A Nested Control Strategy for Single Phase Power Inverter Integrating Renewable Energy Systems in a Microgrid

ABSTRACT:  

In this paper a nested power-current-voltage control scheme is introduced for control of single phase power  inverter, integrating small-scale renewable energy based power generator in a microgrid for both stand-alone and grid-connected modes. The interfacing power electronics converter raises various power quality issues such as current harmonics in injected grid current, fluctuations in voltage across the local loads, voltage harmonics in case of non-linear loads and low output power factor. The proposed nested proportional resonant current and model predictive voltage controller aims to improve the quality of grid current and local load voltage waveforms in grid-tied mode simultaneously by achieving output power factor near to unity. In stand-alone mode, it strives to enhance the quality of local load voltage waveform. The nested control strategy successfully accomplishes smooth transition from grid-tied to stand-alone mode and vice-versa without any change in the original control structure. The performance of the controller is validated through simulation results.

KEYWORDS:

1.      Microgrid

2.      Stand-alone mode

3.      Grid-connected mode

4.      Voltage harmonics

5.      Current harmonics

6.      Proportional resonant control

7.      Model predictive control

SOFTWARE: MATLAB/SIMULINK

BLOCK DIAGRAM:

Fig. 1. Block diagram of MPVC scheme

EXPECTED SIMULATION RESULTS:

Fig. 2(a). Steady state grid voltage, load voltage and grid current waveforms with resistive load

Fig. 3(b). Steady state grid voltage, load voltage and grid current waveforms with non-linear load

Fig. 4. THD values of voltage and current waveforms in grid connected mode

Fig. 5(a). Steady state grid voltage, load voltage and filter current waveforms with resistive load

Fig. 6 (b). Steady state grid voltage and load voltage waveforms with non-linear

Load

Fig. 7. THD values of load voltage waveform in stand-alone mode

Fig. 8(a). Transient state grid voltage, load voltage and grid current waveforms

with change in active power reference

Fig. 9(b). Transient state grid voltage, load voltage and grid current waveforms with change in reactive power reference

Fig. 10(c). Grid voltage, load voltage and grid current waveforms during voltage

Sag

(a)     Transfer from stand-alone to grid-tied mode

(b) Transfer from grid-tied to stand-alone mode

Fig.11. Grid voltage, load voltage, filter inductor current, grid current

Waveforms

(a)     Transfer from stand-alone to grid-tied mode

(b) Transfer from grid-tied to stand-alone mode

Fig.12. Grid current tracking error waveforms

CONCLUSION:

In this paper, a nested proportional resonant current and model predictive voltage controller is introduced for control of single phase VSI integrating a RES based plant in a microgrid. This strategy improves the quality of local load voltage and grid current waveforms with both linear and non linear loads. A non-linear load such as the diode bridge rectifier introduces voltage harmonics, but this scheme is successful in achieving low THD values for inverter local load voltage and grid current simultaneously. Simulation results validates the outstanding performance of the proposed controller in both steady state and transient state operations. A smooth transfer of operation modes from stand-alone to grid-tied and vice versa is also achieved by the nested control scheme without changing the control algorithm.

REFERENCES:

[1] H. Farhangi, "The path of the smart grid,” IEEE Power and Energy Magazine, vol. 8, no. 1, pp. 18-28, Jan/Feb. 2010.

[2] F. Blaabjerg, Z. Chen, and S. B. Kjaer, “Power electronics as efficient interface in dispersed power generation systems,” IEEE Trans. Power Electron., vol. 19, no. 5, pp. 1184–1194, Sep. 2004.

[3] F. Blaabjerg, R. Teodorescu, M. Liserre, and A. V. Timbus, “Overview of control and grid synchronization for distributed power generation systems,” IEEE Trans. on Ind. Electron., vol. 53, no. 5, pp. 1398–1409,  Oct. 2006.

[4] Q. C. Zhong and T. Hornik, "Cascaded Current–Voltage Control to Improve the Power Quality for a Grid-Connected Inverter With a Local  Load," IEEE Transactions on Ind. Electron., vol. 60, no. 4, pp. 1344- 1355, April 2013.

[5] Y Zhilei, X Lan and Y Yangguang, "Seamless Transfer of Single-Phase Grid-Interactive Inverters Between Grid-Connected and Stand-Alone  Modes," IEEE Transactions on Power Electronics, vol. 25, no. 6, pp. 1597-1603, June 2010.

