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Grid Connected Three-Level Converters: Studies for Wave Energy Conversion
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
2014 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

This thesis presents an electrical system analysis of a wave energy converter (WEC) for the objective of grid connection. To transfer the enormous amount of power from waves to the load centers, efficient power electronic systems are essential. This thesis includes the modeling of a buoy–translator dynamics and the modeling of a linear permanent magnet generator along with simulation and experimental validation. Diode bridge rectifiers are considered for rectification to avoid the complex linear generator control at the input side. To reduce the size and the cost of energy storage elements, DC voltage regulation is done using a DC/DC converter.

To achieve smooth and high power, many WECs need to be connected to a common DC link. A neutral point clamped inverter is considered for the DC/AC conversion due to its advantages over conventional topologies. Various pulse width modulation schemes are tested for the inverter to choose the optimum PWM method. The harmonics in the inverter output voltage is derived numerically and compared with simulation and experiment to understand the effect of dead-time in the inverter operation.

Depending on the load current drawn from the inverter, the voltages in the two input capacitors of a three-level neutral point clamped inverter deviates from equilibrium unless the neutral point is grounded. To avoid this voltage imbalance as well as to regulate the DC link voltage a dual output boost converter with pulse delay control is proposed. The modeling, simulation and experiments show an improvement in the compensation voltage using pulse delay control compared to the previously proposed methods in the literature. The synchronous current control and the grid connection of the three-level converter have been accomplished in the laboratory. 

Finally, the three-level power converter system has been tested with a linear permanent magnet generator at Lysekil to analyze the controller requirements.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2014. , 78 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1123
Keyword [en]
Wave energy, power converters, control strategies, grid connection
National Category
Engineering and Technology
Research subject
Engineering Science with specialization in Electronics
Identifiers
URN: urn:nbn:se:uu:diva-218219ISBN: 978-91-554-8875-8 (print)OAI: oai:DiVA.org:uu-218219DiVA: diva2:695395
Public defence
2014-03-26, Häggsalen, Lägerhyddsvägen 1, Angstrom laboratory, Uppsala, 09:15 (English)
Opponent
Supervisors
Available from: 2014-03-04 Created: 2014-02-10 Last updated: 2014-12-02Bibliographically approved
List of papers
1. On the Optimization of Point Absorber Buoys
Open this publication in new window or tab >>On the Optimization of Point Absorber Buoys
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(English)Article in journal (Refereed) Submitted
National Category
Engineering and Technology
Identifiers
urn:nbn:se:uu:diva-218218 (URN)
Available from: 2014-02-10 Created: 2014-02-10 Last updated: 2017-08-24Bibliographically approved
2. Analysis of linear wave power generator model with real sea experimental results
Open this publication in new window or tab >>Analysis of linear wave power generator model with real sea experimental results
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2013 (English)In: IET Renewable Power Generation, ISSN 1752-1416, E-ISSN 1752-1424, Vol. 7, no 5, 574-581 p.Article in journal (Refereed) Published
Abstract [en]

This study presents the dynamic modelling of a linear permanent magnet generator for extracting energy from ocean waves. Translator position, calculated from measured generator voltage, is used as input for the simulation model. Instantaneous power from the simulation model has been compared with the measurements from the Lysekil research site. The power output from the model considering the air gap flux variation is precisely matching with the measured values before core saturation. The generator dynamic model is modified by including the saturation effect. Although a simple mathematical expression is considered for representing the saturation, the model gives accurate power spectrum close to the experimental results. The presented model is a first step towards the system model that can simulate the entire electric system including electric grid. As such, the generator model can be used for further analysis of the wave energy conversion system.

National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering
Research subject
Engineering Science with specialization in Science of Electricity
Identifiers
urn:nbn:se:uu:diva-206866 (URN)10.1049/iet-rpg.2012.0117 (DOI)
Available from: 2013-09-05 Created: 2013-09-05 Last updated: 2017-12-06Bibliographically approved
3. Contour-Based Dead-Time Harmonic Analysis in a Three-Level Neutral-Point-Clamped Inverter
Open this publication in new window or tab >>Contour-Based Dead-Time Harmonic Analysis in a Three-Level Neutral-Point-Clamped Inverter
2015 (English)In: IEEE transactions on industrial electronics (1982. Print), ISSN 0278-0046, E-ISSN 1557-9948, Vol. 62, no 1, 203-210 p.Article in journal (Refereed) Published
Abstract [en]

The term dead time refers to a prime safety factor for most power electronic converter topologies, and it is included either in the control software or in the gate/base driver hardware, depending on the application as well as the control requirements. In this paper, the authors present a comprehensive numerical analysis of dead-time effects on the output voltage of a three-level neutral-pointclamped (NPC) inverter. To incorporate the dead-time effect in the output voltage, 3-D models of three-level carrier pulse width modulation (PWM) methods are modified for two dead-time implementations. Closed-form expressions of inverter phase voltage harmonics for phase opposition disposition (POD) PWM are derived based on the double Fourier series approach and modified contour plots. The harmonic spectra from numerical evaluations, simulations, and experiments for natural sampling (NS), symmetrical regular sampling (SRS), and asymmetrical regular sampling (ARS) are compared to validate the mathematical models. In addition, the fundamental voltage with respect to the dead time and the load phase angle is presented based on analytical results and simulation.

