HIGH GAIN DC- DC CONVERTER USING LPPT TECHNIQUE FOR PV APPLICATION

Transcription

1 Volume 119 No , ISSN: (on-line version) url: HIGH GAIN DC- DC CONVERTER USING LPPT TECHNIQUE FOR PV APPLICATION Karuppiah M*, Karthikumar K*, Arunbalaj A* *Assistant Professor s, Department of Electrical and Electronics Engineering, Veltech, Avadi, Chennai ABSTRACT:-Differential power processing (DPP) systems are an effective architecture for future photovoltaic (PV) power systems that achieve high system efficiency through processing a faction of the full PV power. It achieves distributed local maximum power point tracking (MPPT). The power processed through the DPP converters depends on the string current in the PV-to-bus DPP architecture. The string current must be controlled to minimize the power processed through the DPP converters. A real-time least power point tracking (LPPT) method is proposed to minimize power stress on PV DPP converters. Mathematical analysis shows the unique of the least power point for the total power processed through the system. The perturb-and-observe LPPT method is presented. The method enables the DPP converters to maintain optimal operating conditions. The method helps in reducing the total power loss and converter stress. This work validates through simulation and experimentation that LPPT in the string-level converter successfully operates with MPPT in the DPP converters to maximize output power for the PV-to-bus architecture. Hardware prototypes were developed and tested at 140 and 300 W, and the LPPT control algorithm showed effective operation under steady-state operation and an irradiance step change Keyword: Differential power processing, least power point tracking, photovoltaic (PV) systems, perturb-andobserve (P&O), PV-to-bus (PV bus) architecture 1. INTRODUCTION The various renewable green energy sources, solar photovoltaic (PV) power generation capacity is increasing worldwide. In addition to the construction of large, utility-scale solar farms, rooftop solar systems are also gaining wide acceptance in many areas. For PV power at both the utility and residential scale, cost and efficiency are crucial factors to maximizing watts-per-cost, which is a primary concern for PV system owners.ease of installation with minimal power electronics is also an important consideration. Increasing efficiency, decreasing system cost, and reducing the size of the power converters are all important factors in developing future PV systems. This research focuses on a control algorithm for the recently spotlighted converter architecture for PV systems, called differential power processing (DPP), which shows promise in terms of efficiency, cost, and size improvements the photovoltaic systems are major contributors in the electrical power. These are utilized effectively with interface to the existing systems through DC-DC converters. The major challenge is to extract the power under varying operating conditions which influence the output voltage.isolated converter structures with cascaded configuration enables to achieve high voltage gain.these are used up to several kw applications. The multilevel buck converters proposed are widely used in high frequency DC/DC power conversion. In the conventional boost converters, high voltage ratio is feasible without multistage cascading. The voltage ratios in these are limited by the parasitic elements and switching control used.threelevel boost converters have significant advantage as compared to conventional boost converter. The size of the inductor is reduced and switch voltage rating is half of the output voltage. This reduces the overall size and improves the efficiency in three-level DC- DC converters. However, the voltage balancing across the DC bus capacitors is required due to nonidealities in the components. This is feasible by sensing the voltages across them with corrective feedback through controllers.the current sensing of inductor by dispensing the voltage measurements is feasible to balance the voltages. PV array in solar power conversion system operates at a point having maximum power transfer. It is necessary to track this operating point by using the MPPT control algorithms to maximize the utilization efficiency. Various algorithms for MPPT are reported in the literature and used for the efficient energy conversion process 2. PV string The various power conditioning architectures and converter topologies have been developed to improve system efficiency and maximize output power. The centralized power conditioning architecture, which is considered the first generation, has benefits of high costeffectiveness and low design complexity; however, due to inability of individualized control, the energy efficiency suffers greatly when there is imbalance 195