An Intelligent Fuzzy Sliding Mode Controller for aBLDC Motor

ABSTRACT:  

Brushless DC (BLDC) motors are one of the most widely used motors, not only because of their efficiency, and torque characteristics, but also because they have the advantages of being a direct current (DC) supplied, eliminating the disadvantages of using Brushes. BLDC motors have a very wide range of speed, so speed control is a very important issue for it. Sliding mode control (SMC) is one of the popular strategies to deal with uncertain control systems. The Fuzzy Sliding Mode Controller (FSMC) combines the intelligence of a fuzzy inference system with the sliding mode controller. In this paper, an intelligent Fuzzy Sliding Mode controller for the speed control of BLDC motor is proposed. The mathematical model of the BLDC motor is developed and it is used to examine the performance of this controller. Conventionally PI controllers are used for the speed control of the BLDC motor. When Fuzzy SMC is used for the speed control of BLDC motor, the peak overshoot is completely eliminated which is 3% with PI controller. Also the rise time is reduced from 23 ms to 4 ms and the settling time is reduced from 46 ms to 4 ms by applying FMSMC. This paper emphasizes on the effectiveness of speed control of BLDC motor with Fuzzy Sliding Mode Controller and its merit over conventional PI controller.

KEYWORDS:

1.      BLDC motors

2.      Sliding Mode Control

3.      Fuzzy Sliding Mode controller

4.      PI Controller

SOFTWARE: MATLAB/SIMULINK

BLOCK DIAGRAM:

Fig 1 Block diagram of BLDC speed control.

EXPECTED SIMULATION RESULTS:

Fig 2 Step response with Fuzzy SMC and Fuzzy PI and PI Controllers

Fig 3 Current in the three phases

CONCLUSION:

Fuzzy sliding mode controller for the speed control of BLDC motor is designed and its performance comparison with PI controller is carried out in this paper. Conventionally PI controllers are used for the speed control of BLDC motor and they give moderate performance under undisturbed conditions even though they are very simple to design and easy to implement. But their performance is poor under disturbed condition like sudden changes in reference speed and sudden change in load. The BLDC motor with PI controller shows large overshoot, high settling time and comparatively large  speed variation under loaded condition.

The Fuzzy Sliding Mode Controller combines the intelligence of fuzzy logic with the Sliding Mode technique. The peak overshoot is completely eliminated and the rise time and settling time are improved when Fuzzy SMC is applied for the speed control of BLDC motor. The fluctuation in speed of the motor under loaded condition is also reduced when fuzzy SMC is applied. Thus this controller becomes an ideal choice for applications where very precise and fine control is required.

REFERENCES:

[1] Neethu U., Jisha V. R., “Speed Control of Brushless DC Motor : A Comparative Study”, IEEE International Conference on Power  Electronics, Drives and Energy Systems, Vol. 8, No. 12, 16-19 December 2012, Bengaluru India.

[2] Chee W. Lu, “T orque Controller for Brushless DC Motors”, IEEE Transactions on Industrial Electronics, Vol. 46, No. 2, April 1999.

[3] Tony Mathew, Caroline Ann Sam, ”Closed Loop Control of BLDC Motor Using a Fuzzy Logic Controller and Single Current Sensor”, International Conference on Advanced Computing and Communication Systems (ICACCS), Vol. 2, No. 13, 19-21 December 2013, Coimbatore India.

[4] T . Raghu, S. Chandra Sekhar, J. Srinivas Rao,“SEPIC Converter based – Drive for Unipolar BLDC Motor”, International Journal of Electrical  and Computer Engineering (IJECE), Vol.2, No.2, April 2012, pp. 159- 165.

[5] M. A. Jabbar, Hla Nu Phyu, Zhejie Liu, Chao Bi, “Modelling and Numerical Simulation of a Brushless Permanent – Magnet DC Motor in Dynamic Conditions by Time – Stepping T echnique”, IEEE Transactions on Industry Applications, Vol. 40, no. 3, MAY/JUNE 2004.

