WWW.IJITECH.ORG ISSN 2321-8665 Vol.04,Issue.16, October-2016, Pages:3130-3134 An Interleaved High-Power Flyback Inverter for Photovoltaic Applications B. SATHISH 1, S. NARESH 2, S. RAJESH 3 1 PG Scholar, Dept. of EEE, Siddhartha Institute of Engineering and Technology, Ibrahimpatnam, Hyderabad, India, E-mail: sathishsatya207@gmail.com. 2 Assistant Professor, Dept. of EEE, Siddhartha Institute of Engineering and Technology, Ibrahimpatnam, Hyderabad, India, E-mail: nareshraju.32@gmail.com. 3 Assoc Prof & HOD, Dept. of EEE, Siddhartha Institute of Engineering and Technology, Ibrahimpatnam, Hyderabad, India, E-mail: Rajesh118@gmail.com. Abstract: This paper presents analysis, design, and implementation of an isolated grid-connected inverter for photovoltaic (PV) applications based on interleaved fly back converter topology operating in discontinuous current mode. In today s PV inverter technology, the simple and the lowcost advantage of the fly back topology is promoted only at very low power as micro inverter. Therefore, the primary objective of this study is to design the fly back converter at high power and demonstrate its practicality with good performance as a central-type PV inverter. For this purpose, an inverter system rated at 2 kw is developed by interleaving of only three fly back cells with added benefit of reduced size of passive filtering elements. A simulation model is developed in the piecewise linear electrical circuit simulator. Then, the design is verified and optimized for the best performance based on the simulation results. Finally, a prototype at rated power is built and evaluated under the realistic conditions. The efficiency of the inverter, the total harmonic distortion of the grid current, and the power factor are measured as 90.16%, 4.42%, and 0.998, respectively. Consequently, it is demonstrated that the performance of the proposed system is comparable to the commercial isolated PV inverters in the market, but it may have some cost advantage. Keywords: Fly Back Converter, Harmonics, Interleaved Converters, Photovoltaic (PV) Inverters. I. INTRODUCTION The solar energy is considered as one of the most renewable and freely available source of energy and the candidate to play a greater role in the energy market of the world in the near future [1]. Therefore, the research and development in the solar technology field is in the rise [2] [6], [8] [20], [22] [25]. However, the high cost of the technology still limits its usage globally. The low cost is greatly important for commercialization especially in small electric power systems including the residential applications [2] [6]. Therefore, the primary objective of the study presented in this paper is to contribute to the research and development in the photovoltaic (PV) inverter technology by trying the fly back topology at high power. If it is implemented effectively with good performance, the developed inverter system can be a low-cost alternative to the commercial isolated grid-connected PV inverters in the market. The simple structure of the fly back topology and easy power flow controls with high power quality at the grid interface are the key motivations for this work. The fly back converter is recognized as the lowest cost converter among the isolated topologies since it uses the least number of components. This advantage comes from the ability of the fly back topology combining the energy storage inductor with the transformer. In other type of isolated topologies, the energy storage inductor and the transformer are separate elements. While the inductor is responsible for energy storage, the transformer on the other hand is responsible for energy transfer over a galvanic isolation [7]. The combination of these two components in a fly back topology eliminates the bulky and costly energy storage inductor and therefore leads to a reduction in cost and size of the converter. However, we have to make it clear here that the cost depends on the implementation as much as the selected topology, so not every implementation of the fly back topology leads to a low-cost converter. For this reason, as we try to achieve the high-power implementation of the fly back converter with good performance, which is our primary research contribution, we will also try to preserve the cost advantage during the final implementation step. Practical implementation of a transformer with relatively large energy storage capability is always a challenge. The air gap is where the energy is stored, so a high-power fly back converter design needs a relatively large air gap. As a result of this, the magnetizing inductance is going to be quite small. The aforementioned challenge is actually achieving such a small magnetizing inductance with low leakage inductance. A fly back converter built with a transformer that has large leakage flux and poor coupling will have poor energy transfer efficiency. Mainly for this reason, the fly back converters are generally not designed for high power. As a result, the fly back topology finds a limited role in PV applications only at very low power as micro inverter [10] [13]. In this technology, every PV panel comes with a dedicated energy conversion unit; a micro inverter attached to the output Copyright @ 2016 IJIT. All rights reserved.
