Control for Grid-Connected and Stand-Alone Operations of Three-Phase Grid-Connected Inverter

ABSTRACT:    

This paper describes a simple grid current control method for the grid-connected operation, and inverter voltage control method based on the phase locked loop (PLL) for the intentional islanding operation at the three-phase grid-connected inverter. The PLL controller based on the pq theory with a simple P-controller is used to synchronize the phase of inverter output voltage with a grid voltage at the grid-connected operation or generate a desired inverter output voltage at the islanding operation. The outputs of current controller are connected together to those of voltage controller, in order to prevent a sudden change of the outputs of both controllers during the transfer instant. The simulation and experimental results are carried out to verify the effectiveness of the proposed control strategies.

KEYWORDS:

1.      Distributed generation (DG)

2.      Grid-connected operation

3.      Islanding operation

4.      Phase locked loop (PLL)

5.      Three phase inverter.

SOFTWARE: MATLAB/SIMULINK

BLOCK DIAGRAM:

Figure 1. A control structure of three-phase grid-connected inverter.

EXPECTED SIMULATION RESULTS:

Figure 2. Simulation result for grid current control at grid-connected operation

Figure 3. Simulation result for inverter voltage control at the islanding operation.

CONCLUSION:

This paper described a simple grid current control method for the grid-connected operation, and output voltage control method based on the PLL for the intentional islanding operation at the three-phase grid-connected inverter. The PLL controller based on the pq theory with a simple P-controller which has no steady-state phase error, was used to synchronize the phase of inverter output voltage with a grid voltage or generate a desired voltage. As the outputs of current controller are connected together to those of voltage controller, the grid connected inverter was able to change smoothly from the grid connected operation to islanding operation. The experimental results showed that the proposed control schemes are capable of obtaining the good grid current response and also maintaining the inverter voltage within the desired level. The measured THDs of grid current and output voltage of inverter are only 1.92% and 1.89%, respectively.

REFERENCES:

[1] B. Kroposki, R. Lasseter, T. Ise, S. Morozumi, S. Papathanassiou, and N. Hatziargyriou, “Making Microgrids Work.” IEEE Power & Energy Mag., vol.6, no.3, pp.41-53, May/June, 2008.

[2] H. M. Kojabadi, B. Yu, I. A. Gadoura, L. Chang, and M. Ghribi, “A Novel DSP-Based Current-Controller PWM Strategy for Single Phase Grid Connected Inverters,” IEEE Trans. Power Electron., vol.21, no.4, pp.985-993, July 2006.

[3] I. J. Gabe, V. F. Montagner, and H. Pinheiro, “Design and Implementation of a Robust Current Controller for VSI Connected to the Grid Through an LCL Filter,” IEEE Trans. Power Electron., vol.24, no.6, pp.1444-1452, June 2009.

[4] J. C. Moreno, J. M. Espi. Huerta, P. G. Gil, and S. A. Geonzalez, “A Robust Predictive Current Control for Three-Phase Grid-Connected Inverters,” IEEE Trans. Ind. Electron, vol.56, no.6, pp.1993-2004, June 2009.

[5] K. J. Lee, B. G. Park, R. Y. Kim, and D. S. Hyun, “Robust Predictive Current Control Based on a Disturbance Estimation in a Three-Phase Grid-Connected Inverter,” IEEE Trans. Power Electron., vol.27, no.1, pp.276-283, Jan. 2012.

Active and Reactive Power Control of Single Phase Transformerless Grid Connected Inverter for Distributed Generation System

ABSTRACT:

This paper presents a novel approach by which enhancement in power quality is ensured along with power control for a grid interactive inverter. The work presented in this paper deals with modeling and analyzing of a transformer less grid-connected inverter with active and reactive power control by controlling the inverter output phase angle and amplitude in relation to the grid voltage. In addition to current control and voltage control, power quality control is made to reduce the total harmonics distortion. The distorted current flow can compensate for the disturbance caused by nonlinear load. The Simulation of the grid interactive inverter is carried out in MATLAB/SIMULINK environment and experimental results were presented to validate the proposed methodology for control of transformer less grid interactive inverter which supplies active and reactive power to the loads and also makes the grid current to a sinusoidal one to improve the power factor and reduce the harmonics in grid current. This work offers an increased opportunity to provide distributed generation (DG) use in distribution systems as reliable source of power generation to meet the increased load demand which helps to provide a reasonable relief to the customers and utilities to meet the increasing load demand

KEYWORDS:

1.      Grid interactive inverter

2.      Voltage Controller

3.      Current Controller

4.      THD improvement

5.      Reactive power compensation

6.      Intelligent power module

SOFTWARE: MATLAB/SIMULINK

BLOCK DIAGRAM:

Figure 1: Schematic diagram of grid connected system

Figure 2: grid tie inverter

 EXPECTED SIMULATION RESULTS:

Figure 3: Simulation waveforms of current a) when load is controlled rectifier b) inverter current c) grid current d) the reference current

Figure 4: Power flow graph.

Figure 5: grid voltage, load current & grid current

Figure 6: FFT analysis

Figure 7: load current

Figure 8: Injected current

CONCLUSION:

The simulation of single phase grid interactive inverter has been carried out with non-linear load and the results obtained from the simulations shows that this control technique improves the power quality ie THD and the power factor. The simulation also shows that power transfer of active and reactive power from the inverter to grid is possible. The reactive power required for the load is completely provided from the inverter. The hardware implementation of the interactive inverter has been conducted using real time workshop in the MATLAB Simulink environment. The half wave rectifier is used as load in the hardware implementation. The results show that the controller is capable for reactive power compensation, and maintaining constant voltage at the grid satisfying standard for grid interconnection. That is the THD is lessthan5% 3.74 and the power factor is .9977 which is near to unity. Energy conservation by load management is possible and a reasonable relief to the customer and voltage profile is maintained at the grid. This work can be extended to cascaded inverter configuration and reliability analysis has to be made as a better option for future studies.

REFERENCES:

[1] EPRI-white paper “Integrating Distributed Resources into electric utility systems ”Technology Review.December2001

[2] Thomas Ackerman “Distributed Generation, a definition’’ Electric power system research,57(3),2001,pp195-204

[3] G. Joos, B.T Ooi, D. McGill is, F.D. Galiana, and R. Marceau, “The potential of distributed generation to provide ancillary services,” at IEEE Power Engineering Society Summer Meeting, 16-20 July 2000, vol. 3, pp. 1762 – 1767

[4] Frede Blaabjerg, Zhechen, Soreren Baekhoej Kjaer “Power electronics as efficient interface in dispersed power generation system” IEEE Transactions on Power Electronics vol:19,no.5, sept2004 pp1184-1194

[5] Yong Yang, Yi Ruan, Huan-qing Shen, Yan-yan Tang and Ying Yang; “Grid-connected inverter for wind power generation system” Journal of Shanghai University, Page(s):.51-56, Vol. 13, No 1,Feb,2009.

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.