DESIGN AND ANALYSIS OF STEP UP RESONANT CONVERTER FOR GRID-CONNECTED WITH PV SYSTEM

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1 DESIGN AND ANALYSIS OF STEP UP RESONANT CONVERTER FOR GRID-CONNECTED WITH PV SYSTEM 1 SOHAIL AHMED, 2 MAHAMMAD NAGMA ARSHIN 1 M.Tech, Al-Habeeb College of Engineering and Technology, affiliated to JNTU, Hyderabad, Telangana, India. 2 Assistant Professor, Al-Habeeb College of Engineering and Technology, affiliated to JNTU, Hyderabad, Telangana, India. Abstract-According to this project we are implementing a resonant converter which is suitable for gridconnected renewable energy sources. The converter can achieve high voltage gain by utilizing the LC parallel resonant tank. Solar PV has specific advantages as an energy source: its operation generates no pollution and no greenhouse gas emissions once installed, it shows simple scalability in respect of power needs and silicon has large availability in the Earth s crust. There are characterized by zero-voltage-switching (ZVS) turn-on and nearly ZVS turn-off of main switches as well as zero-current-switching turn-off of rectifier diodes; therefore according to the equivalent voltage stress of the semiconductor devices is lower than other resonant step-up converters. By using simulation result we can verify the operation principle of the proposed converter. Index Terms Renewable energy, resonant converter, soft switching, voltage step-up, voltage stress, PV system. INTRODUCTION Development of renewable power assets is crucial to relieve the pressures of exhaustion of the fossil gasoline and environmental pollution. Photovoltaic (PV) power generation is becoming more promising since the introduction of the thin film PV technology due to its lower cost, excellent high temperature performance, low weight, flexibility, and glass-free easy installation. At present, maximum of the renewable power resources are applied with the shape of ac strength. The generation equipments of the renewable power resources and electricity garage gadgets typically comprise dc conversion tiers and the produced electric power is added to the power grid thru dc/ac tiers, resulting in extra energy loss. Moreover, the not unusual hassle of the renewable strength resources, inclusive of wind and sun, is the large versions of output power, and the relationship of huge scale of the renewable resources to the electricity grid is a huge challenge for the traditional electric equipment, grid structure, and operation. At gift, the voltages over the dc stages in the era equipments of the renewable strength assets are enormously low, inside the range of several hundred volts to numerous thousand volts; subsequently, excessive-strength excessive-voltage step-up dc dc converters are required to supply the produced electrical electricity to the HVDC grid. The three-section topologies, inclusive of 3- segment SAB converter, collection resonant converter, and dual lively bridge converter, which can be more appropriate for excessive-energy programs because of alleviated modern-day stress of every bridge, are also studied and designed for highelectricity excessive-voltage step-up applications [11] [13]. A increase converter is customized by the researchers of Convert an organization to transmit power from ±50 to ±200 kv [22]. To attain the better voltage gain, Enjeti et al. Proposed a a couple of-module shape, which includes a lift converter and a greenback/enhance converter related in input parallel output-series [23]. The scarcity of the RSC-based totally converter is the terrible voltage regulation and the requirement of a large number of capacitors. Jovcic et al. Proposed a singular sort of resonant step-up converter with doubtlessly tender-switching operation, which utilizes thyristors as switches and does now not be afflicted by immoderate transfer stresses and opposite healing issues; moreover, a huge voltage benefit is effortlessly received [26] [28]. A photovoltaic system, also PV system or solar power system, is a power system designed to supply usable solar power by means of photovoltaics. 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, and mounting cabling and other electrical accessories to set up a working system. In this paper, a novel resonant step-up dc dc converter is proposed, which now not most effective can realize gentle switching for fundamental switches and diodes and huge voltage benefit, but also has incredibly lower equivalent voltage pressure of the semiconductor devices and bidirectional magnetized resonant inductor. CONVERTER STRUCTURE AND OPERATION PRINCIPLE The proposed resonant step-up converter is shown in Fig. 1. The converter is composed of an FB switch network, which comprises Q1 through Q4, an

