Grid-Interfacing Converter Systems with Enhanced Voltage Quality

Fei Wang

Contents

• Introduction

• Grid-interfacing systems

• Structure and functionalities

• Control design and implementation

• Conclusions

2

Transition to the future grid

• Improving energy efficiency

• Applying sustainable energy

3

• Growing electricity consumption

• Demanding high-quality electricity

Conventional electricity grid

• Central power station

• Top-down centralized control

• Unidirectional power flow

GenerationTransmission

Distribution

4

Loads

Voltage quality problems

• Harmonics

5

• Dips • Unbalance • Fluctuations

GenerationTransmission

DistributionLoads

Distributed generation in the grid

Energy StoragesDistributed Generation

Distribution networks

Grid-interfacing converters

6

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

8Path to the future grid

9Series-parallel grid-interfacing systems

• Independent distributed sources powered dc bus

10Series-parallel grid-interfacing systems

• Common distributed sources powered dc bus with isolation techniques

11Series-parallel grid-interfacing systems

• An example of coupling the utility grid and a local grid/micro-grid

12Series-parallel grid-interfacing systems

• Adapted series-parallel structure

Common distributed sources powered dc bus

13Reconfiguring system functionalities

• Conventional power quality enhancement

• Unified PQ conditioners (UPQC)

• UPQC + energy storage (batteries, super-capacitors, distributed sources, etc. )

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

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

16Comparison with shunt systems

(a) Series-parallel system

(b) Shunt-connected system

17Control design and implementation

• Employed configuration of the laboratory system

18Control design and implementation

• Overall control structure• Parallel converter

• Series converter

19Control design – parallel converter

• Control diagram of the parallel converter

20Control design – parallel converter

• Instability improvement under no-load conditions

Bode plots of the plant transfer function

Feedforward loop

21Control design – parallel converter

• Selective harmonic regulation

Bode plots of the open-loop transfer function with multiple PR controllers

Multiple PR controllers

22Control design – parallel converter

• Disturbance sensitivity improvement

System sensitivity to current disturbances

Inner current feedback loop

23Control design – series converter

• Control diagram of the series converter

24Control design – series converter

Inverter output voltage to feedback current

when

Bode plots of open-loop transfer function

Laboratory system 25

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

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

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

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

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

30

• Grid interaction control integrating grid-impedance adaptability

• Scaled up grid-interfacing systems for smart-grid research

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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