PhD projects Department of Electric Power Engineering

May 2014

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An overview over PhD Projects 2014

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Department of Electric Power Engineering Faculty of Information Technology, Mathematics and Electrical Engineering

Norwegian University of Science and Technology This report gives an overview of current PhD research projects at the Department of Electric Power Engineering. Currently 27 students are registered in our PhD program. This is a drop from 36 students last year after a steady growth for several years, reflecting the increased general interest in energy and electric power from renewable resources. The department has twelve professors, four associate professors, and five adjunct professors. The number of Postdocs has increased the last few years from zero to five. In addition to the scientific and administrative staff, the department houses a mechanical workshop and an electro technical laboratory. The research activity at the Department is mainly covered by the following tree fields:

• Power Systems • Electrical Power Technology • Energy Conversion

The PhD projects presented here focus on topics from all these areas. The research projects are both theoretical and practical and based on extensive use of our computer and laboratory resources. The projects are also influenced by our collaboration with industry and our neighbour institution SINTEF Energy Research AS. Since the PhD projects represent the main part of the professors’ research, this folder also gives an overview of the entire research activity at the Department. The nominal duration of PhD program is three years of full-time research, of which a half year is devoted to post graduate courses. A typical PhD project, however, lasts for four years, where the additional year is devoted to university/educational duties. For further information about the research projects presented, please contact the individual researcher given by name in this folder. For more information on previous projects, please contact the Department. NTNU, May 2014 Hans Kristian Høidalen Professor

Phd summary 2014

Name Title

Supervisor p

Aanensen, Nina Sasaki

Load Current Interruption in Air for 12/24 kV systems Arne Nysveen 1

Barrera C.R. Alexander

Multi-Domain Optimization Model for Evaluation of Power Density and Efficiency of Wind Energy Conversion Systems

Marta Molinas 2

Duong, Dinh Thuc

Voltage stability monitoring and control based on wide area measurement system

Kjetil Uhlen 3

Endegnanew, Atsede

Stability and control of multi-terminal HVDC transmission Kjetil Uhlen 4

Gebrekiros, Yonas T.

Reserve and Balancing Market Integration in Northern Europe

Gerard L. Doorman

5

Hoff, Bjarte Cascaded model predictive control of power electronic converters

Trond Østrem NUC Lars Norum

6

Holtsmark, Nathalie

Offshore Wind Energy Conversion using High Frequency Transformation and DC Collection

Marta Molinas 7

Kalemba, Lester

Multivariable Control and coordination of Facts Devices Kjetil Uhlen 8

Larsen, Camilla Thorrud

Long-term hydropower scheduling using stochastic dual dynamic programming (SDDP)

Gerard L. Doorman

9

Lindberg, Karen Byskov

The impact of Zero Energy/Emission Buildings on the energy system

Gerard L. Doorman

10

Lotfi, Abbas Transformer Modeling for Low and Mid Frequency Transients

Hans Kristian Høidalen

11

Olsen, Pål Keim Partial discharges in insulation material exposed to combined DC and AC voltage

Frank Mauseth 12

Preda, Traian Nicolae

Stability and Model Reduction of Active Distribution Grids Kjetil Uhlen 13

Ranaweera, Iromi

Energy Storage for Control of Distributed Photovoltaic Power Systems in the Smart Grid

Ole-Morten Midtgard

15

Røkke, Astrid Investigation of permanent magnet synchronous machines with fractional slot windings for use in renewable energy applications

Robert Nilssen 16

Sanchez A., Santiago

Identifying electrical instability in grids dominated by power electronics

Marta Molinas 17

See, Phen Chiak Development of Electricity Market Model Incorporating Offshore Grids and Offshore Wind Farms

Olav B. Fosso 18

Skar, Christian Modeling of the European power system for low emission scenarios

Gerard L. Doorman

19

Soloot, Amir Hayati

Switching Transient in Offshore Wind Farm Hans Kristian Høidalen

20

Støa, Bendik Modelling of Void Formation and PD Activity due to Mass Transport in Mass Impregnated HVDC Subsea Cables

Erling Ildstad 21

Tai Su, Vin Cent Integration of offshore DC grids and onshore AC power networks – Stability and Control

Kjetil Uhlen 22

Toh, Chuen Ling Investigate New Control Architecture for Future Advanced Power Electronics Converter

Lars Norum 23

Tønne, Erling Planning of the future smart and active distribution grids

Kjell Sand 25

Valavi, Mostafa Magnetic Forces and Vibrations in Wind Power Generators

Arne Nysveen 26

Zadeh, Mehdi Karbalaye

Stability Analysis of Distributed Multi-converter Systems under Non-ideal Electrical Conditions

Marta Molinas 27

PhD graduated from 1990

28

Nina Sasaki Aanensen Home Country: Norway Year of Birth: 1987 Email: nina.aanensen@elkraft.ntnu.no Home Page: www.ntnu.no/employees/nina.aanensen Master Degree: MSc Applied Physics and Mathematics, 2011 University: NTNU

Supervisor: Magne Runde Research Group: Electric Power Technology Co-Supervisor(s): Arne Nysveen Project: “Air Insulated Switchgear Technology”

Load Current Interruption in Air for 12/24 kV systems. The problem with using SF6 (sulphur hexafluoride) as insulating gas in high voltage equipment is the large contribution to the greenhouse effect. Therefore, it is desirable to replace the SF6 gas in load break switchgear with air. The challenge is to make the air insulated switchgear in the same geometrical dimensions as the old SF6 breakers, without compromising the current interruption capability. The main goal of this PhD research is to be able to understand the air flow interaction with the electric arc, and how the flow parameters influence the current breaking capability. Different contact and nozzle geometries will therefore be investigated. Using the newly built high current / switchgear laboratory at NTNU, experiments can be conducted with the possibility of changing one parameter at a time. There are currently two PhD students working on this project, myself and Erik Jonsson. In addition to the laboratory work, some computational simulations will be conducted as a supplement to the experimental results. Computational simulations can be a fast and inexpensive tool in the design process of a switchgear manufacturer. The “Air Insulated Switchgear Technology” project is financed by the Norwegian Research

Council with Magne Saxegaard from ABB as project manager.

1

Name: Atsede Gualu Endegnanew Home Country: Ethiopia Year of Birth: 1986 Email: atsede.g.endegnanew@ntnu.no Home Page: http://www.ntnu.no/ansatte/atsede.g.endegnanew

Master Degree: Msc in electric power engineering University: The Norwegian University of Science and Technology Graduation Year: 2010

Supervisor: Kjetil Uhlen Research Group: Power Systems Co-Supervisor(s): Salvatore D’Arco Project: OffshoreDC

Stability and control of multi-terminal HVDC transmission

There are plans for large-scale far offshore wind power plant development in the North Sea region in the future. Large and complex transmission system will be needed to transfer power from offshore wind farms to onshore connection points. Moreover, the transmission system will be used for exchange of power between the different power systems located around the North Sea. Due to the long distances and the large amount of power that will be transferred, the transmission system will consist of HVDC grid with multiple connection points. In order to use multi-terminal HVDC (MTDC) for large scale wind power connection and interconnection of AC systems, there is a need to develop solutions for a stable and secure design and operation of the grid.

The PhD research focuses on stability and control of multi-terminal HVDC grids. A system with both MTDC and detailed multi-machine ac grid will be used to study the dynamic interaction between existing ac grids and the dc transmission grid. Modal analysis study will be used to identify interactions between different modes in ac grids connected via MTDC. In addition, a research will be carried out on control of VSC-MTDC converters in order for the converters to participate and improve stability of AC grids. Illustration of the studied system is shown below.

