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Improving the functionality of synchronous machines using power electronics
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.ORCID iD: 0000-0001-8237-3107
2017 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

With the advent of modern power electronics there is reason to explore what can be achieved when it is applied to a mature technology like synchronous machines. In this text several concepts will be presented on how it is possible to control forces and how to get better performance out of synchronous machines by using power electronics. Methodologies to create radial forces by controlling the field current in a standard series connected rotor winding as well as when the winding is split in to several segments is presented. By segmenting the rotor a resulting force vector can be created to cancel forces due to unbalanced magnetic pull.

It is also shown that inverting the field current with respect to the stator field enables line start of synchronous machines without using damper bars, frequency converters, or starting motors.

Some first results from the installation and testing of an electromagnetic thrust bearing installed in unit U9 in the hydropower station in Porjus are presented. The benefits of the system is increased reliability and higher efficiency of the thrust bearing system.

An evaluation of a 2-stage brushless excitation system was done, different rotating power electronics topologies were tested in the stationary frame connected to a six-phase permanent magnet brushless exciter. The rotating control and measurement system for the power electronics is presented. Potential benefits of the system is that there is no need for brushes to transfer the field current to the rotor winding, fast response time due to actively controlled electronics, independence of the station bus voltage, and reduced maintenance.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2017.
Series
UURIE / Uppsala University, Department of Engineering Sciences, ISSN 0349-8352 ; 352-17L
Keyword [en]
Power electronics, Synchronous machines, Excitation systems, Magnetic thrust bearing, Starting synchronous machines, Split rotor, Rotating electronics, Magnetic fields, Measurement systems, Unbalanced magnetic pull, Harmonics
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Research subject
Engineering Science with specialization in Science of Electricity
Identifiers
URN: urn:nbn:se:uu:diva-333940OAI: oai:DiVA.org:uu-333940DiVA: diva2:1158572
Presentation
2017-12-18, Häggsalen (Å10132), Ångströmslaboratoriet, Lägerhyddsvägen 1, Uppsala, 10:15 (English)
Opponent
Supervisors
Available from: 2018-01-08 Created: 2017-11-20 Last updated: 2018-01-08Bibliographically approved
List of papers
1. Altering Flux Density Harmonics in the Air-gap of Synchronous Machines by Active Control of the Field Current
Open this publication in new window or tab >>Altering Flux Density Harmonics in the Air-gap of Synchronous Machines by Active Control of the Field Current
(English)In: IEEE transactions on industrial electronics (1982. Print), ISSN 0278-0046, E-ISSN 1557-9948Article in journal (Refereed) Accepted
Abstract [en]

Flux density harmonics in the air-gap of electrical machines create distorted voltage waveforms and can induce vibrations. These problems are difficult to mitigate when the machine is already in operation and therefore a lot of effort is made during the design phase to eliminate them. Still, many in-operation machines experience problems related to harmonics and often the solution is to mechanically reinforce and change modal shapes which is expensive and inconvenient. Using a current controlled switched power supply to excite a synchronous machine and adding specific harmonics to the DC-field current it is shown that it is possible to alter the harmonic content of the flux density in the air-gap, and thus affect  voltage harmonics and the exciting magnetic forces. The method is verified and compared with simulations and experiments on an existing electrical machine. 

Keyword
Air-gap flux density, Finite element, Harmonic waves, Noise reduction, Power electronics, Synchronous machines
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Research subject
Engineering Science with specialization in Science of Electricity
Identifiers
urn:nbn:se:uu:diva-333565 (URN)
Available from: 2017-11-15 Created: 2017-11-15 Last updated: 2017-11-20
2. Demonstration of synchronous motor start by rotor polarity inversion
Open this publication in new window or tab >>Demonstration of synchronous motor start by rotor polarity inversion
2017 (English)In: Article in journal (Refereed) Submitted
National Category
Engineering and Technology
Identifiers
urn:nbn:se:uu:diva-328081 (URN)
Available from: 2017-08-16 Created: 2017-08-16 Last updated: 2017-11-20
3. Magnetic thrust bearing for a hydropower unit with a Kaplan turbine
Open this publication in new window or tab >>Magnetic thrust bearing for a hydropower unit with a Kaplan turbine
2017 (English)Manuscript (preprint) (Other academic)
National Category
Engineering and Technology
Identifiers
urn:nbn:se:uu:diva-328082 (URN)
Available from: 2017-08-16 Created: 2017-08-16 Last updated: 2017-11-20
4. Design and Characterization of a Rotating Brushless Outer Pole PM Exciter for a Synchronous Generator
Open this publication in new window or tab >>Design and Characterization of a Rotating Brushless Outer Pole PM Exciter for a Synchronous Generator
Show others...
2017 (English)In: IEEE transactions on industry applications, ISSN 0093-9994, E-ISSN 1939-9367, Vol. 53, no 3, 2016-2027 p.Article in journal, Letter (Refereed) Published
Abstract [en]

