Continuous Conduction Mode SEPIC Converter For Maximum Power Point Tracking Prof. S.R. Kurode Asst.Proffesor in Electrical Engineering, Govt.College Of Engineering, pune. [email protected] Ramesh babu. Darla, [email protected] Student Govt.College Of Engineering,pune. cell at a fixed cell temperature and at a certain radiation.

Abstract: -- The purpose of this paper is to present mathematical analysis of SEPIC Converter in continuous conduction mode to use as maximum power point tracker. The SEPIC converter has one power processing stage, which can be operated in step-up/stepdown mode. A steady state analysis is presented .The converter works with constant frequency and PWM. Design is presented for 60W output model, and MATLAB simulation results are presented. Index terms: SEPICConverter,CCM,MaximumPower Point Tracking.

Fig 1 characteristics of PV cell with R load

1.INTRODUCTION As green house effects and environmental issues become more of a concern, renewable energy is one of the options in reducing pollution. Further more, natural resources used in the production of power are dwindling and becoming more expensive. There are a few types of renewable energies; one of this is the Photovoltaic system (PV). PV module used to more expensive, but in recent years, their price has been slowly dropping and as they become increasingly economical, they will be used in more applications. PV modules output efficiency has also increased in recent years. And will these growths in PV technology; there is no doubt that PV will have a good stand in the future [11]. As PV becomes more affordable, its demand will increase, with this increase PV, it is better to operate PV module at Maximum Power Point (MPP). By operating PV module at MPP we can get maximum output power. And this power used to provide UPS, feeding or selling excess power to grid. There are many applications for PV module. Currently PV modules are used to power up, heater and pump water. Maximum Power Point Fig 1 shows the V vs I characteristic of the solar

If a variable series resistance, R is connected to the cell’s terminals, the operating point is determined by the intersection of the V vs I curve, with the load V vs I characteristics. The characteristic for a resistive load is a straight line with a slope of I/V = 1/R. If the resistance is too small, the cell operates in the region of A to B only (behaving as a constant current source) and if the resistance is too large, the cell operates in the region of C to D of the curve (behaving like a constant voltage source) The Maximum power point, it is the point on the V vs I curve of a cell where power is maximum (power dissipated on the resistive load is maximum), this can be seen in fig 1 from the figure, a maximum power condition is met when Imax meets with Vmax Why the MPPT Maximum Power Point Tracking (MPPT), it is the electronic tracking system to extract maximum available power from solar cells. A MPPT is required to meet the following conditions

1

current above the optimum. The array performance is reflected in both shapes and phase relationships. The product of the derivative p and v is negative if the current is below that for optimum power and positive if the current is above the optimum and zero when the maximum power point is being tracked. Since p v is a chain rule derivative, it is actually equal to dp/dv. This implies that by driving dp/dv to zero, power will be effectively maximized.

Make sure that the system operates close to the Maximum Power Point (MPP) when it is subjected to changing in environmental condition. Provide high conversion efficiency. Maintain tracking for a wide range of variation in environmental conditions. The MPPT consists of two basic components: a switch mode converter and control section. Switch Mode Converter

Incremental Conductance Algorithm [1] This method uses the source incremental conductance method as its MPP search algorithm. It is more efficient than Perturb and Observe method and independent on device physics. The output voltage and current from the source are monitored upon which the MPPT controller relies to calculate the conductance and incremental conductance, and to make its decision (to increase or decrease duty ratio output).

The switch-mode converter allows energy at one potential, stored as magnetic energy in an inductor, and then released at a different potential. By setting up the switch-mode section in various different topologies, like buck converter, boost converter, buck-boost Cuk and SEPIC. Normally, the goal of a switch-mode power supply is to provide a constant output voltage or current. In power trackers, the goal is to provide a fixed input voltage and /or current, such that the array is held at the maximum power point, while allowing the output to match the battery voltage.

2.SEPIC converter (Single Ended Primary Inductor Converter)

MPPT Control Algorithms As said above the converter is required for maximum power point tracker, so a SEPIC topology is selected, because it can be operated in step - up/down modes. The output is also not inverted as in the case of Cuk topology. Voltage conversion is accomplished with out transformers, instead using low cost inductors to transfer energy. The input and output voltages are DC isolated by a coupling capacitor and the converter works with constant frequency PWM [09].

There are many algorithms that are used to control the MPPT. The algorithms that are most commonly used are the perturbation and observation method, dynamic approach method and the incremental conductance algorithm. Perturbation and Observation Method [1] Perturbation and Observation (P&O) method has a simple feedback structure and fewer measured parameters. It operates by periodically perturbing (i.e. incrementing or decreasing) the array terminal voltage and comparing the PV output power with that of the previous perturbation cycle. If the perturbation leads to an increase (decrease) in array power, the subsequent perturbation is made in the same (opposite) direction. In this manner, the peak power tracker continuously seeks the peak power condition. Dynamic Approach Method [8] This method employs the ripple at the array output to maximize the array power by dynamically extrapolate the characteristic of the PV array. The instantaneous behavior of array voltage v, current i and power p can be grouped into three cases: current below that for the optimum power, current near the optimum and

Fig 2 SEPIC Converter Operating Modes Of SEPIC Converter [09] The SEPIC converter can be operated in two modes

2

capacitor C1 transfer its energy to the L2 . The capacitor C1 voltage is considered constant and equal to Vin . The currents Iin and Ilm increases linearly. During this stage D3 is kept blocked and the capacitor Co supplies energy to the load R.

