Grid Integration of Renewable Energy Source Using Single-Phase Bidirectional Multilevel Inverter DG Applications

Transcription

1 Research Paper Grid Integration of Renewable Energy Source Using Single-Phase Bidirectional Multilevel Inverter DG Applications Paper ID IJIFR/ V2/ E1/ 049 Page No Subject Area Electrical & Electronics Engineering Key Words Bidirectional Multilevel Inverter, PV Array, Buck/Boost Converter Topology, Total Harmonic Distortions. M. Nehru 1 M-Tech Scholar, Department of Electrical & Electronics Engineering Anurag Engineering College, Kodad, Telangana, India K. Rajani 2 Assistant Professor, Department of Electrical & Electronics Engineering Anurag Engineering College, Kodad, Telangana, India Abstract With the advancement of power electronics and emergence of new multilevel converter topologies, it is possible to work at voltage levels beyond the classic semiconductor limits, so multi-level inverters have been widely used for high-power high-voltage DG applications. Due to higher number of sources, lower EMI, lower % THD in output voltage and less stress on insulation, they are widely used. This work is focused on integration and operation of a single-phase bidirectional multilevel inverter with two buck/boost maximum power point trackers (MPPTs) for distributed generation applications. In a DG system, a bidirectional 5- level multilevel inverter is required to control the power flow between dc bus and ac grid, and to regulate the dc bus to a certain range of voltages. A droop regulation mechanism according to the inverter inductor current levels to reduce capacitor size, balance power flow, and accommodate load variation is proposed. Since the photovoltaic (PV) array voltage can vary from 0 to 600 V, especially with thin-film PV panels, the MPPT topology is formed with buck and boost converters to operate at the dc-bus voltage around 380 V, reducing the voltage stress of its followed multilevel inverter. The proposed system dynamic analysis is evaluated by using Matlab/Simulink tool and results are presented. 1 Introduction The recent trends in small scale power generation using the with the increased concerns on environment and cost of energy, the power industry is experiencing fundamental changes with more renewable energy sources (RESs) or micro sources such as photovoltaic cells, small wind turbines, and micro turbines being integrated into the power grid in the form of distributed generation (DG). These RES-based DG systems are normally interfaced to the grid through power electronics and energy storage systems [1]. One of the most critical sections of the control system for a distributed generation (DG) unit s interconnection to the utility grid lies within the grid-connected converter s Copyright IJIFR

2 control and protection system. This DG system comprises of PV source, DC/DC converter with MLI topology. Nowadays, a conventional two-stage configuration is usually adopted in the PV inverter systems [4] [8]. Each MPPT is realized with a boost converter to step up the PV-array voltage close to the specified dc-link voltage, as shown in Figure1. The boost converter is operated in by-pass mode when the PV-array voltage is higher than the dc-link voltage, and the inverter will function as an MPPT. However, since the characteristics of PV arrays are different from each other, the inverter operated in by-pass mode cannot track each individual maximum power point accurately, and the inverter suffers from as high-voltage stress as the open voltage of the arrays. The MPPT will switch operation modes between buck and boost when the output voltage of a PV array is close to the dc-bus voltage. The designed controller can switch control laws to achieve smooth mode transition and fulfill online configuration check for the MPPTs, which can be either separate or in parallel connection, to draw the maximum power from the PV arrays more effectively. Figure 1: Configuration of a dc-distribution system Basically Inverter is a device that converts DC power to AC power at desired output voltage and frequency. Demerits of inverter are less efficiency, high cost, and high switching losses. To overcome these demerits, we are going to multilevel inverter. The term Multilevel began with the three-level converter. The conventional voltage source inverters produce an output voltage at the poles with levels +/-Vdc/2, where Vdc is the dc-link voltage, are known as the two-level inverter. To obtain a quality output voltage or a current waveform with a minimum amount of ripple content, they require high-switching frequency along with various pulse-width modulation (PWM) strategies [9]. In highpower and high-voltage applications, these two-level inverters however, have some limitations in operating at high frequency mainly due to switching losses and constraints of device ratings. Among this the most commonly used topologies are neutral-point-clamped (NPC), flying capacitors (capacitor clamped), cascaded H-bridge topology have better features and used in our application. 232

