Analysis and Comparison of Medium Voltage High Power DC/DC Converters for Offshore Wind Energy Systems

ABSTRACT:

Offshore wind farm with an internal medium-voltage dc (MVDC)-grid collection connected HVDC transmission may be an option to harvest offshore wind energy. High-power MV dc/dc converters with high-step-up conversion ratios are the key components for the internal MVDC grid. In this paper, a high efficiency step-up resonant switched-capacitor converter for offshore wind energy system is studied, which is characterized by the soft-switching condition for all switches and diodes. This significantly reduces switching losses and higher switching frequency

is feasible to reduce the overall system volume and weight. The comparisons with other two kinds of topologies are also presented; moreover, the possible specification requirements of high power MV dc/dc converters are analyzed and set. The operation principle of the proposed converter has been successfully verified by simulation and experiment results.

KEYWORDS:

1.      High power

2.      Medium-voltage dc (MVDC) converter

3.      MVDC grid

4.      Offshore wind farm

SOFTWARE: MATLAB/SIMULINK

BLOCK DIAGRAMS:

 Fig.1. Layouts of three kinds of electrical systems for offshore wind farms. (a) AC system. (b) AC/DC system. (c) DC system.

CIRCUIT DIAGRAM:

Fig. 2. Configuration of the proposed ZCS RSC converter.

EXPECTED SIMULATION RESULTS:

 

Fig. 3. Idealized waveforms of Fig. 2.

 

Fig. 4. Simulation waveforms of an 8-level SCR converter.

Fig. 5. Experimental waveforms under full-load condition. (a) V_gs and V_ds of Q₁ , currents of L_p1 and L_{n 1} . (b) V_gs and V_ds of Q₁ , currents of L_p5 and L_p2 . (c) V_gs and V_ds of Q₁ , currents of D_p12 and D_p11 . (d) V_gs and V_ds of Q₁ , currents of D_p52 and D_p51 .

CONCLUSION:

High-power MV dc/dc converters with high-step-up conversion ratios are the key components in MVDC-grid collection systems for offshore wind farms. This paper has studied the possible specification requirements of high power MV dc/dc converters. A high efficiency step-up resonant switched-capacitor converter for offshore wind energy system has been proposed, which significantly reduces switching losses, increases switching frequency and minimizes the overall system volume. The operation principle and detailed design of the main circuit are presented. The experimental results from the prototype have confirmed the feasibility of the proposed converter.

REFERENCES:

[1] P. K. Steimer and O. Apeldoorn, “Medium voltage power conversion technology for efficient windpark power collection grids,” in Proc. IEEE Int. Symp. Power Electron. Distrib. Gener. Syst., Jun. 2010, pp. 12–18.

[2] S. M. Muyeen, R. Takahashi, and J. Tamura, “Operation and control of HVDC-connected offshore wind farm,” IEEE Trans. Sustainable Energy, vol. 1, no. 1, pp. 30–37, Apr. 2010.

[3] O.Martander, “DC grid for wind farms,” Licentiate of Engineering Thesis, Dept. of EPE, Chalmers University of Technology, Landala, Sweden, 2002.

[4] C. Meyer, M. H¨oing, A. Peterson, and R. W. De Doncker, “Control and design of DC grid for offshore wind farms,” IEEE Trans. Ind. Appli., vol. 43, no. 6, pp. 1474–1482, Nov./Dec. 2007.

[5] J. Robinson, D. Jovcic, and G. Jo´os, “Analysis and design of an offshore wind farm using a MV DC grid,” IEEE Trans. Power Deliv., vol. 25, no. 4, pp. 2164–2173, Oct. 2010.

Comprehensive Approach to Modeling and Simulation of Photovoltaic Arrays

ABSTRACT:

This paper proposes a method of modeling and simulation of photovoltaic arrays. The main objective is to find the parameters of the nonlinear I–V equation by adjusting the curve at three points: open circuit, maximum power, and short circuit. Given these three points, which are provided by all commercial array datasheets, the method finds the best I–V equation for the single-diode photovoltaic (PV) model including the effect of the series and parallel resistances, and warranties that the maximum power of the model matches with the maximum power of the real array. With the parameters of the adjusted I–V equation, one can build a PV circuit model with any circuit simulator by using basic math blocks. The modeling method and the proposed circuit model are useful for power electronics designers who need a simple, fast, accurate, and easy-to-use modeling method for using in simulations of PV systems. In the first pages, the reader will find a tutorial on PV devices and will understand the parameters that compose the single-diode PV model. The modeling method is then introduced and presented in details. The model is validated with experimental data of commercial PV arrays.

KEYWORDS:

1.      Array

2.       Circuit

3.       Equivalent

4.       Model

5.      Modeling

6.      Photovoltaic (PV)

7.       Simulation.

SOFTWARE: MATLAB/SIMULINK

CIRCUIT DIAGRAM:

                    

Fig. 1. PV array model circuit with a controlled current source, equivalent resistors, and the equation of the model current (I_m ).

