Control and Grid Synchronization for Distributed Power Generation Systems

1. Control and Grid Synchronization forDistributed Power Generation Systems F. Blaabjerg, R. Teodorescu, M. Liserre,and A. V. Timbus: Overview of Control and Grid Synchronization forDistributed Power Generation Systems, IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 53, NO. 5, OCTOBER 2006 Z.Leonowicz, PhD

2. Renewable energy sources • hydropower and wind energy • photovoltaic (PV) technology • low efficiency • poor controllability of the distributed power generation systems (DPGSs) based on wind and sun

3. Overview • Main DPGS structures, • PV and fuel cell (FC) system • Classificationof wind turbine(WT) systems with regard to the use of powerelectronics • Controlstructures for grid-side converter • Characteristics of control strategies under grid fault conditions • Grid synchronization methods

4. Causes

5. DPGS Control • Input-side controller -extractthe maximum power from the input source • Grid-side controller • control of active power generated to the grid • control of reactive power transfer between the DPGSand the grid • control of dc-link voltage • ensure high quality of the injected power • grid synchronization

6. Topologies of DGPS • Photovoltaics and Fuel Cells – similar topology • Wind Turbines – topology dependent on generator

7. Wind turbines • WT Systems without Power Electronics

8. Wind turbines • WT Systems with Power Electronics • Increased complexity • Higher cost • Better control of power input and grid interaction • Partial Solution

9. WT with full-scale power converter

10. Control Structures for Grid-Connected DGPS • Two cascaded loops • Fastinternal current loop, regulates the grid current • an external voltage loop, controls the dc-link voltage

11. Reference Frames • reference frame transformation module, e.g., abc → dq • PI -controller

12. dq -Control • proportional–integral (PI) controllers • controlled current - in phase with the gridvoltage

13. ab-Control (Clarke transformation) • stationary reference frame • PR proportional –resonant controller

14. ab-Control example • very high gain around the resonance frequency

15. Natural Frame Control (abc control) • PI Controller • PR Controller

16. Power Quality control • Harmonics Compensation Using PI Controllers

17. Harmonics Compensation using PR Controllers • Harmonic compensation by cascading • several generalized integrators tuned to resonate at the desiredfrequency • Nonlinear controllers

18. Control under Grid Faults • Instability of the power system • Stringent exigencies for interconnecting the DPGS 1) Symmetrical fault (no phase shifting)- rare 2) Unsymmetrical fault

19. Control Strategies under Faults • Unity Power Factor Control Strategy • the negative sequence componentgives rise to oscillations (2nd harmonic)

20. Positive-Sequence Control Strategy • follow the positive sequence of the grid voltages • PLL necessary (Synchronous reference frame PLL) • dc-link capacitor should be rated to overcome the second-harmonic ripple • grid currents remainsinusoidal and balanced during the fault

21. Constant Active Power Control Strategy • injecting anamount of negative sequence in the current reference, thecompensation for the double harmonic can be obtained

22. Constant Reactive Power Control Strategy • Reactive power to cancel the double-frequency oscillations • Current vector orthogonal to the grid voltage vector can be found

23. Grid Synchronization Methods • Zero-Crossing Method • simplest implementation • Poor performance(harmonics or impulse disturbances • Filtering of the grid voltages in different reference frames:dq or αβ • difficulty to extract the phase angle (grid variations or faults)

24. PLL Technique • state-of-the-art methodto extract the phase angle of the grid voltages • Better rejection of grid harmonicsand any other kind of disturbances • Problem to overcome grid unbalance

25. Conclusions • Hardware = Full-scale converter • DGPS control = PR controllers • Faults = strategies • Synchronization = PLL