3 1. OPERATIONAL PRINCIPLE & CONTROL LOGIC To achieve the desired performance of the proposed PV inverter system, its operational principle is first presented and the control laws for the inverter operation are then derived. Figure2 shows a configuration of the proposed single-phase bidirectional inverter with two buck/boost MPPTs, which can fulfill either grid-connection mode or rectification mode with PFC. The proposed bidirectional inverter, is a full-bridge configuration, which can fulfill grid connection and rectification with PFC [10]. Figure 2: Configuration of the studied PV inverter system with the buck/boost MPPTs. The inverter senses dc-bus voltage vdc, line voltage vs, and inductor current ils, and uses the variable inductance, which is a function of inductor current, obtained with self-learning algorithm to determine the control for operating the inverter stably. When the output power from PV arrays is higher than load requirement, the dc-bus voltage increases; thus, the inverter is operated in grid-connection mode to inject the surplus power into ac grid. On the other hand, the inverter is operated in rectification mode with PFC to convert ac source to replenish the dc bus. Unlike the unipolar modulation [11]- [13], the deadbeat control laws with a bipolar modulation are derived as follows: (for grid connected mode) (1) (2) (for rectification mode) Where Ts is the switching period and dgc and dre are the duty ratios (controls). A control block diagram of the inverter with bipolar modulation is shown in Figure3. According to the reference current difference (iref (n + 1) iref (n)) and the current error ie (n) between reference current iref (n) and feedback current if b(n), the controller can determine duty ratio d(n + 1) for the (n + 1)th cycle. In (1) and (2), the total current difference ΔiLs can be expressed as follows: 233

4 (3) Figure 3. A control block diagram of the bidirectional inverter with bipolar modulation. Where (4) The controller finely adjusts the duty ratio based on (1) or (2) and keeps with Kp = 1 to compensate the current error ie (n). Feedback gain H is a scaling factor when sensing inductor current ils. According to the duty ratio shown in (1) and (2), the Gc can be determined as follows: (5) Where Ls (ils) is the learned inductance that varies with ils. The plant Gp of the inverter is defined as the transfer function of control d to inductor current ils, which can be derived based on state-space averaging method [19] as follows: (6) Where rl is the equivalent resistance of Ls. Since the load may change abruptly and cause dc-bus voltage to vary beyond the operating range, it requires a regulation mechanism to control the dc-bus voltage to a certain range. The bidirectional inverter will adjust the inductor current command to balance the power and regulate the dc-bus voltage [14]. 2. Operation & Analysis Of Proposed Converter Topology The MPPT topology is formed from a buck converter and a boost converter but with a shared inductor to accommodate wide PV-array voltages from 0 to 600 V. For various PV-array applications, the two MPPTs will be connected separately or in parallel. The MPPT senses PV voltage vpv, dc-bus voltage vdc, and inductor current ilm into the single-chip microcontroller (TMS320LF2406 A) to determine operational mode and duty ratio for tracking the maximum power point accurately. When voltage vpv is higher than vdc, the MPPT is operated in buck mode, and switch M1 is turned ON to magnetize inductor Lm and thus increase inductor current ilm. While switch M1 is turned OFF, inductor Lm releases its stored energy through diodes D1 and D2 in [15]. On the other hand, the MPPT is operated 234

5 in boost mode when voltage vpv is lower than vdc, and switches M1 and M2 are turned ON to magnetize inductor Lm. While switchm2 is turned OFF, inductor Lm releases its stored energy through diode D2. Thus, the control laws can be expressed as follows: (7) (8) To draw maximum power from PV arrays, a perturbation and observation control algorithm for tracking maximum power points is adopted. If the maximum power level of a PV array is higher than the power rating of an MPPT, the two MPPTs will be in parallel operation to function as a single MPPT. Thus, it requires an online configuration check to determine the connection types of the two MPPTs, separately or in parallel. Moreover, if the two MPPTs are in parallel operation, a uniform current control scheme is introduced to equally distribute the PV-array output current to the two MPPTs [16]-[18]. The operational-mode transition control between buck and boost is also presented. In this study, the MPPT controller tracks the maximum output power of a PV array based on the perturbation and observation tracking method. At the beginning, the controller will determine the operation mode of the proposed MPPT. When the MPPT is operated in boost mode, inductor current ilm is equal to output current ipv of the PV array; thus, the output power of the PV array can be expressed as follows: (9) On the other hand, when the proposed MPPT is operated in buck mode, inductor current ilm is equal to output current io ; thus, the output power of the PV array can be expressed as follows: (10) Figure 4: Flowchart of online MPPT configuration check 235

