An Interleaved Flyback Inverter for Residential Photovoltaic Applications Bunyamin Tamyurek and Bilgehan Kirimer ESKISEHIR OSMANGAZI UNIVERSITY Electrical and Electronics Engineering Department Eskisehir, Turkey Tel.: +90 / (222) 239.37.50. E-Mail: btamyurek@ogu.edu.tr, blkm2r@gmail.com Keywords «Harmonics», «Interleaved converters», «Photovoltaic», «Pulse Width Modulation (PWM)». Abstract This study presents the analysis, design, and implementation of a photovoltaic (PV) inverter for residential applications based on interleaved flyback topology operated in discontinuous current mode (DCM). It is expected that the use of solar energy in small electric power system applications will increase largely when the cost of the PV panels and the energy conversion equipment becomes economical for every user. Therefore, the objective of this study is to develop a low-cost inverter system. The cost reduction is achieved by selecting the lowest cost converter topology, simplifying the control system, and making a compact and small size converter. The selected topology and the operating mode are known to yield the lowest component count and so the lowest cost. The paper first performs analysis and design on a 1 kw system then obtains the PLECS and the Simulink models for simulation. Later, using the simulation results, the inverter design and the controller performance are verified and optimized for the given specifications. Lastly, a full-scale prototype is built and evaluated under realistic conditions. Based on the experimental results, the implemented design and the controller can extract the energy from the solar panels with 97% efficiency and transfer it to the grid with high power quality and with 86.14% static efficiency at full load. The total harmonic distortion (THD) of the grid current is measured as 3.68% and the power factor is very close to unity. Consequently, the experimental results demonstrate that the proposed inverter system and its design perform successfully and comply with the existing PV inverter specifications. Introduction The solar energy is considered as one of the most renewable source of energy and has a great potential to play an important 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 today [2]-[8]. However, the high cost of the technology limits its usage globally. Especially, the low cost is greatly important for commercialization in small electric power systems that include the residential applications [9]. This paper proposes a low cost inverter alternative to the existing high cost designs. The cost reduction is achieved using flyback topology operating in discontinuous current mode. The flyback topology is known to be the lowest cost converter among the isolated topologies since it uses the least number of components. This fact comes from the ability of the flyback topology to combine the energy storage inductor (the inductor in the buck-boost converter) with the transformer. In other type of isolated topologies, the inductor and the transformer are separate. While the inductor is responsible for energy storage, the transformer on the other hand is responsible for energy transfer while providing galvanic isolation. The combination of these two components in a flyback topology eliminates the bulky and the costly the energy storage inductor and therefore reduces the overall cost. Now the transformer is required to storage energy, which is not a typical characteristic of a power transformer. In order to distinguish this transformer from the conventional power transformers, it is called flyback transformer.
In order for the flyback transformer to store energy, the magnetizing inductance must be reduced and typically, a large air gap must be inserted. Having to have a relatively large air gap results in large amount of leakage flux and so reduced coupling, and poor energy transfer efficiency. Because of this reasons, the flyback converters are generally not designed for high power. The recommended use of flyback topology is limited below 200 W. Nevertheless, if advanced design techniques are employed, the flyback converter can be used in high power applications as well. One of those methods to push the power limit to higher levels is to employ interleaving technique. In this technique, more than one unit are connected in parallel, but they are equally phase shifted with respect each other. The major advantage of this method is that each unit shares the power equally and does not need any controller for equal sharing. Another major benefit is that the switching frequency ripple elements at voltage and current waveforms are multiplied depending on the number of interleaved units. The last feature allows easy filtering of the ripple components or using smaller sized filtering elements. The ability to use smaller passive elements is very beneficial for reducing the cost and/or obtaining the small sized converter. As mentioned before, the discontinuous mode of operation is preferred and used to simplify the control system, and to obtain always a stable system with fast dynamic response. Contrary to these advantages, the DCM operation is generally a cause for poor efficiency because of the high peak to rms ratio of the waveforms. The discontinuous currents in the DCM operation yield higher rms values compared to the currents in continuous current mode (CCM) case, and therefore more power losses are generated. The interleaving technique can also be a solution to this problem. The discontinuity in the waveforms before and after the common node is reduced and continuity is regained because of the phase-shifted operation of the parallel cells [10]-[11]. Fig. 1 shows the proposed system as a block diagram. Fig. 2 shows the topology of two-cell interleaved flyback PV inverter system, for illustrative purposes. The actual and optimum cell number is determined in the design section of the paper. Decoupling capacitor DCM interleaved flyback converter Full-bridge unfolding inverter CB- Switch PV Source C Grid V pv I grid_ref_pk S 1 S 2 S 3,6 S 4,5 Filter I pv MPPT Grid synchronization and PWM generation I grid V grid Fig. 1: Block diagram of proposed grid-tied flyback based PV inverter system I PV i 1 ni 2 1: n i 2 PV Source + V PV C S 1 + i µ v p L µ S 3 S 5 i o C f L f i grid + v grid S 2 L µ S 4 S 6 Fig. 2: Circuit schematic of flyback PV inverter power stage based on two interleaved cells