Place, publisher, year, edition, pages
IEEE, 2015
Keyword
DC–AC power conversion, dead time, neutral-point-clamped (NPC) inverter, pulse width modulation (PWM), voltage harmonics
National Category
Engineering and Technology
Research subject
Engineering Science with specialization in Science of Electricity
Identifiers
urn:nbn:se:uu:diva-218169 (URN)10.1109/TIE.2014.2327579 (DOI)000346767400022 ()
Funder
SweGRIDS - Swedish Centre for Smart Grids and Energy StorageSwedish Energy AgencyStandUp
Available from: 2014-02-09 Created: 2014-02-09 Last updated: 2017-12-06Bibliographically approved
4. Space vector modulation method to balance the neutral point voltage for a three-level inverter
Open this publication in new window or tab >>Space vector modulation method to balance the neutral point voltage for a three-level inverter
2010 (English)In: Power Electronics, Drives and Energy Systems (PEDES) & 2010 Power India, 2010 Joint International Conference on, IEEE conference proceedings, 2010, 1-6 p.Conference paper, Published paper (Refereed)
Abstract [en]

This paper investigates the capacitor voltage balance scheme for a neutral point clamped inverter. A symmetrical space vector modulation is used to analyze the imbalance in neutral point voltage. This method overcomes the drawbacks of Nearest Three Vector modulation method. The redundancies of different switch configurations for the generation of the same output voltage are used to limit the voltage drift when modulation index is less than half. The deviation in the neutral voltage is further reduced by adjusting the duty cycle distribution variable. The voltage balance scheme is analyzed for linear and non linear loads. The performance of control strategy is also examined for a grid connected inverter.

Place, publisher, year, edition, pages
IEEE conference proceedings, 2010
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:uu:diva-218176 (URN)10.1109/PEDES.2010.5712534 (DOI)978-1-4244-7782-1 (ISBN)
Conference
Joint International Conference on Power Electronics, Drives and Energy Systems (PEDES) & 2010 Power India, 2010
Available from: 2014-02-09 Created: 2014-02-09 Last updated: 2015-06-04
5. Control and Implementation of Three Level BoostConverter for Load Voltage Regulation
Open this publication in new window or tab >>Control and Implementation of Three Level BoostConverter for Load Voltage Regulation
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2013 (English)In: Industrial Electronics Society, IECON 2013 - 39th Annual Conference of the IEEE, IEEE conference proceedings, 2013, 561-565 p.Conference paper, Published paper (Refereed)
Abstract [en]

   The multilevel converters offer significant advantages for high power applications. The use of multilevel DC/DC converters provides improved efficiency for power conversion and transmission at high voltage. This paper investigates the control and implementation of a three level boost converter for regulating the load voltages. A PI controller based switch signal phase delay control (SSPDC) method is used for adjusting the load voltages at equal turn on and turn off time of the converter switches. The circuit simulation is done in Matlab/Simulink. The controller is realized by using the FPGA in Labview/Compact-Rio module. Software waiting loop length control technique is used for implementing the switch signal delay control. The hardware circuit is implemented and tested. The results show a validation of the controller for regulating the voltages. This method can easily be applied for voltage balancing in a three level neutral point clamped inverter where neutral voltage imbalance is always an issue.

Place, publisher, year, edition, pages
IEEE conference proceedings, 2013
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:uu:diva-218177 (URN)10.1109/IECON.2013.6699196 (DOI)
Conference
39th Annual Conference of the IEEE Industrial Electronics Society, IECON 2013
Funder
SweGRIDS - Swedish Centre for Smart Grids and Energy Storage
Available from: 2014-02-09 Created: 2014-02-09 Last updated: 2017-11-01
6. Pulse delay Control for Capacitor VoltageBalancing in a Three Level Boost Neutral PointClamped Inverter
Open this publication in new window or tab >>Pulse delay Control for Capacitor VoltageBalancing in a Three Level Boost Neutral PointClamped Inverter
2015 (English)In: IET Power Electronics, ISSN 1755-4535, E-ISSN 1755-4543, Vol. 8, no 2, 268-277 p.Article in journal (Refereed) Published
Abstract [en]