6 With this control algorithm, the controller tracks the peak power by increasing or decreasing the duty ratio periodically. In this studied PV inverter system, there is a shared auxiliary power supply for the MPPTs and the inverter. Because the switching frequencies of the MPPT (25 khz) and the inverter (20 khz) are different, their switching noises might affect the accuracy of voltage and current sampling, especially under high-power condition. To avoid noise interference, the MPPTs are synchronized with the inverter, and the controller will update the duty ratio of the MPPT power stage every ten line cycles at the zero crossing of the line voltage. Additionally, since the single-phase PV inverter system has a twice line-frequency ripple voltage on the dc bus, this synchronization approach can also eliminate the ripple voltage effect and determine accurate output power of the PV arrays. When the output power of the PV arrays can be determined accurately, the proposed controller can track the maximum power point precisely. In order to track the maximum power point correctly and effectively, a scheme of online MPPT configuration check is proposed. A flowchart of the check algorithm is shown in Figure4. First, the MPPT determines if there is any PV array plugged in or removed from the system by checking voltage vpv for 100 ms. If voltage vpv is higher than the threshold voltage vth, the controller determines that a new PV array is plugged into an MPPT. On the contrary, if voltage vpv is lower than vth, it means that a PV array is removed from an MPPT or there is no PV array. Next, if the input voltages of both MPPTs are very close (within Δv), the MPPT configuration will be determined as a parallel mode. On the contrary, the two MPPTs will be operated in separate mode. Moreover, a parallel verification algorithm is utilized to confirm the MPPT configuration check. The controller will perturb the duty ratio of one MPPT to examine if both MPPT input voltages are still identical to indentify the connection modes. The system controller checks the configuration of the MPPTs every switching cycle. If the PV arrays are connected to the MPPTs separately, as shown in Fig.5, the MPPTs will calculate their PV output power and tune their duty ratios individually. If the maximum power level of a PV array is higher than that of an MPPT, the two MPPTs will be connected to this PV array and operated synchronously, as shown in Figure5. When tracking the maximum PV output power, the MPPTs will sum up their input currents and equally distribute the total current to the two MPPTs based on a uniform current control scheme [19]. There might exist differences between the two MPPTs, such as components, feedback signals, and noise levels, which will result in current imbalance while they are connected in parallel. When a current imbalance occurs, the components with higher current level will suffer from higher temperature and shorter lifetime. Considering the component reliability and thermal problem, a uniform current control scheme is proposed and described as follows. First, we calculate the current difference (Δidiff ) between the two MPPTs to determine if a uniform current control is necessary. If the current difference Δidiff is higher than a threshold value, the controller will vary the duty ratios (Δd) of the MPPTs to achieve equal current distribution. The duty ratios of the two MPPTs are determined as follows: (11) When DPV1 is increased by Δd and DPV2 will be decreased by Δd and vice versa. (12) 236

7 Since the operation range of the dc-bus voltage is limited within V (including ripple voltage) in the dc distribution system, operational-mode transition between the buck and boost modes will be a critical control issue to accommodate a wide PV input voltage variation (0 600 V). When the proposed MPPT is operated in boost mode and voltage vpv is close to vdc, switch M2 is turned OFF and the duty ratio of switch M1 starts to decrease ( Δd) from 100%. With this control scheme, current ipv of the PV array will charge input capacitor Cpv, and voltage vpv can be raised up to a higher level to prevent mode fluctuation problems. On the contrary, switch M1 is continuously turned ON and the duty ratio of switch M2 starts to increase (+Δd) from 0%, when vpv drops toward vdc during buck mode. Therefore, the MPPT can achieve smooth mode transition by tuning the duty ratios of the active switches. A flowchart of the buck/boost mode transition scheme is shown in Figure5. Figure 5:Flowchart of the buck/boost mode transition algorithm. In last few years there is growing interest in multilevel topologies, because of many possibilities of expanding areas of power electronics use. It can also extend the application of power converters to higher voltage and power ratio. Introducing multilevel converters to power conditioning, drives, power generation and power distribution small and medium voltage applications is very promising idea. Multilevel converters synthesize output voltage from more than two voltage levels. Thus, the output signals spectrum is significantly improved in comparison to classical two level converters. Fig.6 shows a five level cascaded H-bridge multilevel inverter. The converter consists of two series connected H-bridge cells which are fed by independent voltage sources. The outputs of the H-bridge cells are connected in series such that the synthesized voltage waveform is the sum of all of the individual cell outputs [20]. 237

