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Voltage unbalance and harmonics compensation for islanded microgrid inverters

Voltage unbalance and harmonics compensation for

islanded microgrid inverters

ABSTRACT:

Voltage source inverters (VSIs) are usually used for all kinds of distributed generation interfaces in a microgrid. It is the microgrid’s superiority to power the local loads continuously when the utility fails. When in islanded mode, the voltage and frequency of the microgrid are determined by the VSIs; therefore the power quality can be deteriorated under unbalanced and non-linear loads. A voltage unbalance and harmonics compensation strategy for the VSIs in islanded microgrid is proposed in this study. This method is implemented in a single synchronous reference frame (SRF) and is responsible for both the voltage unbalance and harmonic compensation. Furthermore, the virtual impedance loop is modified to improve the compensation

effect. The impedance model of the VSI is built to explain the compensation ability of the proposed strategy. The whole control system mainly includes power droop controllers, a modified virtual impedance loop and inner SRF-based voltage unbalance and harmonics compensators. The proposed strategy is demonstrated in detail and validated with simulations and experiments.

SOFTWARE: MATLAB/SIMULINK

 BLOCK DIAGRAM:

                Fig. 1 Schematic and control of DGs interface in an AC microgrid

                                a Typical AC microgrid structure with DGs and loads

                                b Schematic of a VSI as the DG interface

                                 c Power droop control loop of DGs interface

CONCLUSION:

This paper proposes a FPS SRF-based control strategy for voltage unbalance and harmonic compensation of the VSIs used as interfaces in islanded microgrid. The voltage compensation loops are integrated within the power droop loops and the virtual output impedance loop. The proposed strategy is implemented in a single SRF with a PI controller for the voltage’s fundamental component regulation and multi-resonant controller for voltage unbalance and selected harmonics compensation. The impedance model of the DG interface inverter is built when controlled by three different control methods to explain the compensation ability of the proposed strategy, which are the conventional PI voltage controller, the PI plus multi-resonant voltage controller and the PI plus multi-resonant voltage controller with modified virtual impedance loop. The simulation and experimental results of the three different control strategies with balanced load, unbalanced load and diode bridge rectifier load are given to validate the effectiveness of the proposed control strategy.

REFERENCES:

1 Lasseter, R.H.: ‘Certs microgrid’. IEEE Int. Conf. System of Systems Engineering, 2007 (SoSE ’07), 2007, pp. 1–5

2 Lasseter, R.H., Piagi, P.: ‘Extended microgrid using (DER) distributed energy resources’. IEEE Power Engineering Society General Meeting, 2007, pp. 1–5

3 Rocabert, J., Luna, A., Blaabjerg, F., Rodri, X., Guez, P.: ‘Control of power converters in AC microgrids’, IEEE Trans. Power Electron., 2012, 27, (11), pp. 4734–4749

4 Ming, H., Haibing, H., Yan, X., Guerrero, J.M.: ‘Multilayer control for inverters in parallel operation without intercommunications’, IEEE Trans. Power Electron., 2012, 27, (8), pp. 3651–3663

5 Guerrero, J.M., Blaabjerg, F., Zhelev, T., et al.: ‘Distributed generation: toward a new energy paradigm’, IEEE. Ind. Electron. Mag., 2010, 4, (1), pp. 52–64

Control of Reduced-Rating Dynamic Voltage Restorer with a Battery Energy Storage System

Control of Reduced-Rating Dynamic Voltage Restorer with a Battery Energy Storage System

ABSTRACT:

In this paper, different voltage injection schemes for dynamic voltage restorers (DVRs) are analyzed with particular focus on a new method used to minimize the rating of the voltage source converter (VSC) used in DVR. A new control technique is proposed to control the capacitor-supported DVR. The control of a DVR is demonstrated with a reduced-rating VSC. The reference load voltage is estimated using the unit vectors. The synchronous reference frame theory is used for the conversion of voltages from rotating vectors to the stationary frame. The compensation of the voltage sag, swell, and harmonics is demonstrated using a reduced-rating DVR.

KEYWORDS:

1.      Dynamic voltage restorer (DVR)

2.       Power quality

3.      Unit vector

4.      Voltage harmonics

5.       Voltage sag

6.       Voltage swell

SOFTWARE: MATLAB/SIMULINK

BLOCK DIAGRAM:

Fig.1. Schematic of the DVR-connected system.

