ANALYSIS OF ZVS INTERLEAVED LLC RESONANT CONVERTER FOR CURRENT BALANCING IN DC DISTRIBUTION SYSTEM

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1 International Journal of Mechanical Engineering and Technology (IJMET) Volume 10, Issue 02, February 2019, pp , Article ID: IJMET_10_02_177 Available online at ISSN Print: and ISSN Online: IAEME Publication Scopus Indexed ANALYSIS OF ZVS INTERLEAVED LLC RESONANT CONVERTER FOR CURRENT BALANCING IN DC DISTRIBUTION SYSTEM Chandra Sekhar Garlapati Dept.of EEE, GMR Institute of Technology, Rajam, India Ravi Kishore D Dept.of EEE, Godavari Institute of Engineering and technology, Rajahmundry, India ABSTRACT The power conversion efficiency of an isolated ac dc converter is a dominant factor in the overall efficiency of dc distribution systems. To improve the power continuity and power conversion efficiency of the dc distribution system, a three-phase interleaved fullbridge LLC resonant converter employing a voltage source converter which is fed by Photo Voltaic System and Wind Energy System is proposed as the isolated ac dc highfrequency-link power-conversion system. The LLC resonant converter has the advantages of high efficiency, high power density, and low cost. They can be operated at no load condition and at resonance for nominal input voltage. The proposed voltage source converter has the capability of boosting the output voltage without increasing the transformer s turn ratio. Especially, the frequency of the rectifier s output ripple is six times higher than the switching frequency, thereby reducing the output capacitor and the secondary transformer s RMS current. However, the tolerance of the converter s resonant components in each primary stage causes the unbalance problem of output ripple current. It cannot be solved using conventional control techniques since the structure of the three-phase interleaving has the limitations of individual control capability for each converter. To solve the current unbalance problem, a current balancing method is proposed for the output rectifying current. Keywords LLC Resonant Converter, High Frequency, Current balancing, DC Distribution System Cite this Article: Chandra Sekhar Garlapati and Ravi Kishore D, Analysis of Zvs Interleaved Llc Resonant Converter for Current Balancing in Dc Distribution System, International Journal of Mechanical Engineering and Technology, 10(02), 2019, pp editor@iaeme.com

2 Analysis of Zvs Interleaved Llc Resonant Converter for Current Balancing in Dc Distribution System 1. INTRODUCTION The converter which eliminates much of the switching losses associated with fixed frequency by enabling switching at either near - zero voltage or near - zero current is known as resonant converter. The resonant converters can be classified into two types namely, series and parallel resonant converters. The drawback of series resonant converter is that it cannot regulate output at no load condition and output current is high. The disadvantage of parallel resonant converter is circulating current is high and has lower efficiency at reduced loads. The conventional several power delivery architectures which use ac or dc voltage have been presented in. A conventional ac power distribution system of the IDC consists of four power conversion stages with a traditional online uninterrupted power supply (UPS), which employs an ac dc ac double conversion. Compared with the ac distribution system, the dc distribution system does not need several power conversion stages such as the online UPS and the individual power factor correction (PFC) circuit in front of each power supply unit (PSU). Therefore, the dc distribution system for the data centers can reduce the power conversion loss caused by redundant power stages. Furthermore, in order to obtain high-power conversion efficiency of the dc distribution system, the high efficient isolated ac dc converter used in the dc distribution system should decrease its power loss. The galvanic isolation in the power conversion stage is not more popular than the isolation of the server level; however, it is one of interesting research topics of isolation applications for IDC since the safety from an electric shock can be required for the operators who are achieving the maintenance operations of the servers. An isolated conventional ac dc converter consists of two power conversion stages. The primary power conversion stage is generally designed to a non isolated ac dc rectifier for PFC operation. A boost PFC operating under the continuous conduction mode (CCM) is commonly used in ac dc high-power applications. When the boost PFC operates under the CCM, the reverse recovery time of the boost rectifier diode causes the reverse recovery current which increases the power loss. In order to reduce this power loss of the CCM PFC rectifier, the reverse recovery current of the rectifying diode should be reduced. To solve this reverse recovery problem, various soft switching techniques using additional passive or active snubber circuits have been proposed. However, those methods require relatively large number of passive or active components which increase the production cost of the system. Moreover, the complex structure of the rectification circuit with many switching components decreases the reliability of the overall power conversion system. An alternative method to minimize those drawbacks is the use of SiC diodes instead of conventional diodes in the rectification circuit. The SiC diode has very small reverse recovery current. Therefore, it can reduce the power loss caused by the reverse recovery current. Conventional full-bridge diodes used on the front side of the PFC circuit also increase conduction losses. Various bridgeless PFC topologies have been proposed for eliminating the full bridge diode rectifier. Those topologies can get rid of the line-current path and can decrease the conduction loss caused by the full-bridge diode. The power density of an isolated dc dc converter for the dc power distribution systems is one of the significant performance indicators, since the size of the desired system is limited. Therefore, multiple medium power converters connected in parallel, which share load current to increase the amount of power conversion is proper rather than a single large converter employing parallel switching devices with a big isolation transformer. A full-bridge phase shift converter is a frequently selected topology for high power dc dc applications. In order to increase the converter s rated power using parallel operations, the parallel connection methods for the output stages using multiple phase-shift converters have been proposed. However, these topologies cannot accomplish zero-voltage switching (ZVS) under light load conditions and additional filter inductors for the output rectifier are required. The LLC resonant converter is another popular topology because of its outstanding performance such as high-power conversion efficiency, high editor@iaeme.com

