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Modeling, Control and Protection of Low-Voltage DC Microgrids
KTH, School of Electrical Engineering (EES), Electric Power Systems.
2008 (English)Doctoral thesis, comprehensive summary (Other scientific)
Abstract [en]

Current trends in electric power consumption indicate an increasing use of dc in end-user equipment, such as computers and other electronic appliances used in households and offices. With a dc power system, ac/dc conversion within these loads can be avoided, and losses reduced. AC/DC conversion is instead centralized, and by using efficient, fully controllable power-electronic interfaces, high power quality for both ac and dc systems during steady state and ac grid disturbances can be obtained. Connection of back-up energy storage and small-size generation is also easier to realize in a dc power system.

To facilitate practical application, it is important that the shift from ac to dc can be implemented with minimal changes. Results from measurements carried out on common household appliances show that most loads are able to operate with dc supply without any modifications. Furthermore, simple, and yet sufficiently accurate, load models have been derived using the measurement results. The models have been used for further analysis of the dc system, both in steady state and during transients.

AC microgrids have gained research interest during the last years. A microgrid is a part of power systems which can operate both connected to the ac grid, and autonomously in island mode when the loads are supplied from locally distributed resources. A low-voltage dc microgrid can be used to supply sensitive electronic loads, since it combines the advantages of using a dc supply for electronic loads, and using local generation to supply sensitive loads. An example of a commercial power system which can benefit from using a dc microgrid is data center. The lower losses due to fewer power conversion steps results in less heat which need to be cooled, and therefore the operation costs are lowered.

To ensure reliable operation of a low-voltage dc microgrid, well-designed control and protection systems are needed. An adaptive controller is required to coordinate the different resources based on the load-generation balance in the microgrid, and status of the ac grid. The performance of the developed controller has been studied and evaluated through simulations. The results show that it is possible to extend use of the data center dc microgrid to also support a limited amount of ac loads close to the data center, for example an office building.

A protection-system design for low-voltage dc microgrids has been proposed, and different protection devices and grounding methods have been presented. Moreover, different fault types and their impact on the system have been analyzed. The type of protection that can be used depends on the sensitivity of the components in the microgrid. Detection methods for different components have been suggested in order to achieve a fast and accurate fault clearing.

An experimental small-scale dc power system has been used to supply different loads, both during normal and fault conditions. A three-phase two-level voltage source converter in series with a Buck converter was used to interconnect the ac and the dc power systems. Together the converters have large controllability, high power quality performance, and allow bi-directional power flow. This topology can preferably be used together with energy storage. The tests confirm the feasibility of using a dc power system to supply sensitive electronic loads.

Place, publisher, year, edition, pages
Stockholm: KTH , 2008. , xiv, 52 p.
Series
Trita-EE, ISSN 1653-5146 ; 2008:007
Keyword [en]
circuit transient analysis, dc power systems, dispersed storage and generation, load modeling, power conversion, power distribution control, power distribution faults, power distribution protection, power electronics
National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
URN: urn:nbn:se:kth:diva-4666ISBN: 978-91-7178-867-2 (print)OAI: oai:DiVA.org:kth-4666DiVA: diva2:13328
Public defence
2008-04-04, H1, Teknikringen 33, Stockholm, 10:00
Opponent
Supervisors
Note
QC 20100908Available from: 2008-03-10 Created: 2008-03-10 Last updated: 2010-09-08Bibliographically approved
List of papers
1. Efficiency analysis of low- and medium-voltage dc distribution systems
Open this publication in new window or tab >>Efficiency analysis of low- and medium-voltage dc distribution systems
2004 (English)In: 2004 IEEE Power Engineering Society General Meeting: Denver, CO: 6 June 2004 through 10 June 2004, 2004, 2315-2321 p.Conference paper, Published paper (Refereed)
Abstract [en]

In this paper, sustainability aspects connected to the use of dc for power delivery in low- and medium-voltage distribution systems are treated. The efficiency of an example ac system, a dc system and a mixed ac-dc system are calculated and compared. It is shown that, under the assumption of a substantial reduction in semiconductor losses, the total system losses decrease using dc. This means that for the same energy delivered, less energy must be produced from the available sources, thereby indirectly reducing the environmental impact of energy production. Moreover, the dc system seems to lead to better utilization of the HV/MV transformer, so that the same system can be expanded to supply a higher load without changing the transformer.

Keyword
environment, power distribution system, direct current (dc), power electronics, efficiency, losses
National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:kth:diva-8091 (URN)10.1109/PES.2004.1373299 (DOI)000225860300570 ()0-7803-8465-2 (ISBN)
Note
QC 20100908Available from: 2008-03-10 Created: 2008-03-10 Last updated: 2010-09-08Bibliographically approved
2. Load modelling for steady-state and transient analysis of low-voltage dc systems
Open this publication in new window or tab >>Load modelling for steady-state and transient analysis of low-voltage dc systems
2007 (English)In: IET Electric Power Applications, ISSN 1751-8660, Vol. 1, no 5, 690-696 p.Article in journal (Refereed) Published
Abstract [en]

The modelling of loads for low-voltage (LV) DC system studies is treated. Results from measurements on 63 different loads supplied with DC are presented. The measurement results are used to derive simplified load models, which can be used for steady-state and transient analysis of LV DC systems. These load models extend existing standards for load flow and short circuit current calculations.

