Improved control algorithm for gridconnected cascaded H-bridge photovoltaic inverters under asymmetric operating conditions

ABSTRACT:

Here, a single-stage cascaded H-bridge (CHB) inverter is presented for grid-connected photovoltaic (PV) systems. The CHB inverter has separate DC links and allows individual control of PV arrays. The conversion efficiency is high and the harmonic generation is lower than conventional PV inverters. Although the CHB inverter is a good candidate for injection of solar power into grid, its control issues have not been completely solved. One of the main challenges in the CHB inverter is the harmonic generation when the connected PV arrays to the H-bridge cells have different amounts of insolation. This study deals with the asymmetrical operating conditions of PV arrays (or H-bridge cells) in the CHB inverter and presents an analytical equation for determination of cells’ modulation indices based on PV arrays data. Then, a control loop is added to the tracking algorithm of conventional control systems to determine whether an H-bridge cell is in the linear modulation or not. In the case of overmodulation, the corresponding DC link voltage is increased by the controller to bring it back to the linear region. The validity of new method is confirmed by simulations and experiments on a seven-level 1.7 kW CHB inverter.

 

SOFTWARE: MATLAB/SIMULINK

  

BLOCK DIAGRAM:

Grid-connected PV inverter based on the CHB inverter

Fig. 1. Grid-connected PV inverter based on the CHB inverter

 

EXPECTED SIMULATION RESULTS:

 Injected current to grid

Fig. 2. Injected current to grid

(a) By the presented algorithm ,(b) By the proposed strategy in this paper

Fig. 3. Evaluation of proposed control system behaviour before and after applying the new strategy (a) Modulation indices, (b) Modulating waveforms, (c) Arrays DC link voltages and reference values, (d) Total injected power to the grid

Fig. 4. Dynamic behaviour of the proposed control system under change of irradiance level of the first PV array

 

Fig. 5. Dynamic behaviour of the proposed control system under grid voltage swell and non-uniform distribution of irradiances

 

CONCLUSION:

In this paper, a modified control strategy was proposed for the CHB inverter in the grid-connected PV applications. Based on the circuit analysis, a mathematical relation was derived for determination of cells’ operating conditions in the CHB inverter. This relation shows the value of cells’ modulation indices based on the PV system data. Accordingly, a modified control strategy was proposed to extend the operating range of the CHB inverter under heavy mismatching conditions. In this method, the condition of each H-bridge is checked continuously and when a cell enters to the overmodulation region, its voltage is gradually increased to bring it back to the linear region. This modification helps to prevent the interruption of CHB inverter due to extra harmonic generation in the overmodulation region. The proposed method can be easily applied to the already existing control systems to increase their operating range under asymmetric conditions.

 

REFERENCES:

  • Kouro, S., Malinowski, M., Gopakumar, K., et al.: ‘Recent advances and industrial applications of multilevel converters’, IEEE Trans. Ind. Electron., 2010, 57, (8), pp. 2553–2580
  • Bedram, A., Davoudi, A., Balog, R.S.: ‘Control and circuit techniques to mitigate partial shading effects in photovoltaic arrays’, IEEE J. Photovolt.,2012, 2, (4), pp. 532–546
  • Hajizadeh, M., Fathi, S.H.: ‘Fundamental frequency switching strategy for grid-connected cascaded H-bridge multilevel inverter to mitigate voltage harmonics at the point of common coupling’, IET Power Electron., 2016, 9, (12), pp. 2387–2393
  • Kouro, S., Leon, J.I., Vinnikov, D., et al.: ‘Grid-connected photovoltaic systems: an overview of recent research and emerging PV converter technology’, IEEE Ind. Electron. Mag., 2015, 9, (1), pp. 47–61
  • Oliveira, F.M., Oliveira da Silva, S.A., Durand, F.R., et al.: ‘Grid-tied photovoltaic system based on PSO MPPT technique with active power line conditioning’, IET Power Electron., 2015, 9, (6), pp. 1180–1191

Intelligent Maximum Power Tracking and Inverter Hysteresis Current Control of Grid-connected PV Systems

 ABSTRACT:

This paper proposes a maximum power point tracking scheme using neural networks for a grid connected photovoltaic system. The system is composed of a photovoltaic array, a boost converter, a three phase inverter and grid. The neural network proposed can predict the required terminal voltage of the array in order to obtain maximum power. The duty cycle is calculated and the boost converter switches are controlled. Hysteresis current technique is applied on the three phase inverter so that the output voltage of the converter remains constant at any required set point. The complete system is simulated using MATLAB/SIMULINK software under sudden weather conditions changes. Results show accurate and fast response of the converter and inverter control and which leads to fast maximum power point tracking.

