the parallel switch P is turned ON, so the capacitor C is charged to V dc. The switches T3 and T4 are turned ON, so that the output voltage is -Vdc. Thus, Voltage across the capacitor C, Vc = Vdc Voltage across the load / output voltage Vo = - Vdc Level 4 (Vo= - 2Vdc)

Fig. 7(d): Operating Mode of a Hybrid Five-Level Inverter using Switched Capacitor Unit to get Vo= - 2Vdc

In this mode, the parallel switch P is turned OFF and the series switch S is turned ON. The diode D becomes reverse biased and the Capacitor C discharges. The Switches T 3 and T4 are turned ON, so that the output voltage across the load will be -2Vdc. Thus, Voltage across the load / output voltage Vo= - 2Vdc Level 5 (Vo=0) In order to get zero voltage at the output, none of the switches are turned ON. The circuit for this mode will be same as that shown in figure 6.

In this mode, Voltage across the load / the output voltage VO=0.

The operation of the Hybrid Five-Level Inverter using Switched Capacitor Unit is illustrated in table 1. In this table 0 and 1 means OFF and ON switching states of the switches respectively.

Table – 1 Switching Pattern of Hybrid Five Level Inverter Using Switched Capacitor Unit S P T1 T2 T3 T4 Output Voltage 0 1 1 1 0 0 Vdc 1 0 1 1 0 0 2Vdc 0 1 0 0 1 1 -Vdc 1 0 0 0 1 1 -2Vdc 0 0 0 0 0 0 0

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Comparison of the Hybrid Five-Level Inverter using Switched Capacitor Units with the Existing Topologies The comparison of the Hybrid Five-Level Inverter using Switched Capacitor Units with the Existing Topologies is shown in table 2.

Thus from table 2, it is clear that a Hybrid Five-Level Inverter using Switched Capacitor Unit requires less number of components than that required in the conventional topologies. This reduces the size and cost of the overall system. Moreover, it can boost the input voltage. Also the efficiency is increased in this topology.

Table – 2 Comparison of the Hybrid Five-Level Inverter using Switched Capacitor Units with the Existing Topologies Topology Level Switches Diodes Capacitors Maximum Output Voltage Hybrid Multilevel Inverter Using 2n+3 2n+4 N N (N+1)Vdc Switched Capacitor Units Eg 5 6 1 1 2vdc 1 Diode Clamped Mli N 2(N-1) (N-1)(N-2) N-1 Vdc 2 1

Eg

5

8

12

4

Vdc

Capacitor Clamped Mli

N

2(N-1)

0

[(N-1)(N-2)/2]+ N-1

Eg

5

8

0

10

Cascaded H-Bridge Mli Eg

N 5

2(N-1) 8

-

(N-1)/2 2

2 1

Vdc

2 1

Vdc

2

IV. MODULATION TECHNIQUE The modulation technique used in the Hybrid Multilevel Inverter using Switched Capacitor Unit topology is staircase modulation. Figure 8 shows the staircase modulation technique.

Fig. 8: Staircase Modulation Technique

In this modulation scheme, a sine wave of frequency 50Hz and with some appropriate amplitude is compared with constants, having amplitude lesser than that of the sine wave. Whenever the constant is greater than the sine wave, a pulse is generated.

These pulses are then subjected to mathematical or logical operations to generate the switching signals for each of the switches.

The output voltage will be in the form of stepped waveform, the frequency of it will be 50Hz. V. SIMULATION OF HYBRID FIVE-LEVEL INVERTER USING SWITCHED CAPACITOR UNIT The Hybrid Five-Level Inverter using Switched Capacitor unit is simulated using MATLAB/Simulink software. Figure 9 shows the Simulink diagram of Hybrid Five-Level Inverter Using Switched Capacitor Unit. The switches used were MOSFETs. A 1000 F capacitor was used as a switching capacitor and a 1 k resistor was used as load. The switching pulses for the various switches S, P, S1, S2, S3 and S4 were generated using Staircase modulation. Thus the pulses were generated by comparing constant with the sine wave and appropriate MATLAB code is written in the MATLAB function block. A 9V DC is given as the input voltage. Figure 10 shows the Simulink diagram for the generation of switching pulses for the various switches S, P, S 1, S2, S3 and S4. The switching pulses for the different switches and the five-level output voltage of the Hybrid Five-Level Multilevel Inverter using Switched Capacitor Unit is shown in figure 11.

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Fig. 9: Simulink Diagram of Hybrid Five-level Inverter Using Switched Capacitor Unit.

Fig. 10: Simulink Diagram of Switching Pulse Generation of Hybrid Five-Level Inverter using Switched Capacitor Unit.

Fig. 11: Switching Pulses and Five-Level Output Voltage of Hybrid Five-Level Inverter Using Switched Capacitor Unit

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The maximum output voltage was found to be 18V. Figure 12 shows the harmonics spectrum obtained from FFT analysis of the Hybrid Five-Level Inverter using Switched Capacitor Unit.

Fig. 12: Harmonic Spectrum obtained in FFT Analysis of Hybrid Five-Level Inverter Using Switched Capacitor Unit.

VI. HARDWARE IMPLEMENTATION OF HYBRID FIVE-LEVEL INVERTER USING SWITCHED CAPACITOR UNIT Figure 13 shows the experimental setup of Hybrid Five-Level Inverter using Switched Capacitor Unit.

Fig. 13: Experimental Set-up of Hybrid Five-Level Inverter Using Switched Capacitor Unit.

The switches used were MOSFETs-IRFZ44N. The driver for the MOSFETs used was MCT2E. The switching pulses for the various switches were generated using 8051 microcontroller and Arduino by writing a delay program with a frequency of 50Hz. When a 9V DC is applied as input voltage, the maximum output voltage obtained was 18V. Figure 14 shows the input pulses given to the various switches of Hybrid Five-Level Inverter using Switched Capacitor Unit..

The output voltage and the harmonic spectrum obtained is shown in figures 15 and 16 respectively.

Fig. 14: Switching Pulses for the various switches of Hybrid Five Level Inverter using Switched Capacitor Unit generated using Arduino

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Fig. 15: Output Voltage Waveform of Hybrid Five-Level Inverter using Switched Capacitor Unit

Fig. 16: Harmonic Spectrum of a Hybrid Five-Level Inverter using Switched Capacitor unit

VII. CONCLUSION The Hybrid Five-Level Inverter using Switched Capacitor Unit requires only one capacitor, one diode, six power electronic switches and a single DC source. Simulation of a Hybrid Five Level Inverter using Switched Capacitor Unit was done using MATLAB/Simulink software and the output waveforms were observed. The hardware implementation of the Hybrid Five-Level Inverter using Switched Capacitor Unit was done. The pulses to different switches were generated using 8051 microcontroller and an Arduino. It was observed that the harmonics remained same in the simulation as well as in the hardware implementation.

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