Global Research Monthly Update January 2009

Vestas Global Research

# Vestas Global Research is responsible of bringing Vestas' technology leadership to an even higher level. Vestas Global Research is creating the technology breakthroughs transforming the Wind Power Industry of tomorrow by running an extensive portfolio of Research programmes and blue sky projects. The department is responsible for technology opportunities from the early phases in the research front-end, proof-of-concept and technology project until technology integration in product development. Global Research is an integrated part of Vestas Technology R&D.

Your application # Please send your application Innovation Network Supporter, Simon Stacey; sista@vestas.com Alternatively, please feel free to carry out the research and send a copy of the completed document.

Project list

#1 High-frequency electrical transients within wind power plants #2 Validity range of RMS grid models #3 Uncontrolled WTG response #4 Effects of the characterization of internal collector networks in the

performance of simplified models representing wind power plants #5 Design of a discrete - signal comparison tool for its use in model performance

analysis #6 Aggregation study in PSS/E version 31 #7 DFIG represented as a synchronous generator #8 Faults in the grid and their representation in simulation tools #9 DFIG based WTG modelling approaches for load flow studies #10 Grid compliance tool #11 Evaluation of different MicroGrid interconnections #12 Design of energy storage management system for 100 MW wind power plants #13 Wind Power Plant dependability #14 Parallel converter systems and their control #15 Control & Modulation Strategy for Current-Source Inverter #16 Modulation of three-level inverter with common-mode voltage elimination and DC-link balancing #17 Discontinuous DC-link Balancing Modulation Strategy for Three-level aaaaaaiInverters #18 Variable Speed Wind Turbine equipped with Synchronous Generator #19 Expanded slip limits in the double fed asynchronous generator system #20 Wind turbines optimised for VSC-DC transmission. #21 Difference in the grid connection of a conventional- and a wind power plant #22 Study guideline for wind integration studies

#23 Voltage Optimisation of Wind Power Plant Grid Connections #24 Modelling of internal fault control systems #25 Improved Experimental Techniques For Evaluating Through Thickness aaaaaaiiProperties of Composite Wind Turbine Blades #26 Modelling Techniques For characterising the Through Thickness Properties aaaaaaaaof Composites and their Impact on the Design of Wind Turbine Blades #27 Effect of product variation in wind turbine blades #28 Modeling of Environmental Degradation of Composite Bonded Joints #29 Determine Effects of Voids on the Fatigue Strength of Prepreg Carbon Fibre aaaaaaiiLaminates #30 Reactive power control coordination for Wind Power Plant with STATCOM #31 Energy Storage Control for Wind Power Integration Enhancement #32 Control of VSC-Based HVDC Transmission System for Offshore Wind Power Plants #33 Auxiliary power supply in wind power plants

#1 Student Project

Project Title High-frequency electrical transients within wind power plants. Target Group M.Sc.E.E. or B.Sc.E.E. graduate students Background Wind power plants are wind farms, where 10’s or 100’s of turbines are clustered to produce 100’s of MW power output. Such installations must in large operate as conventional power generation. A significant difference is, however, that wind power plants are made up of many individual turbines, interconnected through radials of underground (or subsea) medium-voltage cables. The bundled power comes together in a wind power plant substation, where it is stepped up to high-voltage through the plant transformer, before meeting the transmission grid.

The substation circuit breakers open on commands from the protection relays, or open/close commands from the operator. The turbines contain breakers too, and advanced power electronic frequency converters, which can control the generated output power in milliseconds in response to changes in grid conditions. The “marriage” of intelligent power electronics and fast circuit breakers creates some new interface challenges. F.x. when breakers open (upon a short-circuit) it causes a significant disturbance to the line voltage seen by adjacent turbines, and their control systems must be robust enough to withstand such disturbances. Further, reflections between breaker and turbines can lead to excessive voltages for the installed equipment. To design robust controls and robust equipment, one must first describe the problem correctly. Therefore, representative mathematical models and simulation models are required for the principal components: power electronics, breakers, cables and transformers.

Problem Statement It is of interest to investigate: Influence by circuit breaker open/close manoeuvres on the voltage seen at the

different turbines. Cable properties determining over-voltage seen at turbines. Critical resonance frequencies in cable network. Intelligent protection coordination to minimise disturbances. Intelligent turbine control to recognise grid conditions. Project Content It is recommended that mathematical/simulation models are developed for implementation in PSCAD simulation tool. The project could contain, but is not limited to, the following: A simulation model of wind turbine power circuit, a cable radial, a substation breaker. Mathematical model of high-frequency properties of cables, transformers, breakers. Simulation study of system reaction to faults. Simulation study of worst-case short-circuits leading to high voltages on equipment. Conclusions on feasibility of wind power plant layout.

#2 Student Project

Project Title Validity range of RMS grid models Target Group M.Sc graduate project Background For elaboration of control performance in wind power plants (WPP), a library of Simulink models has been developed within Vestas. The library consists of models representing: Wind turbines (DFIG) Statcoms Switched Caps Transformers Cables A centralized WPP controller All models are based upon iterative procedures solving the stationary load-flow for each element in a WPP configuration rather than solving the full system matrix. This approach has been proven valid around nominal voltage. A special case of interest when designing a WPP is the fault ride-through capability where the remaining voltage during the fault can be down to 20% of the nominal voltage, or even less. In this case the validity of this modelling approach is questionable. The scope of this project is to determine the degree of accuracy at which the control studies can be conducted using this modelling approach. This includes the evaluation of different complexity levels of the component models (real/complex solution or internal current source representation). Problem Statement Define the boundary of validity of using RMS load-flow component models in control studies of wind power plants. Project Content: Model implementation in Simulink (looking into current implementation) Validating model performance against more detailed tools and models (for example

PSCAD) Analytical study to define boundary of validity with respect to LVRT studies Recommendations for modelling future in design studies

#3 Student Project

Project Title Uncontrolled WTG response Target Group: M.Sc graduate project Background Currently the Vestas stability models of the double-fed induction generator (DFIG) wind turbines are based on a simple algebraic representation of the generator in the wind turbine. This means that the model primarily shows the controlled response of the wind turbine (that means the performance as dictated from the wind turbine controller). The scope of this project is to investigate the real generator behaviour and deducting a simplified representation of some of the uncontrolled phenomena in the wind turbine. These could be stator and rotor transient response, magnetisation and converter-blocking due to over-current protection of the converter. Problem Statement To what level can uncontrolled response of a DFIG wind turbine (5'th order model) be represented in an RMS modelling environment? Project Content Understanding of RMS stability model for DFIG (PSS/E V6.0) Understanding of 5'th order generator model and response Translation of uncontrolled generator behavior to RMS model Implementation of model in a test tool (Simulink) Validating model performance against more detailed tools and models (for example

PSCAD)

#4 Student Project

Project Title Effects of the characterization of internal collector networks in the performance of simplified models representing wind power plants Target Group B.Sc.E.E. / M.Sc.E.E. graduate students Background The representation of wind power plants (WPPs) in dynamic and static analysis of interconnected electric power systems includes commonly different simplifications, among them: RMS modelling. The modelling of the wind turbine generators (WTGs) and the rest of

elements integrated in the WPP will mainly represent their response in a certain band-width of interest for the type of study, and comprising phenomena in the range from 1-10Hz to fundamental frequency. Phenomena out of that range will not be accurately represented.