Wind farm

AC grid-1 AC grid-2

2

ReneAlexanderBarreraCardenasHome Country:  Colombia                      

Year of Birth:          1982           

Email:                       rene.barrera@elkraft.ntnu.no 

Home Page:           http://www.ntnu.edu/employees/rene.barrera                             

Master Degree:     MSc Electrical Engineering, 2006 

University:               UIS ‐ Colombia                              

Supervisor:   Marta Molinas                

Research Group:    Electric Power Systems            

Co‐Supervisor(s):   Tor Arne Johansen                          

Project:   NOWITECH WP4                      

Multi‐DomainOptimizationModelforEvaluationofPowerDensityandEfficiencyofWindEnergyConversionSystems

The prospective development of the wind energy conversion systems (WECS) is mainly promoted by demand for higher efficiency and power density. These requirements can be satisfied through the use or development of new topologies, modulation strategies or new semiconductor technologies. The gain in performance improvement is reduced over time, once the new concept or technology has been established. After the basic concept has been adopted, a significant gain in performance can only be achieved by allocating the optimal values of design variables during the design process. In the other hand, by detecting the sensitivity of the system level performance on component parameters, the development of components could be adjusted for maximal impact on the system level.

So to achieve such an optimization first a complete model of the converter circuit must be set, including thermal and magnetic component models. This model could be based on analytical equations, on numerical simulations or on a combination of both. Based on WECS circuit model, an optimization for multiple objectives, efficiency and power density, will be performed. The optimization makes best use of all degrees of freedom of a design and also allows determining the sensitivity of the system performance based on technologies like measurement of the efficiency of the power semiconductors or properties of the magnetic core materials. Furthermore, different topologies can be easily compared and inherent performance limits can be identified.

This project is looking for developing a methodology of multi-domain design to optimize the power density and efficiency of the wind energy conversion system in offshore wind farms. Analytical approaches for designing the main functional elements of a wind energy conversion system will be described and arranged to a linear design process in a first step. Moreover, the linking of the component models, i.e. of the electric, magnetic and thermal design domains and an overall optimization of the respective design variables based on the linked models will be considered and including the coupling of the different domains. 

 WECS Based on medium frequency AC‐link 

 Pareto surface: Efficiency, Power Density and Power to mass ratio 

for WECS solution 10[MW] including parameter variation (Converter Topology, number of modules, AC‐link freq.) 

3

Dinh Thuc Duong Home Country: Vietnam

Year of Birth: 1979

Email: thuc.duong@ntnu.no

Home Page: www.ntnu.no/emploees/thuc.duong

Master Degree: MSc in Electrical Power Engineering, 2011

University: RWTH Aachen, Germany.

Supervisor: Prof. Kjetil Uhlen

Research Group: Electric Power Systems

Co-Supervisor(s): Prof. Olav B. Fosso

Project: Smart transmission grid operation and control (STRONgrid).

Voltage stability monitoring and control based on wide area

measurement system

Voltage stability remains one of the major stability issues in electrical power systems,

especially in those heavily loaded. The initiation of this phenomenon is the operating point

where the maximum power transferred from source to load is hit. Operation beyond this point

will lead to significant voltage drop while current continues increasing. Due to recent voltage

collapse incidents, more attention has been paid to this stability issue, presently focusing on

real time monitoring. However, online indicators of voltage stability and mitigating control

actions are still under investigation and require further work.

PMUPMU

PMU

PMU

This research has resulted in some approaches based on either only PMU measurements

or with information of the system topology to monitor voltage instability at load buses in real

time. The online prototype application has been developed and being tested with real time

data from transmission system.

In the next phase, preventive and corrective control will be examined. The main focus is

to develop online or near-online tools that can assist system operators in securing the system

when it is vulnerable to voltage instability due to high demand or after disturbances.

4

Yonas Tesfay Gebrekiros Home Country: Ethiopia Year of Birth: 1983 Master Degree: MSc. Electrical Engineering University: Delft University of Technology Graduation year: 2009

Research Group: Electric Power Systems

Supervisor: Gerard L. Doorman

Email: Yonas.gebrekiros@elkraft.ntnu.no Phone: +47 73594252 Home Page: http://www.ntnu.no/ansatte/yonas.gebrekiros Reserve and Balancing Market Integration in Northern Europe In the coming few years large scale integration of wind power is expected in the European power system. This is an appreciated progress as seen from the utilization of more renewable energy sources (RES) in the system. This, however, makes the operational planning, forecasting, balancing, and control of the system more challenging. This is due to the intermittency and unpredictability of wind and other RES: subsequently increasing the requirement of balancing reserves in the system. The increasing integration of European electricity markets is an indication towards the long term goal of establishing a single European electricity market. The prospect of exchange of balancing resources will further increase market efficiency. This happens, among others, as a result utilizing cheaper and abundant balancing resources. The thesis will focus on the implementation of market-based exchange of balancing services (procurement of reserves, optimal transmission capacity allocation for reserves exchange and implementation of real-time balancing).

5

Name Bjarte Hoff Home Country: Norway Year of Birth: 1983 Email: bho@hin.no Home Page: http://www.bhfoto.no http://www.ntnu.no/ansatte/bjarte.hoff

Bachelor Degree: Maritime electro automation University: Vestfold University College Graduation Year: 2008

Master Degree: Electrical Engineering University: Narvik University College Graduation Year: 2010

Supervisor: Research Group: Electro mechanical systems, Narvik University College (NUC) Co-Supervisor(s): Trond Østrem (NUC) and Lars Norum (NTNU) Project: Distributed energy systems

Cascaded model predictive control of power electronic converters

Introducing renewable energy as a source of electrical power allows a green environment friendly energy production, compared to fossil fuels. Most of the captured renewable energy cannot be directly connected to the grid, but through the use of power electronic converters.

These converters rely on a control algorithm to achieve good power quality and reduce losses to a minimum.

This thesis investigates the use of new control algorithms by utilizing the ever increasing speed of low-cost digital circuits and processors. Based on a detailed mathematical model, the objectives are to reduce losses, and improve the response and power quality. This will be achieved by a cascaded model predictive controller, never used before in this application.

6

Nathalie Holtsmark

Home Country: Norway Year of Birth: 1986

Master Degree: Electrical Engineering

University: NTNU

Graduation Year: 2010

Research Group: Energy Conversion

Supervisor: Marta Molinas

PhD Start: August 2010

Phone: +47 73594228 Email: Nathalie.Holtsmark@elkraft.ntnu.no Home Page: http://www.ntnu.no/ansatte/nathalie.holtsmark (NO) http://www.ntnu.edu/employees/nathalie.holtsmark (EN) Offshore Wind Energy Conversion using High Frequency Transformation and DC Collection

All wind parks commissioned, under construction or planned as of today (2014) emulate the onshore grid by featuring an internal AC grid with either AC or DC transmission to shore. It is not certain however that the onshore model is still superior when moved off shore. The alternative wind park considered in this work is the series connection of DC wind turbines with an HVDC transmission to shore as shown below. The main motivation for this configuration is that an expensive offshore platform with transformer, AC/DC or DC/DC converter is not required as a DC voltage high enough for transmission is build up by the series connection of turbines. Inside the nacelle, the conversion system is composed of a permanent magnet synchronous generator, a three phase AC-AC converter, a high frequency three-phase transformer for galvanic isolation and possibly voltage step up and finally a diode bridge rectifier for the AC-DC conversion. Three PhD students are working on this project, funded by the Norwegian Research Council and the RENERGI Program. The scope of my PhD work is the AC-AC converter in the conversion system of the wind turbine. The goal is to select the best converter topology in terms of efficiency, volume, thermal performance and make simulation models to demonstrate the operation and control of the selected converter in this wind park configuration. So far results show that the matrix converter, a direct AC-AC converter with no intermediate DC link with energy storage elements, performs well and a prototype is built for experimental verification, see picture.

