Design and Implementation of Step-Up DC-DC Converter by Cascading Cockcroft Walton Voltage Multiplier for Hybrid System | Open Access Journals

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Design and Implementation of Step-Up DC-DC Converter by Cascading Cockcroft Walton Voltage Multiplier for Hybrid System

Alice Hepzibah.A1, Dr. A.Senthil Kumar2 and Ranjith.N3
  1. Associate Professor, Electrical & Electronics Engineering, Rajalakshmi Engineering College, Chennai, India
  2. Professor, Electrical & Electronics Engineering, Velammal Engineering College, Chennai, India
  3. PG Student, Electrical & Electronics Engineering, Rajalakshmi Engineering College, Chennai, India
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Abstract

this paper presents the use of a stepupdc-dc converter by cascading cock croftwalton voltage multiplier for a hybrid system. The hybrid renewable energy system used is a combination of as olar panel and a fuel cell. the conventional method isto use a converter increase the voltage and further uses a transformer to increase the voltage level of the output. The p ro p o s ed converter topology used is boost converter cascaded with an n-stage cockcroft-walton voltage multiplier. the number of stages of the voltage multiplier increases the output level of the stepupdc-dcconverter which drives the load. The converter-voltage multiplier module provides continuous conduction with the voltage stress on cascaded capacitor-diode and switches being reduced. The output voltage obtained can be used in highvoltage applications .the simulation of the entire topology for hybrid system was done using matlab software.

Keywords

Cockcroft-Walton (CW) voltage multiplier, solar panel, fuel cell, step-up DC-DC converter.

INTRODUCTION

In the present scenario throughout the world, everyone is concerned with fossil fuel exhaustion and environmental problems caused by conventional power generation. Hence renewable energy sources are used among which solar panels and wind generators are the most used. The solar panel generally uses a power converter for connecting with the load. The efficiency of photovoltaic (PV) module is too low and output power depends on insolation level and temperature. Therefore the solar or PV panel is used along with a fuel cell (FC) system making it a hybrid renewable energy system. The hybrid renewable energy system is a combination of two or more renewable energy system to obtain higher efficiency than from a single energy power source. The fuel cell used converts an oxidant and fuel into electricity through an electrochemical process. They produce very low emissions and have high operating efficiencies. The converter topology used is a step-up converter which is cascaded with a Cockcroft-Walton (CW) voltage multiplier which increases the voltage level.

II. HYBRID SOLAR PANEL/FUEL CELL SYSTEM

The block diagram of the hybrid solar and fuel cell system with the power converter is show in the Fig. 1. It consists of solar panel, a fuel cell, a boost converter topology, CW voltage multiplier, a pulse generator and load. The solar panel is used as the primary supply and fuel cell as the backup supply. The converter is used to boost up the input voltage from the hybrid solar panel/fuel cell system.
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A.Solar Panel
The solar panel is a linked collection of PV modules, which is in turn made of multiple interconnected solar cells. The cells convert solar energy into electricity by photovoltaic effect. The power one PV module produces is enough to power a residential place. The collection of solar panels is a solar array. The solar array generally requiresan inverter to convert DC power to AC powerso it can be used to supply AC loads. The modules of the solar panel are connected in series to obtain the required voltage and the individual strings are connected in parallelto increase the current. The photons in sunlight thatstrikes the panel are absorbed by semiconducting materials like silicon. The electrons present in the semiconductor material are knocked out from the atoms that cause an electric potential difference. Thus current starts to flow through the silicon to eliminate the potential and hence electricity is captured which is then supplied to the load. The equivalent circuit of solar cell is shown below as in Fig. 2.
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Applying KCL to equivalent circuit,
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Applying KVL to equivalent circuit,
VPVcell = VD - RSIPV(2)
The PN-junction diode characteristics is given by,
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B. Fuel Cell
The fuel cell used is a Proton Exchange Membrane (PEM) fuel cell that consists of an electrolyte which is a small, thin, and light permeable polymeric membrane. The reaction is catalysed using platinum electrodes are used on both sides of the membrane. The hydrogen molecules are supplied at the anode which splits the molecules into electrons and hydrogen protons. The protons pass through the polymeric membrane to the cathode while the electrons are passed round an external circuit to produce electricity. Oxygen is supplied to the cathode which combines with hydrogen to produce water.Hydrogen molecules are supplied to fuel cell directly or from natural gas, methanol or petroleum. Each electrode assembly has two electrodes of anode and cathode with a layer of catalyst which is thin. The desired amount of electrical power can be obtained by combining individual fuel cells to form a fuel cell stack. The output voltage of a single fuel cell is given by the equation,
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The Nernst voltage of the fuel cell is,
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T - temperature (K)
pH2 - partial pressure of the Hydrogen (atm)
pO2 - partial pressure of the oxygen (atm)
The activation potential having anode & cathode is given by,
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The ohmic voltage drop or loss is provided by the following equation,
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Rm - resistance to transfer of protons through membrane
Rc- resistance of the membrane to electron flow
The voltage drop due to mass transport or the concentration voltage is given as
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B - parametric coefficient depending on the fuel cell
J - actual current density of the fuel cell (A/m2)
Jmax- maximum current density of the fuel Cell (A/m2)