Aggregation. The components integrating the WPP (WTGs, lines, switches,

transformers…) are most of the times represented in lumped equivalents, where some reduction is always applied.

The application of these modeling simplifications when representing the internal collector networks in lumped model equivalents comes at the price of some simulation accuracy lost in the performance of the simplified model. The justification for these simplifications is that the performance obtained from the models representing the WPPs in that way will be accurate enough for the studies to perform, while avoiding the excessive calculation demands that would arise if fully-detailed representation was to be used.

Project Content The aspects of the WPP model performance that will be considered are : Contribution to short-circuits in the system. Dynamic response. Static behaviour.

For the identified aspects of the modelling affecting the performance of the WPP model, the following approach will be taken: Description and characterization. Quantification of the impact on model performance and related participating factors.

Sketching ranges of validity for the simplified models. Identifying best practices and solutions to correct the possible impact of the

simplification.

#5 Student Project

Project Title Design of a discrete - signal comparison tool for its use in model performance analysis Target Group M.Sc.E.E. / M.Sc. graduate students Background Validation and benchmark studies of the performance of models require comparing data-sets containing the time - series of the studied magnitudes. This comparison will be in some of the cases between results obtained from different simulation tools, and in some other cases between simulated and measured data. Some existing grid – codes include certain guidelines about how model validation should be performed. Signal comparison in benchmark and validation analysis must be able to produce objective conclusions about similarity between signals. The completion of analysis tasks needs a systematic approach to signal comparison able to provide reproducible results. At the same time, the comparison process must be adapted to: Analysis frequency bandwidth. When analysing EMT phenomena, instantaneous

values are of key interest when assessing signal similarity. For analysis involving RMS values or slower phenomena, on the contrary, average values over relevant time lapses will rather be studied.

Input data. The comparison process must be able to adapt to the data sampling periods of the input signals. In relation to the analysis bandwidth, some requirements must be defined for the time duration and sampling periods of the input data needed to produce valid conclusions on similarity.

The analysis group inside Power Plant R&D is currently developing generic model test specifications. Creating the means to systematically compare signals in a “study-wise” manner is among the list of deliveries inside the “model test specification” item. These means should be in the form of an automated tool capable of processing data exported from different simulation environments to produce conclusions on signal similarity according to pre-established (though fully accessible) criteria related to the analysis region. Problem Statement: It is of interest: To investigate on the existing standards for discrete signal comparison as well

as on the existing needs ( in relation to the frequency bandwidth of the phenomena of study ) inside the analysis group. To design and implement an automated tool capable of performing the requested functions.

#6 Student Project

Project Title Aggregation study in PSS/E version 31 Target Group M.Sc.E.E. graduate students Background PSS/E is a tool for analyzing system stability in bulk power systems. Therefore, wind power plant models normally assume an aggregation of a single turbine to represent an entire wind power plant. Are these assumptions correct in a tool such as PSS/E which is based on positive sequence load flow. Could a differing level of aggregation be used for various studies. It is likely that some studies would benefit from using a smaller number of clusters of WTG perhaps with reduced representation of the bulk power system. A comparison of different power plant topologies would show the performance difference and create sufficient background for the common assumption. As part of this project it could be of interest to analyze also the difference between several simulation platforms, Vestas currently uses Power Factory, Matlab, PSS/E and PSCAD. A systematic characterization of these tools and their differences would be beneficial for system planners who rely on the knowledge gained during the system impact studies. Problem Statement In order to investigate the impact of the basic assumptions behind PSS/E, positive sequence loadflow, it would be interesting to do an aggregation study of a wind power plant with this tool. The comparison of the power plant behaviour to an event for different aggregation topologies can be used to justify the chosen aggregation in this particular tool. By comparing with other tools it can be evaluated if the programs give very different results due to their underlying algorithm. Project Content The project could contain, but is not limited to, the following: Development of a number of aggregation topologies Comparison of the simulated behaviour of the power plant, for different aggregations,

when subjected to a disturbance Comparison of PSS/E version 31 simulations to PowerFactory (perhaps also PSCAD)

simulation

#7 Student Project

Project Title DFIG represented as a synchronous generator Target Group M.Sc.E.E. / M.Sc graduate project Background Double-fed induction generator (DFIG) wind turbines are used mainly by all the wind manufactures. The grid operator knows very well the behaviour of the traditional synchronous generators, and most of the tools used for short circuit calculation used synchronous generators. For this reason is needed to make and equivalent electrical representation of the DFIG into a synchronous one. This means, that the electrical impedances of the DFIG have to be represented as a synchronous one. Finding 3 different representations, depending on the status of the converter: Converter not blocked Converter blocked Crow bar activated

Fig.1 DFIG equivalent circuit diagram represented in rotating coordinates

Fig.2 Synchronous machine d-axis Fig.3 Synchronous machine q-axis

Problem Statement To what level is possible to represent these 3 different behaviours with the

same model? Which is the best way to calculate these equivalents?

Project Content: The project could contain, but is not limited to, the following: Understanding of DFIG Understanding of Synchronous generator Implementation of the models in a simulation tools Validating equivalent model performance using simulation tools and real measurements

#8 Student Project

Project Title Faults in the grid and their representation in simulation tools Target Group M.Sc.E.E. / M.Sc graduate project Background Faults in the grid are the most severe conditions that could happen into the grid. For this reason is important to evaluate the real response of the system when the fault happens. Because the difficulties of making real faults into the system, it is needed to use simulation tools to observe what is the response of the system. For these studies is needed to evaluate the severity of different faults (3 phases, phase to phase, phase to phase to ground and 1 phase to ground) and how they are propagated into different electrical network configurations. As well, it is needed real but simplified networks representation to study the recovery of the voltage in different conditions. Protection devices should be represented in detail to observe the clearance of the faults.

Problem Statement Make a classification of faults conditions following severity. Make a simplified but god enough representation of the grid to study the voltage recovery of the system. Study different breakers configurations and their representation

Project Content The project could contain, but is not limited to, the following: Understanding of DFIG Understanding of Synchronous generator Implementation of the models in a simulation tools PSCAD modelling of the models for the grid, faults and protections

#9 Student Project

Project Title DFIG based WTG modelling approaches for load flow studies Target Group M.Sc graduate project Background Vestas is willing to meet the expectation and demands of its customers who request representative models of Vestas’s wind turbines in different simulation tools and for various types of analysis. The majority of these tools are based on power flow calculation for a given network, and usually running at time steps above 5ms (e.g.: PSS/E, PSLF …etc). This puts the WTG’s electromagnetic response (DFIG system) in question as it needs smaller simulation time step. Problem Statement Different degrees of details can be used to model the generator and its control including the LVRT, so it is important to compare each modelling approach and define its limitation and advantages. Once this overview is achieved, it should be concluded which modelling approach is best suited for the target group of simulation tools. Project Content Overview various approaches for modelling the DFIG system (Generator, Controller &

LVRT). Implement the relevant approaches (models) in Matlab/Simulink Define a test network Build test network (e.g. 5 bus system), including the load flow solver Define the interface of the model to test network (taking into consideration difference of

time steps used for the DFIG system and the one for test network) Compare the performance of the various modelling approach when connected to the test

system at various time steps.