7

Name:Lester.Kalemba Home Country: Zambia Year of Birth: 9.05.76 Email: lester.kalemba@ntnu.no Home Page:

Master Degree: Electric Power Engineering University: NTNU Graduation Year: 2011

Supervisor: Kjetil Uhlen Research Group: Power Systems Co-Supervisor(s): Morten Hovd Project: Stronggrid

Title: MULTIVARIABLE CONTROL AND COORDINATION OF FACTS DEVICES

Abstract: Power Systems are today increasingly stressed mainly due to increased interconnectivity and levels of power exchange, and continual increase in load. To ensure secure operation of this increasingly important infrastructure, the provision of adequate levels of control is necessary. This entails that controls for system stability augmentation, voltage regulation and enhancement of power transfer capabilities must be optimized and coordinated in time and space. FACTS devices can give the much needed additional controllability of various system phenomena. These devices must, however, be coordinated to optimize their utilization and to prevent adverse interaction, and thereby foster an overall improvement in system performance. The development of controllers that enhance system performance and reliability, as well as economics of existing transmission corridors, is, therefore, of great interest. In this PhD Research project we study multivariable control of FACTS and investigate their coordination, for the augmentation of power oscillation damping, transient stability enhancement, secondary voltage regulation and impact on power transfer. In the studies, possible interaction among control loops is examined with a view to optimize the overall control strategy for the overall benefit of improving system performance. The focus is on the use of Static Var Compensators (SVCs) controls, since this is the most widely used FACTS component.

8

Camilla Thorrud Larsen Home Country: Norway Year of Birth: 1980 Email: camilla.t.larsen@elkraft.ntnu.no Home Page: www.ntnu.no/emploees/camillla Master Degree: Siv.ing. Industrial mathematics University: NTNU

Supervisor: Prof. Gerard L. Doorman Research Group: Electric Power Systems Co-Supervisor(s): Birger Mo, Sintef Energy Research

Long-term hydropower scheduling using stochastic dual dynamic programming (SDDP) The objective of long-term scheduling from a global viewpoint is to find a hydro release policy which is coordinated with generation from other sources, to meet the electricity demand at minimum expected costs. For a local system (in a deregulated market), the objective will usually be to find a production schedule that maximize expected profits over the planning horizon. Long-term policies are important for price forecasting, generation scheduling, maintenance planning, investment- and expansion planning, as well as general power system analysis. Moreover, long-term decisions provide targets for the seasonal and short-term scheduling which concerns detailed operation of the hydro system. Long-term scheduling constitutes a multi-stage stochastic problem which in principle can be solved by stochastic dynamic programming (SDP). However, for a detailed model of a system consisting of several reservoirs, and thus many state variables, the dimension of the problem quickly explodes and becomes computationally intractable. One approach to the problem for which SDP can still be used is based on reservoir aggregation and depends on heuristics for disaggregation to address the multi-reservoir aspect realistically. Such models are frequently used by power market participants in the Nordic countries and the commercially available EMPS and EOPS models, developed at Sintef Energy Research, are based on this methodology. A different solution approach, called stochastic dual dynamic programming (SDDP), is a sampling-based approximation technique for solving multi-stage stochastic problems. This method eliminates the need to completely discretize the state space and allows for detailed modeling of multi-reservoir systems. SDDP relies on formal optimization, rather than heuristics and manual calibration, and is currently the state-of-the-art procedure to solve hydro-thermal scheduling problems. The overall objective of this project is to improve the performance of the long-term hydro-thermal scheduling models which utilize the SDDP-methodology. Primarily, the focus will be on the local model, i.e. for a system confined in a geographical area that can be covered by a single power balance equation, and typically owned by a single power producer. The goal is to develop a good and reliable model which will provide a useful tool for hydropower producers in the Nordic market. The main focus in this work will be on how to model and represent the stochastic input parameters in a best possible way, and which is in correspondence with the SDDP framework.

9

Karen Byskov Lindberg Home Country: Norway Year of Birth: 1978 Email: karen.lindberg @elkraft.ntnu.no Home Page: www.ntnu.no/emploees/karen.lindberg Master Degree: MSc Energy and Environmental Engineering, 2004 University: NTNU

Supervisor: Prof. Gerard Doorman Research Group: Electric Power Systems Co-Supervisor(s): Prof. Asgeir Tomasgard and Dr.-Ing. Igor Sartori Project: Part of the two FME-centres Zero Emission Buildings (ZEB) and Centre for Sustainable Energy Studies (CenSES), and co-funded by the Norwegian Water Resources and Energy Directorate (NVE).

The impact of Zero Energy/Emission Buildings on the energy system In the recast of the EU Directive on Energy Performance of Buildings (EPBD) it is stated that by end of 2020 all new buildings shall be “nearly zero energy buildings”. In Norway, the Norwegian white paper on climate efforts, ”Klimameldingen 2012”, stated that technical regulations (TEK) for buildings will be passive standard in 2015, and zero energy standard by 2020.

A zero energy (emission) building is a building which has low energy demand, and which has the capability of producing energy such that its net energy consumption (or net GHG emissions) on a yearly basis is zero. The definition may also be extended to including embodied energy (or GHG emissions) of the building, reflecting energy use (or GHG emissions) from the construction, material use and demolition of the building.

This thesis investigates the impact of introducing such high energy efficient and energy generating buildings, called ZEBs, into an energy system with a high share of intermittent renewable production. What challenges do we face when combining intermittent renewable production with a reduced, and perhaps less flexible, demand? The work is divided into three main parts:

(1) Forecasting hourly demand profiles, divided on heat and electricity, for normal and passive standard non-residential buildings in Norway (schools, offices, nursery homes, hospitals and hotels).

(2) Optimisation of four different ZEB-type buildings in order to find their hourly net electricity load towards the grid, based on the heat and electricity demand profiles found in (1).

(3) Analysing the net load profiles in the power market model, EMPS. Investigating the effects on price formation and Norway’s ability to capacity trading towards Europe.

10

Abbas Lotfi Home Country: Iran Year of Birth: 1983 Email: abbas.lotfi@elkraft.ntnu.no Home Page: http://www.ntnu.edu/employees/abbas.lotfi Master Degree: Electrical Engineering, 2006 University: University of Zanjan

Supervisor: Prof. Hans Kristian Høidalen Research Group: Electric Power Technology Co-Supervisor: Nicola Chiesa, PhD Project: NFR/SINTEF

Transformer Modeling for Low and Mid Frequency Transients The power transformer is an important component in all power systems and its operation is crucial for system reliability. Accordingly modeling of power transformer is very important that is widely and continuously done by researchers and engineers to be used in different levels of power system analysis. This PhD work is mainly aimed to discuss on Power Transformer modeling enabling simulation of low and mid frequency transients. Zero Sequence characteristic of transformers is one of the challenging subjects. At the first step, the works are concentrated on off-core flux path and modeling and extracting an equivalent circuit for this flux path. The figure below shows the equivalent circuit considered for this purpose.

This equivalent circuit is obtained from magnetic circuit of the flux path using duality transformation between magnetic and electric domain and will be only ZS part of complete dual circuit model of transformer. The parameters L1, L2 represent the gap between active-part and the tank; nonlinear Ltank and Rtank represent tank equivalent inductance and losses, respectively. Identification of these parameters by means of test results as well as by use of design information is of ongoing activities. Since the transformer structure is not symmetric from off-core flux path point of view, it is not straight forward to use 2D based methods for calculation of the mentioned parameters. For the A method is presented to calculate inductance and losses of the tank using only 2D-FEM Analysis. In order to identify the parameters through measurements, it is necessary to analytically formulate magnetic penetration properties of the tank material. Eddy currents and hysteresis are two challenging subjects regarding the tank material. The main idea is to introduce a so-called 𝐾𝛿 correction factor. This factor corrects the penetration depth of the material in the non-linear case compared to linear case. Some extra parameters appear for this formulation which should be identified in the parameter identification process.