2 among the strings. The second-generation architecture utilizes a multistring architecture, where each PV string has its own power conditioner. This shows a slight improvement over the centralized architecture, but the efficiency of one entire string still greatly decreases when it experiences partial shading, due to imbalance within a single PV string. A multistring power conditioning system wherein each PV module is supplied from its own dc dc converter, is tied to a common dc bus and then the total power is processed through a single inverter. Such solutions are free from nominal power limitation, mismatch problem of parallel-connected strings, and severe partial shading problems. A gridconnected PV system employing two parallelconnected inverters is proposed by Romero-Cadaval et al.in this approach is that the way in which the inverter switching strategy. One of the inverters processes the power under quasi-square wave strategy, while the other inverter uses pulse width modulation control. The concept of multilevel inverter is well established in the conventional power processing. However, its application in PV is slowly coming up. Three such applications are reported in this Special Section. A single-phase H-bridge multistring topology while the second one is a sixlevel three-phase inverter for stand-alone applications. The third one is focused on the parallel connection of inverters and then feeding power to a common grid. The resulting conversion structure performs as a multilevel power active filter, doubling the power capability of a single voltage source inverter with given voltage and current ratings. These methodologies demonstrated the reduction in the total harmonic distortion through experiments. 3. METHODOLOGY 3.1 LPPT Controller for PV Based High Gain Dc-Dc Converter The proposed LPPT control algorithm is designed to minimize the power processed through the DPP converters, while maintaining effective MPPT of each PV module it shown in block diagram (Fig.1). The LPPT algorithm aims to dynamically track the unique LPP according to the instantaneous PV voltage and current in the system.since the LPP is a minimum point in a convex function, an extremum seeking algorithm can be used. After measuring the current and voltage for each DPP converter, the power processed through each converter is calculated, and then, summed together. The current DPP power sum is compared to the previous calculation. Fig 1 Block diagram If the power decreased, such that the algorithm is moving toward the LPP, the string current is incremented in the same direction; however, if the power increased, the current increment changes direction to continually move toward the LPP. MPPT implementations utilize algorithms that frequently sample panel voltages and currents, then adjust the duty ratio as needed. Microcontrollers are employed to implement the algorithms. Modern implementationsoften utilize larger computers for analytics and load forecasting. The current DPP power sum is compared to the previous calculation. If the power decreased, such that the algorithm is moving toward the LPP, the string current is incremented in the same direction; however, if the power increased, the current increment changes direction to continually move toward the LPP. The MPPT implementations utilize algorithms that frequently sample panel voltages and currents, then adjust the duty ratio as needed. Microcontrollers are employed to implement the algorithms. Modern implementations often utilize larger computers for analytics and load forecasting.the current DPP power sum is compared to the previous calculation. If the power decreased, such that the algorithm is moving toward the LPP, the string current is incremented in the same direction; however, if the power increased, the current increment changes direction to continually move toward the LPP. In double-stage gridconnected PV inverters, the dynamic interactions among the dc/dc and the dc/ac stages and the MPPT are investigated. The detrimental effects, particularly in terms of system efficiency and MPPT 196

