Single-Stage DC-AC Converter for Photovoltaic System

Single-Stage DC-AC Converter for Photovoltaic Systems

ABSTRACT:

This paper presents a DC-AC converter that merges a DC-DC converter and an inverter in a single-stage topology to be used as an interface converter between photovoltaic systems and the electrical AC grid. This topology is based on a full bridge converter with three levels output voltage, where two diodes and one inductor have been added in order to create a Boost converter. The control system of the proposed converter is based on two hysteretic controllers: one for the grid injected current and the other for controlling the panel current. A prototype of the proposed converter including power and control circuits was developed. The MPPT algorithm is not yet implemented and, therefore, to obtain experimental results an additional power supply is used to emulate the PV panel. Theoretical analysis and design criteria are presented together with simulated results to validate the proposed concepts. Experimental results are obtained in a lab prototype to evidence the feasibility and performance of the converter.

SOFTWARE: MATLAB/SIMULINK

 BLOCK DIAGRAM:

Fig.1. Power Grid connection of a PV array by means of a two stages converter.

Fig.2. Proposed converter: single-stage DC-AC converter for PV systems.

EXPECTED SIMULATION RESULTS:

 Fig.3. Experimental results: a) iLR (blue trace) with a gain of 1A/div, iLP (green trace) with a gain of 1A/div; b) grid voltage (magenta trace) with a gain of 50 V/div; c) vCF voltage (red trace) with a gain of 100V/div and iLR (blue trace) with a gain of 1A/div.

      

                                     

      

 Fig.4. Simulation results: a) iLR (blue trace) with a gain of 1A/div, iLP (green trace) with a gain of 1A/div; b) Obsetvation of iLP perturbations; c) verification of the maximum iLR switching frequency value, fSRmax.

CONCLUSION:

The new contribution of this paper consisted in the proposal of a new DC-AC converter for PV systems that includes two Boost converters and a full-bridge inverter in a single-stage topology. The unique restriction imposed by the converter is the minimum VCF voltage which must be greater than the sum of the maximum values of the panel and the grid voltage. Due to the high voltage gain given by the two input boosts, the topology is suitable to operate with low panel voltages. The theoretical concepts introduced in the paper were proved by the preliminary results obtained in the experimental tests of the converter prototype that is still in development. A converter efficiency of 90.5% was achieved. The prototype used to obtain the preliminary experimental results presented in the paper is not yet optimized in terms of layout and power density. It is expected that, in what concerns circuit layout, the reduction of the leakage inductances will result in a significant reduction of the dissipated power which will end up in efficiency increase.

REFERENCES:

[1] Johan H. R. Enslin, Mario S. Wolf, Daniel B. Snyman, and Wernher Swiegers, “Integrated Photovoltaic Maximum Power Point Tracking Converter”, in IEEE Transactions on Industrial Electronics, Vol. 44, no. 6, December 1997.

[2] Soeren B. Kjaer, John K. Pedersen and Frede Blaabjerg, ‘‘A Review of Single-Phase Grid-Connected Inverters for Photovoltaic Modules’’, in IEEE Transactions on Industry Applications, Vol. 41, No. 5, 2005.

[3] Fritz Schimpf and Lars E. Norum, “Grid connected Converters for Photovoltaic, State of the Art, Ideas for Improvement of Transformerless Inverters”, Nordic Workshop on Power and Industrial Electronics, 2008.

[4] Mario Fortunato, Alessandro Giustiniani, Giovanni Petrone, Giovanni Spagnuolo, and Massimo Vitelli, “Maximum Power Point Tracking in a One-Cycle-Controlled Single-Stage Photovoltaic Inverter”, in IEEE Transactions on Industrial Electronics, vol. 55, no. 7, pp. 2684-2693, July 2008.

