ADVANCED HYBRID TRANSFORMER HIGH BOOST DC DC CONVERTER FOR PHOTOVOLTAIC MODULE APPLICATIONS

Transcription

1 ADVANCED HYBRID TRANSFORMER HIGH BOOST DC DC CONVERTER FOR PHOTOVOLTAIC MODULE APPLICATIONS SHAIK ALLIMBHASHA M.Tech(PS) NALANDA INSTITUTE OF ENGINEERING AND TECHNOLOGY G V V NAGA RAJU Assistant professor NALANDA INSTITUTE OF ENGINEERING AND TECHNOLOGY ABSTRACT- Now-a-days due to the shortage of electric power and rising cost of non-renewable energy resources generating electric power from the renewable energy resources such as PV modules are increasing day to day. A nonisolated, high boost ratio hybrid transformer dc dc converter with applications for lowvoltage renewable energy sources proposed in this paper. Due to the high system efficiency and the ability to operate with a wide variable input voltage, the proposed converter is an attractive design for alternative low dc voltage energy sources, such as solar photovoltaic modules and fuel cells. The proposed converter utilizes hybrid transformer to transfer the inductive and capacitive energy simultaneously, achieving high step-up voltage with smaller sized magnetic component, the turn-off loss of switch is minimized, increasing the efficiency of the converter under all load conditions. The input current ripple and conduction losses are also reduced because of the hybrid linear-sinusoidal input current waveforms. The voltage stresses on the active switch and diodes are maintained at a low level and are independent of the changing input voltage over a wide range as a result of the resonant capacitor transferring energy to the output of the converter. By using the simulation results we can analyze the effectiveness of the proposed converter. I. INTRODUCTION In recent years, with fast emerging technological innovations, there is an increasing demand for improved efficiency, reduced size, cost and weight. Integrating the power from the PV module into the existing power distribution infrastructure can be achieved through power conditioning systems (PCS). The double-stage PCS consists of a dc dc conversion stage that is connected to either a low power individual inverter or a highpower centralized inverter that multiple converters could connect to. The dc dc conversion stage of the PCS requires a high efficiency, high boost ratio dc dc converter to increase the low dc input voltage from the PV panel to a higher dc voltage. This project proposes a hybrid transformer DC-DC converter. The proposed converter utilizes the inductive and capacitive energy simultaneously to achieve high boost ratio with the smaller sized magnetic component. This converter utilizes hybrid switching technique. Hybrid switching consists of PWM and resonant power conversions to achieve high boost ratio while maintaining high efficiency. Since resonant operation mode is incorporated into the traditional high boost ratio pulse width modulation converter, switching (turn-off) losses are reduced, increasing the efficiency of the converter under all load conditions. The conduction losses input current ripple are also reduced. The voltage stresses on the active switch and diodes are maintained at a low level. Due to the rising costs and limited amount of nonrenewable energy sources, there is an increasing demand for the utilization of renewable energy sources such as photovoltaic (PV) modules. Typical PCS can be accomplished using a single-stage or a double-stage as shown in Fig. 1 Fig. 1. Typical double-stage PCS architectures with high boost ratio dc dc converters and PWM dc ac inverters for PV applications. (a) Two-state PV module integrated micro inverter. (b) Parallal PV module integrated micro converter with centralized inverter A nonisolated dc dc converter with a high boost ratio would be advantageous for a two-stage PCS [1] because it can be easily integrated with current PV systems while reducing the cost and maintaining a high system efficiency. Due to the different output voltages from the PV panel, it would be beneficial to have a system with a high efficiency over the entire PV voltage range to maximize the use of the PV during different operating conditions. Another important function of the dc dc converter for PV applications is being able to implement maximum power point tracking (MPPT). The ability to implement MPPT for an individual PV panel would ensure that a large cluster of PV could

