An Improved Interleaved High Power Flyback Inverter for Photovoltaic Application

Ms.S.V.Kamble

Assistant Professor,

Department of Electrical Engineering,

SBGI.MIRAJ.

Abstract— This paper present, a grid connected central-type photovoltaic inverter based on the interleaved flyback converter topology. The interleaved flyback converter used to maximize the power level, which can reduce current ripple, reduce the size of the passive component and reduce the overall cost. The existing system the flyback topology used in microinverter at very low power. The main aim of this paper is to design flyback converter at high power and prove its practicality with satisfactory performance as a central-type photovoltaic inverter. The design of flyback inverter rated at 12Kw by using interleaving of 2-stage flyback converter. The combined benefits of flyback converter to diminished size of passive filtering component. This converter works in discontinuous current mode (DCM), it provides the fast dynamic response, easy control and no turn ON losses. The operation principles are discussed in details. The proposed converter is simulated in MATLAB software.

Keywords—Flyback converter, Interleaved flyback converter, Discontinuous current mode (DCM), Maximum power point tracking (MPPT).

I. INTRODUCTION

Recently, the PV source is most significant energy source in the earliest power generation system because it gives light from the sun and it is available everywhere in free of cost[1]. The low cost is important for industrialization as well as residential applications [2]–[6]. Therefore, the main objective of this paper is the advancement in the photovoltaic (PV) inverter technology with the help of flyback inverter. By using this concept, we can get high power by using interleaving technique. The simplest structure of the flyback inverter and simple flow control power with improved power quality output at grid are the main requirements of these system. The flyback converter is having low cost and it requires less number of elements. This is due to the inductor is combined along with the transformer so that the voltage ratios are get multiplied with an advantage of isolation. So that the inductor required for storage of energy and the transformer is responsible for transfer of energy[7]. Here, combination of inductor and transformer can eliminates the bulky system and cost reduction and reduction in size of the converter takes place.

A Flyback converter has constructed by using a transformer which has very high leakage flux and have less transfer efficiency. So, the flyback converter is not suitable for very high power applications.

So,flyback converters are interleaved to make it suitable for high power applications.

Another advantage of interleaving is that the frequency of ripple components is increased in proportion to number of cells which allows easy filtering.

The solar power must be used very properly through this method so maximum power point tracker can be used for the PV system [19].The another important factor for system is selection of mode of operation and it is discontinuous current mode (DCM) which has several advantages mentioned as follows:

· It gives instant dynamic response.

· It has absence of turn on losses.

· The transformer size is small,so cost is low.

· It is easy to control.

The disadvantage of discontinuous current mode(DCM) is higherform factor which is compared to continuous current conduction mode(CCM) which leads to large amount of power losses. The solution for this is,paralleling the device.In this system, we provide interleaving of converters which decreases the peak value of current with high discontinuity. This discontinuity is removed by connecting all cells together at one common point .In the existing system, three interleaved transformers were implemented. It makes circuit more bulky and size of circuit is also increased circuit complexity and cost increase due to use of more components are drawbacks of those system. In the proposed system, size of circuit and complexity is reduced by removing switch and transformer. The capability of existing system can be developed in proposed system by increasing the turns of the transformers.

2. SYSTEM DESCRIPTIONS

The block diagram of the proposed system contains main five blocks. The photovoltaic source is acts as an input source, is fed to the flyback converter integrating decoupling capacitor. DC output of the converter is applied to the full bridge inverter; the inverter converts the DC output of the flyback converter to AC output. The low pass filter is provided to reduce the harmonic content of the inverter output current. The two control techniques are provided (1) To regulate DC input Photovoltaic current and voltage (2) It should also provide control strategy to convert direct current in to alternating current for power injection at the grid interface.

Fig.1 Block Diagram Of Interleaved Flyback Converter System

The maximum power point tracking algorithm is developed to extract the maximum power from the photovoltaic array and is fed to the unfolding full-bridge inverter by using interleaved flyback converter.

1. PV Module:- PV module is formed by series and parallel combination of PV cell, to provide desired value of voltage and current.

2. DC/DC converter:- The DC to DC converter can increase the voltage of the PV system depending on the load requirements. But interleaved flyback inverter boost the voltage fed by the PV panel in rectified ac form which can be synchronized with grid voltage or it can also directly fed to load. The rectified AC output voltage is higher than the grid voltage.

3.DC/AC inverter:-A modulated high-frequency sine PWM is used for the MOSFETs switches to generate the sinusoidal output voltage and current.

4.Filter:-Filter is connected to the output of the full-bridge inverter circuit. It is employed in system to eliminate the harmonics presented in final output.

4.Control system

PWM strategy is employed here to generate control signal which can control switching operation of MOSFET switches.

0. Solar Array

A solar array is a group of solar photovoltaic panels or modules connected electrically together and mounted on a sustainable structure to produce higher amount of power. For this project the main task is to design a stand-alone power generation system for a small load like a house situated on hilly area or for any small load that is not connected to grid network.