 

KEYWORDS:

  1. Neural networks
  2. Grid connected
  3. Maximum power point tracking
  4. Photovoltaic system
  5. Hysteresis control.

 

SOFTWARE: MATLAB/SIMULINK

BLOCK DIAGRAM:

Fig. 1. Block diagram of the grid connected photovoltaic system

 

EXPECTED SIMULATION RESULTS:

 

Fig. 2. Power-voltage curves for the two cases

Fig. 3. (a) Temperature, (b) Irradiance, (c) Output power of the array, (d)Terminal voltage of the array.

Fig. 4. (a) Reference voltage of inverter control, (b) Voltage at inverter’s DC side.

 

CONCLUSION:

This paper presents a complete control scheme for a grid connected photovoltaic system. The whole system was simulated and the controllers were tested. The proposed maximum power point tracking control using neural networks maintains accurately the maximum power and shows fast dynamic response against sudden environmental condition changes or disturbances. Further research can be carried out in the near future to implement a physical model of the system and to compare the applied scheme with other conventional ones.

 

REFERENCES:

  • G. Villalva, J. R. Gazoli and E. Ruppert F. “Analysis and simulation of the P&O MPPT algorithm using alinearized array model”. Power electronics conference, 2009, Brazil.
  • Safri and S. Mekhilef. “Incremental conductance MPPT method for PV systems”. Electrical and Computer Engineering (CCECE). 2011. Canada.
  • I. Sulaiman, T.K. Abdul Rahman, I.Musirin and S.Shaari. “Optimizing Three-layer Neural Network Model for Grid-Connected Photovoltaic output prediction”. Conference on innovative technologies in intelligent systems and industrial applications.2009.
  • Subiyanto, A.Mohamed and M.A.Hannan. “Maximum Power Point Tracking in Grid Connected PV System using A Novel Fuzzy Logic Controller”. IEEE student conference on research and development, 2009.
  • Trishan Esram and Patrick L. Chapman. “Comparison of Photovoltaic Array Maximum Power Point Tracking Techniques. “ IEEE Transactions on energy conversion, Vol.22, NO. 2, 2007.

Single- and Two-Stage Inverter-Based Grid Connected Photovoltaic Power Plants With Ride-Through Capability Under Grid Faults

IEEE TRANSACTIONS ON SUSTAINABLE ENERGY, VOL. 6, NO. 3, JULY 2015

 ABSTRACT Grid-connected distributed generation sources interfaced with voltage source inverters (VSIs) need to be disconnected from the grid under: 1) excessive dc-link voltage; 2) excessive ac currents; and 3) loss of grid-voltage synchronization. In this paper, the control of single and two stage grid-connected VSIs in photovoltaic (PV) power plants is developed to address the issue of inverter disconnecting under various grid faults. Inverter control incorporates reactive power support in the case of voltage sags based on the grid codes’ (GCs) requirements to ride-through the faults and support the grid voltages. A case study of a 1-MW system simulated in MATLAB/Simulink software is used to illustrate the proposed control. Problems that may occur during grid faults along with associated remedies are discussed. The results presented illustrate the capability of the system to ride-through different types of grid faults.

 

KEYWORDS:

  1. DC–DC converter
  2. Fault-ride-through
  3. Photovoltaic (PV) systems
  4. Power system faults
  5. Reactive power support
  6. single and two stage inverter

 

SOFTWARE: MATLAB/SIMULINK

 

BLOCK DIAGRAM:

single and two stage inverter

Fig. 1. Diagram of a single-stage GCPPP

 single and two stage inverter

Fig. 2. Diagram of the two-stage conversion-based GCPPP

 

EXPECTED SIMULATION RESULTS:

Fig. 3. Short-circuiting the PV panels: (a) grid voltages; (b) grid currents; and (c) dc-link voltage when applying a 60% SLG voltage sag at MV side of the transformer.