The cross regulation effect in multi-output DC/DC converters offers a reliable support for the grid integration of multilevel inverters by balancing the capacitor voltages. The capacitor voltage balancing by single input dual output boost converter is often realised by conventional three-level switching scheme. The three-level operation benefits lower inductor ripple current, but it limits the maximum possible compensation voltages. In this study, the entire operating modes of the boost converter is presented and all the possible cases which contribute to the voltage balancing are employed for balancing the capacitor voltages in a three-level neutral point clamped inverter. A proportional-integral controller based duty ratio control and pulse delay control are used for DC link voltage regulation and capacitor voltage balancing. Since the classical state-space averaging technique is not suitable for SIDO converters, inductor current ripple averaging technique is utilised for controller design. The circuit simulation is performed in Matlab/Simulink. The digital controller is realised using the Virtex-5FPGA in Labview/CompactRIO module. Both simulation and experimental results are presented to validate the controller performance.

Keyword
invertors, DC-DC power convertors, PI control, control system synthesis, field programmable gate arrays, digital control, virtual instrumentation, voltage control, delays, pulse delay control, capacitor voltage balancing, three level boost neutral point clamped inverter, multioutput DC-DC converters, cross regulation effect, grid integration, single input dual output boost converter, three-level switching scheme, inductor ripple current, proportional-integral controller, DC link voltage regulation, state-space averaging technique, inductor current ripple averaging technique, controller design, Matlab, Simulink, digital controller, Vertex 5-FPGA, Labview-Compact-Rio module
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Research subject
Engineering Science with specialization in Science of Electricity
Identifiers
urn:nbn:se:uu:diva-218163 (URN)10.1049/iet-pel.2014.0103 (DOI)000349535400011 ()
Available from: 2014-02-09 Created: 2014-02-09 Last updated: 2017-12-06Bibliographically approved
7. Synchronous Current Compensator for a Self Balanced Three-Level Neutral Point Clamped Inverter
Open this publication in new window or tab >>Synchronous Current Compensator for a Self Balanced Three-Level Neutral Point Clamped Inverter
2014 (English)In: Advances in Power Electronics, ISSN 2090-181X, E-ISSN 2090-1828, 620607Article in journal (Refereed) Published
Abstract [en]

This paper presents a synchronous current control method for a three-level neutral point clamped inverter. Synchronous reference frame control based on two decoupled proportional-integral (PI) controllers is used to control the current in direct and quadrature axes. A phase disposition pulse width modulation (PDPWM) method in regular symmetrical sampling is used for generating the inverter switching signals. To eliminate the harmonic content with no phase errors, two first-order low pass filters (LPFs) are used for the dq currents. The simulation of closed-loop control is done in Matlab/Simulink. The Vertex-5 field programmable gate array (FPGA) in Labview/CompactRio is used for the implementation of the control algorithm. The control and switch pulse generation are done in independent parallel loops. The synchronization of both loops is achieved by controlling the length of waiting time for each loop. The simulation results are validated with experiments. The results show that the control action is reliable and efficient for the load current control.

Place, publisher, year, edition, pages
Hindawi Publishing Corporation, 2014
National Category
Engineering and Technology
Identifiers
urn:nbn:se:uu:diva-218168 (URN)10.1155/2014/620607 (DOI)
Funder
SweGRIDS - Swedish Centre for Smart Grids and Energy Storage
Available from: 2014-02-09 Created: 2014-02-09 Last updated: 2017-12-06
8. Direct Predictive Current Control of GridConnected Neutral Point Clamped Inverterfor Wave Power Extraction
Open this publication in new window or tab >>Direct Predictive Current Control of GridConnected Neutral Point Clamped Inverterfor Wave Power Extraction
2010 (English)Conference paper, Published paper (Refereed)
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:uu:diva-218178 (URN)
Conference
International Symposium on Power Electronics Electrical Drives Automation and Motion (SPEEDAM), 2010
Available from: 2014-02-09 Created: 2014-02-09 Last updated: 2014-03-05
9. Predictive current controller for a grid connected three level inverter with reactive power control
Open this publication in new window or tab >>Predictive current controller for a grid connected three level inverter with reactive power control
2010 (English)Conference paper, Published paper (Refereed)
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:uu:diva-218179 (URN)
Conference
12th Workshop on Control and Modeling for Power Electronics (COMPEL), 2010 IEEE
Available from: 2014-02-09 Created: 2014-02-09 Last updated: 2014-03-05

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Citation style
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  • modern-language-association-8th-edition
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  • en-GB
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  • nn-NO
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More languages
Output format
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