2 LC parallel resonant tank, a voltage doubler rectifier, and two input blocking diodes, Db1 and Db2. Fig. 1. Topology of the proposed resonant step-up converter. The steady-state operating waveforms are shown in Fig. 2 and detailed operation modes of the proposed converter are shown in Fig. 3. In order to simplify the analysis of the converter, the following assumptions are made: 1) all switches, diodes, inductor, and capacitor are ideal components; 2) output filter capacitors C1 and C2 are equal and large enough so that the output voltagevo is considered constant in a switching period Ts (a) (a) [t0, t1] In this mode, the energy delivered from Vin to Lr is E = L I I (2) Mode 2 [t1, t3] [See Fig. 3(b)] At t1, Q1 and Q4 are turned off and after that Lr resonates with C r, vc r decreases from Vin, and ilr increases from I1 in resonant form. Taking into account the parasitic output capacitors of Q1 through Q4 and junction capacitor of Db2, the equivalent circuit of the converter after t1 is shown in Fig. 4(a), in which CDb2, CQ1, and CQ4 are charged, CQ2 and CQ3 are discharged. (b) (b) [t1, t3]. Due to C r is much larger than the parasitic capacitances, the voltages across Q1 and Q4 increase slowly. After that, vc r increases in negative direction and ilr declines in resonant form. At t2, vc r = Vin, the voltages across Q1 and Q4 reach Vin, the voltages across Q2 and Q3 fall to zero and the two switches can be turned on under zero-voltage condition. Fig. 2. Operating waveforms of the proposed converter. A. Mode 1 [t0, t1] [See Fig. 3(a)] During this mode, Q1 and Q4 are turned on resulting in the positive input voltage Vin across the LC parallel resonant tank, i.e., vlr = vc r = Vin. I = I + (1) where T1 is the time interval of t0 to t1. (a)

3 (b) Fig. 4. Further equivalent circuits of Mode 2. (a) [t1, t2]. (b) [t2, t3] The voltage across Q1 is kept at Vin. The equivalent circuit of the converter after t2 is shown in Fig. 4(b), in which D2 and D3 are the anti-parallel diodes of Q2 and Q3, respectively. It can be seen that during t1 to t3, no power is transferred from the input source or to the load, and the whole energy stored in the LC resonant tank is unchanged, i.e., L I + C V = L I + C (3) We have i (t) = sin[ω (t t )] + I cos[ω (t t )] v (t) = V cos[ω (t t )] I Z sin[ω (t t )] T = arc sin + arc sin B. Mode 3 [t3, t4] [See Fig. 3(c)] At t3, vc r = Vo/2, DR1 conducts naturally, C1 is charged by ilr through DR1, vc r keeps unchanged, and ilr decreases linearly. At t4, ilr = 0. E = (6) Assuming 100% conversion efficiency of the converter and according to the energy conversation rule, in half-switching period E = E = E (7) C. Mode 4 [t4, t5] [See Fig. 3(d)] At t4, ilr decreases to zero and the current flowing through DR1 also decreases to zero, and DR1 is turned off with zero current switching (ZCS); therefore, there is no reverse recovery. (d) (d) [t4, t5]. Meanwhile, the voltage across Q4 declines from Vo/2. At t5, vc r = Vin, and ilr = I3. In this mode, the whole energy stored in the LC resonant tank is unchanged, i.e., C = L I + C V (8) We have I = I = (9) i (t) = sin[ω (t t )] (10) v (t) = [ ( )] (11) T = arc cos (12) D. Mode 5 [t5, t6] [See Fig. 3(e)] If Q2 and Q3 are turned on before t5, then after t5, Lr is charged by Vin through Q2 and Q3, ilr increases in negative direction, and the mode is similar to Mode 1. (c) (c) [t3, t4]. The time interval of t3 to t4 is T = (4) The energy delivered to load side in this mode is E = (5) The energy consumed by the load in halfswitching period is (e) (e) [t5, t6]. If Q2 and Q3 are not turned on before t5, then after t5, Lr will resonate with C r, the voltage of node A

