Control and Grid Synchronization for Distributed Power Generation Systems Z.Leonowicz, PhD 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

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

 -Control (Clarke transformation) stationary reference frame PR proportional –resonant controller

 -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 double- frequency 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