III. CONVERTER TOPOLOGY FOR HYBRID SYSTEM

The converter circuit consists of the step–up converter topology by which two switches are connected in series having an inductor in between the two switches. The converter also has a capacitor connected in parallel to the switches. The converter circuit output is then given to a CW voltage multiplier which is made of n-stages. The CW voltage multiplier converts AC or DC voltage from a lower level to a higher DC level. The output voltage of CW voltage multiplier depends on the number of stages and voltage across the even capacitors. The advantage is that it is easy to insulate, requires low cost components and the voltage gain is proportional to the number of cascaded stages. It also utilises the output from any stage. The converter topology consists of nine stages. Hence there are 18 capacitors and 18 diodes. Each stage consists of two diodes and two capacitors respectively.The relationship between the supply voltage of the converter and capacitor voltage of CW voltage multiplier converter is given by
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The change in inductor current and capacitor voltage of converter are given by
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The inductance (L) and capacitor(C) of the converter are given by
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The time period of the converter is given as
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The output voltage and output current of CW voltage multiplier are given as
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The change in capacitor voltage of CW voltage multiplier is given by
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The capacitor of the CW voltage multiplier is given by
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A. Operating Principle
Theoperating principle of the converter topology depends on the terminal current ia which is a pulsating form current. The switches are given gate pulse by trigger pulse generator through a drive circuit. The modes of operation are as follows:
Mode-I: S1 and S2 are turned OFF and diodes (D1 to D18) do not conduct.The inductoris charged by input DC from the hybrid system, hence the odd numbered capacitors (C1,C3,C5,C7,C9,C11,C13, C15andC17) float while even numbered
Capacitors (C2, C4, C6, C8, C10,C12, C14, C16 and C18) supply the load.
S1 and S2 are turned ON and the current ia is positive for modes II to X. The inductor is discharged and input source transfers energy to the CW voltage multiplier through the even numbered diodes.
Mode-II: D18isturned ON, at the same time the other diode do not conduct hence C2, C4, C6, C8,C10, C12, C14, C16 and C18gets charged whileC1,C3,C5,C7,C9,C11, C13, C15 and C17are all discharged byia.
Mode-III:D16 is turned ON hence C2, C4, C6, C8,C10, C12, C14 and C16gets charged while C1,C3,C5,C7,C9,C11, C13 and C15 are discharged byia.Hence C18 supplies the load while C17 floats.
Mode-IV:D14 is turned ON hence C2, C4, C6, C8,C10, C12 and C14 gets charged while C1,C3,C5,C7,C9,C11 and C13 are discharged byia. Hence C16 and C18 supply the load while C15 and C17 floats.
Mode-V: D12 is turned ON hence C2, C4, C6, C8,C10 and C12 gets charged while C1,C3,C5,C7,C9 and C11 are discharged by ia. Hence C14, C16 and C18 supply the load while C13, C15 and C17 float.
Mode-VI: D10 is turned ON hence C2, C4, C6, C8 and C10 gets charged while C1,C3,C5,C7 and C9 are discharged by ia.Hence C12, C14, C16 and C18 supply the load while C11, C13, C15 and C17float.
Mode-VII:D8 is turned ON hence C2, C4, C6 and C8 gets charged while C1,C3,C5 and C7 are discharged by ia. Hence C10, C12, C14, C16 and C18 supply the load whileC9, C11, C13, C15and C17 float.
Mode-VIII: D6 is turned ON hence C2, C4 andC6 gets charged while C1,C3 andC5 are discharged by ia. Hence C8, C10, C12, C14, C16 and C18 supply the load while C7, C9, C11, C13, C15 and C17 float.
Mode-IX:D4 is turned ON hence C2 and C4 gets charged while C1and C3 are discharged by ia .Hence C6, C8, C10, C12, C14, C16 and C18supply the load while C5, C7, C9, C11, C13, C15and C17float.
Mode-X:D2 is turned ON hence C2 gets charged while C1 is discharged by ia. Hence C4, C6, C8, C10, C12, C14, C16 and C18supply the load whileC3, C5, C7, C9, C11, C13, C15 and C17floats.
S1 and S2 are turned ON and the current ia is negative for modes XI to XIX. The inductor is discharged and input source transfers energy to the CW voltage multiplier through the odd numbered diodes.
Mode-XI:D17 is turned ON while diodes (D1to D16and D18) do not conduct hence C2, C4, C6, C8,C10, C12, C14 and C16 are all discharged whileC1,C3,C5,C7,C9,C11, C13, C15 and C17 are all charged by ia. Hence C18 supplies the load. Mode-XII: D15is turned ON hence C2, C4, C6, C8,C10, C12 and C14are all discharged while C1,C3,C5,C7,C9,C11, C13 and C15 are charged by ia. Hence C16 and C18supply the load while C17 floats.
Mode-XIII:D13is turned ON hence C2, C4, C6, C8,C10 and C12gets discharged while C1,C3,C5,C7,C9,C11 and C13are all charged by ia. Hence C14, C16 and C18supply the load while C15 and C17floats.
Mode-XIV:D11 is turned ON hence C2, C4, C6, C8and C10are all discharged while C1,C3,C5,C7,C9 and C11are all charged by ia. Hence C12, C14, C16 and C18supply the load while C13, C15 and C17float.
Mode-XV: D9 is turned ON hence C2, C4, C6 and C8 are discharged while C1,C3,C5,C7 and C9 are charged by ia. Hence C10, C12, C14, C16 and C18supply the load whileC11, C13, C15 and C17float.
Mode-XVI:D7is turned ON hence C2, C4 and C6are all discharged while C1,C3,C5 and C7 are charged by ia. Hence C8, C10, C12, C14, C16 and C18supply the load whileC9, C11, C13, C15 and C17 float.
Mode-XVII:D5is turned ON hence C2 and C4 gets discharged while C1,C3 and C5 are charged by ia. Hence C6, C8, C10, C12, C14, C16 and C18supply the load while C7, C9, C11, C13, C15 and C17float.
Mode-XVIII:D3 is turned ON hence C2 is discharged whileC1 and C3 are charged b yia. Hence C4, C6, C8, C10, C12, C14, C16 and C18supply the load whileC5, C7, C9, C11, C13, C15 and C17 are floating.
Mode-XIX:D1 is turned ON hence C1 is charged by ia. Hence all even capacitors supply the load while C3, C5, C7, C9, C11, C13, C15 and C17float.
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IV.SIMULATED CONVERTER TOPOLOGY FOR HYBRID SYSTEM