terminals. For this reason, the technology is also named as ac PV module application [14] [18]. In this practice, many such ac PV modules are connected in parallel to get the desired power output. The maximum harvesting of solar energy in this method is the best since there is a dedicated maximum power point tracker (MPPT) for each PV panel [19]. However, the overall cost of this application is higher compared to the central-type inverter systems. Nevertheless, when advanced design methods are employed effectively, single-stage fly back converters can be designed and used in high power applications as well. Furthermore, the interleaving of these high-power fly back stages (cells) facilitates developing a central-type PV inverter. The added benefit of interleaving is that the frequency of the ripple components (undesired harmonics) at the waveforms are increased in B. SATHISH, S. NARESH, S. RAJESH to synthesize a sinusoidal current with good total harmonic distortion (THD). This makes the implementation of the control system less complex for DSP and allows faster execution time. Contrary to the aforementioned great benefits of the DCM operation, it has several disadvantages as well. In this mode of operation, the current waveforms have higher form factor (high RMS to mean ratio) compared to continuous current mode (CCM). This normally leads to more power losses. So, as a solution, every current carrying path including the switching devices should have low resistivity. Another drawback of DCM operation is the current pulses with large peaks and high amount of discontinuity in the waveforms. Device paralleling is a way to handle the high peak currents. Nevertheless, these disadvantages can be considerably reduced by interleaving of several cells. As a first benefit, the current in each cell will have much less peak but the same amount of discontinuity. However, the discontinuity will be significantly reduced as soon as the cells connect at the common point. All this benefits come from the ability of phase shifted several cells spreading the power flow evenly over the witching cycle with minimum discontinuity at the source and grid side. In brief, the effective interleaving has the potential to solve or greatly reduce the adverse effects of the DCM operation [21]. Fig.1. Block Diagram of the Proposed Grid-Connected PV Inverter System Based on Interleaved DCM Fly Back Converter Topology. Proportion to the number of interleaved cells. This feature facilitates easy filtering of the ripple components or using smaller sized filtering elements. The ability to reduce the size of passive elements is beneficial for reducing the cost and obtaining a compact converter [21]. Fig. 1 shows the block diagram of the proposed inverter system. The results of an earlier work based on the same topology where the primary objective was to prove the concept with a design at 1 kw were presented in [26]. Since the time of that work, there have been major design changes and upgrades in order to process twice more power and at the same to achieve better overall performance. As mentioned before, the choice of operation mode for the converter is discontinuous current mode (DCM). The fundamental motivations for selecting DCM operation are summarized as follows. It provides very fast dynamic response and a guaranteed stability for all operating conditions under consideration. No reverse recovery problem. The diodes exhibits reverse recovery problems in CCM operation which cause noise, electromagnetic interference problems, and additional losses. So, DCM operation eliminates all these complications. No turn on losses. Small size of the transformer. Easy control. No need for a feedback loop for the control of the grid current. Only an open-loop control is enough Consequently, the circuit diagram of the proposed inverter system based on three-cell interleaved DCM fly back converter topology is shown in Fig. 2. In conclusion, this study has developed and presented the technology in full detail to produce a grid-tied, isolated, and central-type inverter based on the fly back converter topology at 2 kw, which is not available in today s PV market. The developed system has performed satisfactorily according to major specifications such as the efficiency and the THD of the grid current. Moreover, the study has developed high-power fly back transformers at 700 W and below with extremely low leakage inductance. We also consider this outcome as the significant research contribution since this technology may lead to the development of different applications where the low cost and simplicity are always an issue. The remainder of this paper is organized as follows. Section II describes the converter topology and defines the operating Principles. Section III performs the analysis of the converter and derives the design equations. Section IV presents the design of the converter in steps. SectionsV and VI give the simulation and the experimental results, respectively. Finally, SectionVII provides the conclusions. II. TYPES OF INVERTERS The basic function of the inverter in a photovoltaic solar power system array is to convert the DC electricity generated by the solar panels into standard AC power. Any photovoltaic system which supplies power to an load must use an inverter to cover the DC power generated into AC power. There are four basic types of inverters commonly used namely Standalone inverters, Grid Tie inverters,bimodal inverters,