Generally, PM machines are used as PMG pre-exciters in 3-stage brushless excitations systems. This paperpresents the design, characterization and prototyping of a rotatingbrushless PM exciter used in a proposed 2-stage excitation systemfor a synchronous generator. The proposed design reduces thenumber of components compared with conventional systems.A comparison with the state-of-the-art conventional excitationsystems is given. The design of a fast-response, or high initialresponse, brushless exciter requires active rectification on therotating frame, replacing the non-controllable diode bridge. Theobjective was to construct an exciter with the capability of a50 Aoutput field current as well as a high value of the available ceilingvoltage and ceiling current. The final exciter was constructed to befitted into an in-house synchronous generator test setup. A finiteelement model of the exciter was validated with experimentalmeasurements. The exciter prototype is also compared with analternative armature design with non-overlapping single-layerconcentrated windings but with the same main dimensions.The paper includes a general design procedure suitable foroptimization of PM brushless exciters that fulfill the requirementsof their synchronous generators and the grid.

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-315594 (URN)10.1109/TIA.2017.2669890 (DOI)
Available from: 2017-02-15 Created: 2017-02-15 Last updated: 2017-11-20Bibliographically approved
5. Comparison of Thyristor Rectifier Configurations for a Six-Phase Rotating Brushless Outer Pole PM Exciter
Open this publication in new window or tab >>Comparison of Thyristor Rectifier Configurations for a Six-Phase Rotating Brushless Outer Pole PM Exciter
Show others...
2018 (English)In: IEEE transactions on industrial electronics (1982. Print), ISSN 0278-0046, E-ISSN 1557-9948, Vol. 65, no 2, 968-976 p.Article in journal (Refereed) Published
Abstract [en]

Recent technological developments have caused a renewed interest in the brushless excitation system. With the application of wireless communication, the conventional diode bridge has been replaced with fully controllable thyristors on the shaft. It offers the same dynamic performance as the conventional static excitation system. The thyristor bridge of the conventional three-phase exciter needs to be controlled with a high firing angle in normal operation in order to fulfill a requirement of both a high ceiling voltage and a high ceiling current. A high firing angle causes high torque ripple to be absorbed by the exciter stator and a low power factor results in a low utilization of the designed exciter. In this contribution, we present a strategy that solves this problem by looking into combinations of thyristor configurations of a double-star six-phase connection of the exciter. Experimental results are used to verify the circuit models implemented for this investigation. A hybrid-mode 12-pulse thyristor bridge configuration seems to be a good solution for implementations in commercial apparatus. An additional switch interconnects two separate thyristor bridges from parallel- to series connection at the rectifier output, and utilizes the advantages of both topologies.

Keyword
Bridge circuits, Generators, Shafts, Thyristors, Topology, Torque, Windings, 12-pulse thyristor rectifiers, Fast-response exciters, active rectification, brushless exciters, hybrid-mode operation, multiphase machines, outer-pole PM machines, rotating exciters, synchronous generators
National Category
Engineering and Technology
Identifiers
urn:nbn:se:uu:diva-328391 (URN)10.1109/TIE.2017.2726963 (DOI)
Available from: 2017-08-22 Created: 2017-08-22 Last updated: 2018-01-08Bibliographically approved
6. Testing of Active Rectification Topologies on a 6-Phase Rotating Brushless Outer Pole PM Exciter
Open this publication in new window or tab >>Testing of Active Rectification Topologies on a 6-Phase Rotating Brushless Outer Pole PM Exciter
Show others...
2017 (English)In: IEEE transactions on energy conversion, ISSN 0885-8969, E-ISSN 1558-0059, Vol. PP, no 99, 1-9 p.Article in journal (Refereed) Epub ahead of print
Abstract [en]

The static exciter is dominating among large grid- connected generators due to the weak dynamic performance of conventional brushless exciters. In this contribution, a six-phase double-star outer pole permanent magnet rotating brushless exciter is evaluated with different active rectification topolo- gies. Both thyristor-based and chopper-based topologies are considered. A high speed response brushless excitation system is obtained by replacing the conventional rotating diode bridge rectifier with the proposed active rectification topologies on the shaft. The given two-stage system generates its own excitation power directly from the shaft, contrary to static exciters. The selection of an appropriate rectification topology could minimize the rotor armature phase currents for a given generator field current. The objective is a high power factor and a high uti- lization of the exciter machine. An optimal rectification topology makes higher ceiling currents possible, improving the transient behavior of the synchronous generator. In this paper, we show that six-phase topologies add complexity, but improve exciter redundancy, increase the available ceiling voltage and reduce the steady state torque ripple. Experimental results are given for validating the models implemented for the analysis.