Converter Operating Principles According to Fig. 2, the PV source is connected to the interleaved flyback converter through a decoupling capacitor. Each flyback converter uses an insulated gate bipolar transistor (IGBT) for switching, a flyback transformer, and a diode at the secondary side. The topology then employs a fullbridge IGBT inverter and a low-pass filter for proper interface to the grid. When the IGBT is turned on, a current flows from the PV source into the magnetizing inductance of the flyback transformer and the energy is stored as magnetic field; no current flows to the output due to the position of the secondary side diode. During the on time of the IGBT, the energy to the output is supplied by the capacitor and the inductor placed at the output stage. When the IGBT is turned off, the energy stored in the magnetizing inductance is transferred into the grid through the transformer windings. The flyback converters in this design are operated in DCM for easy and stable generation of ac currents synchronized to grid frequency at the grid interface. The DCM operation of converter under proper control produces triangular current pulses at every switching period. If the sinusoidal PWM method is used for control, the inverter will always regulate these current pulses into a sinusoidal current in phase with the grid voltage. The flyback converter input current in Fig. 2 has three components: the switching frequency components, the low frequency 100-Hz component, and the average component. The decoupling capacitor placed at the flyback converter input must be sized in such a way that both the low and the high frequency ac components are adequately bypassed and only the average (dc) component of the current is allowed to be delivered by the PV source. The MPPT performance and the waveform quality of the grid current are directly related to the ripple at the PV voltage and poorly affected if this ripple is large. Therefore, especially the low frequency voltage ripple across the PV panel terminals (also the voltage across the flyback converter terminals) must be as small as practically possible. Fig. 2 also shows the flyback converter output current after unfolded by the full-bridge inverter, and the grid current, which is essentially the instantaneous average of. The full-bridge IGBT inverter is responsible for only unfolding the sinusoidally modulated current packs from dc into ac at the right moment of the grid voltage. Since the IGBT inverter is operated at the grid frequency, the switching losses are insignificant. The low pass filter that comes after the IGBT inverter is responsible for supplying a current to the grid with low THD by removing the high frequency harmonics of the pulsed current waveforms. Converter Analysis The initial analysis of the converter is going to be performed over one particular switching period, which is when the grid voltage at its peak value (this instant corresponds to the instant the duty ratio is also at its peak value ). Then, the results will be generalized for the operation of the converter over a full grid period. In addition, the equations to be derived here are for the non-interleaved case. Accordingly, Fig. 3 shows the control signal, flyback transformer primary voltage, and the magnetization current over one switching period. Analysis when Switch is Turned On When the switch is turned on in Fig. 2, the PV voltage is applied to the flyback transformer primary winding. If it is assumed that the PV voltage is constant and current starts from zero initial value (because of the DCM operation), the flyback input current in Fig. 3 can be written as: where is the flyback transformer magnetizing inductance. At the end of the switch on time, the input current reaches to its highest value inside a grid period is given below. (1)
where is the switching frequency. Equation (2) finds the peak value of the largest of the sinusoidally modulated triangular current pulses over one grid period. The area of this triangle also gives the peak value of the 100 Hz component of the flyback input current. (2) (3) The half of this current is the average (dc) current that is drawn from the PV panels. The relationship between the flyback converter parameters and the PV output power can be written as follows. At the design stage, the magnetizing inductance of the flyback transformer is computed using (5) based on the selected switching frequency, the optimum number of the interleaved cells, and the optimum value. Note that the entry for in (5) is the PV voltage at the maximum power point. The parameter in (5) assumes non-interleaved single-cell design. So, when using (5) for interleaved design, the parameter must be divided by the number of cells. (4) (5) Control signal v p S 1 is ON S 1 is OFF t V PV DT s Ts t i µ V grid n DT s i 1 ni 2 DT s T s t Fig. 3: Control signal, flyback transformer primary voltage, and magnetization current Analysis when Switch is Turned Off When the switch is turned off, the flyback transformer primary voltage becomes negative of the grid voltage after divided by the turn ratio, and the magnetizing current in this case is. (6) At the end of the switch off time, the magnetizing current decreases from its peak value to zero linearly as given below.