CONCLUSION:

The operation of a DVR has been demonstrated with a new control technique using various voltage injection schemes. A comparison of the performance of the DVR with different schemes has been performed with a reduced-rating VSC, including a capacitor-supported DVR. The reference load voltage has been estimated using the method of unit vectors, and the control of DVR has been achieved, which minimizes the error of voltage injection. The SRF theory has been used for estimating the reference DVR voltages. It is concluded that the voltage injection in-phase with the PCC voltage results in minimum rating of DVR but at the cost of an energy source at its dc bus.

REFERENCES:

[1] M. H. J. Bollen, Understanding Power Quality Problems—Voltage Sags and Interruptions. New York, NY, USA: IEEE Press, 2000.

[2] A. Ghosh and G. Ledwich, Power Quality Enhancement Using Custom Power Devices. London, U.K.: Kluwer, 2002.

[3] M. H. J. Bollen and I. Gu, Signal Processing of Power Quality Disturbances. Hoboken, NJ, USA: Wiley-IEEE Press, 2006.

[4] R. C. Dugan, M. F. McGranaghan, and H. W. Beaty, Electric Power Systems Quality, 2nd ed. New York, NY, USA: McGraw-Hill, 2006.

[5] A. Moreno-Munoz, Power Quality: Mitigation Technologies in a Distributed Environment. London, U.K.: Springer-Verlag, 2007.

Inner Control Method and Frequency Regulation of a DFIG Connected to a DC Link

Inner Control Method and Frequency Regulation of a DFIG Connected to a DC Link

ABSTRACT:

In this paper, an inner loop for the control and frequency regulation of the doubly fed induction generator connected to a dc link through a diode bridge on the stator is presented. In this system, the stator is directly connected to the dc link using a diode bridge, and the rotor is fed by only a pulse width-modulated (PWM) converter. If compared to the DFIG connected to an ac grid, this system uses one PWM inverter less and a much less expensive diode bridge. Thus, the cost of power electronics is reduced. The application in mind is for dc networks such as dispersed generation grids and microgrids. These networks use several elements that should work together. Usually, these elements are connected with each other by power electronic devices in a common dc link. This paper presents a control system for the inner control loop in order to regulate the torque and the stator frequency. Simulation and experimental results show that the system works properly and is able to keep the stator frequency near the rated value of the machine.

KEYWORDS:

1.      Control

2.       Dc link

3.       Dc microgrids

4.       Doubly fed induction generator

SOFTWARE: MATLAB/SIMULINK

BLOCK DIAGRAM:

Fig.1.Structure of the DFIG-DC. Diode bridge on the stator, PWM converter on the rotor.

 CONCLUSION:

This paper presents a control method for the DFIG connected to a dc link through a diode rectifier on the stator windings. Simulation and experimental results show that it is possible to drive the stator flux at the rated frequency of the machine by using a simple controller that adjusts the rotor d-axis current reference in order to annihilate the orientation error. The method converges to the field orientation and the steady-state frequency error is zero.Agood dynamics is achieved in the electromagnetic torque. The waveforms of the stator current are not sinusoidal, due to the presence of the diode bridge, but have an acceptable harmonic content. The industrial application of this system could be implemented using a 12-pulse rectifier, which reduces not only the torque ripple but also the harmonic content in the rotor currents.

REFERENCES:

 [1] S. Chowdhury, S. P. Chowdhury, and P. Crossley “Microgrids and active distribution networks,” in IET Renewable Energy (Series 6). London, U.K.: The Institution of Engineering and Technology, 2009.

[2] J. A. Pec¸as Lopes, C. L. Moreira, and A. G. Madureira, “Defining control strategies for microgrids islanded operation,” IEEE Trans. Power Syst., vol. 21, no. 2, pp. 916–924, May 2006.

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

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

[5] D. Salomonsson and A. Sannino, “Low-voltageDC distribution system for commercial power systems with sensitive electronic load,” IEEE Trans. Power Del., vol. 22, no. 3, pp. 1620–1627, Jul. 2007.

Performance Investigation of Isolated Wind–Diesel Hybrid Power Systems with WECS Having PMIG

Performance Investigation of Isolated Wind–Diesel Hybrid Power Systems with WECS Having PMIG

ABSTRACT:

This paper presents the automatic reactive power control of isolated wind–diesel hybrid power systems having a permanent-magnet induction generator for a wind energy conversion system and a synchronous generator for a diesel generator set. To minimize the gap between reactive power generation and demand, a variable source of reactive power is used such as a static synchronous compensator. The mathematical model of the system used for simulation is based on small-signal analysis. Three examples of the wind–diesel hybrid power system are considered with different wind power generation capacities to study the effect of the wind power generation on the system performance. This paper also shows the dynamic performance of the hybrid system with and without change in input wind power plus 1% step increase in reactive power load.