1.Discontinuous conduction mode (DCM) 2. Continuous conduction mode (CCM) 1.Discontinuous conduction mode: In the DCM of SEPIC Converter. It has three operating phases a. Feeding phase b. Freewheeling phase c. Restoring phase The wave forms is shown in fig 3 [09]

fig 4 SEPIC Converter when S is ON Mode2: (DT

Abstract: -- The purpose of this paper is to present mathematical analysis of SEPIC Converter in continuous conduction mode to use as maximum power point tracker. The SEPIC converter has one power processing stage, which can be operated in step-up/stepdown mode. A steady state analysis is presented .The converter works with constant frequency and PWM. Design is presented for 60W output model, and MATLAB simulation results are presented. Index terms: SEPICConverter,CCM,MaximumPower Point Tracking.

Fig 1 characteristics of PV cell with R load

1.INTRODUCTION As green house effects and environmental issues become more of a concern, renewable energy is one of the options in reducing pollution. Further more, natural resources used in the production of power are dwindling and becoming more expensive. There are a few types of renewable energies; one of this is the Photovoltaic system (PV). PV module used to more expensive, but in recent years, their price has been slowly dropping and as they become increasingly economical, they will be used in more applications. PV modules output efficiency has also increased in recent years. And will these growths in PV technology; there is no doubt that PV will have a good stand in the future [11]. As PV becomes more affordable, its demand will increase, with this increase PV, it is better to operate PV module at Maximum Power Point (MPP). By operating PV module at MPP we can get maximum output power. And this power used to provide UPS, feeding or selling excess power to grid. There are many applications for PV module. Currently PV modules are used to power up, heater and pump water. Maximum Power Point Fig 1 shows the V vs I characteristic of the solar

If a variable series resistance, R is connected to the cell’s terminals, the operating point is determined by the intersection of the V vs I curve, with the load V vs I characteristics. The characteristic for a resistive load is a straight line with a slope of I/V = 1/R. If the resistance is too small, the cell operates in the region of A to B only (behaving as a constant current source) and if the resistance is too large, the cell operates in the region of C to D of the curve (behaving like a constant voltage source) The Maximum power point, it is the point on the V vs I curve of a cell where power is maximum (power dissipated on the resistive load is maximum), this can be seen in fig 1 from the figure, a maximum power condition is met when Imax meets with Vmax Why the MPPT Maximum Power Point Tracking (MPPT), it is the electronic tracking system to extract maximum available power from solar cells. A MPPT is required to meet the following conditions

1

current above the optimum. The array performance is reflected in both shapes and phase relationships. The product of the derivative p and v is negative if the current is below that for optimum power and positive if the current is above the optimum and zero when the maximum power point is being tracked. Since p v is a chain rule derivative, it is actually equal to dp/dv. This implies that by driving dp/dv to zero, power will be effectively maximized.

Make sure that the system operates close to the Maximum Power Point (MPP) when it is subjected to changing in environmental condition. Provide high conversion efficiency. Maintain tracking for a wide range of variation in environmental conditions. The MPPT consists of two basic components: a switch mode converter and control section. Switch Mode Converter

Incremental Conductance Algorithm [1] This method uses the source incremental conductance method as its MPP search algorithm. It is more efficient than Perturb and Observe method and independent on device physics. The output voltage and current from the source are monitored upon which the MPPT controller relies to calculate the conductance and incremental conductance, and to make its decision (to increase or decrease duty ratio output).

The switch-mode converter allows energy at one potential, stored as magnetic energy in an inductor, and then released at a different potential. By setting up the switch-mode section in various different topologies, like buck converter, boost converter, buck-boost Cuk and SEPIC. Normally, the goal of a switch-mode power supply is to provide a constant output voltage or current. In power trackers, the goal is to provide a fixed input voltage and /or current, such that the array is held at the maximum power point, while allowing the output to match the battery voltage.

2.SEPIC converter (Single Ended Primary Inductor Converter)

MPPT Control Algorithms As said above the converter is required for maximum power point tracker, so a SEPIC topology is selected, because it can be operated in step - up/down modes. The output is also not inverted as in the case of Cuk topology. Voltage conversion is accomplished with out transformers, instead using low cost inductors to transfer energy. The input and output voltages are DC isolated by a coupling capacitor and the converter works with constant frequency PWM [09].

There are many algorithms that are used to control the MPPT. The algorithms that are most commonly used are the perturbation and observation method, dynamic approach method and the incremental conductance algorithm. Perturbation and Observation Method [1] Perturbation and Observation (P&O) method has a simple feedback structure and fewer measured parameters. It operates by periodically perturbing (i.e. incrementing or decreasing) the array terminal voltage and comparing the PV output power with that of the previous perturbation cycle. If the perturbation leads to an increase (decrease) in array power, the subsequent perturbation is made in the same (opposite) direction. In this manner, the peak power tracker continuously seeks the peak power condition. Dynamic Approach Method [8] This method employs the ripple at the array output to maximize the array power by dynamically extrapolate the characteristic of the PV array. The instantaneous behavior of array voltage v, current i and power p can be grouped into three cases: current below that for the optimum power, current near the optimum and

Fig 2 SEPIC Converter Operating Modes Of SEPIC Converter [09] The SEPIC converter can be operated in two modes

2

capacitor C1 transfer its energy to the L2 . The capacitor C1 voltage is considered constant and equal to Vin . The currents Iin and Ilm increases linearly. During this stage D3 is kept blocked and the capacitor Co supplies energy to the load R.

1.Discontinuous conduction mode (DCM) 2. Continuous conduction mode (CCM) 1.Discontinuous conduction mode: In the DCM of SEPIC Converter. It has three operating phases a. Feeding phase b. Freewheeling phase c. Restoring phase The wave forms is shown in fig 3 [09]

fig 4 SEPIC Converter when S is ON Mode2: (DT