An Interleaved High-Power Flyback Inverter for Photovoltaic Applications AC module inverters. In this project we are using Grid Tie Inverters. Popularly known as utility-interactive inverters, these inverters are connected to and work concurrently with the local utility grid. Power from the PV array is first directed to the point of consumption based on the load demand. Any excess power from the solar array which is not consumed is fed back into the utility grid through the power company s electrical meter. These inverters are inbuilt with safety features like anti-islanding. Anti-islanding will shut down the transmission of power from the solar array to the utility in case of any fault or other serious fluctuation in voltage or frequency. III. FLYBACK CONVERTERS In this paper, we have analyzed about the working of fly back converter operating in BCM mode. Conventional converters used like Buck boost have certain disadvantages hence we go for the improved fly back topology in this paper. The fly back converter is used in both AC/DC and DC/DC conversion along with the galvanic isolation between the input and any outputs. The fly back converter is nothing but a buck boost converter with the inductor split to form a transformer, with an additional advantage of isolation. The fly back converters provide isolation between input and output. The main reason for isolation is for safety to prevent any kind of shock hazard when using a piece of equipment. In addition to providing safety, isolation is used to separate sections from one another. controller operates right on the boundary between CCM and DCM. During this mode, the switch turns on and stores just enough charge to replenish the load during the time the switch opens. Thus the switch will turn on again once all the energy stored is transferred to the output load. The point where the switch is turned on and the current begins to ramp in the primary occurs as soon as the current returns to zero in the secondary. since the switching occurs after all the energy is transferred, the operating frequency is dependent on line and load conditions. V. COMPARISON BETWEEN BCM AND CCM DCM operation is very similar to BCM operation because the transformer is not required to store energy across cycles. Attributing to this factor, the two operational modes i.e. BCM and DCM share similar operating characteristics, merits and demerits. Due to this reason a comparison of CCM and BCM would provide more insight into the advantages and disadvantages of each operational strategy. Comparison is first made in terms of losses and then in terms of operational strategy. A. Loss Mechanism There are four major losses which include conduction losses, switching losses, transformer losses and diode losses. Of the four losses diode loss accounts for about half of the accountable loss. When comparing CCM and BCM, certain losses are higher for one than the other. For example in terms of conduction loss, the BCM mode shows higher loss because of its higher relative RMS current levels. On the other hand in terms of switching loss,the loss here is higher for CCM since the strategy operates at higher frequency resulting in a larger loss. It was not clear whether CCM or BCM resulted in higher diode losses. Finally it is difficult to arrive at a conclusion as of which strategy is better in terms of comparison of just the losses. Fig.2. Basic Fly Back Inverter and Its Functioning IV. MODES OF OPERATION The fly back converter usually operates in three modes namely CCM,DCM and BCM. They can exclusively operate in one mode for their load and line range but can also switch between modes of operation. For example, a fly back converter can be designed to operate in CCM for high loads and then transition to DCM for light loads. In this paper we have realized the operation of fly back in BCM mode for high loads and enter DCM at light loads.in the BCM the fly back operates at a variable frequency that is varies with the output load for a given input voltage and output voltage. Entering DCM allows the fly back to handle with light loads eliminating the need to operate at higher frequencies. If the fly back remained in BCM, then the operational frequency might increase to infinity while operating at lighter loads. The name boundary conduction mode comes from the fact that the Fig.3. Proposed System Block Diagram. B. Operational Comparison BCM differs from CCM fact that switching occurs only after all the energy stored in the primary of the transformer is transferred to the secondary. One key advantage gained from switching at a known condition on each cycle is that no oscillator is required. A second operational difference is how the two topologies deal with current limit or short circuit conditions. The BCM mode is inherently suited for short circuit operation because it only switches once the current reaches zero in the secondary.in addition, a similar advantage
relates from the fact that the diode turns off at zero current in BCM. The soft switching of the output diode reduces EMI generated by the turn off of the output diode. For the reasons listed above, BCM provides some advantages over CCM operation in practical circuit applications. VI. WORKING OF THE CIRCUIT The PV panel supplies DC power to the Fly back inverter. The Fly back inverter is a combined block of Fly back converter and Grid tied inverter. The grid tie inverter works in tandem with the load. The grid tie inverter inverts the DC into AC and feeds the AC supply to the load. The operation of the converter in the inverter block is controlled using a MOSFET driver. The driver controls the duty cycle of the MOSFET in the converter circuit thereby controlling the operation of the inverter. The output load current is compared with the reference current in the comparator the comparator output is fed to the controller. Based on the error signal the comparator drives the controller. The controller in turn drives the MOSFET driver. During heavy loads the inverter is made to operate in BCM mode when light loads, it operates in discontinuous conduction mode. Thereby the inverter operates at a wider switching frequency during heavy loads. The MOSFET being a faster switching device and hence the switching losses are reduced. VII. CONCLUSION A central-type PV inverter for small electric power system applications rated at 2 kw is implemented based on the interleaved fly back converter topology. The 2 kw power level is achieved by interleaving of three fly back cells each rated at 700 W. The fly back topology is selected because of its simple structure and easy power flow control with high power quality outputs at the grid interface. The experimental results prove the successful operation of the inverter and compliance to the specifications. 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