National Category
Engineering and Technology
Research subject
Engineering Science with specialization in Science of Electricity
Identifiers
urn:nbn:se:uu:diva-328396 (URN)10.1109/TEC.2017.2746559 (DOI)
Available from: 2017-08-22 Created: 2017-08-22 Last updated: 2017-11-20
7. Failure-Modes Demonstration and Redundant Postfault Operation of Rotating Thyristor Rectifiers on Brushless Dual-Star Exciters
Open this publication in new window or tab >>Failure-Modes Demonstration and Redundant Postfault Operation of Rotating Thyristor Rectifiers on Brushless Dual-Star Exciters
(English)In: IEEE transactions on industrial electronics (1982. Print), ISSN 0278-0046, E-ISSN 1557-9948Article in journal (Refereed) Submitted
Abstract [en]

The excitation system plays a critical role for the operation of synchronous generators. A potential equipment failure impacts the voltage quality for smaller grids. Further, it can cause cost penalties and reduced production for the power plant owner. Recently, a new high-speed response rotating brushless exciter employs remote control of rotating thyristors on the generator shaft. It has lead to need possibilities for improved performance of brushless exciters. This contribution investigates a dual-star outer pole permanent magnet exciter feeding two separate thyristor bridges, which are connected in parallel during normal operation. The possibility of redundant post-fault operation due to open-thyristor or open-phase faults are demonstrated from experimental testing. The system is compared with the faulty performance of the conventional three-phase system. Furthermore, a fault-predicting double-dq exciter model is implemented and validated. In addition, the dangerous effects of a shorted-thyristor fault is investigated. Moreover, a ”skip firing” protection algorithm is briefly demonstrated for fast isolation of such faults, yielding nondestructive postfault recovery. The evidence shows that the dual-star exciter is a competitive choice for the future developments of fault-tolerant brushless exciters.

Keyword
Brushless exciters, double-star windings, dual-port electrical machines, 12-pulse thyristor bridges, open-phase faults, open-thyristor faults, shorted thyristors
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Research subject
Engineering Science with specialization in Science of Electricity
Identifiers
urn:nbn:se:uu:diva-333567 (URN)
Available from: 2017-11-15 Created: 2017-11-15 Last updated: 2017-11-20
8. Design and characterization of a rotating brushless PM exciter for a synchronous generator test setup
Open this publication in new window or tab >>Design and characterization of a rotating brushless PM exciter for a synchronous generator test setup
Show others...
2016 (English)In: Design and characterization of a rotating brushless PM exciter for a synchronous generator test setup / [ed] IEEE Xplore, 2016, 259-265 p.Conference paper, Published paper (Refereed)
Abstract [en]

This paper deals with the characterization and construction of a rotating brushless PM exciter intended for synchronous generator excitation purposes. Traditionally, PM exciters are used as pre-exciters in synchronous generator excitations systems. In order to reduce the number of components and to increase the step time response of the system, a PM exciter is designed as an outer pole PM machine, with permanent magnets on the stator and armature windings on the rotor. The exciter was constructed electrically and mechanically to be fitted into an in-house synchronous generator test setup. A finite element model of the exciter was validated with no-load measurements of voltages and magnetic flux densities. The exciter was then characterized with unsaturated and saturated parameters.

Keyword
Brushless exciters, rotating exciters, permanent magnet machines, machine design, voltage measurement
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-292798 (URN)10.1109/ICELMACH.2016.7732536 (DOI)000390884900037 ()9781509025381 (ISBN)
Conference
International Conference on Electrical Machines 2016 (ICEM'16), Lausanne, SWITZERLAND, SEP 04-07, 2016
Available from: 2016-05-09 Created: 2016-05-09 Last updated: 2017-11-20Bibliographically approved
9. Evaluation of different power electronic interfaces for control of a rotating brushless PM exciter
Open this publication in new window or tab >>Evaluation of different power electronic interfaces for control of a rotating brushless PM exciter
Show others...
2016 (English)Conference paper, Published paper (Refereed)
National Category
Engineering and Technology
Identifiers
urn:nbn:se:uu:diva-328394 (URN)
Conference
42nd annual conference of the IEEE Industrial Electronics Society
Available from: 2017-08-22 Created: 2017-08-22 Last updated: 2017-11-20
10. Active Current Sharing Control Method for Rotating Thyristor Rectifiers on Brushless Dual-Star Exciters
Open this publication in new window or tab >>Active Current Sharing Control Method for Rotating Thyristor Rectifiers on Brushless Dual-Star Exciters
(English)In: Article in journal (Refereed) Submitted
Abstract [en]

