Improved P-F/Q-V And P-V/Q-F Droop Controllers For Parallel Distributed Generation Inverters In AC Microgrid

ABSTRACT:

Distributed generation inverters are generally operated in parallel with P-f/Q-V and P-V/Q-f droop control strategies. Due to mismatched resistive and inductive line impedance, power sharing and output voltage of the parallel DG inverters deviate from the reference value. This leads to instability in the microgrid system. Adding virtual resistors and virtual inductors in the control loop of droop controllers improve the power sharing and stability of operation. But, this leads to voltage drop. Therefore, an improved P-f/Q-V and P-V/Q-f droop control is proposed. Simulation results demonstrate that the proposed control and the selection of parameters enhance the output voltage of inverters.

KEYWORDS:

1.      Distributed generation inverters

2.      Droop control

3.      Microgrid

4.      Output impedance

5.      Virtual resistors

6.      Virtual inductors

SOFTWARE: MATLAB/SIMULINK

 DROOP CONTROL BLOCK DIAGRAM.:

Fig. 1. Droop control block diagram.

EXPERIMENTAL RESULTS:

Fig. 2. Parallel inverter output voltage using P-V/Q-f droop control with virtual resistor under resistive line impedance.

Fig. 3. Active power sharing using secondary control with virtual resistor under resistive line impedance.

Fig. 4. Reactive power sharing using secondary control with virtual resistor under resistive line impedance.

Fig. 5. Parallel inverter output frequency using secondary control with virtual resistor under resistive line impedance.

Fig. 6. Parallel inverter output voltage using secondary control with virtual resistor under resistive line impedance.

Fig. 7. Active power sharing using P-V/Q-f droop control under inductive line impedance.

Fig. 8. Reactive power sharing using P-V/Q-f droop control under inductive line impedance.

Fig. 9. Active power sharing using P-f/Q-V droop control with virtual inductor under inductive line impedance.

Fig. 10. Reactive power sharing using P-f/Q-V droop control with virtual inductor under inductive line impedance. 

Fig. 11. Parallel inverter output frequency using P-f/Q-V droop control with virtual inductor under inductive line impedance.

Fig. 12. Parallel inverter output voltage using P-f/Q-V droop control with virtual inductor under inductive line impedance.

Fig. 13. Active power sharing using secondary control with virtual inductor under inductive line impedance.

Fig. 14. Reactive power sharing using secondary control with virtual inductor under inductive line impedance.

Fig. 15. Parallel inverter output frequency using secondary control with virtual inductor under inductive line impedance.

Fig. 16. Parallel inverter output voltage using secondary control with virtual inductor under inductive line impedance.

Fig. 17. Active power sharing using secondary control with different DG ratings under resistive line impedance.

Fig. 18. Reactive power sharing using secondary control with different DG ratings under resistive line impedance.

Fig. 19. Parallel inverter output frequency using secondary control with different DG ratings under resistive line impedance.

Fig. 20. Parallel inverter output voltage using secondary control with different DG ratings under resistive line impedance.

Fig. 21. Active power sharing using secondary control with different DG ratings under inductive line impedance.

Fig. 22. Reactive power sharing using secondary control with different DG ratings under inductive line impedance.

Fig. 23. Parallel inverter output frequency using secondary control with different DG ratings under inductive line impedance.

Fig. 24. Parallel inverter output voltage using secondary control with different DG ratings under inductive line impedance.

CONCLUSION:

In this paper, analysis of improved P-f/Q-V and P-V/Q-f droop control with secondary control for DG parallel inverters in microgrid is proposed considering line and output impedance. Proportional integral controller is adopted to ensure accurate tracking of the output voltage of the inverter to the reference value and the influence of the controller parameters on the voltage closed loop transfer function and the equivalent output impedance of the inverter is analyzed. In order to match the total output impedance of the inverter and line impedance in parallel, the P-V/Q-f and P-f/Q-V droop control strategy based on the inductive and resistive virtual impedance is adopted to improve the total output impedance of the inverter through the virtual impedance. The proposed P-f/Q-V and P-V/Q-f droop control, adaptively compensates the virtual resistor and inductor voltage drop to improve output voltage amplitude accuracy to the reference value. Simulation results show the rationality and effectiveness of the proposed improved control method.

REFERENCES:

Brabandere, K. D., Bolsens, B., & Van, J. (2007). A voltage and frequency droop control method for parallel inverters. IEEE Transactions on Power Electronics, 22(4), 1107–1115.