3 Chandra Sekhar Garlapati and Ravi Kishore D power density, and its ZVS capability over the entire load range. Multiphase LLC resonant converters have also been developed to reduce the output ripple current. Since an interleaved operation for the LLC resonant converter has been adopted, the output ripple current and the size of the filter capacitors could be reduced. However, those conventional studies have concentrated on the low output voltage and high-output current applications using a center-tapped transformer. The number of turns in the secondary winding used for the center-tap structure is twice the turn number required for the full-bridge structure. Since the output voltage needed for the dc distribution system is relatively high (e.g., 300 to 400 V), the center-tap structure is not suitable for high-output voltage applications. A three-phase interleaved LLC resonant converter employing a Voltage source converter is proposed in this project. The proposed converter consists of three full-bridge LLC resonant converters whose output stage is composed of voltage source converter for each secondary transformer winding. It has ZVS capability over the entire load range similar to the conventional LLC resonant converter. In addition, the proposed voltage source converter can boost the output voltage without increasing the transformer s turn ratio. Especially, the frequency of the rectifier s output ripple is six times higher than the switching frequency, thereby reducing the output capacitor and the RMS current of the transformer s secondary winding. Therefore, the proposed converter is suitable for high-power and high-output voltage applications. However, the imbalance of resonant networks in the LLC resonant converters can cause the unbalance phenomena of output rectifying current. It cannot be solved using conventional control techniques since the structure of the three-phase interleaving has the limitation of individual control capability for each converter. Circuit Diagram of the Proposed Isolated Ac Dc High-Frequency- Link Power-Conversion System is shown in fig 1. Figure.1. Circuit Diagram of the Proposed Isolated Ac Dc High-Frequency-Link Power-Conversion System. 2. LITERATURE SURVEY The system proposed in [1] a simple and accurate design methodology for LLC resonant converters, based on a semi empirical approach to model steady-state operation in the belowresonance region. This has led to simple yet accurate design-oriented model and to a simple step-by-step design procedure that ensures stable operation at no load, ZVS under all operating conditions. The methodologies based on first harmonic approximation (FHA) analysis are much editor@iaeme.com