11

Pål Keim Olsen Home Country: Norway

Year of Birth: 1982

Email: palkeim@ntnu.no

Home Page: www.ntnu.no/ansatte/pal.keim.olsen

Master Degree: MSc Electrical Engineering, 2008

University: NTNU

Supervisor: Ass. Prof. Frank Mauseth

Research Group: Electric Power Systems

Co-Supervisor: Prof. Erling Ildstad

Project: High Voltage AC and DC Subsea Cables for Offshore Wind Farms and Transmission Grids

Partial discharges in insulation materials exposed to combined DC and AC voltage HVAC subsea transmission is limited in terms of distance to shore: at about 50-70 km distance from shore the capacitive load of the subsea cables is too high, taking too much of the current carrying capability of the cables. It has been found that the HVDC system is economical for distances above 70 km from shore, and there is no practical limit in the distance from shore as for HVAC systems. The majority of cables already installed in AC and DC systems are mass impregnated cables, but there is a drive to use dielectric extruded cables for subsea applications. The PhD work focus on the electric degradation phenomena which can occur in electric equipment in an offshore HVDC network, during steady state operation and under faults. In particular, the effect of DC voltage with superimposed AC voltage on partial discharges in the insulation. Simulation of partial discharges in cavities under DC superimposed AC voltage and ageing under such conditions is studied through lab experiments. The main materials to be studied are PEEK, but the model should be relevant for other materials like XLPE for cables and epoxy for converter transformers, generators and switchgear. The results from the work can be used when developing qualification tests and design of long cables and other electric equipment used in offshore HVDC networks.

12

Traian Nicolae Preda Home Country: Romania Year of Birth: 1986 Email: traian.preda@ntnu.no Home Page: http://www.ntnu.edu/employees/traiannp

Master Degree: Power Engineering University: POLITEHNICA University of Bucharest Graduation Year: 2011

Supervisor: Kjetil Uhlen Research Group: Power Systems Co-Supervisor: Dag Eirik Nordgård Project: Optimal infrastructure for seamless integration of distributed generation

Stability and Model Reduction of Active Distribution Grids

In Norway most of the electricity is produced in large scale hydroelectric power plants (98.5%). But in the last years more distributed generation units (subsequently referred to as DG), such as small scale hydropower plants and wind turbines have started to be connected at the distribution system level. Most of these DG units are located in sites with relatively weak grids, low local load and long distances to the transmission system.

As increasing share of the DG leads to changes in the conventional power system structure, the functions of traditional centralized power plants (controlling and stabilizing the power system during faults, damping of power oscillations) must also be performed by DG units. Therefore, the aforementioned DG is expected to play a significant role as generation sources in the Norwegian power system. This will make the control, operation and modelling of the power system more complicated than before, presenting many new challenges in terms of power system stability concerns. This PhD-work is focusing on the integration of DG units into the future power systems from the perspective of power systems stability and grid code requirements.

In the first part, the work addresses important parts of the Active Distribution Grids challenge providing updated knowledge and practical guidelines regarding the proper DG integration under aforementioned technical challenges. Additional grid code requirements are proposed, when the integration of DG units in ADPSs is of concern, as:

1. Small scale hydro unit power plant modelling for small signal stability studies

2. External power system modelling for Low Voltage Fault Ride Through (LV-FRT) integration studies of DG units

3. Impact of voltage phase angle on LV-FRT integration studies of DG units

13

As the grade of DG penetration increases in many distribution grids, their representation as simple PQ buses it will be outdated. Therefore, simpler but still accurate models will be needed to be used by the TSOs, for transmission system purpose studies.

Therefore, the second part of this work investigates methods to produce dynamic equivalents of Norwegian particular ADPSs. The focus is primarily on dynamic equivalents used for small signal and transient stability studies.

To produce dynamic equivalents of ADPSs for small signal stability, two classical methods are investigated:

1. Slow coherency concept

2. Synchronic Modal Equivalencing (SME)

The PhD work proposes two new methods to compute the dynamic equivalents for offline studies, which are based on:

1. model parameter identification

2. state space identification

As usually, the topology of distribution grids it is not well documented by the TSOs the methods used for only studies are sometimes useless. Therefore, measurement based methods which can be used for the modal identification and reduction of the ADPSs will be investigated, as Eigensystem Realization Algorithm (ERA).

In terms of dynamic equivalents used for transient stability, the PhD work investigates the coherency concept for the case of small scale hydro units and the influence of the automatic voltage regulator on this concept. Classical methods (Ward and Extended Ward) to produce dynamic equivalents for transient stability are used.

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ExternalPower System

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Vx A x B uy C x D u

State Space Representation

Thevenin Equivalent

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Figure 1. Model Reduction of an Active Distribution Grid for Small Signal Stability.

14

Iromi Ranaweera Home Country: Sri Lanka Year of Birth: 1986 Email: iromi.ranaweera@ntnu.no Home Page: www.ntnu.no/ansatte/iromi.ranaweera

Master Degree: MSc in Renewable Energy University: University of Agder Graduation Year: 2013

Supervisor: Ole-Morten Midtgård Research Group: Electric Power Systems Co-Supervisor(s): Kjetil Uhlen Project: NTNU

Energy Storage for Control of Distributed Photovoltaic Power Systems in the Smart Grid One of the major goals of the smart grid is, accommodating a large number of distributed generators in the low voltage distribution network. PV technology is one of the fastest growing renewable technologies today, not only in large scale applications, but as small distributed generators connected to the LV distribution grid. However, when the penetration level of PV power increases, new non-traditional problems start to arise in the distribution grid. The increasing number of distributed PV generators in the LV network can create reverse power flow in the distribution network resulting overvoltage along the distribution feeders during high PV generation hours. This may impose constraints on the installation of PV systems in the distribution system. Not only that, the intermittency of solar irradiance can result step changes in the power output of the PV plant with significant magnitudes which can create voltage fluctuations at the point of common coupling. In addition to these, higher PV penetration can create problems such as; overloading the distribution feeder, difficulty in voltage control in the distribution line due to reverse power flow, and poor operating power factor of the transformer. One of the feasible solutions to the problems caused by the high PV penetration and intermittent generation of distributed PV generators is the use of energy storage. Small scale energy storage technologies include batteries, super capacitors, flywheels, hydrogen technology (electrolyzer and fuel cell) and super conducting magnetic energy storage. However, recently battery type energy storage has become more popular in residential applications with PV generators.

This research is aimed at developing effective control methods and novel operating strategies, for control of grid connected distributed photovoltaic power systems using energy storage units in the smart grid framework. The main focus will be given to mitigating overvoltage problems and voltage fluctuations at PCC, PV power output ramp rate control, and time of use of energy storage in response to the electricity price for increasing the economic benefits from the PV/battery system. Both coordinated control methods for distributed PV/battery units and the local control method for individual PV/battery/converter units will be investigated.

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Astrid Røkke Home Country: Norway Year of Birth: 1982 Email: astrid.roekke@ntnu.no Home Page: www.ntnu.ni/employees/astridr

Master Degree: MSc Electrical engineering University: NTNU Graduation Year: 2007

Supervisor: Robert Nilssen Research Group: Energy conversion Co-Supervisor(s): Arne Nysveen Project: SmartMotor

Investigation of permanent magnet synchronous machines with fractional slot windings for use in renewable energy applications The main purpose is to identify the best configurations of PM machines for renewable energy converters. Special focus within renewables is on marine current turbines.