2 maintain maximum power output from each panel without interfering with the other panels in the system. The major consideration for the main power stage of the converter in being able to implement an accurate MPPT is that the input current ripple of the converter has to be low. Of the high boost ratio nonisolated dc dc converter topologies published, the uses of coupled-inductor and switched capacitor are attractive for use in a simple high boost ratio converter due to the fact that only a single low voltage active switch is required for the topologies. The converter s leakage energy from the coupledinductor was recycled reducing the losses of the system. However, the output diode stress for this converter was similar to that of a traditional II. PROPOSEDCONVERTERTOPOLOGY AND OPERATIONANALYSIS Fig. 3 shows the circuit diagram of the proposed converter. C in is the input capacitor; HT is the hybrid transformer with the turns ratio 1:n;S1 is the active MOSFET switch;d1 is the clamping diode, which provides a current path for the leakage inductance of the hybrid transformer whens1 is OFF, Cc captures the leakage energy from the hybrid transformer and transfers it to the resonant capacitor Cr by means of a resonant circuit composed ofcc, Cr, Lr, and Dr; Lr is a resonant inductor, which operates in the resonant mode; and Dr is a diode used to provide an unidirectional current flow path for the operation of the resonant portion of the circuit. Fig. 2. High step-up dc dc converters using coupledinductor and switched-capacitor techniques. (a) Highstep coupled-inductor robust dc-dc converter.(b) High step-up dc-dc converter with coupled-inductor and switched-capacitor. flyback converter, i.e., higher than the output dc bus voltage. Another drawback of the converter was that there was a high input current ripple due to the fact that there was no direct energy transfer path when the MOSFET was OFF. By adding a switched-capacitor in series with the energy transformer path, a new improved high boost ratio dc dc converter with coupledinductor and switched capacitor, as shown in Fig. 2(b), was introduced [11]. With the switchedcapacitor inserted between the primary side and secondary side of the coupled-inductor, the boost ratio was increased and the output diode voltage stress was reduced closer to that of the output dc bus voltage. However, the magnetic core was not fully utilized because it functioned more as an inductor than as a transformer. Light load efficiency of the converter is also reduced because switching losses were more dominant under light load conditions. The conduction losses in the transformer are greatly reduced because of the reduced input current RMS value through the primary side. The voltage stress of the active switch is always at a low voltage level and independent of the input voltages. Fig. 3. Proposed high step-up dc dc converter with hybrid transformer Cr is a resonant capacitor, which operates in the hybrid mode by having a resonant charge and linear discharge. The turn-on of Dr is determined by the state of the active switchs1. Do is the output diode similar to the traditional coupled-inductor boost converter and Co is the output capacitor. Ro is the equivalent resistive load. The five operation modes are briefly described as follows. [t0,t1], [see Fig.4]:In this period, MOSFETS1is ON, the magnetizing inductor of the hybrid transformer is charged by input voltage, Cr is charged by Cc, and the secondary-reflected input voltage n Vin of the hybrid transformer together by the resonant circuit composed of secondary side of the hybrid transformer, Cr, Cc, Lr, and Dr. The energy captured by Cc is transferred to Cr, which in turn is transferred to the load during the off-time of the MOSFET. converter with hybrid transformer. (a)t0-t1

3 [t1,t2], [see Fig. 4(b)]: At timet1, MOSFET S1 is turned OFF, the clamping dioded1is turned ON by the leakage energy stored in the hybrid transformer during the time period that the MOSFET is ON and the capacitor Cc is charged which causes the voltage on the MOSFET to be clamped. [t4,t0], [see Fig. 4(e)]:The MOSFETS1is turned ON at time t4. Due to the leakage effect of the hybrid transformer, the output diode current io will continue to flow for a short time and the output diode Do will be reversed biased at timet0; then the next switching cycle starts. The boost ratio Mb can be obtained by three flux balance criteria for the steady state. converter with hybrid transformer. (b)t1-t2 [t2,t3], [see Fig. 4(c)]:At timet2, the capacitorcc is charged to the point that the output diode Do is forwarded biased. The energy stored in the magnetizing inductor and capacitor Cr is being transferred to the load and the clamp dioded1 continues to conduct while Cc remains charged. converter with hybrid transformer..(c)t2-t3. [t3,t4], [see Fig. 4(d)]:At timet3, diode D1 is reversed biased and as a result, the energy stored in magnetizing inductor of the hybrid transformer and in capacitor Cr is simultaneously transferred to the load. During the steady-state operation, the charge through capacitor Cr must satisfy charge balance. The key waveform of the capacitor Cr current shows that the capacitor operates at a hybrid-switching mode, i.e., charged in resonant style and discharged in linear style. converter with hybrid transformer (d)t3-t4 converter with hybrid transformer. (e)t4-t5 The first flux balance on the magnetizing inductor of hybrid transformer requires that in steady state V = (1). Second, according to flux balance on the resonant inductor during on-time V = nv + V = n + V (2) The last flux balance that governs the circuit is voltage-second balance of the magnetizing inductor in the hybrid transformer for the whole switching period. V D = (1 D) (3) By substituting (2) into (3), the boost conversion ratio can be obtained M = = (4) The conversion ratio is similar to the conventional boost converter except that the turns ratio terminals added, so the traditional duty ratio control method that is applied for a standard boost converter can also be applied to the proposed converter. The current in MOSFETS1is the sum of the resonant current and linear magnetizing inductor current as shown in Fig. 5. There are two distinctive benefits that can be achieved by the linear and resonant hybrid mode operation. The first benefit is that the energy is delivered from source during the capacitive mode and inductive mode simultaneously. Compared to previous coupled-inductor high boost ratio dc dc converters with only inductive energy delivery, the dc current bias is greatly reduced, decreasing the size of the magnetics. Second, the turnoff current is decreased, which causes a reduction in the turn-off switching losses.