The simulation of the converter for the hybrid system is done using MATLAB software. The solar panel is used as the primary supply. The simulated solar panel of converter topology is shown as in Fig. 4.
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The solar panel has a subsystem which contains the design equations based on the equivalent circuit of a solar cell.
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The output waveforms and results obtained can be used so that it is easier to design the prototype model. The waveform of the output voltage of solar panel is shown below in Fig.6.
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The fuel cell is used as a back-up to the solar panel. The fuel cell and the solar panel are connected in parallel. Therefore only one will be used at a time. The simulated fuel cell is shown
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The first subsystem is the calculation of the Nernst voltage (Enernst). The simulated subsystem1 for calculating the Nernst voltage is shown in Fig. 8.
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The second subsystem of the fuel cell is calculation of the actual voltage (Vact). The simulated subsystem for calculating the actual voltage is shown in Fig. 9.
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subsystem of the fuel cell is for calculating the ohmic voltage drop (Vohmic) and the concentration voltage (Vconc). The simulated subsystem is shown as in Fig. 10.
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The simulated fuel cell has one output waveform which is the output voltage shown as in Fig.11.
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The simulated circuit consists of two output waveforms of the output voltage and output current.From the waveforms it can be observed that after a certain point the steady state is achieved.The simulated converter for the hybridsystem is show in Fig. 12.
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V. CONCLUSION

In this paper, the solar panel and fuel cell were designed and simulated initially using MATLAB software. The boost converter cascaded with Cockcroft-Walton voltage multiplier was designed and then integrated with the solar panel and fuel cell with solar panel as the primary source and fuel cell as the backup. Then the entire setup was simulated using MATLAB software. From the simulation results it was observed that, the proposed circuit for the hybrid system is suitable for high voltage applications. The boost converter for Cockcroft-Walton voltage multiplier depends on the capacitance value and number of stages of the voltage multiplier. In future work,for the same or any number of stages of the voltage multiplier used the voltage level can be improved by changingthe capacitance value.

References