8 Figure 6 : Five cascaded H-bridge Multi level inverter Where the output voltage of the first cell is labelled V1 and the output voltage of the second cell is denoted by V 2. There are five level of output voltage ie 2V, V, 0, -V, -2V.The main advantages of cascaded H-bridge inverter is that it requires least number of components, modularized circuit and soft switching can be employed. But the main disadvantage is that when the voltage level increases, the number of switches increases and also the sources, this in effect increases the cost and weight. The cascaded H-bridge multilevel inverters have been applied where high power and power quality are essential, for example, static synchronous compensators, active filter and reactive power compensation applications, photo voltaic power conversion, uninterruptible power supplies, and magnetic resonance imaging. Furthermore, one of the growing applications for multilevel motor drive is electric and hybrid power trains. 3. Matlab/Simulink Modeling And Results Here simulation is carried out in several cases, in that Proposed PV System Fed Buck/Boost Bi-Directional Inverter for Grid Connected System. Proposed PV System Fed Buck/Boost Bi-Directional Multilevel Inverter for Grid Connected System. Case 1: Proposed PV System Fed Buck/Boost Bi-Directional Inverter for Grid Connected System. Figure 7: Matlab/Simulink Model of Proposed Grid Connected PV Tied Inverter with Buck/Boost Converter Cell. 238

9 Figure 8. (ILm1, ILm2) Inductor Currents, of Proposed Grid Connected PV Tied Inverter with Buck/Boost Converter Cell Figure 9. (ILm1, ILs) Inductor Currents, (Vdc) DC Link Voltage, (Vpv) PV Output Voltage of Proposed Grid Connected PV Tied Inverter with Buck/Boost Converter Cell Figure 10. ILs Inductor Currents Proposed Grid Connected PV Tied Inverter with Buck/Boost Converter Cell under Sudden Load Condition. 239

10 Figure 11: Inverter Output Voltage of the Proposed Grid Connected PV Tied Inverter with Buck/Boost Converter Figure 12 : THD Analysis of Output Voltage of Proposed Grid Connected PV Tied 3-level Inverter with Buck/Boost Converter, attain THD as 50.49% Case 2: Proposed PV System Fed Buck/Boost Bi-Directional Multilevel Inverter for Grid Connected System. Figure 13: Matlab/Simulink Model of Proposed Grid Connected PV Tied with 5-Level Multilevel Inverter with Buck/Boost Converter Cell. 240

11 Fig.14 5-Level Output Voltage & Grid Voltage of Proposed Grid Connected PV Tied with 5-Level Multilevel Inverter with Buck/Boost Converter Cell. Fig.15 THD Analysis of Output Voltage of Proposed Grid Connected PV Tied 5-level Multilevel Inverter with Buck/Boost Converter, attain THD as 28.32%. 4. Conclusion Relatively small power generations such as small wind or solar system, would be an approach to penetrate renewable to the power systems. Small renewable energy sources are connected to the low side of the distribution systems. In this paper, a single-phase bidirectional multilevel inverter with two buck/boost MPPTs has been designed and implemented. The inverter controls the power flow between dc bus and ac grid, and regulates the dc bus to a certain range of voltages. A droop regulation mechanism according to the inductor current levels has been proposed to balance the power flow and accommodate load variation. Since the PV-array voltage can vary from 0 to 600 V, the MPPT topology is formed with buck and boost converters to operate at the dc-bus voltage around 380 V, reducing the voltage stress of its followed inverter. Increment of levels gets good quality nature of output voltage, which reduces the load side filter, low THD values with optimal quality of voltage, for 3-level THD value is 50.49% and for 5-level MLI topology attain 28.32%, more number of levels increases THD goes to drastically reduces. Integration and operation of the overall multilevel inverter 241