Fig.2 Formation of solar Module and solar PV Array

For this kind of loads design such a system that uses the power generated from PV Array and convert it into AC for AC loads or stores it in storage element with efficiently and paralleling supplies the load.

0. Maximum Power Point Tracking [MPPT]

The current and voltage of the Photovoltaic source is fluctuating throughout the day due to changing temperature and solar irradiation. The maximum power is extracted at only one operating point such a point, where, the value of current and voltage are maximum known as maximum power point. There are so many algorithms for MPPT but P & O algorithm is used because of its simplicity. Its unsettle duty ratio of flyback converter and it also adjust the DC link current and voltage among the photovoltaic array and flyback converter to extract maximum power. The fig. (6). shows flow chart of the P & O algorithm. The PV panel power can be computed measure photovoltaic current and voltage. The duty ratio can be reduced by changing small amount of voltage and power but the change should be positive. The duty ratio can be increase by keeping the power positive and change in voltage is negative. The appropriate duty ratio can be calculated by using MPPT algorithm and firing pulses of flyback converter is generated.

1. Perturb and Observe

The current and voltage of the Photovoltaic source is fluctuating throughout the day due to changing temperature and solar irradiation. The maximum power is extracted at only one operating point such a point, where, the value of current and voltage are maximum known as maximum power point. There are so many algorithms for MPPT but P & O algorithm is used because of its simplicity. Its unsettle duty ratio of flyback converter and it also adjust the DC link current and voltage among the photovoltaic array and flyback converter to extract maximum power. fig. 3. shows flow chart of the P & O algorithm. The PV panel power can be computed measure photovoltaic current and voltage. The duty ratio can be reduced by changing small amount of voltage and power but the change should be positive. The duty ratio can be increase by keeping the power positive and change in voltage is negative. The appropriate duty ratio can be calculated by using MPPT algorithm and firing pulses of flyback converter is generated.

Fig.3 Flow chart of P& O Method

0. Flyback Converter:-

In a flyback topology the main switch and flyback transformer are connected in series. Energy fed by PV module is stored in an inductor which is connected in parallel with flyback transformer which boosts up the voltage. The flyback transformer not only boosts up the voltage it also provides isolation between PV module and load or grid. The inductor configuration is used in flyback converter is divided in to two parts to form a transformer. it is also a buck-boost converter this topology provides high voltage conversion ratio by multiplying turn ratio of the transformer it is also capable of providing isolation. This configuration consists of magnetizing inductance ‘Lm’ with transformer of turn ration N1/N2. The losses in the transformer are negligible so losses are neglected. The mode of operation of this converter depends on magnitude of magnetizing inductance.

Fig.4 Flyback converter

The MOSFET is used as a switch. The primary side of the transformer is connected to the input supply. The magnetic flux is increased and potential is stored in it .Reversed biased condition of the output diode results in negative voltage induced in it. Output filter capacitor is required to supply the energy to the load. the topology in which MOSFET is turn off automatically current and flux in the primary side are reduced and the diode on the secondary side is forward biased results in positive voltage induced in secondary side of the transformer. The transformer contains stored energy is successfully supplied to the load. The flyback converter has many advantages which successfully eliminate the requirement of inductor filter by reducing cost and it is also capable of providing filtered output. When MOSFET is turned on the output capacitor discharges energy to the load by delivering load current, where its value should be high in this converter.

1. Operation of proposed system:-

The flyback converter is used in both AC/DC and DC/DC conversion with galvanic isolation between input and many outputs. The flyback converter is a buck-boost converter with the inductor split to form a transformer, so that the voltage ratios are multiplied with an additional advantage of isolation. The flyback converter is an isolated power converter.

Fig. 5. Circuit diagram of the PV inverter system based on two cell interleaved flyback converter topology.

As shown in fig. 5 the circuit configuration of proposed converter. The photovoltaic array as an input source, the photovoltaic current (Ipv) is applied to the two stage interleaved flyback converter through the decoupling capacitor (C).The decoupling capacitor is provided to eliminate harmonics present in the photovoltaic current and also provide balance to the system. The low on-state resistance power electronic switch (MOSFET) is used in the primary side of the flyback converter as a flyback switch. During turn-on period of MOSFET switches S1, S2 and S3, a current from photovoltaic array is passed through the magnetizing inductance (Lm) and primary winding of the flyback converter. The main function of magnetizing inductance is to store the magnetic field energy. During turn-on period of switches they can’t flow through the secondary side due to the current blocked by the reversed biased diode. During this period the energy supplied to the grid by Cf and Lf. During turn-off period of switches, the magnetic field store energy of the magnetizing inductance is delivered to the grid in the form of current. So, the operation of the above configuration based on the flyback inverter and behaves as a voltage controlled current source. The proposed converter operated in DCM mode and provides easy as well as steady state generation at AC current at the grid interface. While operating of proposed converter in the DCM mode with open loop control generates triangular current pulses for each switching period. If SPWM method is used for control the proposed system can provide sinusoidal current in phase with grid voltage.The input current of the flyback converter as shown in fig 6 and output current of the flyback converter after inverter as shown in fig.7.