(7) Where is the ratio of the time that takes the magnetizing current to reset when the grid voltage is at its peak, and can be computed by equating the Volt-second area across the primary voltage as below. (8) Knowing allows finding the peak of the grid current, which is again the area of the largest of the triangular current pulses within a grid period. Following gives this peak value. (9) Comparing (5) and (9) also verifies the fact that average power from the PV panels equal to the active power transferred to the grid assuming an ideal converter. (10) Assuming and using (8), the following finds the flyback transformer turn ratio. (11) The final step in the flyback transformer design is to determine the air gap length. This parameter can be found using the following. Where is the number of turns of the winding, is the permeability of air and is the cross sectional area of the magnetic core. Converter Design Since this work is intended mainly for the residential applications and small electric power systems, the power rating is selected as 1 kw. Table I gives the list of design parameters and the specifications used for the design of the proposed PV inverter system. The following subsections present the design decisions and the design steps in detail. Table I: Design Specifications Design parameters Specifications PV model BP365 65 W Open circuit voltage and short circuit current 21.7 V, 3.99 A Voltage and current at maximum power 17.6 V, 3.69 A PV panel group 4 PV modules in a string and 4 strings in parallel Total maximum dc power from PV panel group 1040 W MPPT energy harvesting efficiency >97% Inverter static efficiency >85% Single-phase, nominal 220 V and 50 Hz Grid characteristics 185 V 240 V rms voltage range 45.5 Hz 54.5 Hz frequency range Grid current THD <5% Grid side power factor >0.99 (12)
Design of Photovoltaic Stage In this study, we have used the Simulink and the PLECS simulation programs for verification of our design work and later for improving its performance. Therefore, for convenience, we base our design on the PV panels used during the simulation studies, and they are given in Table I. It is expected that the design should always work with different PV modules manufactured by different companies as long as the PV voltage and power range is matched. Hence, the PV source selected for the current design uses four BP365 PV modules in a string and four strings in parallel yielding a maximum power of 1040 W at the PV terminals. As mentioned above, the experimental setup will use different PV modules but provide the same rated output power. Design of PV Inverter Power Stage The decoupling capacitor is an important component of the power stage that controls the voltage ripple at the flyback converter input. As mentioned in the analysis section, smaller the voltage ripple, smaller the grid current THD. However, too small ripple means a very large value of capacitance; thus, some compromise must be made between ripple and size. The value of the decoupling capacitor is determined as 9400 based on the simulation studies. Because of DCM operation, the turn on switching losses are eliminated; this is an advantage, but switches are faced to high peak current stress. In addition, the parasitic inductances and leakage inductance of the flyback transformer cause large voltage spikes across the switches during turn off if a clamp is not employed. Therefore, we prefer using insulated gate bipolar transistors (IGBT) in this application because of their ruggedness under high current and voltage stress. Since the choice of switch is the IGBT and the switching method is hard switching, the switching frequency is selected as 25 khz in order to achieve high efficiency along with smaller magnetics. The number of interleaved cells is selected as three based on the following two reasons. The switching frequency ripple (also the harmonics) both at voltage and current waveforms at common point becomes 75 khz (three times the switching frequency due to phase shifting). This frequency is high enough for easy filtering of the switching frequency harmonics and allows smaller sized passive components. Moreover, realization of a three-phase flyback transformer is economical and practical. As mentioned before, the flyback transformers have to store large amount of energy in their air gaps and transfer it to the output through magnetic coupling at every switching cycle. Therefore, during the design process, the strategies that firstly create the most effective energy storage mechanism and secondly the most optimum and efficient energy transfer path must be employed. Consequently, it is advantageous to make greater than for efficient energy transfer through the transformer to the output. So, the peak duty ratio is selected as 0.29 for the worst case, which is when the grid voltage is at the minimum. Using 262 V for, 70.5 V for, and 0.29 for in (11), the turns ratio of the transformer is found as 9. Using 25 khz for, 347 W for (1040 W is divided by 3 because of 3 cell interleaving), and 0.29 for in (5), the magnetizing inductance of the flyback transformer is calculated as 12. For this design, we selected to use ferrite core made by Ferroxcube. The selected core has 840 cross sectional area. Using 840 for and 2 for and 12 for in (12), the air gap length is found as 0.352. In order to obtain practically the lowest leakage inductance, we have employed the following techniques, which are mostly described in the literature. 