#10 Student Project

Project Title Grid compliance tool Target Group M.Sc.E.E. graduate students Background When someone wants to connect a wind power plant to the public grid you normal have to comply with a set of requirement defined in a so called grid code. Often you want to have a quick indication of how you comply with a certain grid code. Therefore Vestas has developed a simple tool to do a gap analysis between the actual turbine type and the grid code requirement related to: Frequency range Power factor range Voltage control Power control Fault ride through (FRT) The tool is implemented in Excel and even though the tool calculate on dynamics items as frequency control, runback, voltage control and FRT, the calculations is based on static load flow considerations. The tool is based on the following aggregated model of a wind power plant:

Vestas want to continue the development of this tool also to include simplified gap analysis on: Short circuit conditions inside the wind farm Harmonics contents Temporary overvoltage phenomenon (TOV)

Problem Statement It is of interest to investigate: Possibility to development simplified mathematical equations and still keep

a certain accuracy in the calculations Project Content The project could contain, but is not limited to, the following: Development of mathematical equations to simplify calculation short circuit conditions,

harmonics and TOV Demonstrates calculations on the simplified model Validation of method by simulations in PSCAD or similar tool

#11 Student Project

Project Title Evaluation of Different MicroGrid Interconnections Target Group M.Sc.E.E. or B.Sc.E.E. graduate students Background The installation of wind power generation has increased dramatically in recent years. To interconnect different size of wind farms to distribution systems forms a new type of power system, the MicroGrid. A MicroGrid is a small integrated power network consists of interconnected loads and distributed generation, which is usually connected to a conventional utility grid by tie lines. The idea of using MicroGrid is to provide energy suppliers and consumers with efficient and reliable electric power delivery services. On the other hand, there are various challenges associated with the implementation of MicroGrid. The connection of wind farms to system network will have significant impact on the voltage and frequency stability of the conventional power grid due to uncertainty of wind power generation output. In general, several technical problems remain in the operation of the grid, like steady state and transient over-voltage or under-voltages at the points of connection, protection malfunctions, increase in short circuit levels and power quality problems. To analyze the related problems, an evaluation study for different MicroGrid interconnection is required. An investigation of system stability is possible only if the variations of the loads and productions are taken into account. This can be achieved applying probabilistic techniques like the probabilistic load flow (PLF). PLF requires modeling of loads and power productions as probability density functions and provides the complete spectrum of all probable values of the bus voltages and power flows in the study period with their respective probabilities taking into account generation and load uncertainties and correlations and topological variations. Fig1 shows the concept of different interconnection for MicroGrid assumed in this study.

Fig 1 MicroGrid Interconnection

Problem Statement It is of interest to investigate the impacts of different interconnections on: System supply reliability Voltage variation at Point of Common Coupling Transmission losses inside MicroGrid and large power grid Project Content The project could contain, but is not limited to, the following: Simulation models for different MicroGrid interconnection, including wind farm,

microGrid and power grid. Contingency study of system supply reliability using probabilistic techniques. Simulation study of complete spectrum of all probable values of bus voltage and power

losses. Conclusions and suggestions on different MicroGrid interconnections.

#12 Student Project

Project Title Design of energy storage management system for 100 MW wind power plants Target Group B.Sc.E.E. graduate students Background Wind power plants are a group of turbines in the same location for production of electric power. Such installations should provide a same performance as conventional power plants to integrate them to the power systems. However, the wind power plants are different from the conventional power plants because the wind power is intermittent. Wind power plants operate when the wind blows, and their power levels vary with the strength of the wind. Due to the intermittency and non-controllability of wind power plants, it is a major concern to maintain the balance between demand and supply for the electric power system with wind power plants. Moreover, the wind regime may not coincide with the demand. In other words, the wind speed may be the greatest when the system demand is lowest. This will bring out both environmental and economic issues. To absorb the excessive output of wind power plants, the conventional power plants should operate at suboptimal operating point. It will increase the pollution emission and reduce the economic return of conventional power plants. Therefore, it is essential to use energy storage technologies to store excess energy and to return that energy to the grid when the output of wind power plants drops off and the demand is high. Energy storage is a technology that long ago achieved a place in modern society. There are presently a plethora of energy storage technologies available, ranging in terms of applicability, type and cost. There are three main types of energy storage technologies: kinetic, electrochemical and electromagnetic. However, no one technology is necessarily applicable for the integration of the wind power plants to power grid. It is necessary to analyze the performance of wind power plants with energy storage management system and the incurred cost to choose the right energy storage technology or the combination of energy storage technologies. Problem Statement It is of interest to investigate: Wind speed modelling Wind power plants modelling Survey of existing energy storage techniques Objective of energy storage management system Control scheme design for the candidate storage techniques Performance analysis of wind power plants with the selected energy storage

techniques Cost analysis of the selected energy storage management system

Project Content The project could contain, but is not limited to, the following: A probabilistic model of wind speed. Mathematical model of 100 MW wind power plants. Mathematical model for wind power plants combined with energy storage systems. Cases studies of performance and cost for wind power plants with energy storage

systems. Conclusions on the design of energy storage management system for 100 MW wind

power plants.

#13 Student Project

Project Title Wind Power Plant dependability Target Group B.Sc.E.E. graduate students Background As the percentage of total system load supplied by wind power increases the dependability of the power produced by wind power plants becomes important to maintaining system security. Traditional power plants have very well understood characteristics with respect to start-up times, dispatchability and reliability. If wind power plants are going to supply a large percentage of a system’s load the system planners and operators will need tools to predict the dependable contribution of wind power plants in the long term as well as the short term. One way to make the power produced by a wind power plant more dependable is by adding energy storage systems. Unfortunately energy storage systems are expensive and relatively inefficient. Problem Statement: It must be determined exactly how much energy storage is required and how

the dependability of supply is affected.

Project Content A: Model a wind power plant with 10+ wind turbine generators. Use Monte-Carlo analysis to determine the energy and power ratings of the energy storage system required to meet specified dependability targets. Over time the average wind speed in the wind farm can be modelled as a Rayleigh distribution with wind speeds at individual generators normally distributed around this average. Examples of possible dependability targets include a 50% capacity factor and having the ability to deliver rated power 1 hour with a 90% certainty. B: Given an arbitrary hourly load curve that needs to be met with a specified reliability and given a cost ratio of wind turbine generation to energy storage use Monte Carlo simulation to calculate the optimum make-up of a wind power plant. Assume 50% losses in the energy storage system. The ideal mixture of low wind and high wind generators for use with different sized energy storage systems could also be investigated.

#14 Student Project

Project Title Parallel converter systems and their control Target Group M.Sc.E.E. graduate students Background The latest year’s development have shown that wind power turbines are increasing in power rating and are erected in isolated areas such as offshore turbines. High demands concerning power quality and grid support has laid the way for use of power electronics in wind turbines. Demands to reliability and cost of downtime are increasing due to the increasing power rating and the low accessibility of the turbines. On the other hand is the steadily increasing power rating making paralleling of power components prudent, which are decreasing the reliability due to the increase in number of power components. A way to handle this problem is to look into the possibilities of paralleling power systems such that they are redundant and autonomies, illustrated on figure 1.

