Download Control and Grid Synchronization for Distributed Power Generation

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