Chandorkar, M. C., Divan, D. M., & Adapa, R. (1993). Control of parallel connected inverters in standalone ac supply systems. IEEE Transactions on Industry Applications, 29(1), 136–143.

Chengshan, W., Zhaoxia, X., & Shouxiang, W. (2009). Multiple feedback loop control scheme for inverters of the microsource in microgrids. Transactions of China Electro Technical Society, 24(2), 100–107 [in Chinese].

Chengshan, W., Zhangang, Y., Shouxiang, W., & Yanbo, C. (2010). Analysis of structural characteristics and control approaches of experimental microgrid systems. Automation of Electric Power Systems, 34(1), 99–105 [in Chinese].

Chowdhury, A. A. S., & Agarwal, K. (2003). Dono Koval Reliability modeling of distributed generation in conventional distribution systems planning and analysis. IEEE Transactions on Industry Applications, 39(5), 1493–1498.

A Novel Control Strategy for a Variable Speed Wind Turbine with a Permanent Magnet Synchronous Generator

ABSTRACT:

This paper presents a novel control strategy for the operation of a direct drive permanent magnet synchronous generator (PMSG) based stand alone variable speed wind turbine. The control strategy for the generator side converter with maximum power extraction is discussed. The stand alone control is featured with output voltage and frequency controller capable of handling variable load. The potential excess of power is dissipated in the damp resistor with the chopper control and the dc link voltage is maintained. Dynamic representation of dc bus and small signal analysis are presented. Simulation results show that the controllers can extract maximum power and regulate the voltage and frequency under varying wind and load conditions. The controller shows very good dynamic and steady state performance.

KEYWORDS:

1.      Permanent magnet synchronous generator

2.      Maximum power extraction

3.      Switch-mode rectifier

4.      Variable speed

5.      Wind turbine

6.      Voltage and frequency control

SOFTWARE: MATLAB/SIMULINK

 BLOCK DIAGRAM:

 Figure 1. Control Structure of a PMSG based standalone variable speed wind turbine.

 EXPERIMENTAL RESULTS:

Figure 2. Response of the system for a step change of wind speed from 10 m/s

to 12 m/s to 9 m/s to 10 m/s.

Figure 3. Optimum torque and generator torque.

Figure 4. Turbine mechanical input power and Electrical output power.

Figure 5. Voltage and current responses at a constant load.

Figure 6. Frequency response, DC link voltage and modulation index at

a constant load.

Figure 7. Voltage and current responses when load is reduced by 50%.

Figure 8. Frequency response, DC link voltage and modulation index when load is reduced by 50%.

CONCLUSION:

Control strategy for a direct drive stand alone variable speed wind turbine with a PMSG is presented in this paper. A simple control strategy for the generator side converter to extract maximum power is discussed and implemented using Simpower dynamic system simulation software. The controller is capable to maximize output of the variable speed wind turbine under fluctuating wind. The load side PWM inverter is controlled using vector control scheme to maintain the amplitude and frequency of the inverter output voltage. It is seen that the controller can maintain the load voltage and frequency quite well at constant load and under varying load condition. The generating system with the proposed control strategy is suitable for a small scale standalone variable speed wind turbine installation for remote area power supply. The simulation results demonstrate that the controller works very well and shows very good dynamic and steady state performance.

REFERENCES:

[1] Müller, S., Deicke, M., and De Doncker, Rik W.: ‘Doubly fed induction genertaor system for wind turbines’, IEEE Industry Applications Magazine, May/June, 2002, pp. 26-33.

[2] Polinder H., Van der Pijl F. F. A, de Vilder G. J., Tavner P. J.:  "Comparison of direct-drive and geared generator concepts for wind turbines," IEEE Trans. on energy conversion, 2006, . 21, (3), pp. 725- 733.

[3] Chan T. F., and Lai L. L., "Permanenet-magnet machines for distributed generation: a review," Proc. IEEE power engineering annual meeting, 2007, pp. 1-6.

[4] De Broe M., Drouilhet S., and Gevorgian V.: “A peak power tracker for small wind turbines in battery charging applications,” IEEE Trans. Energy Convers. 1999, 14, (4), pp. 1630–1635.

[5] Datta R., and Ranganathan V. T.: “A method of tracking the peak power points for a variable speed wind energy conversion system,” IEEE Trans. Energy Convers., 1999, 18, (1), pp. 163–168.