[5] Yeong-Chau Kuo, Tsorng-Juu Liang, and Jiann-Fuh Chen, “Novel Maximum-Power-Point-Tracking Controller for Photovoltaic Energy Conversion System”, in IEEE Transactions on Power Electronics, vol. 48, no. 3, pp. 594-601, June 2001

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Study of Induction Motor Drive with Direct Torque Control Scheme and Indirect Field Oriented Control Scheme Using Space Vector Modulation

Study of Induction Motor Drive with Direct Torque Control Scheme and Indirect Field Oriented Control Scheme Using Space Vector Modulation

ABSTRACT:

Induction motors are the starting point to design an electrical drive system which is widely used in many industrial applications. In modern control theory, different mathematical models describe induction motor according to the employed control methods. Vector control strategy can be applied to this electrical motor type in symmetrical three phase version or in unsymmetrical two phase version. The operation of the induction motor can be analyzed similar to a DC motor through this control method. With the Joint progress of the power electronics and numerical electronics it is possible today to deal with the axis control with variable speed in low power applications. With these technological projections, various command approaches have been developed by the scientific community to master in real time, the flux and the torque of the electrical machines, the direct torque control (DTC) scheme being one of the most recent steps in this direction. This scheme provides excellent properties of regulation without rotational speed feedback. In this control scheme the electromagnetic torque and stator flux magnitude are estimated with only stator voltages and currents and this estimation does not depend on motor parameters except for the stator resistance. In this dissertation report conventional DTC scheme has been described. Induction motor has been simulated in stationary d-q reference frame and its free acceleration characteristics are drawn. Conventional DTC scheme has been simulated with a 50 HP, 460V, 60 Hz induction motor. Literature review has been done to study the recent improvements in DTC scheme which somehow is able to overcome the drawbacks of conventional one. The space vector modulation technique (SVPWM) is applied to 2 level inverter control in the vector control based induction motor drive system, thereby dramatically reducing the torque ripple. Later in this project space vector PWM technique will be applied to DTC drive system to reduce the torque ripple.

SOFTWARE: MATLAB/SIMULINK

BLOCK DIAGRAM:

Fig.1 Block diagram of conventional DTC scheme for IM drives

EXPECTED SIMULATION RESULTS:

Fig.2  Electromagnetic torque

Fig.3 Rotor speed

Fig.4 Stator current

Fig.5 d-axis stator flux

Fig.6 q-axis stator flux

For TL = 2 Nm

Fig.7 Electromagnetic torque

Fig.8 Rotor speed

Fig.9 Trajectory of d axis and q axis stator flux in stationary reference frame

Fig.10 Electromagnetic torque

Fig.11 Rotor speed

Fig.12 d-axis stator flux

Fig.13 q-axis stator flux

Fig.14 d-axis stator current

Fig.15 q-axis stator current

Fig.16 Mean value of Phase voltage of inverter

Fig.17 Line voltage output of inverter

Fig.18 Electromagnetic torque

Fig.19 Rotor speed

Fig.20 q-axis stator flux

Fig.21 d-axis stator flux

CONCLUSION:

For any IM drives, Direct torque control is one of the best controllers proposed so far. It allows decoupled control of motor stator flux and electromagnetic torque. From the analysis it is proved that, this strategy of IM control is simpler to implement than other vector control methods as it does not require pulse width modulator and co-ordinate transformations. But it introduces undesired torque and current ripple. DTC scheme uses stationary d-q reference frame with d-axis aligned with the stator axis. Stator voltage space vector defined in this reference frame control the torque and flux. The main inferences from this work are:

1. In transient state, by selecting the fastest accelerating voltage vector which produces maximum slip frequency, highest torque response can be obtained.

2. In steady state, the torque can be maintained constant with small switching frequency by the torque hysteresis comparator by selecting the accelerating vector and the zero voltage vector alternately.

3. In order to get the optimum efficiency in steady state and the highest torque response in transient state at the same time, the flux level can be automatically adjusted.

4. If the switching frequency is extremely low, the control circuit makes some drift which can be compensated easily to minimize the machine parameter variation.

The estimation accuracy of stator flux is very much essential which mostly depends on stator resistance because an error in stator flux estimation will affect the behavior of both torque and flux control loops. The torque and current ripple can be minimized by employing space vector modulation technique.

REFERENCES:

[1] B. K.Bose. 1997. Power Electronics and Variable Frequency Drives. IEEE Press, New York.

[2] Kazmierkowski, R.Krishnan, Blaabjerg, Control in Power Electronics, Selected Problems.