Figure 1. Problem Statement: The focus of the project can be laid on the design of the control for the converter systems or the hardware design of the converter systems.

- With focus on the control of the converter systems the main objectives is to design an autonomies controller that ensures load sharing between the converter systems.

With focus of the hardware design the main objective is to identify and design how to parallel the converter systems. The topology of the converter is a B6 DC to AC inverter. Project Content

- Literature study: collection and evaluation of information on paralleling of converter systems

- Compare different approaches of controlling parallel converters - Design, implemented and test a controller that ensure load sharing

Focus on the controller can again be laid on the load side or the supply side, depending on interest. For the load side a possible embodiment is shown on figure 2.

Figure 2. The setup uses a six phase induction motor together with two Danfoss VLT’s and a DSpace development system. If the focus of the control is on the supply side a possible setup is shown on figure 3.

Figure 3 The setup uses two Danfoss VLT’s and a DSpace development system. Contents: Hardware focus

- Literature study: collection and evaluation of information on paralleling of converter systems

- Compare different approaches of paralleling converters - Design, implemented and test a paralleling principle and protection strategies

A possible setup is shown on figure 4, where the project will be to design the contents of the block marked with the question mark.

Figure 4.

#15 Student Project

Project Title Control & Modulation Strategy for Current-Source Inverter Target Group M.Sc.E.E. or B.Sc.E.E. graduate students Background Two features of variable-speed drives for high power industrial applications which have received much attention in recent years are (i) increased voltage levels and (ii) improved quality of the motor terminal voltage waveform. As the drive power increases, so does the preferred voltage level. The voltage-source converter has been the dominant topology in low-voltage drives (<1kV), using forced-commutated power devices as MOSFETs and IGBTs. In certain higher power medium-voltage drives GCTs (gate-commutated thyristors) have been used successfully, showing high power handling capability and robustness. The limited voltage rating of power semiconductors has forced a development of multi-level MV converters. The design and control of such drives is well documented in literature. Recently, the GCT (in its reverse-blocking implementation) has also found use in current-source converters. The type of circuit shown in Figure 1 is particularly interesting as it allows a high power handling at high output voltages (6kV) while keeping high quality waveforms with a modest switching frequency (~500Hz). However, because the concept is new, very little research has been documented concerning control and design of this type of converter. An increased understanding of control properties, main circuit ratings and device selection is needed to judge which applications such a drive is suitable for.

Figure 1: Example schematic diagram of a 4-quadrant current-source inverter using series-connected Symmetrical Gate-Commutated Thyristors to achieve output voltage levels of 6kV.[Allen-Bradley] Problem Statement: It is of interest to investigate: Pulse-width modulation and control schemes achieving low switching losses, low harmonics, small filters and good dynamics.

Project Content It is recommended that simulation models and experimental hardware are developed together. The project could contain, but is not limited to, the following:

A simulation model of main circuit and control system, including component loss calculation.

PWM schemes and their impact on converter losses, passive component ratings, harmonics in output voltage.

Schemes for control of machine torque and flux. Coordinated control of line-side and machine-side converters. Impact from control scheme, filters and the low switching frequency on dynamic

performance. Design, construction and testing of a laboratory small-scale demonstrator circuit

(based on MOSFETs and diodes, for example) with associated control hardware and software.

#16 Student Project

Project Title Modulation of three-level inverter with common-mode voltage elimination and DC-link balancing Target Group M.Sc.E.E. or B.Sc.E.E. graduate students Background Conventional two-level voltage source inverters (VSI) generate common-mode voltage within the motor windings, which may cause motor failures due to bearing currents. Further, due to the capacitive coupling between the stator winding and the grounded motor frame, a common mode leakage current will flow, resulting in significant common-mode EMI. By use of a three-level inverter, c.f. Fig 1. and by use of only six of the active switch vectors, the common-mode voltage can be eliminated. (The common-mode voltage elimination is achieved at the expense of a reduction in the voltage transfer ratio, which becomes 0.866.) This modulation scheme was proposed by [1] where the voltage levels within the three-level inverter were achieved from independent DC-sources. However, in three-level converter structures, where the voltage levels are obtained by series connected capacitors, a DC-link voltage problem might occur by which an excessive high voltage might be applied to the switching devices (only for the topology in Fig. 1a). Further, the three-level converter might be unable to synthesize the reference voltage if too large voltage unbalance occurs. Hence, besides avoiding the common mode voltage, the modulation of the three-level converter also has to address the voltage unbalance between the upper and the lower switches.

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Fig. 1. Three-level neutral point clamped inverters. a) Conventional topology. b) Modified

topology. [1] Haoran Zhang and Annette von Jouanne and Alan Vallace, Multilevel inverter modulation schemes to eliminate common-mode voltages, Transaction on industry applications, Vol. 36, No. 6, pp. 1645-1653, 2000 Problem Statement: Based on the problems listed above, the problem statement for this project becomes:

Development of a modulation scheme that addresses both common-mode voltage elimination and DC-link balancing.

Project Content Besides paying attention to the stated problem, the project could/should include the following issues: Hardware design of the three-level inverter Development of a simulation model to test different modulation strategies before

implementation. d-SPACE or DSP implementation of the modulation/control of the three-level converter. Comparison of the emitted common-mode EMI from conventional modulation schemes

and the developed modulation scheme. Analysis of the DC-link unbalance problem.

#17 Student Project

Project Title Discontinuous DC-link Balancing Modulation Strategy for Three-level Inverters Target Group M.Sc.E.E. or B.Sc.E.E. graduate students Background Since the introduction of the Neutral Point Clamped (NPC) inverter, c.f. Fig. 1a, this inverter has mainly been applied for high voltage- and low switching frequency power conversion applications. However, progressing advance in computational power processors and in solid state switching devices, such as the IGBT, makes the NPC inverter applicable also in high switching frequency application. Considering low switching frequency applications (fsw < 1 kHz), a lot of research has been concerned about calculating optimal switching patterns to eliminate low order harmonics in the output voltage. As the switching frequency increases, research on harmonic voltage elimination recedes while problems like reduction of switching losses becomes more urgent. A simple method to reduce the switching losses of a three-level converter is to employ the discontinuous modulation schemes known from the conventional two-level voltage source inverter (VSI). However, a non-modified adoption of the discontinuous two-level VSI modulation schemes is only functional when the voltage-levels in the three-level converter is built from separate DC-sources. When series capacitors are used to divide the DC-link voltage, three-level inverters (multi-level inverters in general) have a voltage unbalance problem due to the following reasons: Unequal capacitor values due to manufacture tolerances. Unequal loading of the capacitors due to unintended switching delays. Unequal loading of the capacitors due to e.g. non-linear loads containing even order

harmonics. Transient operation of the converter,

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Fig. 1. Three-level neutral point clamped inverters. a) Conventional topology. b) Modified

topology.

Problem Statement: Based on the problems listed above, the problem statement for this project becomes: Development of a discontinuous modulation scheme that addresses the DC-link unbalance problem. Project Content Besides paying attention to the stated problem, the project could/should include the following issues: Hardware design of the three-level inverter Analysis of the DC-link unbalance problem. Development of a simulation model to test modulation strategies before implementation. d-SPACE or DSP implementation of the control of the three-level converter.