4 Fig. 5. Key waveforms for different operation stages III. ANALYSIS ANDADVANTAGES OF THE PROPOSEDCONVERTER A. Fixed Voltage Stresses of the Power Devices Voltage stresses for all the power devices of the converter are determined in this section to select power devices with the proper rating and all the results are with respect to the output dc voltage. From the circuit diagram oft0tot1andt1tot2in Fig. 4, respectively, the voltage stresses for MOSFETS1and clamping dioded1are obtained. V = V = = (5) From the circuit diagram oft0tot1andt2tot3in Fig. 4, one obtains the voltage stress of diode resonant diodedr and output diodedo V = V = V V = V = ( ) (6) From (5) and (6), it is obvious that all the voltage stresses of the switches are independent of input voltage and load conditions. In other words, all the voltage stresses of the switches are optimized based on the output voltage and the turns ratio of the transformer. The resonant periodtr and the resonant frequency are given by T = 2π L C (7) f = (8) If the constant on-time controltonis used, chooseton=1/2tr so that the resonant diode can turn OFF at zero-current condition and conduction loss can be minimized. In the experimental implementation of the hybrid transformer, the leakage inductance of the hybrid transformer should be considered, so that the total resonant inductance is expressed as follows. L, = L + L + n L (9) wherellrs is the secondary side leakage inductance and Llrp is the primary side inductance of the hybrid transformer. The resonant capacitance Cr is composed by Cr and Cc in series. Normally, we choose Cr Cc so that the voltage stress of the MOSFET can be clamped well. The optimal operation mode is the constant PWM on-time Ton control with variable frequency, however, traditional PWM control method is applicable to the proposed converter as described. B. Analysis of Energy Transfer The simplified waveforms for energy transfer analysis are shown in Fig. 6. In order to analyze the energy transfer feature from the low voltage dc energy source to the high-voltage dc bus, it is necessary to solve the equivalent circuit in Fig. 3(a) subject to the initial conditions imposed by the previous PWM OFF-time interval given by i (0) = 0 (10) v (o) = v (11) Where Δv Cr is the ripple of the resonant capacitor Cr. Fig. 6. Waveforms for energy transfer analysis

5 The resonant solutions are obtained as i (t) = i sin 2πf. t (12) v (t) = v cos 2πf. t (13) v = R. i (14) Where RN is characteristic impedance given by R = (15) For PWM off-time interval, the discharge equations of the resonant capacitorcr1are given by v = (16) I = (17) = Where ILm sec is the average linear magnetizing current referred to secondary side of the hybrid transformer, Io is the average output current, Po is the output power, and Vo is the output voltage. This feature helps improve the converter efficiency over a wide input voltage range by decreasing the conduction losses which are more dominant at low-input voltages and reducing the switching losses that are more dominant at high-input voltages. The proposed converter works using the resonant sinusoidal charge mode, while a conventional non resonant converter works using the linear charge mode. For a fixed output power and given input voltage, the average input currents Iin for these two converters shown in Fig. 8 are equal. The switching losses for a dc dc converter are directly proportional to the switching current given by the fixed conversion voltages. The main switching loss then becomes the turn-off switching losses. As a result, the leakage inductance design of the coupledinductor has a tradeoff between the conversion ratio and a higher turn-off switching current. This is perfect for an application where a low profile PVmodule-integrated dc dc converter is needed. D. Two-Phase Interleaved Extension In order for the proposed converter to be used in higher power level conversion applications, the interleaving method applicable to the traditional high boost ratio PWM dc dc converter can be employed, as shown in Fig. 9. Fig. 7. Kr versus Vin curve. C. Advantages Over Conventional Non resonant High Step-Up Converter Current Popular methods used to achieve high boost ratio for non isolated dc dc converters consist of using coupled-inductor and switched-capacitor techniques. The converter presented in this paper utilizes hybrid-switching technique combing PWM and resonant power conversions to achieve a high boost ratio while maintaining a high efficiency. The advantages gained from using the hybrid-switching operation will be illustrated in this section. The input currents for the resonant sinusoidal charge mode and the PWM linear charge mode are comparatively illustrated in Fig. 8. Fig. 9. Two-phase extension for proposed converter This gives the advantages of standard interleaved converter systems such as low-input current ripple, reduced output voltage ripple, and lower conduction losses. The difference between standard interleaved converters and the proposed interleaved converter is that the clamping capacitorcc can also be shared by the interleaved units reducing the total number of components in the system. Using the phase-shift method of control, the current ripple through the clamping capacitor Cc is reduced as a result the capacitance needed for Cc is also reduced. TABLE I DESIGNPARAMETERS Fig. 8. Input current comparison between resonant mode and linear mode. (a) Resonant mode. (b) Linear mode.