Power Quality Analysis of a PV fed Seven Level Cascaded H-Bridge Multilevel Inverter

ABSTRACT:  

Efficient DC to DC and DC to AC converters play a vital role in the reliable performance of standalone and grid connected photovoltaic systems. This paper deals with DC to AC conversion by a seven level cascaded H-bridge multilevel inverter for a standalone photovoltaic system. The PV fed seven level cascaded H-bridge multilevel inverter is analyzed in two ways: 1) with equal voltage sources as input to the H bridges and 2) with unequal voltage sources as input. A comparative study of the total harmonic distortion reduction in the PV fed multilevel inverter system with and without equal voltage sources as input is carried out. It is observed that with unequal voltage sources, the total harmonic distortion is increased than that with equal voltage sources as input to the PV fed seven level cascaded H-bridge multilevel inverter. Further, the study attempts to show that with an LC filter at the output stage of the multilevel inverter, the total harmonic reduction is significantly reduced and the power quality of the PV fed multilevel inverter system is highly improved. Results are verified using simulations done in MATLAB/Simulink environment.

KEYWORDS:

1.      Photo voltaic Array (PV Array)

2.      Cascaded Multilevel Inverter

3.      Pulse Width Modulation (PWMJ

4.      Total Harmonic Distortion (THD)

SOFTWARE: MATLAB/SIMULINK

CIRCUIT DIAGRAM:

Fig.1. Seven level Cascaded H-bridge multilevel inverter

EXPECTED SIMULATION RESULTS:

Fig.2. (a)Seven level cascaded MLI output voltage (b) Harmonic spectrum of the output voltage

Fig.3.(a) Seven level cascaded MLI output current (b) Harmonic

spectrum of the output current

Fig.4. (a) Output voltage of MLI with LC filter (b) Harmonic spectrum of

the output voltage with LC filter

Fig.5. (a) Output current of MLI with LC filter (b) Harmonic spectrum of

the output current with LC filter

Fig.6. (a) Output voltage of seven level multilevel inverter with unequal

voltage sources (b) Harmonic spectrum of the output voltage

Fig.7. (a) Output Current of seven level MLI with unequal voltage sources

(b) Harmonic spectrum of the output current

Fig.8. (a) Output voltage of seven level cascaded MLI with unequal voltage sources and LC filter (b) Harmonic spectrum of the output voltage with LC filter

Fig.9. (a) Output current of seven level cascaded MLI with unequal voltage sources and LC filter (b) Harmonic spectrum of the output current with LC filter

CONCLUSION:

In this paper, an analysis of a seven level cascaded H bridge multilevel inverter for a standalone photovoltaic system is carried out 1) with equal voltage sources as input to the H-bridges and 2) with unequal voltage sources as  input. It is found that when equal voltage values are fed as input to the H-bridges of the multilevel inverter, there is a reduction in the total harmonic distortion of the MLI output when compared to that with unequal voltage sources as its input. It is also observed that with an LC filter at the output stage of the multilevel inverter in both the scenarios, the total harmonic reduction is significantly reduced and the power quality of the PV fed multilevel inverter system is highly improved.

REFERENCES:

[1] Venkatachalam, Jovitha Jerome and J. Karpagam, "An experimental investigation on a multilevel inverter for solar energy applications," International Journal of Electrical Power and Energy Systems, 2013, pp.157-167.

[2] Ebrahim Babaei, Mohammad Farhadi and Farshid Najaty, "Symmetric and asymmetric multilevel inverter topologies with reduced switching devices," Electric Power Systems Research, 2012, pp. 122- 130.

[3] Jia-Min Shen, Hurng-Liahng Jinn-Chang Wu and Kuen-Der, "Five-Level Inverter for Renewable Power Generation System, IEEE transactions on energy conversion," 2013, pp.257-266.

[4] Hui Peng, Makoto Hagiwara and Hirofumi Akagi, "Modeling and Analysis of Switching-Ripple Voltage on the DC Link  between a Diode Rectifier and a Modular Multilevel Cascade Inverter (MMCI)," IEEE transactions on power electronics, 2013, pp.75-84.

[5] Javier Chavarria, Domingo Bie!, Francesc Guinjoan, Carlos Meza and Juan J. Negroni, "Energy-Balance Control of PV  Cascaded Multilevel Grid-Connected Inverters Under LevelShifted and Phase-Shifted PWMs," IEEE transactions on industrial electronics, 2013, pp.98-111.