Keyword
Electric current measurement, Electric loads, Mathematical models, Short circuit currents, Transient analysis, Load models, Low voltage DC systems
National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:kth:diva-8092 (URN)10.1049/iet-epa:20060418 (DOI)000249908600006 ()2-s2.0-34548029928 (Scopus ID)
Note
QC 20100908Available from: 2008-03-10 Created: 2008-03-10 Last updated: 2010-09-08Bibliographically approved
3. Comparative Design and Analysis of dc-Link-Voltage Controllers for Grid-Connected Voltage-Source Converter
Open this publication in new window or tab >>Comparative Design and Analysis of dc-Link-Voltage Controllers for Grid-Connected Voltage-Source Converter
2007 (English)In: 2007 IEEE Industry Applications Conference 42nd Annual Meeting, IAS, 2007, 1593-1600 p.Conference paper, Published paper (Refereed)
Abstract [en]

Electric power conversion is used when loads require a voltage with a different magnitude and frequency than the supplying grid. In most cases the power conversion is made in two steps: one rectifier and one converter. The design and operation of the rectifier is of great importance for the interaction with the utility grid. By using a Voltage-Source Converter with a controllable dc-link voltage, a higher power quality can be obtained than using a simple diode rectifier.

In this paper, two designs of a dc-link-voltage controller are analyzed and tested. The parameter sensitivity, frequency response and load-disturbance rejection are investigated and compared. The response of the voltage controllers are also improved by utilizing feed forward. Both the measured dc-load current and a calculated dc-load current, obtained with an observer, are used. The performance of the controllers is evaluated through simulations and laboratory tests.

Series
IEEE Industry Applications Society Annual Meeting, ISSN 0197-2618
Keyword
Control theory, Disturbance rejection, Electric load dispatching, Frequency response, Military data processing, Power converters, Power quality, Sensitivity analysis, Voltage control, Annual meetings, Dc-link, DC-link voltages, Design and analysis, Design and operation, Diode rectifiers, Electric powers, Feed forward (FF), Grid-connected, Laboratory testing, Load currents, Observer (OBS), Parameter sensitivities, Power conversions, Utility grids, Voltage controllers, Voltage-source-converter (VSC)
National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:kth:diva-8093 (URN)10.1109/IAS.2007.246 (DOI)000260446901004 ()2-s2.0-47849097989 (Scopus ID)
Note
QC 20100908Available from: 2008-03-10 Created: 2008-03-10 Last updated: 2010-09-08Bibliographically approved
4. Centralized ac/dc power conversion for electronic loads in a low-voltage dc power system
Open this publication in new window or tab >>Centralized ac/dc power conversion for electronic loads in a low-voltage dc power system
2006 (English)In: 37th IEEE Power Electronics Specialists Conference 2006, PESC'06, 2006, 2043-2049 p.Conference paper, Published paper (Refereed)
Abstract [en]

In this paper, an interface to be used as a centralized ac/dc converter for electronic loads is presented. The proposed interface is tested both in a simulation software and in a lab setup. The performance of the interface is studied during load transients (connection and disconnection), and during a grid transient, i.e. an unbalanced voltage dip. The result shows that the proposed interface does not affect the utility power quality in terms of low-frequency harmonics and has voltage-dip ride-through capability.

Series
IEEE Power Electronics Specialists Conference Records, ISSN 0275-9306
Keyword
Computer simulation, DC power transmission, Power quality, Transients, Electronic loads
National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:kth:diva-8094 (URN)10.1109/PESC.2006.1712251 (DOI)000245402504020 ()2-s2.0-42449102554 (Scopus ID)0-7803-9716-9 (ISBN)
Note
QC 20100908Available from: 2008-03-10 Created: 2008-03-10 Last updated: 2011-10-10Bibliographically approved
5. Comparison of different solutions for emergency and standby power systems for commercial consumers
Open this publication in new window or tab >>Comparison of different solutions for emergency and standby power systems for commercial consumers
2006 (English)In: INTELEC 2006: 28th Annual International Telecommunications Energy Conference, 2006Conference paper, Published paper (Refereed)
Abstract [en]

In this paper a new improved method to evaluate the design of commercial power systems is described. The power system is divided into seven design criteria and each criterion Is evaluated separately. The aim with the method is to identify problems with the present design which can be modified to improve the performance of the system. The method is applied to four sensitive commercial consumers: substation; hospital; voice and data communication facility; and data center. They are all equipped with emergency and standby power systems. Different requirements and solutions for each systems are analyzed and presented. The study shows that the voice and data communication facility and especially the data center both have possibilities of improvements. The data center power systems has a potential to improve the efficiency, and therefore also to reduce the energy cost, improve the availability, and to use the local energy source and energy storage to sell power to the utility grid during peak load.