Fig. 4. Short-circuiting the PV panels: (a) overall generated power; (b) injected active power; and (c) reactive power to the grid.

Fig. 5. Turning the dc–dc converter switch ON: (a) grid voltages; (b) grid currents; and (c) dc-link voltage when applying a 60% SLG voltage sag at the MV side.

Fig. 6. Control of the dc–dc converter to produce less power under voltage sag: (a) grid voltages; (b) grid currents; (c) dc-link voltage; (d) input voltage of the dc–dc converter; (e) estimated duty cycle; and (f) actual duty cycle under a 3LG with 45% voltage sag at MV side.

Fig. 7. Control of the dc–dc converter to produce less power under voltage sag: (a) grid voltages under a 3LG with 45% voltage sag at MV side; (b) related grid currents for G = 300 W/m2; and (c) related dc-link voltage; (d) grid voltages under an SLG with 65% voltage sag at theMV side; (e) related grid currents for G = 1000 W/m2; (f) related dc-link voltage; (g) related grid currents under G = 300 W/m2; and (h) related dc-link voltage.”

single and two stage inverter

CONCLUSION

Performance requirements of GCPPPs under fault conditions for single and two stage grid-connected inverters have been addressed in this paper. Some modifications have been proposed for controllers to make the GCPPP ride-through compatible to any type of faults according to the GCs. These modifications include applying current limiters and controlling the dc-link voltage by different methods. It is concluded that for the single-stage configuration, the dc-link voltage is naturally limited and therefore, the GCPPP is self-protected, whereas in the two-stage configuration it is not. Three methods have been proposed for the two-stage configuration to make the GCPPP able to withstand any type of faults according to the GCs without being disconnected. The first two methods are based on not generating any power from the PV arrays during the voltage sags, whereas the third method changes the power point of the PV arrays to inject less power into the grid compared with the prefault condition. The validity of all the proposed methods to ride-through voltage sags has been demonstrated by multiple case studies performed by simulations.

 

REFERENCES

  1. Trilla et al., “Modeling and validation of DFIG 3-MW wind turbine using field test data of balanced and unbalanced voltage sags,” IEEE Trans. Sustain. Energy, vol. 2, no. 4, pp. 509–519, Oct. 2011.
  2. Popat, B. Wu, and N. Zargari, “Fault ride-through capability of cascaded current-source converter-based offshore wind farm,” IEEE Trans. Sustain. Energy, vol. 4, no. 2, pp. 314–323, Apr. 2013.
  3. Marinopoulos et al., “Grid integration aspects of large solar PV installations: LVRT capability and reactive power/voltage support requirements,” in Proc. IEEE Trondheim Power Tech, Jun. 2011, pp. 1–8.
  4. Islam, A. Al-Durra, S. M. Muyeen, and J. Tamura, “Low voltage ride through capability enhancement of grid connected large scale photovoltaic system,” in Proc. 37th Annu. Conf. IEEE Ind. Electron. Soc. (IECON), Nov. 2011, pp. 884–889.

A Function Based Maximum Power Point Tracking Method for Photovoltaic Systems

ABSTRACT:

In this paper a novel maximum power point tracking (MPPT) algorithm based on introducing a complex function for photovoltaic systems is proposed. This function is used for determination of the duty cycle of the DC-DC converter in PV systems to track the maximum power point (MPP) in any environment and load condition. It has been suggested based on analyzing the expected behavior of converter controller. The function is formed by a two-dimensional Gaussian function and an Arctangent function. It has been shown that contrary to many algorithms which produce wrong duty-cycles in abrupt irradiance changes, the proposed algorithm is able to behave correctly in these situations. In order to evaluate the performance of method, various simulations and experimental tests have been carried out. The method has been compared with some major MPPT techniques with regard to start-up, steady state and dynamic performance. The results reveal that the proposed method can effectively improve the dynamic performance and steady state performance simultaneously.