12 system have been discussed in detail, which contributes to DG applications significantly and simulation results are conferred. 5. References [1] J. M. Carrasco, L. G. Franquelo, J. T. Bialasiewicz, E. Galvan, R. C. P. Guisado, Ma. A. M. Prats, J. I. Leon, and N. Moreno-Alfonso, Power-electronic systems for the grid integration of renewable energy sources: a survey, IEEE Trans. Ind. Electron., vol. 53, no. 4, pp , Aug [2] L. N. Khanh, J.-J. Seo, T.-S. Kim, and D.-J. Won, Power-management strategies for a grid-connected PV-FC hybrid system, IEEE Trans. Power Deliv., vol. 25, no. 3, pp , Jul [3] Y. K. Tan and S. K. Panda, Optimized wind energy harvesting system using resistance emulator and active rectifier for wireless sensor nodes, IEEE Trans. Power Electron., vol. 26, no. 1, pp , Jan [4] J.-M. Kwon, K.-H. Nam, and B.-H. Kwon, Photovoltaic power conditioning system with line connection, IEEE Trans. Ind. Electron., vol. 53, no. 4, pp , Aug [5] J. Selvaraj and N. A. Rahim, Multilevel inverter for grid-connected PV system employing digital PI controller, IEEE Trans. Ind. Electron., vol. 56, no. 1, pp , Jan [6] F. Gao, D. Li, P. C. Loh, Y. Tang, and P. Wang, Indirect dc-link voltage control of two-stage singlephase PV inverter, in Proc IEEE ECCE, 2009, pp [7] A. Nami, F. Zare, A. Ghosh, and F. Blaabjerg, A hybrid cascade converter topology with seriesconnected symmetrical and asymmetrical diode clamped H-Bridge cells, IEEE Trans. Power Electron., vol. 26, no. 1, pp , Jan [8] S. Dwari and L. Parsa, An efficient high-step-up interleaved dc dc converter with a common active clamp, IEEE Trans. Power Electron., vol. 26, no. 1, pp , Jan [9] J.-M. Shen, H.-L. Jou, and J.-C.Wu, Novel transformerless gridc onnected power converter with negative grounding for photovoltaic generation system, IEEE Trans. Power Electron., vol. 27, no. 4, pp , Apr ] T. Kitano, M. Matsui, and D. Xu, Power sensorless MPPT control scheme utilizing power balance at DC link System design to ensure stability and response, in Proc. 27th Annu. Conf. IEEE Ind. Electron. Soc., 2001, vol. 2, pp [12] Y. Chen and K. M. Smedley, A cost-effective single-stage inverter with maximum power point tracking, IEEE Trans. Power Electron., vol. 19, no. 5, pp , Jun [13] Q. Mei, M. Shan, L. Liu, and J. M. Guerrero, A novel improved variable step-size incrementalresistance MPPT method for PV systems, IEEE Trans. Ind. Electron., vol. 58, no. 6, pp , Jun [14] A. K. Abdelsalam, A. M. Massoud, S. Ahmed, and P. N. Enjeti, High-performance adaptive perturb and observe MPPT technique for photovoltaic-based microgrids, IEEE Trans. Power Electron., vol. 26, no. 4, pp , Apr [15] P. Mattavelli, A closed-loop selective harmonic compensation for active filters, IEEE Trans. Ind. Appl., vol. 37, no. 1, pp , Jan./Feb [16] X. Yuan, W. Merk, H. Stemmler, and J. Allmeling, Stationary-frame generalized integrators for current control of active power filters with zero steady-state error for current harmonics of concern under unbalanced and distorted operating conditions, IEEE Trans. Ind. Appl., vol. 38, no. 2, pp , Mar./Apr [17] J. Allmeling, A control structure for fast harmonics compensation in active filters, IEEE Trans. Power Electron., vol. 19, no. 2, pp , Mar [18] C. Lascu, L. Asiminoaei, I. Boldea, and F. Blaabjerg, High performance current controller for selective harmonic compensation in activ power filters, IEEE Trans. Power Electron., vol. 22, no. 5, pp , Sep [19] D. De and V. Ramanarayanan, A proportional + multiresonant controller for three-phase four-wire high-frequency link inverter, IEEE Trans. Power Electron., vol. 25, no. 4, pp , Apr

13 [20] R. C ardenas, C. Juri, R. Pen na, P.Wheeler, and J. Clare, The application of resonant controllers to four-leg matrix converters feeding unbalanced or nonlinear loads, IEEE Trans. Power Electron., vol. 27, no. 3, pp , Mar Biographies Mr. M. Nehru is currently pursuing as M.Tech in power electronics & electric drives at Anurag Engineering College, Kodad, Nalgonda (Dt), Telangana, India. His areas of interests are Power Electronics, Power Converters, and Electrical Machines Ms K.Rajani is presently working as an Assistant Professor in Department of Electrical and Electronics Engineering in Anurag Engineering College,Kodad. She has 7 years of teaching experience. She completed her B.Tech in EEE from Madhira Institute of science and Technology,Kodad in 2006 and M.Tech in Power Electonics specialization from Ellenki College of Engineering and Technology, Hyderabad in Her area of interests are Application of Power electronic devices in Power Systems for the power quality improvement, HVDC Transmission. 243