4 va will increase from zero and the voltage of node B vb will decay from Vin; zero-voltage condition will be lost if Q2 and Q3 are turned on at the moment. Therefore, Q2 and Q3 must be turned on before t5 to reduce switching loss. The operation modes during [t6, t10] are similar to Modes 2 4, and the detailed equivalent circuits are shown in Fig. 3(f) (h). During [t6, t10], Q2 and Q3 are turned off at almost zero voltage, Q1 and Q4 are turned on with ZVS, and DR2 is turned off with ZCS. fault pass through input side, and vice versa. The comparison of different nonisolated converter topologies is listed in Table I. Table I Comparison Of Different Non isolated Converter Topologies (f) (g) (h) (f) [t6, t8]. (g) [t8, t9]. (h) [t9, t10]. ANALYSIS AND DESIGN OF THE CONVERTER A.Voltage Rating and DC Fault Response According to the analysis of Section II, the voltage stresses of Q1 and Q2 are the input voltage Vin, the voltage stresses of Q3 and Q4 are half of the output voltage, i.e., Vo/2, the voltage stresses of Db1 and Db2 are Vo/2 Vin. The total voltage stress of the primary semiconductor devices is 2Vo, which is half of that in [26] [29]. As shown in Fig. 1, the proposed converter can block an output fault and prevent the Voltage Balance between C1 and C2 The previous analysis is based on the assumption that voltages across C1 and C2 are, respectively, half of output voltage. Provided that V c1 = Vc2, for example, Vc1 > Vo/2 > Vc2, according to the operation principle of Fig. 2, the peak current of i c at t3 will be smaller than that at t8, which means that the average current flowing into C1 will be smaller than the average current flowing into C2. Vice versa, i.e., Vc1 increases and Vc2 decreases under the presumption that Vc1 < Vo/2 < Vc2.. B. Analysis of the Converter From Fig. 2, we have T + T + T + T = (13) Combining (1), (2), and (14) yields V I T = + V T (14) From (19), we have T = (15) From (16), the gain of Vo/Vin is expressed as (16) = ( ) It can be seen that the gain of Vo/Vin is impacted by the parameters of the resonant tank (Lr and Cr ) and the time interval of t4 to t5, which is a part of switching period; hence, in other words, the gain is impacted by Lr, Cr, and the switching frequency. Several important conclusions are obtained 1) For any given voltage gain (larger than 2) and the resonant tank parameters Lr and Cr, there must be a T4 to meet (21), which implies that for given Lr and Cr, the voltage gain can be infinite if the switching frequency range is not taken into account.

5 2) For given voltage gain, the larger the ωr, the shorter the T4; an example is shown in Fig. 5, which means that the switching frequency will be higher. Insulated-gate bipolar transistors (IGBTs) are taken as the main switches and fsmax is set to be 5 khz. However we are indicates that Ts is an implicit function associated with Lr and the concrete analytic solution of Ts cannot be obtained. With the help of mathematical analysis software Maple, we can obtain the curves between L r and Ts under different input voltages as shown in Fig. 6. Fig. 5. Voltage gain versus ωr and T4. 3) For given voltage gain and ωr, although T4 is constant, but the expressions of T1, T2, and T3 are related to Lr or Cr, which means that different pairs of Lr and Cr impact the switching frequency of the proposed converter. Substituting I = (17) Substituting the following equation in the above I = (18) the resonant frequency of Lr and Cr, i.e., f = (19) It can be seen that the switching frequency is equal to the resonant frequency under unloaded condition. Actually, it can be seen from Fig. 2 that T1 = T3 = 0 under unloaded condition because there is no energy input and output if the converter is assumed to be lossless. And if I o > 0, then both T1 and T3 are larger than zero; thus, the switching frequency is lower than the resonant frequency; the heavier the load, the lower the switching frequency. Therefore, the maximum switching frequency of the converter is f = f (20) From the analysis of Section II, it can be seen that to realize zero-voltage turn-on of the