EXPECTED SIMULATION RESULTS:

                  

Fig. 2. P –V curves plotted for different values of R_s and R_p .

                     

Fig. 3. P_max,m versus V for several values of R_s > 0.

                                    

Fig. 4. I–V curves plotted for different values of R_s and R_p .

                                  

                          

Fig. 5. P_max = f (R_s ) with I = I_mp and V = V_mp.

        

                   

Fig. 6. I–V curve adjusted to three remarkable points.

                        

Fig. 7. P –V curve adjusted to three remarkable points.

                      

Fig. 8. I–V model curves and experimental data of theKC200GT solar array at different temperatures, 1000 W/m² .

        

                 

Fig. 9. I–V model curves and experimental data of theKC200GT solar array at different irradiations, 25⁰C.

CONCLUSION:

This paper has analyzed the development of a method for the mathematical modeling of PV arrays. The objective of the method is to fit the mathematical I–V equation to the experimental remarkable points of the I–V curve of the practical array. The method obtains the parameters of the I–V equation by using the following nominal information from the array datasheet: open circuit voltage, short-circuit current, maximum output power, voltage and current at the MPP, and current/temperature and voltage/temperature coefficients. This paper has proposed an effective and straightforward method to fit the mathematical I–V curve to the three (V, I) remarkable points without the need to guess or to estimate any other parameters except the diode constant a. This paper has proposed a closed solution for the problem of finding the parameters of the single-diode model equation of a practical PV array. Other authors have tried to propose single-diode models and methods for estimating the model parameters, but these methods always require visually fitting the mathematical curve to the I–V points and/or graphically extracting the slope of the I–V curve at a given point and/or successively solving and adjusting the model in a trial and error process. Some authors have proposed indirect methods to adjust the I–V curve through artificial intelligence and interpolation techniques . Although interesting, such methods are not very practical and are unnecessarily complicated and require more computational effort than it would be expected for this problem. Moreover, frequently in these models R_s and R_p are neglected or treated as independent parameters, which is not true if one wishes to correctly adjust the model so that the maximum power of the model is equal to the maximum power of the practical array. An equation to express the dependence of the diode saturation current I₀ on the temperature was proposed and used in the model. The results obtained in the modeling of two practical PV arrays have demonstrated that the equation is effective and permits to exactly adjust the I–V curve at the open-circuit voltages at temperatures different from the nominal. Moreover, the assumption I_pv ≈ I_sc used in most of previous works on PV modeling was replaced in this method by a relation between I_pv and I_sc based on the series and parallel resistances. The proposed iterative method for solving the unknown parameters of the I–V equation allows to determine the value of I_pv , which is different from I_sc . This paper has presented in detail the equations that constitute the single-diode PV I–V model and the algorithm necessary to obtain the parameters of the equation. In order to show the practical use of the proposed modeling method, this paper has presented two circuit models that can be used to simulate PV arrays with circuit simulators. This paper provides the reader with all necessary information to easily develop a single-diode PV array model for analyzing and simulating a PV array. Programs and ready-to-use circuit models are available for download at: http://sites.google.com/site/mvillalva/pvmodel.

REFERENCES:

[1] A. S. Sedra and K. C. Smith, Microelectronic Circuits. London, U.K.: Oxford Univ. Press, 2006.

[2] H. J. M¨oller, Semiconductors for Solar Cells. Norwood, MA: Artech House, 1993.

[3] A. L. Fahrenbruch and R. H. Bube, Fundamentals of Solar Cells. San Francisco, CA: Academic, 1983.

[4] F. Lasnier and T. G. Ang, Photovoltaic Engineering Handbook. New York: Adam Hilger, 1990.

[5] “Photovoltaic systems technology,” Universit¨at Kassel, Kassel, Germany, 2003.

A Five Level Inverter Topology with Single DC Supply by Cascading a Flying Capacitor Inverter and an H-Bridge

ABSTRACT:

In this paper, a new three-phase, five-level inverter topology with a single-dc source is presented. The proposed topology is obtained by cascading a three-level flying capacitor inverter with a flying H-bridge power cell in each phase. This topology has redundant switching states for generating different pole voltages. By selecting appropriate switching states, the capacitor voltages can be balanced instantaneously (as compared to the fundamental) in any direction of the current, irrespective of the load power factor. Another important feature of this topology is that if any H-bridge fails, it can be bypassed and the configuration can still operate as a three-level inverter at its full power rating. This feature improves the reliability of the circuit. A 3-kW induction motor is run with the proposed topology for the full modulation range. The effectiveness of the capacitor balancing algorithm is tested for the full range of speed and during the sudden acceleration of the motor.