KEYWORDS:

1.      Permanent-magnet induction generator (IG) (PMIG)

2.       Static synchronous compensator (STATCOM)

3.       Synchronous generator (SG)

4.       Wind–diesel hybrid system

SOFTWARE: MATLAB/SIMULINK

 BLOCK DIAGRAM:

 Fig 1.Single line diagram of an isolated wind–diesel hybrid power system.

CONCLUSION:

Reactive power control of isolated wind–diesel hybrid power systems has been investigated when WECS uses PMIG for power generation. The WECSs are interconnected to diesel generation-based grid for the enhancement of capacity and fuel saving. The system also comprises STATCOM for reactive power support during steady-state and transient conditions. A mathematical model of the system has been derived for investigating the dynamic performance of the system. For comparison of performance with the existing systems, WECS has also been considered with IG for power generation. Three examples of wind–diesel systems with different wind power generation capacities have been considered for study. It has been observed that the STATCOM effectively stabilizes the oscillations in less than 0.01 s, caused by disturbances in reactive power load and in input wind power. As steady-state condition is reached, the STATCOM provides the additional reactive power required by the system. It has also been observed that, as the unit size of the wind-power generation decreases, the value of the optimum gain setting increases. The W-D systems with PMIG have the added advantage of reduction in the size of the STATCOM but have comparable transient performance when W-D system uses IG for power generation. The PMIG also has higher efficiency than the IG. Therefore, PMIGs are very good options for W-D systems than IG.

REFERENCES:

 [1] J. K. Kaldellis, Stand-Alone and Hybrid Wind Energy Systems: Technology, Energy Storage and Applications. Cambridge, U.K.: Wood head Publ. Ltd., 2011.

[2] R. Hunter and G. Elliot, Wind–Diesel Systems, A Guide to the Technology and Its Implementation. Cambridge, U.K.: Cambridge Univ. Press, 1994.

[3] H. Nacfaire, Wind–Diesel and Wind Autonomous Energy Systems. London, U.K.: Elsevier Appl. Sci., 1989.

[4] T. K. Saha and D. Kastha, “Design optimization and dynamic performance analysis of a standalone hybrid wind diesel electrical power generation system,” IEEE Trans. Energy Convers., vol. 25, no. 4, pp. 1209–1217, Dec. 2010.

[5] R. Pena, R. Cardenas, J. Proboste, J. Clare, and G. Asher, “Wind–diesel generation using doubly fed induction machines,” IEEE Trans. Energy Convers., vol. 23, no. 1, pp. 202–214, Mar. 2008.

Reactive Power Control of Permanent-Magnet Synchronous Wind Generator with Matrix Converter

Reactive Power Control of Permanent-Magnet Synchronous Wind Generator with Matrix Converter

ABSTRACT:

In this paper, the reactive power control of a variable speed permanent-magnet synchronous wind generator with a matrix converter at the grid side is improved. A generalized modulation technique based on singular value decomposition of the modulation matrix is used to model different modulation techniques and investigate their corresponding input reactive power capability. Based on this modulation technique, a new control method is proposed for the matrix converter which uses active and reactive parts of the generator current to increase the control capability of the grid-side reactive current compared to conventional modulation methods. A new control structure is also proposed which can control the matrix converter and generator reactive current to improve the grid-side maximum achievable reactive power for all wind speeds and power conditions. Simulation results prove the performance of the proposed system for different generator output powers.

KEYWORDS:

1.      Matrix converter

2.       Permanent-magnet synchronous generator (PMSG)

3.       Reactive power control

4.       Singular value decomposition (SVD) modulation

5.       Variable-speed wind generator

SOFTWARE: MATLAB/SIMULINK

BLOCK DIAGRAM:

Fig.1. Simplified control block diagram of a PMSG.

CONCLUSION:

In this paper, a new control strategy is proposed to increase the maximum achievable grid-side reactive power of a matrix converter-fed PMS wind generator. Different methods for controlling a matrix converter input reactive power are investigated. It is shown that in some modulation methods, the grid-side reactive current is made from the reactive part of the generator-side current. In other modulation techniques, the grid-side reactive current is made from the active part of the generator-side current. In the proposed method, which is based on a generalized SVD modulation method, the grid-side reactive current is made from both active and reactive parts of the generator-side current. In existing strategies, a decrease in the generator speed and output active and reactive power, will decrease the grid-side reactive power capability. A new control structure is proposed which uses the free capacity of the generator reactive power to increase the maximum achievable grid-side reactive power. Simulation results for a case study show an increase in the grid side reactive power at all wind speeds if the proposed method is employed.