Tidal power is an area of development, predicted to produce a significant amount of energy within the next few decades. The aim of the project is to develop methods and tools to optimize Permanent Magnet generators for low speed high torque tidal applications. Advanced numerical analysis software will be used to model coupled problems including thermal, magnetic, electric and mechanical quantities. This will form a new basis for optimization. The analysis will take into account 3D phenomena, time dependency and motion.

The optimization should take into account the practical constraints for a set of relevant tidal power cases. The optimization will be conceptual, meaning that choices such as the selection of winding type and layout are a part of the synthesis. The variable space should be as wide as possible, avoiding sub-optimal solutions. The optimization should also handle several objectives such as efficiency, cost and lifetime.

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Santiago Sánchez Acevedo Home Country: Colombia Year of Birth: 1982 Email: Santiago.sanchez@elkraft.ntnu.no Home Page: Master Degree: MSc Electrical Engineering, 2008 University: Universidad Tecnológica de Pereira

Supervisor: Marta Molinas Research Group: Electric Power Systems Co-Supervisor(s): Tor Arne Johansen Project:

Identifying electrical instability in grids dominated by power electronics The main purpose of the research is to identify the phenomena that affect the stability of a micro-grid fed by multiple renewable energy sources and non-renewable energy sources with power electronics systems. The future distribution systems will be affected by the bidirectional current flow due to the distributed generation growth. Hence, the classical stability tools and classical system dynamic assumptions are not sufficient to design the new grids dominated by power electronics. The grids will present AC, DC or hybrid AC-DC behavior in multiple nodes. The stability analysis can be realized in the different domains (i.e. time/frequency). Where a criteria is used to identify the instability occurrence. As final objective the project will establish a guideline for the design of a micro-grid that ensures a safe operation region.

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Phen Chiak See Home country: Malaysia Year of birth: 1983 Email: phen.chiak.see@elkraft.ntnu.no Home page: Master Degree: ME. Mechanical Engineering, 2008 University: University of Technology, Malaysia Supervisor: Olav Bjarte Fosso Research group: Electric Power Systems Co-supervisor(s): Marta Molinas Project: NOWITECH WP4 Development of Electricity Market Model Incorporating Offshore Grids and Offshore Wind Farms Integrated electricity market is a key factor for the successful implementation of single European electricity market. It allows bid made in the wholesale electricity market of interconnected countries to be assessed by their counterparts. This significantly enhances the competition and the security of supply in the region. Nevertheless, it requires good liquidity of physical flow of electricity power as well as the availability of synchronized (trade-friendly) financial rules.

Although the creation of synchronized financial rules is possible between the participating countries, the development of the integrated electricity market is often limited by the existence of transmission constraints. Often, efficient congestion management is a challenge faced by stakeholders in deregulated electricity market. In this regard, we study the electricity market by taking into account the inter-connection of countries that implement different market rules. Also, we investigate how offshore wind farms and offshore grids help in lightening the transmission congestion issues that hinders the liquidity in power flow.

We construct a computer program for simulating the integration of cross-continental electricity market in the North Sea countries. The program is called Power System Economics and Electricity Market (PSEEM), which partly embraces codes and concepts previously developed by Olav Fosso. It is intended for demonstrating the interactions between electricity markets that implement different pricing rules, and how this influences the global benefit (price convergence, intuitiveness of flow, etc.) in the connected countries.

The PhD work is expected to complete by July 31, 2015. We thank NOWITECH and NTNU for all supports given in this research.

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Christian Skar Home Country: Norway Year of Birth: 1985 Email: christian.skar@ntnu.no Home Page: www.ntnu.no/employees/christian.skar Master Degree: MSc Applied Mathematics, 2010 University: NTNU

Supervisor: Gerard Doorman Research Group: Electric Power Systems Co-Supervisor(s): Asgeir Tomasgard (IØT) Project: LinkS

Modeling of the European power system for low emission scenarios Research objectives: Develop a model for calculating optimal long-term expansion of the European power system taking into account climate mitigation strategies given by the Global Climate Assessment Model (GCAM). The capacity expansion model will provide results such as country-wise investments in production capacity, investments in transmission capacity between countries and a resulting production mix for aggregate technologies. These results will be used to suggest optimal ways of implementing GCAM strategies along with related costs. In addition we aim at identifying possible problematic features in GCAM scenarios, for instance scenarios where high RES penetration result in much unserved demand. This can then be used to adjust assumptions in GCAM simulations. Figure 1 shows some sample results from the capacity expansion model when run for different GCAM policy scenarios.

Figure 1: Expansion model results for GCAM policy scenarios

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Amir Hayati Soloot Home Country: Iran

Year of Birth: 1984

Email: amir.h.soloot@elkraft.ntnu.no

Home Page: http://www.ntnu.edu/employees/amir

Master Degree: Electric Power Engineering, 2009

University: Iran Uni. of Sci. and Tech. (IUST)

Supervisor: Hans Kristian Høidalen

Research Group: Electric Power Technology

Co-Supervisor(s): Bjørn Gustavsen

Project: NOWITECH WP4

Switching Transient in Offshore Wind Farm

In order to connect the offshore wind turbines, large undersea cable connections are required. Since each wind turbine has a step-up transformer, a row of Offshore Wind Farm (OWF) composed of cable-transformer sections which are linked in series. Wind Turbine Transformers (WTTs) can be exposed to dielectric failures, internal insulation damage as well as external one due to switching overvoltages

The aims of this PhD study are:

1- Study and simulation of switching transient phenomena in a row of OWF. The focus is on the potential of resonance overvoltage on WTT terminals within energization for various OWF configurations. The effect of protective devices such as surge arresters and RC filters are also investigated.

2- Development of the High Frequency (HF) modelling of WTTs with available winding designs based on RLC latter model and the analysis of resonance overvoltages along transformer winding. A 500 kVA transformer equipped with layer, disc and pancake windings and voltage taps along the windings is designed and manufactured to validate the HF model of WTT with experiments.

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Bendik Støa Home Country: Norway Year of Birth: 1987 Email: bendik.stoa@ntnu.no Home Page: www.ntnu.no/employees/bendik.stoa Master Degree: MSc Industrial Mathematics, 2012 University: NTNU

Supervisor: Erling Ildstad Research Group: Electric Power Technology Co-Supervisor(s): Magne Runde Project: Load cycling and radial mass flow in mass impregnated HVDC subsea cables

Modelling of Void Formation and PD Activity due to Mass Transport in Mass Impregnated HVDC Subsea Cables Mass impregnated HVDC cables have been, and still remains, the state-of-the-art technology for transmission of large amounts of energy over long sea crossings. During a certain part of the type tests for such cables, in the cooling phase of the load cycling procedure, a reduction of dielectric strength of up to 50% has been reported. It is suggested that breakdowns under these conditions is caused by discharges in gas-filled cavities that has formed and grown in the insulation. The insulation consists of around 200 layers of paper tapes that are wound around the conductor, with small butt gap channels between them. After being applied and dried, the papers are impregnated with a high-viscosity mineral-oil based compound. Then a lead sheath is extruded onto the insulation, and it is cooled down to room temperature. During this process the oil will contract so that it is not able to fill all the available space, and cavities are formed. Loading the cable will in turn expand the oil, leading to radial flow of oil when all the cavities are filled. The goal of the PhD project is to develop a mathematical model describing radial flow in mass impregnated HVDC cable insulation. The model will, together with experimental investigations on full-scale cable samples, be used to increase our understanding of the processes governing the dielectric strength of such insulation. Hopefully, the work will contribute to better exploitation of existing cables, and assist in the design of new projects.

