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Grid-Interfacing Converter Systems with Enhanced Voltage Quality
Fei Wang
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Contents
• Introduction
• Grid-interfacing systems
• Structure and functionalities
• Control design and implementation
• Conclusions
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Transition to the future grid
• Improving energy efficiency
• Applying sustainable energy
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• Growing electricity consumption
• Demanding high-quality electricity
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Conventional electricity grid
• Central power station
• Top-down centralized control
• Unidirectional power flow
GenerationTransmission
Distribution
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Loads
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Voltage quality problems
• Harmonics
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• Dips • Unbalance • Fluctuations
GenerationTransmission
DistributionLoads
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Distributed generation in the grid
Energy StoragesDistributed Generation
Distribution networks
Grid-interfacing converters
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AC Bus
DC/AC
Micro-grid
Loads
Loads
Utility Grid
Shunt ConverterDC/DC
AC/DC/AC
Data Bus
Energy Storage
DC/AC
Loads
Loads
Series Converter
DC/AC
Distributed Source / Energy Storage
Solar Panel
Wind Turbine
Path to the future grid 7
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8Path to the future grid
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9Series-parallel grid-interfacing systems
• Independent distributed sources powered dc bus
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10Series-parallel grid-interfacing systems
• Common distributed sources powered dc bus with isolation techniques
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11Series-parallel grid-interfacing systems
• An example of coupling the utility grid and a local grid/micro-grid
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12Series-parallel grid-interfacing systems
• Adapted series-parallel structure
Common distributed sources powered dc bus
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13Reconfiguring system functionalities
• Conventional power quality enhancement
• Unified PQ conditioners (UPQC)
• UPQC + energy storage (batteries, super-capacitors, distributed sources, etc. )
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14Reconfiguring system functionalities
• Circuit presentation of the proposed grid-interfacing system
Subscripts:
+, - : positive and negative sequence;
1: fundamental components
h: harmonics
1pv
1pv
phv
pZ
1ldi
1ldi
ldhi ldZ
1si
1si
shi
sZ1gv
1gv
ghv
gZ
Series Converter
Parallel Converter
Loads
Grid
si
pv
ldi
Loads
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15Reconfiguring system functionalities
1pv
1pv
phv
pZ
1ldi
1ldi
ldhi ldZ
1si
1si
shi
sZ1gv
1gv
ghv
gZ
Series Converter
Parallel Converter
Loads
Grid
si
pv
ldi
Loads
• Multi-level control objectives
- Level 1: Maintaining good voltage quality for local loads Dispatching power within the local grid (micro-grids)
- Level 2: Active power filtering function
- System Level: Grid interactive control, grid support, power transfer
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16Comparison with shunt systems
(a) Series-parallel system
(b) Shunt-connected system
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17Control design and implementation
• Employed configuration of the laboratory system
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18Control design and implementation
• Overall control structure• Parallel converter
• Series converter
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19Control design – parallel converter
• Control diagram of the parallel converter
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20Control design – parallel converter
• Instability improvement under no-load conditions
Bode plots of the plant transfer function
Feedforward loop
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21Control design – parallel converter
• Selective harmonic regulation
Bode plots of the open-loop transfer function with multiple PR controllers
Multiple PR controllers
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22Control design – parallel converter
• Disturbance sensitivity improvement
System sensitivity to current disturbances
Inner current feedback loop
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23Control design – series converter
• Control diagram of the series converter
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24Control design – series converter
Inverter output voltage to feedback current
when
Bode plots of open-loop transfer function
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Laboratory system 25
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Experimental results 26
• Output voltages of the parallel converter (tested under a single-phase nonlinear load)
(a) WITHOUT low-order harmonic compensation
(b) WITH low-order harmonic compensation
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Experimental results 27
• Emulation of active filter function with low-order harmonic current injection
Frequency spectra of the phase current
Series-converter current
Injected series voltage
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Experimental results 28
• Test under a distorted grid condition
Distorted grid voltage (6.7%THD) Output voltage of the series-converter
Output voltage of the parallel converter Currents delivered to the grid
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Experimental results 29
• Test under unbalanced voltage dips
Grid voltage (phase a & b dip to 80%) Output voltage of the series-converter
Output voltage of the parallel converter Currents delivered to the grid
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and recommendationsConclusions
• All common grid disturbances at the distribution level can be
mitigated by the proposed approach
•The voltage quality can be improved at both user and grid side,
combing with distributed power generation
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• Grid interaction control integrating grid-impedance adaptability
• Scaled up grid-interfacing systems for smart-grid research
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References
a.Selected papers
1. Wang, F., Benhabib, M.C., Duarte, J.L., Hendrix, M.A.M., Sequence-decoupled controller for three-phase grid-connected inverters. IEEE Applied Power Electronics Conference and Exposition (APEC), 2009, pp. 121-127.
2. Wang, F., Benhabib, M.C., Duarte, J.L., Hendrix, M.A.M., High performance stationary frame filters for symmetrical sequences or harmonics separation under a variety of grid conditions. IEEE APEC, 2009, pp. 1570-1576.
3. Wang, F., Duarte, J.L., Hendrix, M.A.M., Ribeiro, P. F., Modeling and analysis of grid harmonic distortion impact of aggregated DG Inverters. IEEE Transactions on Power Electronics, in press, Oct. 2010.
4. Wang, F., Duarte, J.L., Hendrix, M.A.M., Design and analysis of active power control strategies for distributed generation inverters under unbalanced grid faults. IET Generation, Transmission & Distribution, vol. 4, iss. 8, pp. 905-916, Aug. 2010.
5. Wang, F., Duarte, J.L., Hendrix, M.A.M., Pliant active and reactive power control for grid-interactive converters under unbalanced voltage dips. IEEE Transactions on Power Electronics, in press, May 2010.
10. Wang, F., Duarte, J.L., Hendrix, M.A.M., Control of grid-interfacing inverters with integrated voltage unbalance correction. IEEE Power Electronics Specialists Conference (PESC), 2008, pp. 310-316.
11. Wang, F., Duarte, J.L., Hendrix, M.A.M., Grid-interfacing converter systems with enhanced voltage quality for smart grid application - concept and implementation. IEEE Transactions on Power Electronics, submitted for review, Oct. 2010.
12. Wang, F., Duarte, J.L., Hendrix, M.A.M., Reconfiguring grid-interfacing converters for power quality improvement. IEEE Young Researchers Symposium IAS/PELS/PES Benelux Chapter, 2008, pp. 1-6. b. Ph. D. dissertation
Wang F., Flexible operation of grid-interfacing converters in distribution networks: bottom-up solutions to voltage quality enhancement. Eindhoven University of Technology, 2010.
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