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Control and Grid Synchronization for Distributed Power Generation Systems F. Blaabjerg, R. Teodorescu, M. Liserre, and A. V. Timbus: Overview of Control and Grid Synchronization for Distributed Power Generation Systems, IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 53, NO. 5, OCTOBER 2006 Z.Leonowicz, PhD 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 Overview 1. Main DPGS structures, 2. PV and fuel cell (FC) system 3. Classification of wind turbine (WT) systems with regard to the use of power electronics 4. Control structures for grid-side converter 5. Characteristics of control strategies under grid fault conditions 6. Grid synchronization methods Causes DPGS Control • Input-side controller -extract the maximum power from the input source • Grid-side controller 1. control of active power generated to the grid 2. control of reactive power transfer between the DPGS and the grid 3. control of dc-link voltage 4. ensure high quality of the injected power 5. grid synchronization Topologies of DGPS • Photovoltaics and Fuel Cells – similar topology • Wind Turbines – topology dependent on generator Wind turbines • WT Systems without Power Electronics Wind turbines • WT Systems with Power Electronics – Increased complexity – Higher cost – Better control of power input and grid interaction • Partial Solution WT with full-scale power converter Control Structures for Grid-Connected DGPS • Two cascaded loops – Fast internal current loop, regulates the grid current – an external voltage loop, controls the dc-link voltage Reference Frames • reference frame transformation module, e.g., abc → dq • PI -controller dq -Control • proportional–integral (PI) controllers • controlled current - in phase with the grid voltage ab-Control (Clarke transformation) • stationary reference frame • PR proportional –resonant controller ab-Control example • very high gain around the resonance frequency Natural Frame Control (abc control) • PI Controller • PR Controller Power Quality control • Harmonics Compensation Using PI Controllers Harmonics Compensation using PR Controllers • Harmonic compensation by cascading • several generalized integrators tuned to resonate at the desired frequency • Nonlinear controllers 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 Control Strategies under Faults • Unity Power Factor Control Strategy • the negative sequence component gives rise to oscillations (2nd harmonic) 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 remain sinusoidal and balanced during the fault Constant Active Power Control Strategy • injecting an amount of negative sequence in the current reference, the compensation for the double harmonic can be obtained Constant Reactive Power Control Strategy • Reactive power to cancel the doublefrequency oscillations • Current vector orthogonal to the grid voltage vector can be found 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) PLL Technique • state-of-the-art method to extract the phase angle of the grid voltages • Better rejection of grid harmonics and any other kind of disturbances • Problem to overcome grid unbalance Conclusions • • • • Hardware = Full-scale converter DGPS control = PR controllers Faults = strategies Synchronization = PLL