3 performance, of the oscillations of the PV array voltage, taking place at the second harmonic of the grid frequency, are evidenced. The use of a proper compensation network acting on the error signal between a reference signal provided by the MPPT controller and a signal proportional to the PV array voltage is proposed. A control technique based on a compensation network together with the design guidelines able to cancel the PV voltage oscillations is proposed. LCL filters provides good harmonic attenuation but are prone to resonance that can lead to stability issues. Liu et al. propose a two-current-loop control composed of an outer grid-current loop and a filter-capacitor-current inner loop. The control design is difficult as two feedbacks cannot provide complete information of a three-order LCL filter PV panel A photovoltaic system, also solar PV power system, or PV system, is a power system designed to supply usable solar power by means of photovoltaic. It consists of an arrangement of several components, including solar panels to absorb and convert sunlight into electricity, a solar inverter to change the electric current from DC to AC, as well as mounting cabling and other electrical accessories to set up a working system in the circuit diagram fig.2. It may also use a solar tracking system to improve the system's overall performance and include an integrated battery solution, as prices for storage devices are expected to decline. Strictly speaking, a solar array only encompasses the ensemble of solar panels, the visible part of the PV system, and does not include all the other hardware, often summarized as balance of system (BOS). Moreover, PV systems convert light directly into electricity and shouldn't be confused with other technologies, such as concentrated solar power or solar thermal, used for heating and cooling. The solar panels which generate photocurrent, a solar PV system is made up of many components charge controller, inverter, batteries all connected by wires. None of these are 100 percent efficient. For instance, every 100 amp-hour drawn from the battery requires putting in about amp-hours of charge into it. Inverters are typically only 85 percent efficient and likewise for charge controller. Cables offer electrical resistance to the flow of current which is substantial because of rather low DC voltages involved. The people are try keeping it low by working with higher DC voltages where possible and yet keep wiring cost and resistance losses low (say less than 3%). Therefore, in order to account for all such losses the modules must put in some extra energy into the system; not merely the daily watt-hour load. A load is directly connected to the solar panel, the operating point of the panel will rarely be at peak power. The impedance seen by the panel derives the operating point of the solar panel. Thus by varying the impedance seen by the panel, the operating point can be moved towards peak power point. Since panels are DC devices, DC-DC converters must be utilized to transform the impedance of one circuit (source) to the other circuit (load). Changing the duty ratio of the DC-DC converter results in an impedance change as seen by the panel. At a particular impedance (or duty ratio) the operating point will be at the peak power transfer point. The I-V curve of the panel can vary considerably with variation in atmospheric conditions such as radiance and temperature. Therefore it is not feasible to fix the duty ratio with such dynamically changing operating conditions.pv array in solar power conversion system operates at a point having maximum power transfer. It is necessary to track this operating point by using the MPPT control algorithms to maximize the utilization efficiency. Various algorithms for MPPT are reported in the literature and used for the efficient energy conversion process Fig 1 Circuit diagram 3.3 PV bus bidirectional DPP Fly-back converter system The fly-back converter is used in both AC/DC and DC/DC conversion with galvanic 197

4 isolation between the input and any 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. When driving for example a plasma lamp or a voltage multiplier the rectifying diode of the boost converter is left out and the device is called a fly-back transformer. The schematic of a flyback converter can be seen in (Fig..3). It is equivalent to that of a buckboost converter, [1] with the inductor split to form a transformer. Therefore the operating principle of both converters is very close. When the switch is closed, the primary of the transformer is directly connected to the input voltage source. The primary current and magnetic flux in the transformer increases, storing energy in the transformer. The voltage induced in the secondary winding is negative, so the diode is reverse-biased (i.e., blocked). The output capacitor supplies energy to the output load. circuitry, although the output voltages have to be able to match each other through the turns ratio. Also there is a need for a controlling rail which has to be loaded before load is applied to the uncontrolled rails, this is to allow the PWM to open up and supply enough energy to the transformer. The flyback converter is an isolated power converter. The two prevailing control schemes are voltage mode control and current mode control (in the majority of cases current mode control needs to be dominant for stability during operation). Both require a signal related to the output voltage. There are three common ways to generate this voltage. The first is to use an optocoupler on the secondary circuitry to send a signal to the controller. The second is to wind a separate winding on the coil and rely on the cross regulation of the design. The third consists on sampling the voltage amplitude on the primary side, during the discharge, referenced to the standing primary DC voltage. Fig.3 Fly-back converter When the switch is opened, the primary current and magnetic flux drops. The secondary voltage is positive, forward-biasing the diode, allowing current to flow from the transformer. The energy from the transformer core recharges the capacitor and supplies the load. The operation of storing energy in the transformer before transferring to the output of the converter allows the topology to easily generate multiple outputs with little additional 3.4 DPP fly-back converter The fly-back converter is a buckboost 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. The two prevailing control schemes are voltage mode control and current mode control (in the majority of cases current mode control needs to be dominant for stability during operation). Both require a signal related to the output voltage. There are three common ways to generate this voltage.the leakage inductance current of the primary winding finds a low impedance path through the snubber diode to the snubber capacitor. It can be seen that the diode end of the snubber capacitor will be at higher potential. To check the excessive voltage build up across the snubber capacitor a resistor is put across it. Under steady state this resistor is meant to dissipate the leakage flux energy. The power lost in the snubber circuit reduces the overall efficiency of the fly-back type circuit. Differential power processing (DPP) systems are among the most effective architectures for photovoltaic (PV) power systems because they are highly efficient as a result of their distributed local maximum power point tracking ability, which allows the fractional processing of the total generated power. However, DPP systems require a highefficiency, high step-up/down bidirectional converter with broad operating ranges and galvanic isolation. This study proposes a single, magnetic, 198