4 Analysis of Zvs Interleaved Llc Resonant Converter for Current Balancing in Dc Distribution System simpler but due to lack of accuracy especially in below-resonance region it is not used. The system in [2] proposes an approximate analysis of LLC resonant converter with capacitive filter operating above and below resonance. An equivalent AC resistance model of the rectifier valid for discontinuous as well as continuous conduction modes is proposed. The DC voltage conversion ratio is then obtained using the fundamental harmonic approximation analysis method. Here Fundamental Harmonic Analysis method has been used. Here LLC can operate with wide load range and can achieve step-up as well as step-down voltage conversion. Since it is applicable to off-line converters it has relatively large DC holdup and output capacitor need to provide output voltage support in case of line fault of light load to full load transients. This reduces the power density and has drawback of alleviating fast voltage control. The project in [3] proposed Simulation and interleaved converters using switched capacitor are considered as a better solution for fuel cell systems due to high conversion efficiency. In the proposed interleaved converter, the front end inductors are magnetically cross-coupled to improve the electrical performance and reduce the weight and size. Also, switched capacitors are used to improve the voltage gain of the converter. 3. PROPOSED AC-DC HIGH FREQUENCY-LINK POWER- CONVERSION SYSTEM The proposed ac dc high-frequency-link power-conversion system is composed of three bridgeless PFC rectifiers and a three-phase interleaved LLC resonant converter. Fig. 1 shows the schematic of the proposed isolated ac dc converter. To improve power conversion efficiency, the CCM bridgeless boost PFC rectifier using the sic diodes has been designed. The input power source of the proposed ac dc converter is three-phase four-wired ac. Therefore, three 3.3 kw (380 V/8.7 A) PFC boost rectifiers have been designed for 220 Vac input voltage. Each of the rectifiers is controlled by a commercial analog controller. In addition, the three-phase interleaved full-bridge LLC resonant converter using the Y-connected rectifier is proposed for high efficiency dc dc power conversion and galvanic isolation. The proposed converter is controlled by a single digital signal processor (DSP). The proposed current balancing algorithm is also implemented by the same DSP. The detailed circuit operations of the proposed converter will be discussed in this Section. Figure. 2. Schematic of the Proposed Three-Phase Interleaved LLC Resonant Converter with DSP Control. Fig. 2 shows the schematic of the proposed dc dc power conversion stage. The proposed converter has three full-bridge LLC resonant converters whose output stage is composed of three editor@iaeme.com

5 Chandra Sekhar Garlapati and Ravi Kishore D arms of Voltage Source Converter for each secondary transformer winding. The Y-connected rectifier can boost the output voltage without increasing the transformer s turn ratio. In addition, the frequency of the rectifier output ripple is six times higher than the switching frequency, thereby reducing the output capacitor and the RMS current of the transformer s secondary winding. Due to these advantages, the proposed converter is suitable for high power and highoutput voltage applications. The proposed converter can be controlled in the same manner as the conventional singlephase full-bridge LLC converter. The converter s output voltage can be controlled using a conventional pulse frequency modulation (PFM) technique; however, the phase difference for each converter s switching signal is 2π/3. The steady-state equation of the output voltage, Vout, can be derived as follows: Figure. 3. Theoretical operating waveforms of the Proposed Converter Where n is the turn ratio of the transformer as n = N1/N2 and M is the resonant gain, respectively. From (1), the output Voltage of the proposed converter can be doubled due to the Voltage Source Converter, compared with the input output voltage Ratio of the single-phase LLC resonant converter. Fig. 3. Shows the theoretical operating waveforms of the proposed Converter in a steady state. The operation of the proposed Converter can be divided into 12 operating modes. In this project, Representative operation modes such as Mode 1 (t0 -t1) and Mode 2 (t1 -t2) will be explained. In Mode 1, the master converters Switches Q1 and Q4 turn ON under the ZVS condition and the magnetizing current Im1 increases in the positive direction. In addition, the slave converter s switches Q6, Q7, Q9, and Q12 continue in their ON state and their resonances are still in progress. During this period, the rectifying diodes Do1, Do4, and Do5 turn ON and all of the primary side s energy in the three converters is transferred to the secondary rectifying Stage. In mode 2, when the primary current, Ir3, meets the magnetizing Current Im3, the resonance in the primary stage ends And the energy is transferred from the primary to the secondary Stage. At this time, Do5 is editor@iaeme.com