 

KEYWORDS:

  1. Gaussian-Arctangent Function Based MPPT
  2. Maximum Power Point Tracking
  3. Photovoltaic Systems
  4. Variable Perturbation Frequency

SOFTWARE: MATLAB/SIMULINK

BLOCK DIAGRAM:

Fig. 1. Electrical scheme of the system under test.

EXPECTED SIMULATION RESULTS:

 

Fig. 2. Output power of PV for battery load in startup test.

(a)

(b)

(c)

(d)

Fig. 3. The output power and duty cycle in step irradiance change for: (a) VSSINC, (b) LCASF method (c) Fuzzy method and (d) Proposed method.

Fig. 4. Response of algorithms to load change.

(a)

(b)

(c)

(d)

Fig. 5. Response of GAF-VPF algorithm to changes in (a) , (b) , (c) and (d) k.

CONCLUSION:

In this paper a new MPPT algorithm named Gaussian-Arctangent Function-Based (GAF) method was proposed. The method is based on introducing a complex function formed by multiplying a two-dimensional Gaussian function with an Arctangent function. This function is used for generating an adaptive perturbation size. In addition, variable perturbation frequency has been utilized for computing the time of applying the next duty cycle. Simulation results and experimental measurements confirm the attractiveness and superiority of the proposed method with respect to some well-known MPPT methods such as variable step-size Incremental Conductance, load-current adaptive step-size and perturbation frequency (LCASF) and Fuzzy method. The algorithm behaves robustly in case of load variation and measurement noise. The other advantage of proposed method is its simplicity of design. It does not require exact tuning of so many parameters. The only system-dependent constants required for controller setup are open-circuit voltage and short-circuit current and standard condition. Although, the computational cost of proposed method is higher than methods like P&O and Incremental Conductance, it can be easily implemented in low cost micro-controllers. All in all, these features make it well-suited for tracking uncommonly fast irradiance variations like mobile solar applications.

REFERENCES:

[1] Moacyr Aureliano Gomes de Brito, Luigi Galotto, Jr., Leonardo Poltronieri Sampaio, Guilherme de Azevedo e Melo, and Carlos Alberto Canesin, „Evaluation of the Main MPPT Techniques for Photovoltaic Applications”, IEEE Trans. Ind. Electron., vol. 60, no. 3, pp. 1156-1167, March 2013.

[2] C. Hua, J. Lin, and C. Shen, “Implementation of a DSP-controlled photovoltaic system with peak power tracking,” IEEE Trans. Ind. Electron., vol. 45, no. 1, pp. 99–107, Feb. 1998.

[3] A.R Reisi, M.H.Moradi, S.Jamasb, “Classification and comparison of maximum power point tracking techniques for photovoltaic system: A review”, Renewable & Sustainable Energy Reviews, vol. 19, pp. 433-443, March 2013.

[4] Qiang Mei, Mingwei Shan, Liying Liu, and Josep M. Guerrero, “A Novel Improved Variable Step Size Incremental-Resistance MPPT Method for PV Systems”, IEEE Trans. Ind. Electron., vol. 58, no. 6, pp. 2427-2434, June 2011.

[5] N. Femia, G. Petrone, G. Spagnuolo, and M. Vitelli, “Optimization of perturb and observe maximum power point tracking method,” IEEE Trans. Power Electron., vol. 20, no. 4, pp. 963–973, July 2005.

 

Full Soft-Switching High Step-Up Current-Fed DC-DC Converters with Reduced Conduction Losses

 

ABSTRACT:

Two variants of the full soft-switching high step-up DC-DC converter are proposed. The main advantage of the converters is the minimized conduction losses by the use of the four-quadrant switches and a specific control algorithm. Simulation was performed to verify the principle of operation and to estimate the losses.

 KEYWORDS:

  1. DC-DC power converters
  2. Photovoltaic systems
  3. Soft switching
  4. Step-up
  5. Isolated

 SOFTWARE: MATLAB/SIMULINK

 CIRCUIT DIAGRAM:

 

Fig. 1. Full soft-switching high step-up DC-DC converter

Fig. 2. Proposed converter topology with four four-quadrant switches.