Series
International Telecommunications Energy Conference-INTELEC, ISSN 0275-0473
Keyword
Consumer electronics, Cost effectiveness, Data communication systems, Electric substations, Energy utilization, Voice/data communication systems, Commercial consumers, Commercial power systems
National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:kth:diva-8095 (URN)10.1109/INTLEC.2006.251673 (DOI)000245995800015 ()2-s2.0-42749104145 (Scopus ID)1-4244-0430-4 (ISBN)
Note
QC 20100908Available from: 2008-03-10 Created: 2008-03-10 Last updated: 2010-09-08Bibliographically approved
6. An adaptive control system for a dc microgrid for data centers
Open this publication in new window or tab >>An adaptive control system for a dc microgrid for data centers
2008 (English)In: IEEE transactions on industry applications, ISSN 0093-9994, E-ISSN 1939-9367, Vol. 44, no 6, 1910-1917 p.Article in journal (Refereed) Published
Abstract [en]

In this paper, an adaptive control system for a dc microgrid for data centers is proposed. Data centers call for electric power with high availability, and a possibility to reduce the electric losses and, consequently, the need for cooling. High reliability can be achieved by using local energy sources, and by using a dc power system, the number of conversion steps, and therefore also the losses, can be reduced. The dc microgrid can also supply closely located sensitive ac loads during outages in the ac grid. The proposed dc microgrid can be operated in eight different operation modes described here, resulting in 23 transitions. The control system coordinates the operation of converters, sources, and switches used in the dc microgrid. The control system is tested in the simulation software package PSCAD/EMTDC, and the results of the most interesting transitions are presented. The results show that it is possible to use the proposed dc microgrid to supply sensitive electronic loads and also, during ac-grid outages, supply closely located sensitive ac loads. To reduce the current transients experienced by grid-connected ac/dc converters, fast grid-outage detection and fast switches are required.

Keyword
Adaptive control, circuit transient analysis, computer facilities, dc power systems, power conversion, power distribution control, power distribution faults, power electronics
National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:kth:diva-8096 (URN)10.1109/TIA.2008.2006398 (DOI)000261308700031 ()2-s2.0-57049162052 (Scopus ID)
Note
QC 20100908. Uppdaterad från accepted till published (20100908).Available from: 2008-03-10 Created: 2008-03-10 Last updated: 2010-09-08Bibliographically approved
7. Protection of low-voltage dc microgrids
Open this publication in new window or tab >>Protection of low-voltage dc microgrids
2009 (English)In: IEEE Transactions on Power Delivery, ISSN 0885-8977, E-ISSN 0885-8877, Vol. 24, no 3, 1045-1053 p.Article in journal (Refereed) Published
Abstract [en]

In this paper, a low-voltage (LV) dc microgrid protection system design is proposed. The LV de microgrid is used to interconnect distributed resources and sensitive electronic loads. When designing an LV de microgrid protection system, knowledge from existing dc power systems can be used. However, in most cases, these systems use grid-connected rectifiers with current-limiting capability during dc faults. In contrast, an LV dc microgrid must be connected to an ac grid through converters with bidirectional power flow and, therefore, a different protection-system design is needed. In this paper, the operating principles and technical data of LV dc protection devices, both available and in the research stage, are presented. Furthermore, different fault-detection and grounding methods are discussed. The influence of the selected protection devices and grounding method on an LV dc microgrid is studied through simulations. The results show that it is possible to use available devices to protect such a system. Problems may arise with high-impedance ground faults which can be difficult to detect.

Keyword
Circuit transient analysis, DC power systems, power distribution faults, power distribution protection, power electronics
National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:kth:diva-8097 (URN)10.1109/TPWRD.2009.2016622 (DOI)000267765100009 ()2-s2.0-67650686103 (Scopus ID)
Note
QC 20100908. Uppdaterad från submitted till published (20100908).Available from: 2008-03-10 Created: 2008-03-10 Last updated: 2010-09-08Bibliographically approved
8. Low-voltage dc distribution system for commercial power systems with sensitive electronic loads
Open this publication in new window or tab >>Low-voltage dc distribution system for commercial power systems with sensitive electronic loads
2007 (English)In: IEEE Transactions on Power Delivery, ISSN 0885-8977, E-ISSN 1937-4208, Vol. 22, no 3, 1620-1627 p.Article in journal (Refereed) Published
Abstract [en]

In this paper, the use of dc power system to supply sensitive electronic loads is treated. First, general design issues regarding dc power systems are discussed, and then the measurement results from a scaled laboratory setup are presented. The results show that it is possible to supply sensitive electronic loads through an ac/dc interface, and to keep them online during grid transients. The use of a dc power system to supply sensitive electronic loads will have lower losses compared with a conventional ac uninterruptible power-supply solution due to fewer power conversion steps.

Keyword
DC power system, electronic equipment, emergency power supplies, harmonic distortion, power quality, power system transients
National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:kth:diva-8098 (URN)10.1109/TPWRD.2006.883024 (DOI)000247605900042 ()2-s2.0-34548030939 (Scopus ID)
Note

QC 20100908

Available from: 2008-03-10 Created: 2008-03-10 Last updated: 2017-06-14Bibliographically approved

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