6 Analysis of Zvs Interleaved Llc Resonant Converter for Current Balancing in Dc Distribution System turned OFF, while Do1 and Do4 Remain ON. During these periods, only two converters transfer their energy in the primary stage to the output. The other operations can be explained as the same manner as Mode 1 and 2. Mode (1, 2) repeats to Mode (3, 4), Mode (5, 6), Mode (7, 8), Mode (9, 10), and Mode (11, 12) with different switches and Rectifier diodes. 4. SIMULATION RESULTS Figure. 4: Simulink Design of ZVS Three-Phase Interleaved LLC Resonant Converter with Voltage Source Converter Figure. 5: Pulses of switches Q1-Q12 Figure. 6: DC Voltage output of ZVS Interleaved Boost Converters In order to verify the effect of the gain fluctuation according to the variation of the second converter s resonant inductor Lr2, show the simulation results of the rectifying current under unbalanced conditions of the resonant components according to light and heavy load conditions, respectively. Under the light load condition of 8-kW output power, the peak to peak ripple current editor@iaeme.com

7 Chandra Sekhar Garlapati and Ravi Kishore D of the rectifier under the balanced condition is 0.66 A as shown in Fig. 10. In the case of Fig. 9, Lr2 is 10% larger than the original resonant inductance of 120μH. The second converter s resonant gain is smaller than other converters resonant gain because of the large resonant inductance. As shown in Fig. 9, the peak to peak ripple current of the rectifier was measured to be 1.47 A. The secondary side rectifying current of the second converter IB has the smallest value among other converter s rectifying current due to the decreased resonant gain.in the case of Fig. 11, Lr2 is 10% smaller than the original resonant inductance. The second converter s resonant gain is higher than other converters resonant gain. As shown in Fig. 11, the peak-to-peak ripple current of the rectifier was measured to be 1.69 A and IB has the largest value among the other converter s rectifying current because of the increased resonant gain. Figure. 7: Current waveforms of Resonance inductors and Magnetizing currents of HFT Figure.8: Currents of secondary winding of the Transformer Figure. 9: Irec and Io in decreased 10% of Lr case editor@iaeme.com

8 Analysis of Zvs Interleaved Llc Resonant Converter for Current Balancing in Dc Distribution System Figure. 10: Irec and Io in balanced of Lr case Figure. 11: Irec and Io in increased 10% of Lr case 5. CONCLUSION In power sector the power demand is increasing day to day. It is better to depend on renewable energy sources, because, by using non renewable energy sources we cannot reach the total power demand. We can get unlimited power from renewable energy sources and also those renewable energy sources are ecofriendly. We can say that solar and wind are best renewable sources because they can available any place on the earth. With the given reference and modeling of the Three phase ZVS interleaved boost LLC resonant converter with Voltage Source Converter fed by photo voltaic system and wind energy system has been analyzed with all graphical representation. The variation of the output rectifier current is been observed with change in the resonance inductor value with 10% change. The output voltage at the load is observed to be 300V with very less ripple at 0.45% and the conduction losses are been eliminated with the use of high switching of the inverter. Consequently, the power conversion efficiency and power continuity is improved. REFERENCES [1] S. gong Jiang, G. Hua Liu, W. Wang, and D. guo Xu, "Research on bridgeless support PFC with delicate exchanging," in Proc. Veh. Power Propuls. Conf., 2009, pp [2] Vellanki Mehar Jyothi, T. Vijay Muni, S V N L Lalitha An Optimal Energy Management System for PV/Battery Standalone System, International Journal of Electrical and Computer Engineering, Volume 6, No 6, December [3] T. Vijay Muni, K. Venkata Kishore, Experimental Setup of Solar-Wind Hybrid Power System Interface to Grid System. International Journal for Modern Trends in Science and Technology, Vol 2, Issue 1, January editor@iaeme.com

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