KEYWORDS:

1.      Flying capacitor (FC)

2.       H-bridge

3.       Induction motor drive

4.       Multilevel inverter

SOFTWARE: MATLAB/SIMULINK

 CIRCUIT DIAGRAM:

 Fig 1 Proposed three- phase power circuit formed by the connection of a three phase flying capacitor inverter with H-bridge in series

 EXPECTED SIMULATION RESULTS:

 Fig. 2. Phase voltage V_AN , phase current IA and capacitor voltage ripple for different modulation indexes for phase A: ∆VC 1 = 5 V/div; ∆VC 2 = 10 V/div; IA = 2 A/div. (a) 10 Hz with modulation index of 0.2 (V_{A N} = 50 V/div, time = 20 ms/div). (b) 20 Hz with modulation index of 0.4 (V_AN = 100 V/div, time =10 ms/div). (c) 30 Hz with modulation  index  of  0.6  (V_AN  = 100 V/div, time = 10 ms/div). (d) 40 Hz with modulation index of 0.8 (V_{A N} = 100 V/div, time = 5 ms/div).

Fig. 3. Pole voltage V_AO , phase current IA and capacitor voltage ripple for different modulation indexes for phase A: ∆VC 1 = 5 V/div; ∆VC 2 = 10 V/div; IA = 2 A/div. (a) 10 Hz with modulation index of 0.2 (V_AO = 50 V/div, time = 20 ms/div). (b) 20 Hz with modulation index of 0.4 (V_AO = 100 V/div, time = 10 ms/div). (c) 30 Hz with modulation index of 0.6 (V_AO = 100 V/div, and time = 10 ms/div). (d) 40 Hz with modulation index of 0.8 (V_AO = 100 V/div, time = 5 ms/div).

       

 Fig. 4. Rapid acceleration of motor from 10 to 40 Hz in 5.5 s. Capacitor voltage remains constant. V_AN (phase voltage): 200 V/div, IA (phase current): 2 A/div, VC 1 (VD C /2 capacitor DC voltage): 100 V/div, VC 2 (VD C /4 capacitor DC voltage): 100 V/div, and time scale: 1 s/div.

      Fig. 5.  Capacitor balancing operation. The balancing logic has been disabled at T1. C1 balancing has been enabled at T2 and C2 balancing has been en- abled at T3. V_AN (phase voltage): 200 V/div, IA  (phase current): 2 A/div, VC 1(VD C /2 capacitor DC voltage): 100 V/div, VC 2 (VD C /4 capacitor DC voltage): 100 V/div, and time scale: 2 s/div.

CONCLUSION:

In this paper, a new three-phase f ve-level inverter topology with a single-dc source has been proposed. This configuration is formed by cascading a three-level FC inverter and capacitor-fed H-bridges. The key advantages of this topology compared to the conventional topologies include reduced number of devices and simple control. An important feature of this inverter is the ability to balance the capacitor voltages irrespective of load power factor. Another advantage of this inverter is that if one of the H-bridge fails, it can operate as a three-level inverter at full power rating by bypassing the H-bridge. This feature of the inverter improves the reliability of the system The proposed configuration has been analyzed and experimentally verifie for various modulation indexes and frequencies by running a 3-kW squirrel cage induction motor in V/f  control mode, at no load. The working of the capacitor balancing algorithm has been tested. The stable operation of the inverter for various modulation indexes and stability of the inverter voltage levels during rapid acceleration have been validated  experimentally

REFERENCES:

[1] L. G. Franquelo, J. Rodriguez, J. I. Leon, S. Kouro, R. Portillo, M.A.M.  Prats,  “The  age  of multilevel  converters  arrives,”  IEEE  Ind.  Electron.  Magazine, vol. 2, no. 2, pp. 28–39, June.2008.

[2] S.  Kouro,  M.  Malinowski,  K.  Gopakumar,  J.  Pou,  L.  G.  Franquelo,  B.Wu, J. Rodriguez, M. A. Perez, and J. I. Leon, “Recent Advances and  Industrial  Applications  of  Multilevel  Converters,”  IEEE  Trans.  Ind.  Electron.,vol. 57, no. 8, pp. 2553–2580, Aug. 2010.

[3] A. Nabae,  I.  Takahashi,  and H. Akagi,  “A  new  neutral-point-clamped  PWM inverter,” IEEE Trans. Ind. Appl., vol. IA-17, no. 5, pp. 518–523,  Sep. 1981. 

[4] M. Marchesoni, M. Mazzucchelli, and S. Tenconi, “A non-conventional  power  converter  for  plasma  stabilization,”  in  Proc.  IEEE  19th  Annu.  Power  Electron.  Spec. Conf.  (PESC’88) Rec., Apr.  11–14,  vol.  1,  pp.  122–129.

[5] Z.  Du,  L.M.  Tolbert,  J.  N.  Chiasson,  B.  Ozpineci,  H.  Li,  and  A.  Q.  Huang,  “Hybrid  cascaded H-bridges multilevel motor drive  control  for  electric vehicles,” in Proc. IEEE 37th Power Electron. Spec. Conf., Jun.  18–22, 2006, pp. 1–6.