Control and Grid Synchronization Dist Power Gen
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Transcript of Control and Grid Synchronization Dist Power Gen
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Control and Grid Synchronization forDistributed 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
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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
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Overview
1. Main DPGS structures,2. PV and fuel cell (FC) system3. 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 conditions6. Grid synchronization methods
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Causes
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DPGS Control
• Input-side controller -extract the maximum power from the input source
• Grid-side controller1.control of active power generated to the
grid2.control of reactive power transfer
between the DPGS and the grid3.control of dc-link voltage4.ensure high quality of the injected power5.grid synchronization
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Topologies of DGPS
• Photovoltaics and Fuel Cells – similar topology
• Wind Turbines – topology dependent on generator
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Wind turbines
• WT Systems without Power Electronics
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Wind turbines
• WT Systems with Power Electronics– Increased complexity– Higher cost– Better control of power input and grid interaction
• Partial Solution
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WT with full-scale power converter
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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
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Reference Frames
• reference frame transformation module, e.g., abc → dq
• PI -controller
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dq -Control
• proportional–integral (PI) controllers• controlled current - in phase with the
grid voltage
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-Control (Clarke transformation)
• stationary reference frame
• PR proportional –resonant controller
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-Control example
• very high gain around the resonance frequency
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Natural Frame Control (abc control)• PI Controller• PR Controller
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Power Quality control
• Harmonics Compensation Using PI Controllers
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Harmonics Compensation using PR Controllers
• Harmonic compensation by cascading• several generalized integrators tuned
to resonate at the desired frequency
• Nonlinear controllers
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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
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Control Strategies under Faults
• Unity Power Factor Control Strategy
• the negative sequence component gives rise to oscillations (2nd harmonic)
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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
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Constant Active Power Control Strategy
• injecting an amount of negative sequence in the current reference, the compensation for the double harmonic can be obtained
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Constant Reactive Power Control Strategy
• Reactive power to cancel the double-frequency oscillations
• Current vector orthogonal to the grid voltage vector can be found
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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)
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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
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Conclusions
• Hardware = Full-scale converter• DGPS control = PR controllers• Faults = strategies• Synchronization = PLL