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My Name: Vin Cent Tai (Su) Home Country: Malaysia Year of Birth: 1983 Email: vin.tai@elkraft.ntnu.no Home Page: www.ntnu.no/emploees/vinct Master Degree: MSc Aerospace Engineering, 2009 University: Brunel University

Supervisor: Kjetil Uhlen Research Group: Electric Power Systems Co-Supervisor(s): Marta Molinas Project: OffshoreDC WP5

Integration of offshore DC grids and onshore AC power networks – Stability and Control

Offshore wind power has become one of the major focuses of many researchers and energy companies worldwide in tapping the energy from the renewable source. There has been a lot of research on the voltage-source based high-voltage direct current (VSC-HVDC) technology in recent years. This technology has proven to be the best solution for long distance power transmissions. It enables the wind energy generated offshore to be transmitted to the onshore power networks effectively. However, the integration of larger and larger amount of wind power has posed a challenge for power system planning and operation. For instance, the injection of large amount of offshore wind power tends to increase the rate of the frequency change in the mainland system, which makes the system unstable. Many research have studied the VSC-HVDC technology in terms of its impact on the stability of AC power systems, little has been done in the way of multi-terminal VSC-HVDC (MT-VSC-HVDC). As the power system grows larger and larger, the problem of stability is becoming more and more important. In view of this, the research work therefore is intended to increase the understanding of the static and dynamic behaviors of the multi-area power network interconnected by MT-VSC-HVDC systems. As different configuration can result in different requirements for system control, protection, and management, detailed analysis on different system configurations will also be carried out. Various offshore HVDC grid topologies (meshed and radial) will be investigated, together with various faults scenarios and their impacts on both the off-shore and on-shore AC grids. Control strategies that would improve the grids' stability and mitigate faults will also be proposed.

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Chuen Ling Toh Home Country: Malaysia Year of Birth: 1979 Email: chuen.ling.toh@elkraft.ntnu.no Home Page: --

Master Degree: MSc Electrical Engineering University: Universiti Teknologi Malaysia Graduation Year: 2005

Supervisor: Research Group: Lars E. Norum Co-Supervisor(s): -- Project: Integration of electrical power, propulsion and control in future energy efficient marine power system (Advanced control of PE converter)

Investigate New Control Architecture for Future Advanced Power Electronics Converter

Different range of power converters will be highly demanded due to the fast emerging of distributed generation in recent years. In order to shorten the development cost and time, Power Electronic Building Blocks (PEBBs) is highly recommended for future power converter design. Complex converter can be integrated easily with standard PEBBs to meet high power density. However, with the increasing number of PEBBs, the conventional control interface between the controller and PEBBs will become much more complex. A large number of wires will be required to deliver the control, measurement and status signals. Therefore, distributed control has been introduced to simplify the wiring system. The exchanged information will be packed into data frame format and transmit within the ring. However, data transmission delays must be accurately compensated to ensure all PEBBs able to initialize new duty cycle simultaneously. Besides, communication cable redundancy must be enabled to increase the ring reliability.

This research has first studied and investigated the prospect to implement an established industrial network for internal monitoring and control of a power converter. A set of basic communication requirements has been developed to evaluate some of the potential industrial networks. As a result, EtherCAT is recognized as the most potential control network in this research. It maintains approximately ± 20 ns synchronization jitter and supports single fault tolerant with its cable redundancy. Then, PEBB prototype is built with EtherCAT FB1130 Piggy Back Controller board couples as Slave Communication Controller. This thesis has also proposed a simple redundancy controller for cascaded multilevel converter. When PEBB failure is detected, this controller will either swap the backup PEBB with the defective unit or it will degrade the system level automatically.

A Modular Multilevel Converter (MMC) with 2 kHz switching frequency is setup for experiment verification. MMC’s power cells are enhanced with PEBB prototypes which offer onboard sensor measurements, data conversions and communication interface. Xilinx Zynq ZC702 Evaluation board is employed as a master controller. Level Shifted Pulse Width Modulation and capacitor voltage balancing control schemes are implemented within the Programmable Logic of XC7020. The feasibility of the MMC control strategies and the redundancy controller has been fully validated.

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Three ring behavioral tests are conducted to study the MMC performance by assuming EtherCAT network is used for internal monitoring and control of the converter. The results prove that MMC will operate normally if all PEBBs manage to initialize new duty cycle simultaneously with low jitter (approximately ± 20 ns).

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Erling Tønne Home Country: Norway Year of Birth: 1965 Email: erling.toenne@nte.no Home Page: Master Degree: MSc Electric Power Engineering, 1991 University: NTNU

Supervisor: Kjell Sand Research Group: Electric Power Systems Co-Supervisor(s): Jan A Foosnæs Project: Active distribution grids – concepts, architecture and functionality

Planning of the future smart and active distribution grids The electric power system is undergoing a profound change driven by a number of needs. There’s the need for environmental compliance and energy conservation. We need better grid reliability while dealing with an ageing infrastructure. We need improved operational efficiencies and customer service. The changes that are happening are particularly significant for the electricity distribution grid, where “blind” and manual operations, along with the electromechanical components, will need to be transformed into a “smart grid.” This transformation will be necessary to meet environmental targets, to accommodate a greater emphasis on demand response, and to support distributed generation, electric vehicles and storage capabilities. These needs and changes present the power industry with the biggest challenge it has ever faced. On one hand, the transition to a smart grid has to be evolutionary to keep the lights on; on the other hand, the issues surrounding the smart grid are significant enough to demand major changes in power systems operating philosophy. The Norwegian distribution networks have been developed over many years and have a relatively small amount of active elements, such as generators and demand side management. They are instead dominated by passive elements, principally uncontrolled loads. The focus on integration of renewable energy sources into the electricity system leads to a significant growth in the amount of distributed generation (DG) in the system. The loads will become more dynamic and controllable due to more active response from customers and the expected large introduction of electronic control and regulation systems. At the same time the introduction of advanced metering systems (AMS, smart meters) will provide the network owner with a lot more data.

The objective of my PhD-work is to find, test out and adapt methods for power system planning for the next generation active distribution grid. I am focusing on methods using scenarios and use-cases.

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Mostafa Valavi Home Country: Iran Year of Birth: 1985 Email: mostafa.valavi@elkraft.ntnu.no Home Page: www.ntnu.no/emploees/mostafa.valavi Master Degree: MSc Electrical Engineering, 2010 University: University of Tehran

Supervisor: Prof. Arne Nysveen Research Group: Electric Power Technology Co-Supervisor(s): Prof. Robert Nilssen Project: NOWITECH WP2

Magnetic Forces and Vibrations in Wind Power Generators In direct-driven permanent magnet (PM) generator systems, gearbox is eliminated and

maintenance works can be reduced substantially. This is a clear advantage particularly in offshore wind farms. In direct driven PM generators, the nominal speed is very low, leading to very high number of poles and very large diameter. In this application, utilizing concentrated windings and using fractional slot PM machines can be very advantageous. In this type of machines, it is possible to keep the number of slots relatively low. PM machines with concentrated windings have several significant advantages over the machines with distributed windings, such as high efficiency, low cogging torque, short end-windings and manufacturing advantages. However, the vibration level in machines with non-overlapping concentrated windings can be significantly higher than conventional machines. It is mainly due to presence of low order harmonics in the radial magnetic forces. In addition, machines with high number of poles have a large diameter and short stator length and therefore moderate mechanical stiffness. Due to the mentioned facts, it is important to investigate magnetic forces and vibration in direct-driven PM generators. In this project, the first step is to calculate and harmonic analysis of magnetic flux density in the airgap of the machine using finite element method. Radial and tangential magnetic forces can be computed using Maxwell’s stress tensor. Radial magnetic forces are the main cause of the magnetic vibration in electrical machines. Influence of some design parameters and working conditions on distribution of forces is investigated. A structural finite element analysis is then performed to predict the vibration spectrum. Vibration measurement has been done on a 50kW low speed PM generator in Wind Lab at the Department of Electrical Power Engineering. Investigation of forces and vibrations in radial flux machines, first part of the project, is finished and the second part with focus on multiple airgap machines has been just started.