6 Fig. 10. Simulation waveforms of current of the resonant capacitorcr1, voltage of switchm1, and input current with Po =220 W,Vo =400 V,Vin = 30 V, andfs =88 khz (a) (b) (a) (c) (b) (c) (d) Fig. 12. Simulation waveforms of switch voltage, output diode voltage, input current, and current of resonant capacitor of the proposed converter with 30- V input and 400-V output under different output power level: (a) 30 W, (b) 110 W, (c) 160 W, and (d) 220 W. (d) Fig. 11. Simulation waveforms of switch voltage, output diode voltage, input current, and current of resonant capacitor of the proposed converter with 20- V input and 400-V output under different output power level: (a) 30 W, (b) 110 W, (c) 160 W, and (d) 220 W (a)

7 (b) (c ) (d) Fig. 13. simulation waveforms of switch voltage, output diode voltage, input current, and current of resonant capacitor of the proposed converter with 45- Vinput and 400-V output under different output power level: (a) 30 W, (b) 110 W, (c) 160 W, and (d) 220 W TABLE I I COMPONENTSSELECTION Fig. 15. Conversion efficiency versus output power for different input Voltages Fig. 16. Weighted CEC efficiency at different input voltages V. CONCLUSION An attempt is made to develop and analyse A High-Boost Ratio Hybrid Transformer DC-DC Converter for Photovoltaic Application with low dc input voltage. In this paper a high boost ratio dc dc converter with hybrid transformer suitable for alternative dc energy sources with low dc voltage input is proposed. The concept of achieving high efficiency due to reduction in voltage stresses on switch and compactness in the size is the main paradigm in the present day Power Electronic Industries. This paper presents highly efficient high boost ratio hybrid transformer DC-DC converter for photovoltaic module applications with following features and benefits: This converter transfers the capacitive and inductive energy simultaneously to increase the total power. Delivery reducing losses in the system. The conduction loss in MOSFET is reduced as a result of the low-input RMS current and switching loss is reduced with a lower turn-off current. With these improved performances, the converter can maintain high efficiency under low output power and low-input voltage conditions. A high boost ratio dc dc converter with hybrid transformer suitable for alternative dc energy sources with low dc voltage input is proposed in this paper. These results were independent of the input voltage level. The conversion efficiencies from 30 to 220 W are higher than 96% and the peak efficiency is 97.4% under 35-V input with 160-W output power. REFERENCES [1] J.-S. Lai, Power conditioning circuit topologies, IEEE Ind. Electron. Mag., vol. 3, no. 2, pp , Jun [2] S. B. Kjaer, J. K. Pedersen, and F. Blaabjerg, A review of single-phase grid-connected inverters for photovoltaic modules, IEEE Trans. Ind. Appl., vol. 41, no. 5, pp , Sep./Oct [3] F. Blaabjerg, Z. Chen, and S. B. Kjaer, Power electronics as efficient interface in dispersed power generation systems, IEEE Trans. Power Electron., vol. 19, no. 5, pp , Sep [4] Y. Xue, L. Chang, S. B. Kjaer, J. Bordonau, and T. Shimizu, Topologies of single-phase inverter for

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