EXPECTED SIMULATION RESULTS:

 Fig. 3. Simulated voltage and current waveforms of S1 (a), S2 (b), S7.1 (c), S5 (d) for the proposed converter topology with a single four-quadrant switch

CONCLUSION:

The proposed converters allow soft-switching of the both inverter and rectifier switches without any auxiliary passive elements and clamping circuits.

As seen from simulation results, the topology with a single four-quadrant switch has higher efficiency than the topology with four four-quadrant switches, but at the same time, it has few disadvantages that could affect the final choice of topology:

– Step-up factor is slightly lower than in the topology with four four-quadrant switches;

– The switching interval e (and the symmetrical interval in another half-period) must be of strictly right duration, which is equal to the time of current redistribution between switches S4 and S2. The shorter duration of this interval will result in high switching losses and, in extreme cases, can lead to damage of the switch S4. The significantly longer duration will result in current increase through the switch S2 and eventually may result in the boost inductor saturation.

– The original topology and the topology with four four quadrant switches does not have the problem with the longer duration of this switching interval and so they have lower requirements to the control system in dynamic mode. This means that proposed converter with four four-quadrant switches allows robust soft-switching commutation, which is hard to achieve in galvanically isolated current-fed DCDC converters.

The main disadvantage of the topologies is the presence of four switches in series in the inverter stage on the path of the current flow during the energy transfer interval. This leads to the conduction losses higher than in the conventional phase shifted full-bridge topology. Nevertheless the switching losses are lower due to the introduced soft-switching. It means that switching frequency could be increased while maintaining the efficiency at acceptable level.

Future work will be devoted to the experimental verification of the proposed converters and further control algorithm optimization.

 REFERENCES:

[1] A. Blinov, D. Vinnikov, and V. Ivakhno, “Full soft-switching high stepup dc-dc converter for photovoltaic applications,” 2014 16th European Conference on Power Electronics and Applications (EPE’14-ECCE Europe), pp. 1–7, Aug 2014.

[2] Y. Sokol, Y. Goncharov, V. Ivakhno, V. Zamaruiev, B. Styslo, M. Mezheritskij, A. Blinov, and D. Vinnikov, “Using the separated commutation in two-stage dc/dc converter in order to reduce of the power semiconductor switches’ dynamic losses,” Energy Saving. Power Engineering. Energy Audit, 2014.

[3] A. Blinov, V. Ivakhno, V. Zamaruev, D. Vinnikov, and O. Husev, “Experimental verification of dc/dc converter with full-bridge active rectifier,” 38th Annual Conference on IEEE Industrial Electronics Society (IECON 2012), pp. 5179–5184 , Oct 2012.

[4] R.-Y. Chen, T.-J. Liang, J.-F. Chen, R.-L. Lin, and K.-C. Tseng, “Study and implementation of a current-fed full-bridge boost dc-dc converter with zero-current switching for high-voltage applications,” IEEE Transactions on Industry Applications, vol. 44, no. 4, pp. 1218–1226, July 2008.

[5] J.-F. Chen, R.-Y. Chen, and T.-J. Liang, “Study and implementation of a single-stage current-fed boost pfc converter with zcs for high voltage applications,” IEEE Transactions on Power Electronics, vol. 23, no. 1, pp. 379–386, Jan 2008.

Model Predictive Control of PV Sources in A Smart DC Distribution System Maximum Power Point Tracking and Droop Control

 

ABSTRACT:

In a dc distribution system, where multiple power sources supply a common bus, current sharing is an important issue. When renewable energy resources are considered, such as photovoltaic (PV), dc/dc converters are needed to decouple the source voltage, which can vary due to operating conditions and maximum power point tracking (MPPT), from the dc bus voltage. Since different sources may have different power delivery capacities that may vary with time, coordination of the interface to the bus is of paramount importance to ensure reliable system operation. Further, since these sources are most likely distributed throughout\ the system, distributed controls are needed to ensure a robust and fault tolerant control system. This paper presents a model predictive control-based MPPT and model predictive control-based droop current regulator to interface PV in smart dc distribution systems. Back-to-back dc/dc converters control both the input current from the PV module and the droop characteristic of the output current injected into the distribution bus. The predictive controller speeds up both of the control loops, since it predicts and corrects error before the switching signal is applied to the respective converter.

