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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
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) 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
Causes
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
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
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