switches, the minimum duty cycle of the converter is D = T T (21) As shown in Fig. 2 and the previous analysis, the minimum duty cycle also is the effective duty cycle of the converter, during which the primary current flows through the main switches. According to (5), the time interval ΔT of t1 to t2 is T = arc sin (22) The maximum duty cycle of the converter is D = (23) A. Design of the Converter A 5 MW, 4 kv (±10%)/80 kv step-up converter is taken as an example to design the parameters. Fig. 6. Curves between Lr and Ts under different input voltages. It can be seen that for given Vo and Lr, the lower the input voltage Vin, the lower the switching frequency, and for given input voltage range, the smaller the Lr, the narrower the variation of switching frequency. From (14), one can obtain the curves between Lr and I0 under different input voltages, as shown in Fig. 7. Fig. 7. Curves between Lr and I0, I1 under different input voltages. Through (22) and (25), one can obtain the curves between Lr and I1 under different input voltages, as shown in Fig. 7. It can be seen that the input voltage has little influence on I0 and I1, because as shown in (14) and (22), the larger the voltage gain, the lesser the influence of the input voltage on I0 and I1. From Fig. 7, it can be seen that the smaller the Lr, the larger the I0 and I1, which means that switches and diodes have large peak currents and it is harmful for the device choice, while larger L r is helpful for the device choice Fig. 8. Curves of switching frequency versus output power under different input voltages.

6 After the choice of the resonant parameters, the relationship between the switching frequency and power load is depicted in Fig. 8 with (25). Thus, to realized ZVS for the switches, the duty cyclecan be the any value in the range of , as the shaded area shown in Fig. 9. shows simple scalability in respect of power needs and silicon has large availability in the Earth s crust SIMULATION RESULTS In order to verify the operation principle and the theoretical analysis, a converter is simulated with PLECS simulation software and the detailed parameters are listed in table 2. Fig. 9. Curves of Dm in and Dm ax versus output power under different input voltages. PV SOURCE Photovoltaics (PV) covers the conversion of light into electricity using semiconducting materials that exhibit the photovoltaic effect, a phenomenon studied in physics, photochemistry, and electrochemistry. The dynamic model of PV cell is shown in below Fig.6. Fig:11 Block diagram of simulation Fig 10.Equivalent electrical circuit of the PV cell. The basic equation describing the I -V characteristic of a practical PV cell is I = I I I = I I e (27) where I D is the saturation current of the diode, Q is the electron charge, A is the curve fitting constant (or diode emission factor), K is the Boltzmann constant and T is the temperature on absolute scale. 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. A typical photovoltaic system employs solar panels, each comprising a number of solar cells, which generate electrical power. PV installations may be ground-mounted, rooftop mounted or wall mounted. The mount may be fixed, or use a solar tracker to follow the sun across the sky. Solar PV has specific advantages as an energy source: its operation generates no pollution and no greenhouse gas emissions once installed, it (a)

7 (b) Fig. 12. Steady-state simulation results under different load conditions when Vin = 4 kv. (a) 5 MW. (b) 1 MW. Fig. 12 shows the simulation results at the output power of 5 and 1 MW (Vin = 4 kv), respectively. As the figure shows, the voltage stress of Q1 and Q2 is 4 kv, the voltage stress of Q3 and Q4 is 40 kv, the voltage stress of Db1 and Db2 is 36 kv, and the peak voltage across the LC resonant tank is 40 kv. Table II Simulation Parameters The switching frequencies of the converter at 5 and 1 MW are 2.3 and 4.4 khz, respectively. It can be seen that the output voltage is regulated to be constant and the switching frequency fs changes from 2.3 to 2.5 khz. The simulation results match well with Fig. 8 and the control strategy of variable frequency with constant duty