A new high speed response dual-star brushless rotating exciter has been recently proposed, which utilizes two rotating thyristor rectifiers in a hybrid-mode topology. However, dissymmetries tend to occur in large-scale apparatus, which ultimately results in an undesired unbalanced loading of the topology. Moreover, the topology provides a possibility for compensation via asymmetrical firing, which serves as a promising solution to be investigated. This letter proposes an active current sharing adjustment method between the parallel thyristor bridges. The method improves controllability and performance compared with the alternative ”skip firing” approach. In addition, the method serves as an alternative to the conventional current-balancing interphase transformer employed on the rectifier outputs for general large direct current applications.

Keyword
AC generator excitation, asymmetrical firing, hybrid-mode 12-pulse thyristor rectifier, brushless exciters
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Research subject
Engineering Science with specialization in Science of Electricity
Identifiers
urn:nbn:se:uu:diva-333568 (URN)
Available from: 2017-11-15 Created: 2017-11-15 Last updated: 2017-11-20
11. Initial Performance Tests of a Permanent Magnet Thrust Bearing for a Hydropower Synchronous Generator Test-Rig
Open this publication in new window or tab >>Initial Performance Tests of a Permanent Magnet Thrust Bearing for a Hydropower Synchronous Generator Test-Rig
2016 (English)Conference paper, Oral presentation with published abstract (Refereed)
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:uu:diva-284944 (URN)
External cooperation:
Conference
AIM 2016
Available from: 2016-04-19 Created: 2016-04-19 Last updated: 2017-11-20
12. Performance tests of a permanent magnet thrust bearing for a hydropower synchronous generator test-rig
Open this publication in new window or tab >>Performance tests of a permanent magnet thrust bearing for a hydropower synchronous generator test-rig
2017 (English)In: ACES Journal, Vol. 32, no 8, 704-711 p.Article in journal (Refereed) Published
Abstract [en]

Permanent magnets are an attractive material to be utilized in thrust bearings as they offer relatively low losses. If utilized properly, they have a long service lifetime and are virtually maintenance free. In this contribution, we communicate the results of the tests performed on a permanent magnet thrust bearing that was custom built and installed in a hydropower synchronous generator test-rig. Tridimensional finite element simulations were performed and compared with measurements of axial force. Spin down times and axial force ripple have also been measured. We found good correspondence between the measurements and the simulations.

National Category
Engineering and Technology
Identifiers
urn:nbn:se:uu:diva-328078 (URN)000410166500010 ()
Funder
SweGRIDS - Swedish Centre for Smart Grids and Energy StorageStandUp
Available from: 2017-08-16 Created: 2017-08-16 Last updated: 2017-12-11Bibliographically approved
13. Spin-down Losses and Vibration Analysis of a Flywheel Energy Storage System
Open this publication in new window or tab >>Spin-down Losses and Vibration Analysis of a Flywheel Energy Storage System
(English)In: Energies, ISSN 1996-1073, E-ISSN 1996-1073Article in journal (Refereed) Submitted
Abstract [en]

An integrated flywheel prototype designed to store 868 W h at 30 000 rpm with a novel radial flux core-less electric machine was constructed. The rotor was suspended contactlessly on active radial magnetic bearings and passive axial magnetic bearings. This paper describes the system in detail. The rotor was found to be weakly damped (even at standstill) and the magnetic bearings were used to specifically compensate particular eigenfrequencies. The levitation system was then used to experimentally evaluate the vibrations of the rotor, and compared to FEM studies. Spin-down tests were conducted to evaluate levitation system performance. 

Keyword
flywheel energy storage, spin-down losses, vibrations
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Research subject
Engineering Science with specialization in Science of Electricity
Identifiers
urn:nbn:se:uu:diva-319522 (URN)
Funder
StandUp
Available from: 2017-04-06 Created: 2017-04-06 Last updated: 2017-11-29

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