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Mehdi Karbalaye Zadeh Home Country: Iran Year of Birth: 1982 Email: mehdi.zadeh@ntnu.no Home Page: http://www.ntnu.edu/employees/mehdi.zadeh

Master Degree: MSc in Electrical Engineering, 2010 University: University of Tehran, Tehran, Iran

Supervisor: Marta Molinas Research Group: Electric Power Systems Co-Supervisor: Kjetil Uhlen Project: Smart Grid

Stability Analysis of Distributed Multi-converter Systems under Non-ideal Electrical Conditions

Power system stability is the most important pre-requisite when designing electrical grids integrated with a large number of power electronics converters. Due to their non-linear and complex characteristics, modeling and stability analysis of such systems is complex. This research focuses on the development of a stability analysis tool that can accurately capture the potential phenomena linked to the interaction between converter embedded control algorithm and the characteristics of the electrical grid. This interaction will be investigated under non-ideal regimes in which power electronics converters will most likely operate in the electrical grids of the future. Performance of the grid under such influences is investigated during normal and emergency conditions to assess to what extent they can lead to instability of the system and consequent system outage. Considering the complex and nonlinear characteristics of power electronics dominated systems, the stability analysis is approached by the nonlinear system dynamics. Bifurcation theory is used in order to identify slow scale and fast scale instabilities as well as chaotic behavior of the system. In this context, the non-linear impact of the control architecture on the stability of PWM converters is modeled and analyzed based on nonlinear criteria. The ultimate goal is to develop a stability analysis tool that can be used for defining design specifications of individual converters that can ensure the stability of the system.

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PhD graduated at Department of Electric Power Engineering, NTNU, from 1990

Year Name Title Supervisor 2014 Jonsson, Erik Load Current Interruption in Air for Medium Voltage Ratings Runde, Magne

Kile, Håkon Evaluation and Grouping of Power Market Scenarios in Security of Electricity Supply Analysis

Uhlen, Kjetil

2013 Gjerde, Sverre Skalleberg

Analysis and Control of a Modular Series Connected Converter for a Transformerless Offshore Wind Turbine

Undeland, Tore

Vrana, Til Kristian System Design and Balancing Control of the North Sea Super Grid Fosso, Olav Bjarte

Larsen, Pål Johannes Energy Savings in Road Lighting Correct Lighting at all times and every condition

Hansen, Eilif Hugo

Aigner, Tobias System Impacts from Large Scale Wind Power Gjengedal, Terje Fosso, Olav Bjarte

Nguyen, Dung van Experimental studies for streamer phenomena in log oil gaps Høidalen, Hans Kr.

Jafar, Muhammad Transformer-Less Series Compensation of Line-Commutated Converters for Integration of Offshore Wind Power

Molinas, Marta

Torres Olguin, Raymundo

Grid Integration of Offshore Wind Farms using Hybrid HVDC Transmission Control and Operational Characteristics

Undeland, Tore

Wei, Yingkang Propagation of Electromagnetic Signal along a Metal Well in an inhomogeneous Medium

Norum, Lars

2012 Yordanov, Georgi Hristov

Characterization and Analysis of Photovoltaic Modules and the Solar Resource Based on In-Situ Measurements in Southern Norway

Norum, Lars

Haileselassie, Temesgen Mulugeta

Control, Dynamics and Operation of Multi-terminal VSC-HVDC Transmission Systems

Uhlen, Kjetil

Abuishmais, Ibrahim SiC Power Diodes and Junction Field-Effect Transistors Undeland, Tore

Zhang, Shujun Percussive Drilling Application of Translation Motion Permanent Magnet Machine

Nilssen, Robert

Ruiz, Alejandro Garces Design, Operation and Control of Series-connected Power Converters for Offshore Wind Parks

Molinas, Marta

Jaehnert, Stefan Integration of Regulating Power Markets in Northern Europe Offshore Wind

Doorman, Gerard

Tesfahunegn, Samson Fuel Cell Assisted Photo Voltaic Power Systems Undeland, Tore

Farahmand, Hossein Integrated Power System Balancing in Northern Europe Models and Case Studies

Doorman, Gerard

Suul, Jon Are Control of Grid Integrated Voltage Source Converters under Unbalanced. Conditions – Development of an On-line Frequency-adaptive Virtual Flux-based Approach

Undeland, Tore

2011 Marvik, Jorun Irene Fault localization in medium voltage distribution networks with distributed generation

Høidalen, Hans Kr

28

Krøvel, Øystein Design of Large Permanent Magnetized Synchronous Electric Machines – Low Speed, High Torque Machines – Generator for Direct Driven Wind Turbine –Motor for Rim Driven Thruster

Nilssen, Robert

Chen, Anyuan Investigation of PM machines for downwhole applications Nilssen, Robert

2010 Chiesa, Nicola

Power Transformer Modeling for Inrush Current Calculation

Høidalen, Hans Kr. Danielsen, Steinar Electric Traction Power System Stability. Low-frequency

interaction between advanced rail vehicles and a rotary frequency converter

Fosso, Olav Bjarte

Nordgård, Dag Eirik Risk Analysis for Decision Support in Electricity Distribution System Asset Management

Wangensteen, Ivar

Greiner, Christopher Johan

Sizing and Operation of Wind-Hydrogen Energy Systems Gjengedal, Terje

2009 Eek, Jarle Power System Integration and Control of Variable Speed Wind Turbines

Gjengedal, Terje

Kulka, Arkadiusz Sensorless Digital Control of Grid Connected Three Phase Converters for Renewable sources

Undeland, Tore

Guidi, Giuseppe Energy Management Systems on Board of Electric Vehicles, Based on Power Electronics

Undeland, Tore

2008 Pedersen, Per Atle Forces Acting on Water Droplets in Electrically Energized Oil Emulsions; Observations and Modelling of Droplet Movement Leading to Electrocoalenscence

Ildstad, Erling

Østrem, Trond Reliable Electric Power Conversion for Connecting Renewables to the Distribution Network

Norum, Lars

Skjellnes, Tore Digital Control of Grid Connected Converters for Distributed Power Generation

Undeland, Tore

Næss, Bjarne Idsøe Operation of Wind Turbines with Doubly Fed Induction Generators During and After Line Voltage Distortions

Undeland, Tore

Belsnes, Michael M. Optimal Utilization of the Norwegian Hydropower System Fosso, Olav Bjarte

Helseth, Arild Modelling Reliability of Supply and Infrastructural Dependency in Energy Distribution systems

Holen, Arne T.

2007 Di Marzio, Giuseppe Secure Operation of Regional Electricity Grids in Presence of Wind Power Generation

Fosso, Olav Bjarte

Gullvik, William

Modeling, Analysis and Control of Active Front End (AFE) Converter

Norum, Lars Andreassen, Pål Digital Control of a Zero Voltage Switching Inverter for distributed

Generation of Electrical Energy Undeland, Tore

Hoff, Erik Stjernholm

Status and Trends in Variable Speed Wind Generation Topologies Norum, Lars

Løken, Espen Multi-Criteria Planning of Local Energy Systems with Multiple Energy Carriers

Holen, Arne T.