cycle is validated. Fig. 13. Calculated power losses distribution The calculated efficiency is around 97.2% and the losses distribution is depicted in Fig. 13. It can be observed that the dominant part of the power losses is the conduction loss of diodes and switches. CONCLUSION In this paper a novel resonant dc dc converter is proposed, to get the high step-up voltage gain and it is suitable for high-power high-voltage applications. Solar PV has specific advantages as an energy source: its operation generates no pollution and no greenhouse gas emissions once installed, it shows simple scalability in respect of power needs and silicon has large availability in the Earth s crust. therefore The resonant capacitor is employed to achieve zero-voltage turn-on and turn-off for the active switches and ZCS for the rectifier diodes. The analysis demonstrates that the converter can operate at any gain value (> 2) with proper control; however, the parameters of the resonant tank determine the maximum switching frequency, the range of switching frequency, and current ratings of active switches and diodes. The converter is controlled by the variable switching frequency. Moreover the operation principle of the converter and parameters selection of the resonant tank is verified by the simulation result. REFERENCES [1] CIGRE B4-52 Working Group, HVDC Grid Feasibility Study. Melbourne, Vic., Australia: Int. Council Large Electr. Syst., [2] A. S. Abdel-Khalik, A. M. Massoud, A. A. Elserougi, and S. Ahmed, Optimum power transmission-based droop control design for multiterminal HVDC of offshore wind farms, IEEE Trans. Power Syst., vol. 28, no. 3, pp , Aug [3] F. Deng and Z. Chen, Design of protective inductors for HVDC transmission line within DC grid offshore wind farms, IEEE Trans. Power Del., vol. 28, no. 1, pp , Jan [4] F. Deng and Z. Chen, Operation and control of a DC-grid offshore wind farm under DC transmission system faults, IEEE Trans. Power Del., vol. 28, no. 1, pp , Jul

8 [5] C. Meyer, Key components for future offshore DC grids, Ph.D. dissertation, RWTH Aachen Univ., Aachen, Germany, pp. 9 12, [6] W. Chen, A. Huang, S. Lukic, J. Svensson, J. Li, and Z. Wang, A comparison of medium voltage high power DC/DC converters with high step-up conversion ratio for offshore wind energy systems, in Proc. IEEE Energy Convers. Congr. Expo., 2011, pp [7] L. Max, Design and control of a DC collection grid for a wind farm, Ph.D. dissertation, Chalmers Univ. Technol., Goteborg, Sweden, pp , [8] Y. Zhou, D. Macpherson, W. Blewitt, and D. Jovcic, Comparison of DCDC converter topologies for offshore wind-farm application, in Proc. Int. Conf. Power Electron. Mach. Drives, 2012, pp [9] S. Fan, W. Ma, T. C. Lim, and B. W. Williams, Design and control of a wind energy conversion system based on a resonant dc/dc converter, IET Renew. Power Gener., vol. 7, no. 3, pp , [10] F. Deng and Z. Chen, Control of improved fullbridge three-level DC/DC converter for wind turbines in a DC grid, IEEE Trans. Power Electron., vol. 28, no. 1, pp , Jan Mahammad Nagma Arshin received her B.Tech. Degree in Electrical & Electronics Engineering from Jawaharlal Nehru Technological University, Hyderabad, India, 2013 & M.Tech. Degree in Electrical & Electronics Engineering(power and industrial drives) from Jawaharlal Nehru Technological University, Hyderabad, India, in and working as an Assistant Professor in Al-Habeeb College of Engineering and Technology, Chevella, Telangana, India. She has published a number of papers in various national & international journals & conferences. Her research areas are electrical power systems and industrial drives. Assistant Professor, Dept.of EEE, Al-Habeeb College of Engg.&Tech,Chevella, nagmaarshin@gmail.com SOHAIL AHMED Completed B.E in Electrical & Electronics Engineering in 2015 from OSMANIA UNIVERSITY, HYDERABAD and Pursuing M.Tech in electrical and electronics engineering(power and industrial drives) from Al- Habeeb College of Engineering and Technology, affiliated to JNTU, Hyderabad, Telangana, India. Area of interest includes Power Electronics, Electrical Power Systems, Industrial Drives. id: sohailahmed4310@gmail.com