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Grid-Interfacing Converter Systems
with Enhanced Voltage Quality
Fei Wang
Contents
1
• Introduction
• Grid-interfacing systems
• Structure and functionalities
• Control design and implementation
• Conclusions
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Transition to the future grid
2
• Growing electricity consumption
• Demanding high-quality electricity
• Improving energy efficiency
• Applying sustainable energy
Conventional electricity grid
3
Generation
Transmission
Distribution
Loads
• Central power station
• Top-down centralized control
• Unidirectional power flow
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Voltage quality problems
4
Generation
Transmission
Distribution
Loads
• Harmonics • Unbalance • Fluctuations • Dips
Distributed generation in the grid
5
Distribution networks
Grid-interfacing
converters
Distributed Generation
Energy Storages
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Path to the future grid
Micro-grid
6
Series Converter
Utility Grid
Shunt Converter
Solar Panel
DC/DC
Wind Turbine
DC/AC
AC/DC/AC
Loads
Energy
Storage
DC/AC
DC/AC
Loads
Loads
Loads
Distributed Source /
Energy Storage
AC Bus
Data Bus
Path to the future grid
7
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Series-parallel grid-interfacing systems
8
• Independent distributed sources powered dc bus
Series-parallel grid-interfacing systems
9
• Common distributed sources powered dc bus with
isolation techniques
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Series-parallel grid-interfacing systems
10
• An example of coupling the utility grid and a local
grid/micro-grid
Series-parallel grid-interfacing systems
11
• Adapted series-parallel structure
Common distributed sources powered dc bus
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Reconfiguring system functionalities
12
• Conventional power quality enhancement
• Unified PQ conditioners (UPQC)
• UPQC + energy storage (batteries, super-capacitors, distributed
sources, etc. )
Reconfiguring system functionalities
13
• Circuit presentation of the proposed grid-interfacing system
is 1
Grid
is 1
Zg
is
ish
v g1
Zs
Series
Converter
v g1
v gh
ild
Zp
v p1
Loads
vp
v p1
Loads
Parallel
Converter
v ph
ild 1 ild 1 ildh
Z ld
Subscripts:
+, - : positive and negative sequence;
1: fundamental components
h:
harmonics
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Reconfiguring system functionalities
14
• 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
is 1
Grid
is 1
Zg
is
ish
v g1
Zs
v p1
Series
Converter
v g1
v gh
ild
Zp
Loads
vp
v p1
Loads
Parallel
Converter
v ph
ild 1 ild 1 ildh
Z ld
Comparison with shunt systems
15
(a) Series-parallel system
(b) Shunt-connected system
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Control design and implementation
16
• Employed configuration of the laboratory system
Control design and implementation
17
• Overall control structure
• Parallel converter
• Series converter
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Control design – parallel converter
18
• Control diagram of the parallel converter
Control design – parallel converter
19
• Instability improvement under no-load conditions
Feedforward loop
Bode plots of the plant
transfer function
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Control design – parallel converter
20
• Selective harmonic regulation
Multiple PR controllers
Bode plots of the open-loop transfer
function with multiple PR controllers
Control design – parallel converter
21
• Disturbance sensitivity improvement
Inner current feedback loop
System sensitivity to current disturbances
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Control design – series converter
22
• Control diagram of the series converter
Control design – series converter
Inverter output voltage to
feedback current
23
Bode plots of open-loop transfer function
when
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Laboratory system
24
Experimental results
25
• 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
26
• Emulation of active filter function with low-order harmonic
current injection
Injected series voltage
Series-converter current
Frequency spectra of
the phase current
Experimental results
27
• Test under a distorted grid condition
Distorted grid voltage (6.7%THD)
Output voltage of the parallel converter
Output voltage of the series-converter
Currents delivered to the grid
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Experimental results
28
• 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
Conclusions and recommendations
29
• 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
• Grid interaction control integrating grid-impedance adaptability
• Scaled up grid-interfacing systems for smart-grid research
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References
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a.Selected papers
1. Wang, F., Benhabib, M.C., Duarte, J.L., Hendrix, M.A.M., Sequence-decoupled controller for three-phase gridconnected 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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