1) Longer coil and core heights; 2) reduced number of layers so that less space between the layers; 3) sandwiched windings in order to reduce the magnetic field inside the window area; 4) distributed air gaps along the core structure to reduce fringing flux and improve coupling [11]. The air gap is divided into seven sections and distributed along the transformer core structure, each gap being 0.05. At the last stage, the converter employs an IGBT bridge operating at the grid frequency. This bridge is responsible for converting the dc secondary currents into ac, and therefore provides an interface to the
grid through a low-pass filter. The filter is responsible for removing the switching frequency components of the sinusoidally modulated currents. The switching frequency of each flyback cell is 25 khz. Therefore, the ripple frequency of current waveform at the output of the inverter is 75 khz due to the interleaving. So, the corner frequency of the low-pass filter is selected as 7.5 khz. Design of Control System The control system is required to achieve two important control jobs at the same time. While it is harvesting the maximum power available in the solar cells, it must pump that power into the utility grid with high power quality. For the first job, it should regulate a proper dc current and voltage at the PV interface for maximum energy harvesting. For the second job, it must provide control to convert the dc current that comes from the panels and continuously regulated for the MPPT purpose into ac current at the grid interface for power injection. In addition, this ac current should be synchronized with the grid frequency, should have low harmonic distortion and a power factor close to unity. Fig. 4 shows the Simulink and PLECS models of the inverter system including the controller. Fig. 5 and 6 shows the details of the controller and the power stage, respectively. Fig. 4: Simulink and PLECS models of the proposed inverter system including signal conditioning, DSP based controller, and power stage blocks Fig. 5: Simulink model of control system Because of its implementation simplicity, perturb and observe (P&O) method is selected as the maximum power point tracking (MPPT) algorithm [12]. Based on the measured and values, the MPPT block in Fig. 5 generates the proper control signal that will produce the peak value of the duty ratio. Similar to the voltage modulation ratio used to regulate the magnitude of the output voltage in a voltage source inverter application; the signal generated by the MPPT controller block in Fig. 5 gives the current modulation ratio information in this application. As seen in Fig. 5, for sinusoidal current modulation, the output of the MPPT block is multiplied by the PLL output, which is a sinusoidal waveform with unity gain and synchronized to the grid voltage. The whole control system is implemented in TMS320F28335 Texas Instrument s DSP Controller. Fig. 6 shows the PLECS model of the inverter power stage used to test and evaluate the performance of the controller and the overall converter design.
Fig. 6: PLECS model of the power stage of the PV inverter system Simulation Results Fig. 7a shows the simulated PV module output power, the power delivered to the grid, and the peak value of the duty ratio (also the current modulation ratio) generated by the P&O MPPT algorithm for three different Sun levels, and Fig. 7b shows the simulated waveforms of the grid voltage and current. Based on Figs. 7a and 7b, the simulated MPPT achieves a tracking performance of 98.85% and a tracking time of less than 0.1 s. Furthermore, the waveforms show the success of the controller and the DCM mode flyback topology in achieving the high quality power transfer into the grid. (a) (b) Fig. 7: (a) Simulated PV module output power (red), power delivered to the grid (green), and the peak value of duty ratio (also the current modulation ratio) generated by the P&O MPPT algorithm for three different Sun levels (bottom trace), (b) simulated waveforms of the grid voltage and current Experimental Results An experimental set up as shown in Fig. 8 has been built to evaluate the real time performance of the proposed inverter system. Fig. 9a shows the experimental readings taken by high performance 3193 Hioki power analyzer. The description of the parameters shown on the monitor of the power analyzer is as follows:,, and are the measured PV voltage, current, and power at the maximum power point, respectively. In addition, the parameters,,,,,,, and are the measured grid voltage, current, active power, reactive power, apparent power injected into grid, power factor, and phase shift between the grid current and the voltage, respectively. Finally, and are measured percentage THD values of the grid current and the voltage, respectively.