#18 Student Project

Project Title Variable Speed Wind Turbine equipped with Synchronous Generator Target Group M.Sc.E.E. or B.Sc.E.E. graduate students Background Previously, wind turbines were sited on an individual basis or in small concentrations making it most economical to operate each turbine as a single unit. To day and in the future, wind turbines will be sited in remote areas (including off shore sites) and in large concentrations counting up to several hundreds of MW installed power. This opens up new technical opportunities for designing and controlling the wind turbines but at the same time increasing the demands to reliability, availability and grid impact. One promising solution for operating a large concentration of wind turbines is shown in the figure below:

HVDC

Grid

MainConverter

Each turbine generator is a synchronous generator (either wound or equipped with permanent magnets). The generator is loaded with a three-phase diode rectifier. To control the power flow from each turbine and to obtain a certain speed range, the voltage from the diode-bridge is boosted to the voltage-level of the common DC-link. The system in this project proposal has the following features compared to constant speed solutions:

Each wind turbine can be controlled individually, thereby tracking the point of maximum power extraction.

Transient power fluctuations caused by e.g. tower shadow can be reduced by using the opportunity to store energy in the rotating parts of the wind turbine.

Impacts on the grid during connection of the park and grid failures is very low and is controlled by the main converter.

The power factor of the whole park is controllable and controlled by the main converter.

Each turbine can be built without a gearbox by use of a multi pole generator.

Problem Statement: The problem statement of this project is:

Design and control of the rectifier part (incl. Boost converter) for one turbine unit.

Project Content Besides paying attention to the stated problem, the project could/should include the following issues:

Control of the main inverter. Development of simulation tool usable for designing the rectification part. Simulation of a wind turbine park based on the present concept.

#19 Student Project

Project Title Expanded slip limits in the double fed asynchronous generator system Target Group M.Sc. E.E. or B.Sc.E.E. graduate students Background In a modern wind turbine, frequency converters are often used to regulate the speed of the asynchronous generator to have the optimal performance of the turbine, and make a constant power output. A well known configuration is the double fed asynchronous generator system, where the converter is connected to the rotor of the generator via slip rings, and only handles part of the total power generated to the grid. This configuration is shown in the figure, neglecting dashed lines (K1-K2).

Usually two voltage levels from the high voltage transformer are accessible. One connected to the stator (V1) and one connecting the converter (V2). The voltage level at the stator is higher to reduce the current, and the converter lower because of the voltage limitations of the converter. The possible generator speed variation (slip limits) in the above system is depended on the electrical parameters of the generator, and the maximum output voltage of the converter. At low wind speeds high slip is necessary to retain connection to the grid, and therefore expanded slip limits is desirable. A possibility to increase the slip limits is to connect the stator to the lower voltage level (V1) in these low wind situations, instead of (V2). This shown in the diagram with K1 and K2. Problem Statement: Based on the information listed above, the problem statement for this project becomes: Investigate the advantages / disadvantages in connecting the stator in the double fed asynchronous generator system to the lower voltage level.

ASG 3 R

K1

K2 V1

V2

Project Content Besides paying attention to the stated problem, the project could/should include the following issues:

Basic regulation and control of a wind turbine. Theory of the asynchronous machine, transformer and converter. General understanding of the double fed asynchronous generator system. Calculation and / or simulations of power flow. Protection functions (fx. short circuit and over voltage). Investigation of components for implementation. Test in laboratory or test rig.

#20 Student Project

Project Title Wind turbine optimised for VSC-DC transmission. Target Group M.Sc.E.E. or B.Sc.E.E. graduate students Background Wind energy installations are faced with demands from the grid operators regarding frequency and voltage variations. Increased demands for controllability have pushed the technologies both within the wind turbine and in the power transmission system. To meet the desired controllability, either synchronous generators with full-scale frequency converters or doubly-fed induction generators with slip-recovery frequency converters are favoured in the wind turbines. Meanwhile, the AC transmission systems are reinforced with static VAr compensation and other FACTS devices. Such topologies will remain suited for connection of wind parks to relatively strong grids, low power and short transmission distances. However, for particular wind parks and transmission systems, it may be advantageous to use DC rather than AC for power transmission from wind park to transmission grid. In this case, HVDC transmission using voltage-source converters (VSC) is suitable. The HVDC-VSC is just as controllable as the wind turbine itself, and it decouples the AC transmission system from the wind park AC grid. Hence, all the uncontrolled frequency & voltage variations normally present at the turbine terminals disappear. As a consequence, a less complex wind turbine may be used, for example the rotor current controlled (RCC) induction generator. One operation mode that may be envisaged is that the HVDC-VSC controls voltage and frequency to match the mean wind conditions, while each RCC turbine thus only requires a limited speed range.

Substation w/ SVC

High voltageAC transmission

Full-scale turbine

Figure 1: Full-scale wind turbine and ac transmission with static VAr compensation.

Substation w/ HVDC

High voltageDC transmission

RCC turbine

Figure 2: Rotor current controlled wind turbine with dc transmission.

Problem Statement: It is of interest to investigate:

RCC induction generator turbine control, and performance with variable stator frequency and voltage. Necessary RCC wind turbine speed range when connected to HVDC-VSC. Energy production comparison between DC and AC transmission solutions. Wind park response to faults in backbone transmission grid.

Project Content It is recommended that simulation models and experimental hardware are developed together. The project could contain, but is not limited to, the following:

A simulation model of the wind turbine power control for a full-scale frequency converter turbine and for the RCC turbine.

A simplified simulation model of the main circuit and HVDC transmission control. A theoretical model for computation of yearly energy production. Simulation study of variable-speed WT performance and coordinated control

between HVDC converter and WT. Simulation study of system reaction to faults in transmission system and in WT

system. Conclusions on feasibility of simple turbines for DC transmission.

#21 Student Project Project Title: Difference in the grid connection of a conventional- and a wind power plant Target Group: M.Sc graduate project Brackground: Whenever a new power plant is considered the Transmission System Operator (TSO) conducts grid connection studies. With these studies the TSO investigate the interaction between the power grid and the new power plant. This covers the system impact on the power grid and the dynamic behavior of the power plant, both of which are vital to identify and understand before deciding whether or not the power plant can be connected to the power grid and if countermeasures are necessary. Power plants cover the full range of fuels such as fossil, hydro, nuclear, wind and solar etc. Each type of these plants has their own specific electrical and operational behavior as well as different control implementations. It is proposed to investigate the difference in grid impact in an example power grid by studying the grid connection of a conventional power plant and a wind power plant. The example power grid should be the IEEE 12-bus benchmark power system which is widely accepted as a benchmark for power system dynamics. The power plant is chosen in size applicable to this grid such that it does show impact on the grid behavior. The grid connection studies should be conducted in a chosen simulation tool.

Problem Statement: Set up and conduct a grid connection study of a power plant based on the IEEE 12-bus benchmark power system, a standard IEEE power plant benchmark model and a manufacturer Wind Power Plant (WPP) model. Investigate the differences in grid impact between the two grid connection study cases.