[3] Takahashi Isao, Noguc hi Tos hihiko, „‟A New Quick-Response and High-Efficiency Control Strategy of an Induction Motor‟‟, IEEE Transactions on Industry Applications, Vol. IA-22 No-5, Sept/Oct 1986.

[4] Thomas G.Habetler, Francesco Profumo, Michele Pastorelli and Leon M. Tolbert “Direct Torque Control of IM us ing Space Vector Modulation” IEEE Transactions on Industry Applications, Vol.28, No.5, Sept/Oct 1992.

[5] E.Bassi, P. Benzi, S. Buja, “A Field Orientation Scheme for Current-Fed Induction Motor Drives Based on the Torque Angle Closed-Loop Control” IEEE Transactions on Industry Applications, Vol. 28, No. 5, Sept./ Oct. 1992.

24-Pulse Rectifier Realization By 3-Phase To Four 3-Phase Transformation Using Conventional Transformers

24-Pulse Rectifier Realization By 3-Phase To Four 3-Phase

Transformation Using Conventional Transformers

ABSTRACT:

 A 24-pulse rectifier has been designed for high voltage, low current applications. Four 3-phase systems are obtained from a single 3-phase source using novel interconnection of conventional single- and 3-phase transformers. From two 30º displaced 3-phase systems feeding two 6-pulse rectifiers that are series connected, a 12-pulse rectifier topology is obtained. Thus, from the four 3-phase systems that are displaced by 15º two 12-pulse rectifiers are obtained that are cascaded to realize a 24-pulse rectifier. Phase shifts of 15º and 30º are made using phasor addition of relevant line voltages with a combination of single-phase and three-phase transformers respectively. PSCAD based simulation and experimental results that confirm the design efficacy are presented.

 SOFTWARE: MATLAB/SIMULINK

 BLOCK DIAGRAM:

Figure 1 24-pulse rectifier realized by transforming a single 3-phase system to four 3-phase systems using conventional single- and three-phase transformers

EXPECTED SIMULATION RESULTS:

     

Figure 2 Input line voltages Va0b0, Vb0c0 and Vc0a0 at diode bridge I

Figure 3 Input line voltages Va30b30, Vb30c30 and Vc30a30 at diode bridge II

Figure 4 Input line voltages Va15b15, Vb15c15 and Vc15a15 at diode bridge III

Figure 5 Input line voltages Va45b45, Vb45c45 and Vc45a45 at diode bridge IV

 

Figure 6 Line current in phase a of y0 winding of Yy0d1 main transformer

Figure 7 Line current in phase a of d1 winding of Yy0d1 main transformer

Figure 8 Six-pulse dc output voltage of diode bridge, DBI

Figure 9 DC 6-pulse output voltage of diode bridge, DBII

Figure 10 DC 12-pulse output voltage by cascading diode bridges I and II

Figure 11 Six-pulse dc output voltage of diode bridge, DBIII

Figure 12 DC 6-pulse output voltage of diode bridge, DBIV

Figure 13 DC 12-pulse output voltage by cascading diode bridges III and IV

 

Figure 14 DC 24-pulse voltage by cascading DBI, DBII, DBIII and DBIV

Figure 15 Line current in phase a of Y winding of Yy0d1 main transformer

Figure 16 Line current in phase a of Y winding of Yy0d1 main transformer

Figure 17 Panned view of 24-pulse dc voltage

 Figure 18 24-pulse dc voltage

 Figure 19 Experimental set up        

CONCLUSION:

A 24-pulse rectifier is realized by conventional transformers that meets the theoretical harmonic

and ripple estimates.

REFERENCES:

 [1] IEEE Recommended Practices and Requirements for Harmonics Control in Electric Power Systems, IEEE Std. 519, 1992.

[2] Electromagnetic Compatibility (EMC)—Part 3: Limits-Section 2: Limits for Harmonic Current Emissions (Equipment Input Current (16A per Phase), IEC1000-3-2, Dec., 1995.

[3] Draft-Revision of Publication IEC 555-2: Harmonics, Equipment for Connection to the Public Low Voltage Supply System, IEC SC 77A, 1990.