Ericson, Torgeir Short-term electricity demand response Finden, Per

Mauseth, Frank Charge accumulation in rod-plane air gap with covered rod

Nysveen, Arne

2006 Maribu, Karl Magnus

Modeling the Economics and Market Adoption of Distributed Power Generation

Wangensteen, Ivar

Catrinu, Maria Decision-Aid for Planning Local Energy Systems. Application of Multi-Criteria Decision Analysis

Holen, Arne T

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2005 Hellesø, Svein Magne Dynamic analysis and monitoring of power transmission cables using fibre optic sensors

Runde, Magne

Lund, Richard Multilevel Power Electronic Converters for Electrical Motor Drives Nilsen, Roy

Bjerkan, Eilert High Frequency Modeling of Power. Transformers - Stresses and Diagnostics

Høidalen, Hans Kr

Vogstad, Klaus-Ole A system dynamics analysis of the Nordic electricity Market: The transition from fossil fuelled toward a renewable supply within a liberalized electricity market

Faanes, Hans H.

2004 Øvrebø, Sigurd Sensorless control of Permanent Magnet Synchronous Machines Nilsen, Roy

Kristiansen, Tarjei Risk Management in Electricity Markets Emphasizing Transmission Congestion

Wangensteen, Ivar

Korpås, Magnus Distributed Energy Systems with Wind Power and Energy Storage Holen, Arne T.

2003

Botterud, Audun Long Term Planning in Restructured Power Systems: Dynamic Model- ling of Investments in New Power Generation under Uncertainty

Wangensteen, Ivar

Ettestøl, Ingunn

Analysis and modelling of the dynamics of aggregate energy demand

Wangensteen, Ivar

2002 Kolstad, Helge Control of an Adjustable Speed Hydro Utilizing Field Programmable Devices

Nilsen, Roy

Norheim, Ian Suggested Methods for Preventing Core Saturation Instability in HVDC Transmission Systems

Undeland, Tore

Warland, Leif A Voltage Instability Predictor using Local Area Measurements. VIP++

Holen, Arne t.

Ruppert, Christopher Thermal Fatigue in Stationary Aluminium Contacts Runde, Magne

2001 Larsen, Tellef Juell Daily Scheduling of Thermal Power Production in a Deregulated Electricity Market

Wangensteen, Ivar

Kleveland, Frode Optimum Utilization of Power Semiconductors in High-power High- frequency Resonant Converters for Induction Heating

Undeland, Tore

Myhre, Jørgen Chr. Electrical Power Supply to Offshore Oil Installations by High Voltage Direct Current Transmission

Undeland, Tore

2000 Oldervoll, Frøydis Electrical and thermal ageing of extruded low density polyethylene insulation under HVDC conditions

Ildstad, Erling

Doorman, Gerard Peaking capacity in Restructured Power Systems Faanes, Hans H. Øyvind Skarstein

Hystad, Jan Transverse Flux Generators in Direct-driven Wind Energy converters

Nilssen, Robert

Pleym, Anngjerd EMC in Railway Systems. Coupling from Catenary System to Nearby Buried Metallic Structures.

Olsen, Karstein J.

1999 Gjerde, Oddbjørn Systemanalyser av skipselektriske anlegg Olsen, Karstein J.

Evenset, Gunnar Cavitation as a Precursor to Breakdown of Mass-Impregnated HVDC Cables

Ildstad, Erling

Hvidsten, Sverre Nonlinear Dielectric Response of Water Treed XLPE Cable Insulation

Ildstad, Erling

Pálsson, Magni Tor Converter control design for Battery Energy Storage systems applied in autonomous wind/diesel systems

Faanes, Hans H.

Warland, Geir Flexible transfer limits in an open power market. Congestion versus risk of interruption.

Faanes, Hans H.

30

1998 Hans Kristian Høidalen

Lightning-induced overvoltage in low-voltage systems. Sletbak, Jarle

Selvik, Eirik Information models as basis for computer-aided tools. Arnesen, Odd

Huse, Einar Ståle Power generation scheduling A free market based procedure with reserve constraints included.

1997 Bjørn Harald Bakken Technical and economic aspects of operation of thermal and hydro power systems.

Faanes, Hans H.

Ole-Morten Midtgård Construction and assessment of hierarchal edge elements for three- dimensjonal computations of eddy currents.

Nilssen, Robert

Qing Yu Investigation of dynamic control of a unified power flow controller by using vector control strategy.

Norum, Lars

1996 Gerd Hovin Kjølle Power supply interruption costs: Models and methods incorporating time dependent patterns.

Holen, Arne T.

Tom Fagernes Nestli Modelling and Identification of Induction Machines Norum, Lars

Bjørn Sanden XLPE cable insulation subjected to HVDC stress. Space charge, conduction and breakdown strenth

Ildstad, Erling

Gisle Johannes Torvetjønn

Switchmode Powersupplies Optimum topologies and magnetic components

Undeland, Tore

1995 Lars Arne Aga A Laboratory Platform for Theoretical and Experimental Research on Rotor Flux Oriented Control of Motor Drives.

Norum, Lars

Knut Styve Hornnes A Model for Coordinated Utilization of Production and Transmission Facilities in a Power System Dominated by Hydropower

Faanes, Hans H.

Rolf Ove Råd Converter Fed Sub Sea Motor Drives Raphael, Henry

1994 Snorre Frydenlund A study of voltage stresses in ARC furnace transformers due to switching operations

Nilssen, Robert

Anne Cathrine Gjærde Multifactor Ageing of Epoxy - The Combined Effect of Temperature and Partial Discharge

Sletbak, Jarle

Arne Nysveen A Hybrid Fe-Be Method for Time Domain Analysis of Magnetic Fields Involving Moving Geometry

Nilssen, Robert

Feng Xu Power System Security Assessment. Identification of Critical Counting encies and Outage Distance by a Zone Filter

Holen, Arne T

1993 Bjørn Alfred Gustavsen

A study of overvoltages in high voltage cables with emphasis on sheath overvoltages.

Sletbak, Jarle

Svein Thore Hagen AC breakdown strength of xlpe cable insulation Ildstad, Erling

Olve Mo Time Domain Simulation and Modelling of Power Electronics Circuit. Development of a Simulation Tool

Nilssen, Robert

Terje Rønningen Internal faults in oil-filled distribution transformers. Fault mechanisms and choice of protection

Rein, Asgaut

Gorm Sande Computation of Inducted Currents inThree Dimensions Nilssen, Robert

1992 Per Hveem Computer Aided Learning, Simulations, and Electrical Motor Drives.

Norum, Lars

Ståle Johansen Energy resource planning a conceptual study of a multi objective problem.

Faanes, Hans H.

31

Astrid Petterteig Development and Control of a Resonant DC-link Converter for Multiple Motor Drives

Undeland, Tore

Bendik Storesund Resonant overvoltage transients in power systems Sletbak, Jarle

1991 Jonny Nersveen Kvalitetskriterier og helhetlig planlegging av innendørs belysningsanlegg.

Bjørseth, Hans H.

Torbjørn Strømsvik Kraftelektronikk som kilde til forstyrrelser i fordelingsnettet. Johannesen, Arne

Alf Kåre Ådnanes High Efficiency, High Performance Permanent Magnet Synchronous Motor Drives

Undeland, Tore

1990 Eilif Hugo Hansen Bruk av kunstig lys og lysmanipulering for styrt produksjon av laksefisk.

Bjørseth, Hans H.

Guijun Yao Modelling, Dynamic Analysis and Digital Control of PWM Power Converters

Norum, Lars

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