KEYWORDS:

  1. DC microgrid
  2. Droop control
  3. Maximum power point tracking (MPPT)
  4. Model predictive control (MPC)
  5. Photovoltaic (PV)
  6. Photovoltaic systems

 SOFTWARE: MATLAB/SIMULINK

CIRCUIT DIAGRAM:

image001

Fig. 1. Multiple-sourced dc distribution system with central storage.

EXPECTED SIMULATION RESULTS:

image002

Fig. 2. Ideal bus voltage and load power as system impedance increases and loads are interrupted to prevent voltage collapse. (a) Bus voltage decreases in response to increased system impedance at t1 to reach the operating point on the new P–V curve at t2 . The new bus voltage is below the UVP limit, so control action cause load to be shed, moving to a new operating point on the same P–V curve at t3 with a higher bus voltage. (b) Load power in the system changes as point-of-load converters are turned OFF to reduce total system load when the bus voltage drops below the UVP.

image003

Fig. 3. Response of dc bus voltage to step changes in the power drained by load.

image004

Fig. 4. Response of dc bus voltage and output power to imbalanced input PV sources

image005

Fig. 5. Response validation of dc bus voltage to step changes in the power drained by load.

image006

Fig. 6. Response validation of dc bus voltage and output power to imbalanced input PV sources.

image007

Fig. 7. Response of dc bus voltage and output power to the input PV sources of Fig. 7.

CONCLUSION:

 High efficiency and easy interconnection of renewable energy sources increase interests in dc distribution systems. This paper examined autonomous local controllers in a single-bus dc microgrid system for MPP tracked PV sources. An improved MPPT technique for dc distribution system is introduced by predicting the error at next sampling time using MPC. The proposed predictive MPPT technique is compared to commonly used P&O method to show the benefits and improvements in the speed and efficiency of the MPPT. The results show that the MPP is tracked much faster by using the MPC technique than P&O method.

In a smart dc distribution system for microgrid community, parallel dc/dc converters are used to interconnect the sources, load, and storage systems. Equal current sharing between the parallel dc/dc converters and low voltage regulation is required. The proposed droop MPC can achieve these two objectives. The proposed droop control improved the efficiency of the dc distribution system because of the nature of MPC, which predicts the error one step in horizon before applying the switching signal. The effectiveness of the proposed MPPT-MPC and droop MPC is verified through detailed simulation of case studies. Implementation of the MPPT-MPC and droop MPC using dSPACE DS1103 validates the simulation results.

REFERENCES:

[1] Z. Peng, W. Yang, X. Weidong, and L. Wenyuan, “Reliability evaluation of grid-connected photovoltaic power systems,” IEEE Trans. Sustain. Energy, vol. 3, no. 3, pp. 379–389, Jun. 2012.

[2] W. Baochao, M. Sechilariu, and F. Locment, “Intelligent DC microgrid with smart grid communications: Control strategy consideration and design,” IEEE Trans. Smart Grid, vol. 3, no. 4, pp. 2148–2156, Dec. 2012.

[3] R. Majumder, “A hybrid microgrid with DC connection at back to back converters,” IEEE Trans. Smart Grid, vol. 5, no. 1, pp. 251–259, Jun. 2013.

[4] R. Lasseter, A. Akhil, C. Marnay, J. Stephens, J. Dagle, R. Guttromson, A. S. Meliopoulous , R. Yinger, and J. Eto, “Integration of distributed energy resources. The CERTS microgrid concept,” U.S. Dept. Energy, Tech. Rep. LBNL-50829, 2002.

[5] T. Esram and P. L.Chapman, “Comparison of photovoltaic array maximum power point tracking techniques,” IEEE Trans. Energy Convers., vol. 22, no. 2, pp. 439–449, Jun. 2007.