[4] Bhim Singh, B. N. Singh, A. Chandra, Kamal Al-Haddad, Ashish Pandey, and D. P. Kothari, “A Review of Three-Phase Improved Power Quality AC-DC Converters”, IEEE Trans. Ind. Electron., vol. 51, No. 3, June 2004, 641-660.

[5] S. Choi, “New pulse multiplication technique based on six pulse thyristor converters for high power applications,” IEEE Trans. Ind. Appl., vol. 38, no. 1, pp. 131–136, Jan./Feb. 2002.

Implementation Of Perturb And Observe MPPT Of PV System with Direct Control Method Using Buck And Buck boost Converters

Implementation Of Perturb And Observe MPPT Of PV System with Direct Control Method Using Buck And Buck boost Converters

ABSTRACT:

The Maximum Power Point Tracking (MPPT) is a technique used in power electronic circuits to extract maximum energy from the Photovoltaic (PV) Systems. In the recent decades, photovoltaic power generation has become more important due its many benefits such as needs a few maintenance and environmental advantages and fuel free. However, there are two major barriers for the use of PV systems, low energy conversion efficiency and high initial cost. To improve the energy efficiency, it is important to work PV system always at its maximum power point. So far, many researches are conducted and many papers were published and suggested different methods for extracting maximum power point. This paper presents in details implementation of Perturb and Observe MPPT using buck and buck-boost Converters. Some results such as current, voltage and output power for each various combination have been recorded. The simulation has been accomplished in software of MATLAB Math works.

KEYWORDS

1.      Maximum Power Point Tracking

2.       Perturb and Observe

3.      DC-DC Converters

4.      Photovoltaic System

SOFTWARE: MATLAB/SIMULINK

BLOCK DIAGRAM:

Figure 1. PV module and dc/ dc converter with MPPT

                               

Figure 2. Block diagram of a PV array connected to the load

EXPECTED SIMULATION RESULTS:

Figure 3. Output current, voltage and power of PV panel (insolation changed from 400 to 200 w/ m² at a time of 4.915 sec.)

                                       

Figure 4. Output current, voltage and power of buck converter with P&O algorithm

(Insolation changed from 400 to 200 w/ m² at a time of 4.915 sec.)

                                       

Figure 5. output current, voltage and power of PV panel (Insolation changed from 400 to 200 w/ m² at a time of 5.017 sec.)

                                       

Figure 6. Output current, voltage and power of buck-boost converter with P&O algorithm

(Insolation changed from 400 to 200 w/ m² at a time of 5.017 sec.)

CONCLUSION:

P&O MPPT method is implemented with MATLAB-SIMULINK for simulation. The MPPT method simulated in this paper is able to improve the dynamic and steady state performance of the PV system simultaneously. Through simulation it is observed that the system completes the maximum power point tracking successfully despite of fluctuations. When the external environment changes suddenly the system can track the maximum power point quickly. Both buck and buck-boost converters have succeeded to track the MPP but, buck converter is much more effective specially in suppressing the oscillations produced due the use of P&O technique.

REFERENCES:

[1] A.P.Yadav, S. Thirumaliah and G. Harith. “Comparison of MPPT Algorithms for DC-DC Converters Based PV Systems” International Journal of Advanced Research in Electrical, Electronics and Instrumentation Engineering Vol. 1, Issue 1, July 2012.

[2] Y.-H.Chang and C.-Y. Chang, "A Maximum Power Point Tracking of PV System by Scaling Fuzzy Control," presented at International Multi Conference of Engineers and Computer Scientists, Hong Kong, 2010.

[3] S.Mekhilef, "Performance of grid connected inverter with maximum power point tracker and power factor control," International Journal of Power Electronics, vol.1, pp. 49-62.

[4] M.E.Ahmad and S.Mekhilef, "Design and Implementation of a Multi Level Three-Phase Inverter with Less Switches and Low Output Voltage Distortion," Journal of Power Electronics, vol. 9, pp. 594- 604, 2009.

[5] H.N.Zainudin and S. Mekhilef, "Comparison Study of Maximum Power Point Tracker Techniques for PV Systems" Proceedings of the 14th International Middle East Power Systems Conference (MEPCON’10), Cairo University, Egypt, December 19-21, 2010.