5 highefficiency, high step-up/down bidirectional DC DC converter. The proposed converter is composed of a bidirectional flyback and a bidirectional isolated switched-capacitor cell, which are competitively cheap. The output terminals of the flyback converter and switched-capacitor cell are connected in series to obtain the voltage step-up. In the reverse power flow, the converter reciprocally operates with high efficiency across a broad operating range because it uses hard switching instead of soft switching. The genuine on off interleaved energy transfer at the transformer core and windings, thus providing an excellent utilization ratio. The dynamic characteristics of the converter are analyzed for the controller design. Finally, a 240 W hardware prototype is constructed to demonstrate the operation of the bidirectional converter under a current feedback control loop. To improve the efficiency of PV system, the maximum power point tracking method is applied to the proposed converter. 3.5 LPPT The optimal string current for LPPT changes based on the PV module s operating point, which will vary during MPPT. In contrast, each PV module s MPP is not affected by string current variation. Thus, after each perturb step for the MPPT, multiple LPPT steps are needed to for the string current to reach the new optimal point in steady state before the MPPT algorithm s next step. The ability of these two P&O algorithms (LPPT and MPPT) to work simultaneously. Least power point tracking (LPPT) is a technique that grid connected inverters, solar battery chargers and similar devices use to get the maximum possible power from one or more photovoltaic modules. A charge regulator or battery regulator limits the rate at which electric current is added to or drawn from electric batteries. It prevents overcharging and may protect against overvoltage, which can reduce battery performance or lifespan, and may pose a safety risk. The three-level boost converter is used to interface the PV system for maximization of the power extraction. The new maximum power point tracking algorithm based on the golden section search method is implemented. This algorithm shows the better dynamic response with the faster convergence without any oscillations while tracking. The voltage balancing of the DC bus is executed through the PI controller and performance is observed to be satisfactory. 3.6 LPPT algorithm LPPT algorithm for the PV bus DPP architecture that aims to minimizes the power processed through the high step-up, DPP flyback converters. The LPPT algorithm calculates the instantaneous power processed through the DPP converters and controls the string current to operate at the least power point of the DPP converters power curve. The LPP will either be a single point that is equal to one of the PV currents or a continuous unique set of points that includes at least two PV currents. The proposed LPPT algorithm uses a P&O extremum seeking algorithm and was designed to work simultaneously with a P&O MPPT algorithm. The time scale of the LPPT algorithm is significantly shorter than that of the MPPT algorithm to ensure that the algorithms P&O cycles do not interfere with each other.the proposed LPPT algorithm is a realtime feedback control method for PV bus DPP converters, which minimizes the total power processed in the DPP converters in order to reduce power conversion losses. The LPPT method works in conjunction with MPPT to achieve maximum system output, where individual PV power output is maximized and power losses through the converter is minimized. The PV bus DPP converters used in this study are bidirectional flyback converters with a 1:4 turn ratio in the coupled inductor. As the electric power supplied by solar arrays depends on the isolation, temperature and array voltage, it is necessary to control the operating points to draw the maximum power of the solar array. The object of this paper is to investigate the maximum power tracking algorithms which were often used to compare the tracking efficiencies for the system operating under different controls. Besides, different type DC/DC converters were designed to evaluate the converter performance. A simple method which combines a discrete time control and a PI compensator is used to track the maximum power points (MPPs) of the solar array. The implementation of the proposed converter system was based on a digital signal processor 4. SIMULATION RESULTS Fig.5 PV panel l output 199