Project Content:

Wind turbine modelling for stability studies Wind power plant understanding (control systems, system dynamics, different WTG

types) Conventional power plant understanding (control systems, system dynamics) Modelling of power plants and power systems in a simulation tool (PSS/E,

PowerFactory, PSCAD) Dynamic system analysis (Wind turbines, conventional power plants and power

systems)

#22 Student Project Project Title: Study guideline for wind integration studies Target Group: M.Sc graduate project Background: The steady state and dynamic behaviour of a Wind Power Plant (WPP) is usually simulated using an aggregated wind turbine model implemented in an adequate model of the entire/part of the electrical power grid. Transmission System Operators use different commercial simulation software tools for such studies for instance PSS/E, Power Factory, PSLF etc. Regardless of the tool, as well as the models of the WPP and utility grid, all studies are conducted with the purpose of evaluating the power system dynamics after the grid connection of the new WPP. However, exactly which grid connection studies should be conducted? Which signals/parameters should be monitored when evaluating the obtained results? A guideline specifying the order of grid connection studies and the need for the individual studies is of great interest to WPP manufacturers and Transmission System Operators. This could elevate the understanding between the involved parties and improve the development of future Wind Power Plants and the simulation models. The project could collect information and experience from TSO’s regarding procedures for conduction grid connection studies. How are these studies linked to the grid code requirements? Should additional studies be added to their portfolio to encompass the future needs and grid code requirements? A catalogue of the conducted and required studies is then created from which a systematic treatment could lead to a general guideline/best practise for such grid connection studies. Once the guideline is developed it will be possible to implement the IEEE 12-bus benchmark power system in a chosen simulation tool and then use this to demonstrate a systematic WPP integration study. Problem Statement:

To collect present experiences from national/international TSO’s with respect to grid connection studies. To create a generic grid connection study guideline for WPP integration studies based on present experiences/best practise

Set up and conduct a WPP grid connection study in a chosen simulation tool based on the IEEE 12-bus benchmark power system and a manufacturer WPP model.

Project Content:

Wind turbine models for stability studies (WTG types) Wind power plant understanding (control systems, system dynamics) Modelling of power systems and WPPs in a simulation tool (PSS/E, PowerFactory,

PSCAD) Dynamic system analysis (WPP and power systems)

#23 Student Project Project Title Voltage Optimisation of Wind Power Plant Grid Connections Target Group M.Sc or above Background The efficiency of a Wind Power Plant (WPP) is key to a project’s viability; it must be as efficient as possible in order to ensure high site productivity. Reducing transition loss at connection points is an area researched extensively, thus the modeling of different systems, internal and external to the WPP is fundamental to understanding their different characteristics and effects. In order to optimize the voltage of WPP’s, voltage control should be carried out at the same voltage as the minimization of electrical losses, this enables the grid connection voltage to be controlled and optimized. There are various strategies available to achieve this, each will effect the WPP in different ways; modeling such strategies enables us to identify the best method to apply to WPP’s. To ensure the accuracy of models, real parameters should be used where possible. Problem Statement Create a modelling tool, using PSCAD software, to identify the characteristics and effects of various voltage optimisation methods applicable to a 100MW installed capacity WPP. Project Content: Bibliography search for other similar or relevant strategies Create a modelling tool in PSCAD Define function costs to minimize the implementation costs of such a system into a wind

power plant. Model a centralized WPP voltage controller Define different and relevant scenarios to simulate. Draw conclusions and make recommendations

#24 Student Project Project Title Modelling of internal fault control systems Target Group M.Sc or above Background It’s important that when a fault occurs within a WPP it is quickly identified so it can be rectified and the plant can continue working. If this is not achieved, there will be a level of downtime when the turbine is not working, thus reducing the efficiency of the WPP. Relevant control of the Wind Turbine Generator (WTG) is critical; the fault identification will enable the system to understand the severity of the fault, thus whether the turbine can continue to operate in its existing state. Downtime should be a last resort. Problem Statement Create a modelling tool to assess applicable fault identification and WTG control systems to improve the efficiency of wind power plants. Project Content: Bibliography search for other similar or relevant strategies Create a modelling tool in PSCAD including a doubly fed generator with converters,

controllers, DC link and protections. Set up the WPP protections and its controls. Model a centralized WPP voltage controller Define different and relevant scenarios to simulate. Define and simulate different strategies Draw conclusions on each strategy and make recommendations

#25 Student Project Project Title Improved Experimental Techniques For Evaluating Through Thickness Properties of Composite Wind Turbine Blades Target Group M.Sc or above Background Wind turbine blades are designed using finite element models that require the specification of material properties in the through-thickness direction. Currently, these properties are estimated using test results from uni-directional materials, applying assumptions such as transverse isotropy. The increasing complexity of fabrics used and the need to reduce design tolerances, to facilitate light-weight design, requires that improved testing procedures are developed, which enable through-thickness properties to be more accurately defined. Problem Statement Design a system for accurately identifying the through-thickness properties of composite wind turbine blades. Project Content: In order to address this need, the student/s will be expected to: Investigate current materials used for composite wind turbine blades and the through-

thickness properties (e.g. strength, elastic modulus, poisson’s ratio) that are of primary importance in the design process.

Evaluate the benefits and limitations of current experimental techniques for defining through-thickness properties.

Propose modifications to existing techniques and if necessary, novel new methods for improved measurement accuracy.

Conduct the recommended techniques on sample materials and evaluate their accuracy. Consider whether any modifications to the recommended techniques are required, in

order to evaluate the properties of materials with enhanced through-thickness strength (e.g. stitched and 3-D woven fabrics).

Consider scaling effects and the impact of material and manufacturing imperfections on through-thickness properties, when transferring small-scale coupon results to a full-scale wind turbine blade.

Achieving these aims will involve close liaison with Vestas, a multinational wind turbine blade design and manufacturing company, who will supply appropriate materials and test standards. Although this project is focused on experimental techniques, it is envisaged that the student/s will collaborate with a parallel project, addressing finite element modeling requirements in relation to characterizing through-thickness properties.

#26 Student Project Project Title Modelling Techniques For characterising the Through Thickness Properties of Composites and their Impact on the Design of Wind Turbine Blades Target Group M.Sc or above Background This project is similar to #25; here there is an additional focus on the impacts of the improved modeling techniques. Wind turbine blades are designed using finite element models that require specification of material properties in the through-thickness direction. Currently, these properties are estimated from test results for uni-directional materials, applying assumptions such as transverse isotropy. The increasing complexity of fabrics used and the potential to reduce design tolerances through improved modeling techniques, requires the accuracy of these assumptions to be investigated and, where appropriate, new methods developed. It is also necessary to identify the ways in which such methods may affect the design of wind turbine blades. Problem Statement Create new modelling techniques for through-thickness material property identification and discuss the likely impacts of such a methodology on turbine blade design. Project Content: In order to address this need, the student/s will be expected to: Investigate current finite element modeling techniques used in the design of composite

wind turbine blades, from micro-mechanical material models through to full-scale blade models.

Investigate methods for estimating through-thickness properties (e.g.strength, elastic modulus, poisson’s ratio). For example, transverse isotropy is a reasonable assumption for estimating uni-directional material properties but inadequate for bi-axial mat, which might require a micro-mechanical/unit cell model.

Validate the proposed methods against actual test data. It is envisaged that this will be possible through collaboration with a parallel project investigating experimental techniques for defining through-thickness properties.

Assess how sensitive finite element results are to selected through-thickness properties using appropriate benchmarks.