REFERENCES:

 [1] P. W.Wheeler, J. Rodríguez, J. C. Clare, L. Empringham, and A.Weinstein, “Matrix converters: A technology review,” IEEE Trans. Ind. Electron., vol. 49, no. 2, pp. 276–288, Apr. 2002.

[2] L. Zhang, C. Watthanasarn, and W. Shepherd, “Application of a matrix converter for the power control of a variable-speed wind-turbine driving a doubly-fed induction generator,” Proc. IEEE IECON, vol. 2, pp. 906–911, Nov. 1997.

[3] L. Zhang and C.Watthanasarn, “A matrix converter excited doubly-fed induction machine as a wind power generator,” in Proc. Inst. Eng. Technol. Power Electron. Variable Speed Drives Conf., Sep. 21–23, 1998, pp. 532–537.

[4] R. CárdenasI, R. Penal, P. Wheeler, J. Clare, and R. Blasco-Gimenez, “Control of a grid-connected variable speed wecs based on an induction generator fed by a matrix converter,” Proc. Inst. Eng. Technol. PEMD, pp. 55–59, 2008.

[5] S. M. Barakati, M. Kazerani, S. Member, and X. Chen, “A new wind turbine generation system based on matrix converter,” in Proc. IEEE Power Eng. Soc. Gen. Meeting, Jun. 12–16, 2005, vol. 3, pp. 2083–2089.

Control Of Parallel Multiple Converters For Direct-Drive Permanent-Magnet Wind Power Generation Systems

Control Of Parallel Multiple Converters For Direct-Drive Permanent-Magnet Wind Power Generation Systems

ABSTRACT:

This paper proposes control strategies for mega watt level direct-drive wind generation systems based on permanent magnet synchronous generators. In the paper, a circulating current model is derived and analyzed. The parallel-operation controllers are designed to restrain reactive power circulation and beat frequency circulation currents caused by discontinuous space vector modulation. The control schemes do not change the configurations of the system consisting of parallel multiple converters. They are easy to implement for modular designs and large impedance required to equalize the current sharing is not needed. To increase the system reliability, a robust adaptive sliding observer is designed to sense the rotor position of the wind power generator. The experimental results proved the effectiveness of the control strategies.

KEYWORDS:

1.      Circulation currents

2.       Parallel multiple converters

3.       Permanent magnet synchronous generators (PMSGs)

4.      Wind power

SOFTWARE: MATLAB/SIMULINK

BLOCK DIAGRAM:

Fig.1. High-Power Direct-Drive PMSG Wind Generator System Connected To The Power Grid.

CONCLUSION:

This paper has comprehensively addressed the control issues of parallel three-phase PWM converters for the permanent magnet wind power generation systems. The major accomplishments and some conclusions are summarized in the following.

1) A peak current model of zero-sequence currents has been derived and analyzed for the three-phase PWM converters in parallel connection.

2) A zero-sequence current control scheme has been adapted to reject the zero-sequence current inside an individual converter.

3) An adaptive observer has been integrated with parallel operation control experimentally. The performance of position sensorless control of the generator has been greatly enhanced and the reliability has been increased.

4) Zero-sequence currents have been successfully suppressed for the back-to-back converters with parallel connection. Large impedance needed to equalize the current sharing has been removed.

5) Experimental verification of the control of the three-phase PWM converters in parallel confirms the good performance and promising features of the proposed directly driven permanent magnet synchronous power generation system.

REFERENCES:

[1] T. Kawabata And S. Higashino, “Parallel Operation Of Voltage Source Inverters,” Ieee Trans. Ind. Appl., Vol. 24, No. 2, Pp. 281–287, Mar./Apr. 1988.

[2] J. Holtz, W. Lotzkat, And K. H. Werner, “A High-Power Multi-Transistorinverter Uninterruptable Power Supply System,” Ieee Trans. Power Electron., Vol. 3, No. 3, Pp. 278–285, Jul. 1988.

[3] L. H.Walker, “10mwgto Converter For Battery Peaking Service,” Ieee Trans. Ind. Appl., Vol. 26, No. 1, Pp. 63–72, Jan./Feb. 1990.

[4] S. Fukuda And K. Matsushita, “A Control Method For Parallel-Connected Multiple Inverter Systems,” Presented At The 7th Int. Conf. Power Electron. Variable Speed Drives, London, U.K., 1998.

[5] X. Kun, F. C. Lee, D. Boroyevich, Y. Zhihong, And S. Mazumder, “Interleaved Pwm With Discontinuous Space-Vector Modulation,” Ieee Trans. Power Electron., Vol. 14, No. 5, Pp. 906–917, Sep. 1999.