6 Fig.4 Simulation diagram Fig.6 PV panel 2 output Fig.7 DC output 200

7 5. CONCLUSION Fig.8 Gate Signal 1 Fig.9 Gate Signal 2 The methodology of LPPT is it has high efficiency and reduces noise and better performance in the system. Its applications are the low-power switchmode power supplies and low-cost multiple-output power supplies in high voltage supply.the high efficiency DC-DC converter suitable for medium to large scale distributed PV applications is proposed. High efficiency is achieved by means of partial power processing as well as by coordinating the operation of the interleaved channels of the converter. The output of the converter being a fixed DC-bus also simplifies the MPPT implementation.the Converter was fully tested for performance parameters including the efficiency, mode switching operation and MPPT performance for the static and dynamic conditions REFERENCE Trans. Power Electron., vol. 28, no. 6, pp , Jun [5] G. R. Walker and P. C. Sernia, Cascaded dc-dc converter connection of photovoltaic modules, IEEE Trans. Power Electron., vol. 19, no. 4, pp , Jul [6] M.Karuppiah, M.Sanyasi Rao And K.Karthikumar, Improved Transformer Based Full Bridge Inverter, International Journal of Engineering Research And Technology(IJERT) Volume No.9,Issue No.8,Mar [7] M.Karuppiah, K.Karthi Kumar and A.Arunbalaj The Residential Dc Power Distribution with Multi-Input Single- Control Systems, International Journal Of Engineering Trends And Technology (IJETT) Volume 17, Issue 4, Nov [8] K.Karthi Kumar and M.Karuppiah PV & Wind Hybrid Output Power Generation and Control Fluctuation By Using Battery Energy Storage Station, International Journal Of Trend In Research And Development Volume No.02,Issue No.2, Mar-April [9] K.Karthi Kumar and M.Karuppiah Automatic Constant Wattage Auto Transformer Twin Test Station Using Mitsubishi PLC, International Journal Of Trend In Research and Development Volume No.02, Issue No.2, Mar-April [10] K.Karthi Kumar M.Karuppiah and A.Arunbalaj A Novel Relay Used For Fault Detection and Isolation in Distribution Networks Containing of Several DGS, International Journal of Trend and Development Volume 2(3), Issn: [11] K.Fathima T.Mary Saranya S.Hema and M.Karuppiah Step-Up Converter For Load Variation Using Zero Voltage Switching, International Journal Of Applied Engineering Research (IJAER), Volume No.09, Issue No.24, December [12] M.Karuppiah K.Karthikumar A.Arunbalaj L.Dineshbabu and S.Krishnakumar Improved DC Power Distribution with Multi input Single Control System using Boost Converter, International Journal Of Applied Engineering Research (IJAER), Volume No.10, Issue No.3, Jan [13] K.Karthikumar V Senthilkumar M Karuppiah A Arunbalaj and S Krishnakumar, Modeling of Solar Panel as Genralised Structure, International Journal Of Applied Engineering Research (IJAER), Volume No.14, Issue No.3, Feb [1] P. Fairley, Big solar s big surge, IEEE Spectrum, vol. 52, no. 1, pp , Jan [2] A. Ahmed, L. Ran, S. Moon, and J.-H. Park, A fast pv power tracking algorithm with reduced power mode, IEEE Trans. Energy Convers., vol. 28, no. 3, pp , Sep [3] D. Thayalan, H.-S. Lee, and J.-H. Park, Low-cost high-efficiency discrete current sensing method using bypass switch for PV systems, IEEE Trans. Instrum. Meas., vol. 63, no. 4, pp , Apr [4] P. S. Shenoy, K. A. Kim, B. B. Johnson, and P.T.Krein, Differential power processing for increased energy production and reliability of photovoltaic systems, IEEE 201

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