Evaluate the potential impact of material and manufacturing imperfections when transferring through-thickness properties for small scale test coupons to full-scale blade models.

If time allows, evaluate the potential weight saving achievable through the use of new materials with enhanced through-thickness strength (e.g. stitched and 3-D woven fabrics).

Achieving these aims will involve close liaison with Vestas, a multinational wind turbine blade design and manufacturing company, who will provide advice on current standard modeling procedures and appropriate benchmark test cases.

#27 Student Project Project Title Effect of product variation in wind turbine blades Target Group M.Sc or above Background Product quality is of great importance in the manufacture of wind turbine blades. However, even in the most tightly controlled manufacturing processes, variations from design values will occur and a good understanding of the effect of these variations on the performance of wind turbine blades is critical for blade development. Problem Statement The objective of this project is to investigate how variations within design tolerances for material positioning in key areas of a wind turbine blade will affect blade performance. Project Content: This project will approach the problem in three stages: A general review, and investigation if necessary, will be performed to assess the best

methodology for addressing manufacturing variations in the design process. This methodology will then be applied to key areas of the wind turbine blade to assess

the effect of certain manufacturing tolerances. The methodology followed could be as below:

o Variations in manufacture for key areas of the blade, such as position of structural foams, will be assessed and quantified.

o The selected areas will be modeled using finite element techniques to accurately represent blade behavior.

o Limits of variation observed during manufacture will be incorporated into the model and the effect on blade performance in relation to design parameters will be assessed.

Recommendations for design improvements to reduce sensitivity to positional variations are an additional outcome for the project

#28 Student Project Project Title Modeling of Environmental Degradation of Composite Bonded Joints Target Group M.Sc or above Background Due to harsh operating environments, cyclic loading, and lack of condition monitoring, the strength of the adhesive interface and the constituent materials in a composite bonded joint may degrade. A project to model, predict, and validate, the strength of adhesive joints subject to moisture, contamination, and extreme temperatures may be used to establish accurate safety factors. It is expected that the project would involve analysis and mechanical testing of representative bonded joints. Problem Statement Create a comprehensive modeling system to predict the strength of adhesive joints when subjected to adverse climatic conditions. Project Content: To ensure the success of this project, the following areas should be covered: Research the poor conditions that lead to the degradation of turbine blades and the

characteriscs of the damage caused. Identify a modeling method to predict the damage likely in certain conditions. Draw conclusions from the research carried out; is the modeling method accurate

enough to validate the strength of turbine blades.

#29 Student Project Project Title Determine Effects of Voids on the Fatigue Strength of Prepreg Carbon Fibre Laminates Target Group M.Sc or above Background Current tolerances have been established based on experience and qualitative assessment. A project to investigate the reduction in durability of prepreg carbon fibre spar caps including voids may be used to determine defect tolerances based on quantitative analysis. An outcome of the project would be a validated model to predict the fatigue life of a laminate with embedded voids. Problem Statement The objective of this research project is to identify a modeling system that can simulate the fatigue characteristics of prepreg composites with embedded voids. Project Content: The following should appear in this project in order to achieve a comprehensive report: Discussion of existing research and development in this field, highlighting the issues

experienced in the industry as a result of prepreg voids. A model should be created to identify the likely levels of composite fatigue when voids

are present in the material. Draw conclusions from the research carried out; does the presence of prepreg voids

significantly reduce the life expectancy of the composite material?

#30 Student Project Project Title Reactive power control coordination for Wind Power Plant with STATCOM Target Group M.Sc graduate project Background Renewable energy and especially wind power becomes important player on the energy market. It is expected that by 2030 20-30% of the total generation capacity installed in wind turbines. Moreover, there is tendency to build large units consisting of tens of wind turbines of the same type sharing the common coupling to the grid, as it is shown on Figure 1. Such Wind Power Plants (WPP) have power range from tens to hundreds of MWs. However all power plants, beside power generation, should be also able to take control actions that allow maintaining grid stability. For this reason transmission system operators impose requirements that WPPs must fulfil before they are allowed to connect to the grid.

Figure 1. Exemplary WPP layout with STATCOM at PCC Particularly to secure voltage stability of the system, all grid codes state reactive power that WPP must be able to produce at point of connection to the grid (Point of Common Coupling (PCC)). Individual Variable Speed Wind Turbines (VSWT) are capable of meeting this requirements. However, in some cases overall WPP reactive power capability is not enough to comply with the grid codes. One of the solutions is installing STATCOM at PCC. This forms flexible, but complex control system (see Figure 1). Reactive power command for the WPP must be dispatched between STATCOM and wind turbines. Therefore good control coordination is required. Moreover STATCOM can provide additional feature which is power oscillations damping. Complex WPP+STATCOM layout shown in Figure 1 can be represented in small-scale on dSPACE platform as it is shown if Figure 2. In this setup one converter represents aggregated WPP up to collector bus and second converter represents STACOM. Both converters are controlled by DSP controller. Reactance between two converters represents connection line form the WPP collector bus to the point of common coupling (PCC). Whole system is coupled to the grid through a transformer.

LC F

ilter

LC F

ilter

Line

ST

AT

CO

M

WP

P

Couplingtrafo

Grid

DC

So

urce

dS

PA

CE

P, Q Q

V

Figure 2.Small-scale WPP+STATCOM dSPACE laboratory setup Problem Statement The main objective of this project is to develop control method for small scale representation of wind power plant with STATCOM, which would ensure optimal reactive power flow with respect to the losses and provide appropriate reactive current injection during grid transients. Project Content: Project should cover following topics: Aggregated model of WPP employing full-scale converter turbines (up to collector bus) STACOM modeling Consideration of connection length influence on reactive power profile Consideration of active power production influence on reactive power profile Consideration of short-term overloading capability of converters for fault ride-through Power oscillation damping with STATCOM Development of reactive power control method for WPP with STATCOM for steady

state and transients Validation of the developed control method on dSPACE setup

Contact persons: Andrzej Adamczyk, ana@iet.aau.dk Lars Helle, lah@vestas.com

#31 Student Project Project Title Energy Storage Control for Wind Power Integration Enhancement Target Group M.Sc graduate project Background Installed worldwide wind energy capacity increases about 30% per year. In consequence wind penetration level in power system is considered to significantly increase in near future. Due to increased penetration and nature of the wind, especially its intermittency, partly unpredictability and variability, wind power can put the operation of power system into risk. This can lead to problems with grid stability, reliability and the energy quality. One of the possible solutions can be an addition of energy storage into wind power plant. It will not only enable further increase of wind penetration in the grid but it will also bring nearer WPPs to the behavior of conventional power plants. However Energy Storage with WPPs is a relatively new topic with a very small number of existing installations worldwide.

Transformer Grid

P*

P

TSOPower

reference

Pref

DCSource

Wind Simulator

DC Load

PWPP

Figure 1. Small-scale WPP with Energy Storage Small-scale WPP with Energy Storage can be represented in dSPACE platform as it is shown in Figure 1. In this setup one converter represents aggregated WPP up to collector bus and second converter represents Energy Storage. Both converters are controlled by dSPACE. Whole system is coupled to the grid through a transformer. The idea is to control Energy Storage in such a manner to assure constant power flow to the grid for the specific time periods. During “discharging” process power from the ES inverter will flow to the grid while in “charging” process energy is dissipated in the DC load. Battery state of charge is calculated and controlled in DSpace. For the WPP converter, reference power can be calculated based on Wind simulator implemented in DSpace.