Fig. 6. Input current of flyback converter over a one grid period.

Fig. 7. Output current of flyback converter after full-bridge inverter..

The unfolding full-bridge inverter is only responsible for unfolding the sinusoidally modulated DC current packs in to AC at the right moment of the grid voltage. The grid frequency is used to operate the inverter switches. The grid frequency is low. So, the turn-on and turn-off losses of IGBT are negligible. The conduction losses are considered due to the high on-state resistance power electronic switches (IGBT) of the inverter. The control signal for flyback converter’s MOSFET switch, primary side voltage of flyback transformer (Vp), magnetizing current (im).

1. Simulation and results:

To verify the design as well as to determine hardware requirements,simulations should be done. Like,current ratings,power required etc.can be easily getting from the simulation results.Fig.5.shows simulation model used for the simulation studies.

4.1 Simulation Parameters

Parameter

Symbol

Value

Unit

PV voltage

Vpv

88

V

PV current

Ipv

22

A

Decoupling capacitor

C

9400

µF

Magnetizing inductance

Lm

8

µH

Primary winding turns

Np

4

-

Secondary winding turns

Ns

18

-

Filter inductance

Lf

250

µH

Filter capacitance

Cf

1

µF

Switching frequency

Fs

40

kHz

Number of interleaved cell

Ncell

2

-

Grid frequency

F

50

Hz

Grid voltage

V

240

V

Table 4.1 Simulation Parameter

Following parameters are used for to simulation of proposed system.

4.2 Model of Proposed system

In order to test the performance of the proposed converter, the system shown in fig. 8 is simulated.

Fig.8 MATLAB Model for the proposed interleaved flyback inverter

4.3 Simulation Results

Fig.9. PV voltage

Fig.10. PV current

Fig.11. Diode voltage and current

Fig.12.MOSFET current and voltage

Fig.13.Grid Output current & voltage

Fig.14. THD Analysis for grid current

Fig.15. THD Analysis for grid voltage

In this way, we have getting proper results of proposed system from the simulation.

2. Hardware development

In this paper, we describes details about the prototype model of interleaved flyback inverter using the driving circuit of MOSFET and controlled by PIC Controller [PIC16F877A] controller. And also it includes the hardware results.

Following fig.16 shows the experimental setup of proposed interleaved flyback inverter for PV application. This experimental setup consist of Solar panel, Driver circuit, Transformer, Rectifier diode, Capacitor filter, Single inverter circuit, PIC16F877 controller board, Relay and load.

Fig.16. Experimental set up

Fig 6.16 shows the experimental setup of interleaved flyback inverter for PV applications. The solar panel is used as input source, To use a 12Watt solar panel these converts light energy to electrical energy. And this electrical energy is used.

Depend upon solar radiations intensity, harmonics will generate at input side,So switching stresses may occur. To eliminate we are using Flyback converter technology.The switch,inductor,diode and transformer is the combination of flyback converter.The Flyback switch creates more stresses and this can be overcome by Flyback inductor,All harmonics are dropped at inductor,So at the secondary side of transformer,there are no any losses or harmonics ,so we have getting pure ac.

Controller driver is used for controlling Flyback converter.When MOSFET switches are triggered then only Flyback converter will operate and this can be triggered by using programming in PIC16F877.Here we need to program for 6 pins in port c having PWM pins.Microcontroller operates at 5V DC.SO we can step down 230 V to 12V ac for that.Then we rectified 12 V ac to 12 V dc by using diode rectifier.After that 12V DC is filtered by capacitor.So we can get pure DC .After that by using regulator, we can regulate 5V DC constant and this can be given to microcontroller.In this part ,we are using power supply, microcontroller IC,crystal oscillator to generate frequency.

But MOSFETS operates or triggered at 9V-20V.So we have to amplify 5V to 9V.For that we are using drivers.We can amplified it by using two transistors CK100[PNP] and 2N22[NPN].It gives output to switches. If amplification gets reverses,then microcontroller will damage ,so to avoid that we are using optocouplers.In this way,we have getting the proper results which are matched with simulation results.

Fig.17. Output of DSO showing inverter output and grid

3. Conclusion

Thus an interleaved flyback inverter has become an important component because it can operate light loads. The conduction losses can be reduced in switches by using these concept. Ultimately, an improved interleaved high power flyback interleaved high power flyback inverter is best solution for all the photovoltaic applications. It having efficiency of 90.16%, THD of 4.46% and power factor of 0.998 is very advantageous when compared to other inverters. The effectiveness of this proposal is confirmed by the simulation result.

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I.