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DC Distributed Power Systems - Analysis, Design and Control for a Renewable Energy System

Karlsson, Per LU (2002)
Abstract
Renewable energy systems are likely to become wide spread in the future due to environmental demands. As a consequence of the dispersed nature of renewable energy systems, this implies that there will be a distributed generation of electric power. Since most of the distributed electrical energy sources do not provide their electric power at line frequency and voltage, a DC bus is a useful common connection for several such sources. Due to the differences in output voltage among the sources, depending on both the type of source and their actual operating point, the sources are connected to the DC power system via power electronic converters. The intention behind the presented work is not to replace the existing AC power system, but to... (More)
Renewable energy systems are likely to become wide spread in the future due to environmental demands. As a consequence of the dispersed nature of renewable energy systems, this implies that there will be a distributed generation of electric power. Since most of the distributed electrical energy sources do not provide their electric power at line frequency and voltage, a DC bus is a useful common connection for several such sources. Due to the differences in output voltage among the sources, depending on both the type of source and their actual operating point, the sources are connected to the DC power system via power electronic converters. The intention behind the presented work is not to replace the existing AC power system, but to include local DC power systems. The AC and DC power systems are connected at some points in the network. The renewable energy sources are weak compared to the present hydro power and nuclear power plants, resulting in a need of power conditioning before the renewable energy is fed to the transmission lines. The benefit of such an approach is that power conditioning is applied on a central level, i.e. at the interface between the AC and DC power systems.



Present DC transmission systems are discussed and investigated in simulations. Then, different methods for load sharing and voltage control are discussed. Especially, the voltage droop control scheme is examined thoroughly. Since the droop control method does not require any high-speed communication between sources and loads, this is considered the most suitable for DC distributed power systems. The voltage feed back design of the controller also results in a specification of the DC bus capacitors (equivalents to DC link capacitors of single converters) needed for filtering. If the converters in the DC distribution system are equipped with capacitors selected from this design criterion and if the DC bus impedance is neglected, the source converters share the total load equally in per unit.



The same DC distribution bus configuration is studied in a wind power application. Especially the dynamic properties of load-source interactions are highlighted. They are interesting since the sources are considered weak for a distributed power system. This is illustrated with simulations where the power is fed from wind turbines only and constant power loads are controlled at the same time as the DC bus voltage level.



Personal safety and prevention of property damage are important factors of conventional AC power systems. A grounding scheme for the DC distribution system together with algorithms for detection of ground faults, are presented. The proposed method detects ground faults on both the AC and DC sides and is extended to cover short circuit faults with a minor work effort.



Experimental verifications follow the theoretical investigations introduced above. First, dynamic properties are studied and the behavior predicted from theoretical analysis and simulations is verified. Then, load sharing is investigated. Also in this investigation, the experimental results agree with the simulated. (Less)
Please use this url to cite or link to this publication:
author
supervisor
opponent
  • Professor Boroyevich, Dushan, Virginia Tech
organization
publishing date
type
Thesis
publication status
published
subject
keywords
Energiforskning, fault detection, load sharing, wind power, DC distributed power system, voltage control, Energy research, Elektronik och elektroteknik, VSC, Electronics and Electrical technology
pages
200 pages
publisher
IEA-LTH, Box 118, SE-221 00 LUND, SWEDEN,
defense location
Room M:B of the M-building of Lund Institute of Technology, Lund University
defense date
2002-12-20 10:15
external identifiers
  • other:ISRN: LUTEDX/(TEIE-1033)/1-200/(2002)
ISBN
91-88934-25-X
language
English
LU publication?
yes
id
0963f6fc-c896-451f-963a-8260652ea152 (old id 20667)
date added to LUP
2007-05-28 10:45:21
date last changed
2016-09-19 08:45:12
@phdthesis{0963f6fc-c896-451f-963a-8260652ea152,
  abstract     = {Renewable energy systems are likely to become wide spread in the future due to environmental demands. As a consequence of the dispersed nature of renewable energy systems, this implies that there will be a distributed generation of electric power. Since most of the distributed electrical energy sources do not provide their electric power at line frequency and voltage, a DC bus is a useful common connection for several such sources. Due to the differences in output voltage among the sources, depending on both the type of source and their actual operating point, the sources are connected to the DC power system via power electronic converters. The intention behind the presented work is not to replace the existing AC power system, but to include local DC power systems. The AC and DC power systems are connected at some points in the network. The renewable energy sources are weak compared to the present hydro power and nuclear power plants, resulting in a need of power conditioning before the renewable energy is fed to the transmission lines. The benefit of such an approach is that power conditioning is applied on a central level, i.e. at the interface between the AC and DC power systems.<br/><br>
<br/><br>
Present DC transmission systems are discussed and investigated in simulations. Then, different methods for load sharing and voltage control are discussed. Especially, the voltage droop control scheme is examined thoroughly. Since the droop control method does not require any high-speed communication between sources and loads, this is considered the most suitable for DC distributed power systems. The voltage feed back design of the controller also results in a specification of the DC bus capacitors (equivalents to DC link capacitors of single converters) needed for filtering. If the converters in the DC distribution system are equipped with capacitors selected from this design criterion and if the DC bus impedance is neglected, the source converters share the total load equally in per unit.<br/><br>
<br/><br>
The same DC distribution bus configuration is studied in a wind power application. Especially the dynamic properties of load-source interactions are highlighted. They are interesting since the sources are considered weak for a distributed power system. This is illustrated with simulations where the power is fed from wind turbines only and constant power loads are controlled at the same time as the DC bus voltage level.<br/><br>
<br/><br>
Personal safety and prevention of property damage are important factors of conventional AC power systems. A grounding scheme for the DC distribution system together with algorithms for detection of ground faults, are presented. The proposed method detects ground faults on both the AC and DC sides and is extended to cover short circuit faults with a minor work effort.<br/><br>
<br/><br>
Experimental verifications follow the theoretical investigations introduced above. First, dynamic properties are studied and the behavior predicted from theoretical analysis and simulations is verified. Then, load sharing is investigated. Also in this investigation, the experimental results agree with the simulated.},
  author       = {Karlsson, Per},
  isbn         = {91-88934-25-X},
  keyword      = {Energiforskning,fault detection,load sharing,wind power,DC distributed power system,voltage control,Energy research,Elektronik och elektroteknik,VSC,Electronics and Electrical technology},
  language     = {eng},
  pages        = {200},
  publisher    = {IEA-LTH, Box 118, SE-221 00 LUND, SWEDEN,},
  school       = {Lund University},
  title        = {DC Distributed Power Systems - Analysis, Design and Control for a Renewable Energy System},
  year         = {2002},
}