Problem Statement The main objective of this project is to develop control method for small scale representation of wind power plant with Energy Storage, which would ensure constant power flow for the specific time periods. Project Content: Project should cover following topics: Aggregated model of WPP employing full-scale converter turbines (up to collector bus) Energy Storage modeling Case study with specific parameters Development of power control method for WPP with ES Validation of the developed control method in dSPACE setup

Contact persons: Maciej Swierczynski, mas@iet.aau.dk Lars Helle, lah@vestas.com

#32 Student Project Project Title Control of VSC-Based HVDC Transmission System for Offshore Wind Power Plants Target Group M.Sc graduate project Background The growth in demand for renewable energy sources is, nowadays, a reality. Due to the fast development, more and more energy is required which results in an effort to provide energy to the entire world. Wind turbines have a large participation in the market of energy, which represents that this kind of source is the fastest growing renewable electricity technology. However, due to the random behaviour of the wind, the connection with the power system requires some kind of energy processing. Due the increase of size, noise and visual pollution, off-shore applications of wind power plants, clusters of wind turbines, have been located in off-shore places, that is, in areas out of the sea. The first off-shore wind farm was implemented in 1995 in the Kattegat Sea off the Coast of Denmark. This wind farm consists of 10 Vestas 500 kW pitch controlled wind turbines. Although off-shore applications are suitable according to improve the land occupations, some challenges related to construction, installation, safety and energy transmission appear. The last case requires a great effort in order to become the off-shore application viable, mainly involving great distances. On the context of wind energy, researches have been developed in order to promote improvements to the acknowledgement or to solve some inherent problems which do not present a reasonable solution. One topic related to this technology is the transmission system to drive the energy from wind turbines, located in distant offshore zones to the mainland using high voltage and direct current, called HVDC. The main advantage is related with the transmission losses and cost which are lower than those presented by the traditional AC transmission system. In the beginning, HVDC systems were based on line commutated converters. With this configuration, large amount of energy could be processed. However, high harmonic content was presented and a full controllability was not achieved. With the advances of the power semiconductor devices, as GTOs, IGBTs, IGCTs and IEGTs for example, the use of voltage source converters in high voltage and high power applications became possible. Today, converter using pulse width modulation can be found in HVDC systems and the problems related to harmonic and controllability issues, presented in the classic HVDC, can be decreased. This type of system consists in two voltage source converters (VSCs), each in its own bus, and connected by a DC transmission line as showed in the figure below.

Figure 1: VSC-based HVDC transmission systems scheme.

Each converter is able to control the AC voltage in each bus and DC link Capacitor voltage, as well the active power between the power systems. Reactive power issues can also be supported. Applications in offshore wind farms have a similar layout; but a wind farm is placed in one of the AC terminals as showed in the following figure.

Figure 2: VSC-based HVDC transmission systems for offshore wind farms.

This association brings to the system many challenges involving variable speed wind turbine control, active and reactive power control, power oscillations and stability issues. Problem Statement The project aims to develop good understandings about the control strategies regarding VSC-HVDC transmission and its applications in renewable energy systems. It is intended that in the end of the project the students acquire a qualitative and quantitative knowledge, being able to identify the field of application of the technology, advantages and disadvantages, control schemes and topics related to the grid connection requirements. Analyze control issues under unbalanced voltages and loads and some kind of disturbances will also be covered. Simulations tools such as PLECS and Matlab can be used and a prototype in the green power laboratory will be tested in order to link the theoretical analysis and the results acquired in the real world. Project Content: Project should cover following topics: VSC-HVDC modeling Validation via computational tools (PLECS, Matlab/Simulink) Validation via small-scaled prototype and DSpace platform

Contact persons: Rodrigo da Silva rds@iet.aau.dk Lars Helle lah@vestas.com Remus Teodorecu ret@iet.aau.dk Pedro Rodriguez pro@iet.aau.dk

#33 Student Project Project Title Auxiliary power supply in wind power plants. Target Group M.Sc.E.E. / M.Sc. Graduate students Background A wind power plant consists of multiple wind turbines, connected to a common power collector grid, normally at an AC voltage level of 10-35 kV. Within each turbine, this network connects to the turbine transformer via the main switchgear. The low-voltage side of the turbine transformer connects to the main power generation (generator + frequency converter) but also to the turbine auxiliary supply. If the turbine generates power from the wind, some of this is consumed internally by the turbine itself. If wind is low, the turbine draws power from the collector network, hence from the ac grid. The use of a common AC LV connection in the turbine for main power generation and for auxiliary power supply poses some challenges: AC grid disturbances (voltage and frequency deviation, Low Voltage Ride Through, temporary over-voltage, harmonics etc) also impact the auxiliary supply. This aux supply must therefore be rated and dimensioned for these non-ideal supply conditions. It would be advantageous to provide an “uninterrupted” auxiliary power supply to each turbine. The turbine auxiliary power consumption is a function of the operating conditions: The pitch actuator activity, the yaw motor activity, gearbox oil pump activity, cooling/heating, aviation lights, etc. The power consumption pattern in one turbine is normally not controlled in order to reduce: (i) the total energy consumption by auxiliary functions; (ii) the peak power demand by auxiliary functions. It would be desirable to reduce the energy losses in auxiliary components, and reduce the rating of the turbine aux supply system. In periods where the plant is disconnected from the transmission network, the turbines may still be supplied from on-site generation (diesels). These generators can feed a single turbine or multiple turbines, through the collector network. Again, to reduce the rating of on-site diesel generation, it is of interest to reduce the peak power demanded by the collected number of turbines. This requires coordinated control of individual turbine auxiliary functions. In future wind power plants, the auxiliary power could be supplied to each turbine through a dedicated auxiliary power network, distinct from but in parallel with the main power collection network. Hence, each turbine would have a main power generation output and an auxiliary power supply input. This could indeed be the case in future (offshore) wind plants where each turbine supplies dc on its output rather than ac, to reduce the total power conversion loss, when the entire wind plant is connected to the transmission network via so-called HVDC transmission. Problem Statement Is it technical and economical feasible to design a reliable and dedicated cable network for auxiliary power supply in a wind power plant with many wind turbines and which will not be affected by contingencies and voltage dips in the collector grid or the transmission grid to which the wind power plant is connected. Project Content: Determine the auxiliary power requirement for a large wind power plant. This involves analysis of load patterns (auxiliary power consumption) during different operation conditions of the wind power plant. Analyse and define the main technical requirements/specifications for a cable distribution system for auxiliary power supply to each wind turbine. Design a wind power plant auxiliary power conditioning system for elimination of frequency/voltage deviations which may occur due to disturbances in the AC collector grid or in the AC power grid. The auxiliary conditioner system may include:

Uninterrupted Power Supplies, active/ passive filters etc. Power consumption management to reduce total energy consumption and peak

power consumption by auxiliary functions. Energy storage to reduce peak power demand of the auxiliary supply (peak shaving.)

Contact persons: Rodrigo da Silva rds@iet.aau.dk Lars Helle lah@vestas.com Remus Teodorecu ret@iet.aau.dk