Based on the results given in Fig. 9a, the dc power is measured as 1007.7 W. The PV panel group is configured in such way that it supplies 1040 W without an MPPT, which is also the value used in the design and during the simulation studies. Under these conditions, the energy harvesting efficiency of the MPPT algorithm is calculated as 97%. The power delivered to the grid is measured as 868 W; the static efficiency of the inverter system is therefore measured as 86.14%. The THD of the grid current and grid voltage are measured as 3.68% and 3.65%, respectively. The THD of the grid current is well below 5% specification even under distorted grid voltage. This result demonstrates the effectiveness of using pure sinusoidal control signal, which is generated by the PLL algorithm, for pulse width modulation of the switch control signals. Moreover, the power factor is measured as 0.9956. Finally, Fig. 9b shows the grid voltage and the grid current measured by the TPS2024 Tektronix oscilloscope. The results show that proposed inverter and control system provides high power quality output at the grid interface. All results demonstrate the success of the inverter system and controller. In addition, they fulfill the design specifications and comply with the standards. Decoupling capacitor bank 3-cell interleaved flyback converter 3-phase flyback transformer IGBT based full-bridge inverter Analog signals magnitudescaling card TMS320F28335 DSP Experimenter Kit Fig. 8: Experimental setup of the proposed inverter system (a) (b) Fig. 9: Experimental results: (a) readings taken by 3193 Hioki power analyzer, (b) grid voltage (yellow) and grid current (blue)
Conclusion This paper presents analysis, design, and implementation of a photovoltaic inverter with galvanic isolation for residential applications up to 1 kw power. The main contribution of this work is that the proposed inverter system tries to lower the cost and the size of the converter in order to contribute to the commercialization of solar technology. These are achieved by our topology selection, simpler controller requirement, and compact design. Building the inverter system based on the flyback converter topology offers the lowest cost since it requires the least number of components, operating in the discontinuous current mode enables very simple and always stable control system, and finally three-cell interleaved operation allows compact flyback transformer construction. The energy harvesting efficiency of the MPPT controller and the inverter are measured as 97% and 86.14% at full power, respectively. In addition, the THD of the grid current is measured as 3.68% and the power factor is 0.9956. Consequently, the experimental results demonstrate the successful operation of the inverter and compliance to the specifications. References [1] Solar Energy, (2013, June 15). Available: http://www.conserve-energy-future.com/solarenergy.php. [2] Hu H., Harb S., Fang X., Zhang D., Zhang Q., Shen Z. J., and Batarseh I.: A Three-port Flyback for PV Microinverter Applications With Power Pulsation Decoupling Capability, IEEE Transactions on Power Electronics, vol. 27, no. 9, pp. 3953-3964, September 2012. [3] Nanakos A. C., Tatakis E. C., and Papanikolaou N. P.: A Weighted-Efficiency-Oriented Design Methodology of Flyback Inverter for AC Photovoltaic Modules, IEEE Transactions on Power Electronics, vol. 27, no. 7, pp. 3221-3233, July 2012. [4] Kim Y.H., Kim J.G., Ji Y.H., Won C.Y., and Lee T. W.: Flyback inverter using voltage sensorless MPPT for AC module systems, in 2010 International Power Electronics Conference (IPEC), 2010, pp. 948-953. [5] Housheng Z.: Research on MPPT for Solar Cells Based on Flyback Converter, in 2010 International Conference on Intelligent Computation Technology and Automation (ICICTA), 2010, pp. 36-39. [6] Chen Y. M. and Liao C. Y.: Three-port flyback-type single-phase micro-inverter with active power decoupling circuit, in IEEE 2011 Energy Conversion Congress and Exposition (ECCE), 2011, pp. 501-506. [7] Shimizu T., Wada K., and Nakamura N.: Flyback-type single-phase utility interactive inverter with power pulsation decoupling on the DC input for an AC photovoltaic module system, IEEE Transactions on Power Electronics,vol.21, no.5, pp.1264-1272, Sept. 2006. [8] Kasa N., Iida T., and Chen L.: Flyback inverter controlled by sensorless current MPPT for photovoltaic power system, IEEE Trans. Ind. Elec., vol. 52, no. 4, pp. 1145-1152, 2005. [9] Li Y. and Oruganti R.: A Low Cost Flyback CCM Inverter for AC Module Application, IEEE Transactions on Power Electronics, vol. 27, no. 3, pp. 1295-1303, March 2012. [10] Gao M., Chen M., Mo Q., Qian Z., and Luo Y.: Research on output current of interleaved-flyback in boundary conduction mode for photovoltaic AC module application, in IEEE 2011 Energy Conversion Congress and Exposition (ECCE), 2011, pp. 770-775. [11] Tamyurek B. and Torrey D. A.: A three-phase unity power factor single-stage ac dc converter based on an interleaved flyback topology, IEEE Trans. Power Electron., vol. 26, no. 1, pp. 308 318, Jan. 2011. [12] Esram T. and Chapman P. L.: Comparison of photovoltaic array maximum power point tracking techniques, IEEE Trans. Energy Conversion, vol. 22, no. 2, pp. 439 449, June 2007.