In order to transmit massive amounts of power generated by remotely located power plants, especially offshore wind farms, and to balance the intermittent nature of renewable energy sources, the need for a stronger high voltage transmission grid is anticipated. Due to limitations in AC power transmission the most likable choice for such a grid is a high voltage DC (HVDC) grid. However, the concept of the HVDC grid is still under active development as different technical challenges exist, and it is not yet possible to construct such a DC grid. This paper deals with prospects and technical challenges for the future HVDC SuperGrids. Different topologies for a SuperGrid and the possibility to use modular multilevel converters (M2Cs) are presented. A comprehensive overview of different sub-module implementations of M2C is given. An overview of short circuit behaviour of the M2C is also given, as well as a discussion on the choice between cables or overhead lines and DC-side resonance issues.

In this paper a modular multilevel converter (M2C) basedmulti-terminal direct current (MTDC) system is proposed forthe connection of offshore wind farms. Each M2C ismodelled with 36 sub-modules per arm with a total of 216sub-modules consisting of half bridges. An open-loopconverter control method is employed for the M2Cs. Powersynchronizationcontrol is used instead of a phase-locked loop(PLL) for synchronization. A voltage controller isimplemented with power-synchronization control as an innerloop. By means of numerical simulations in PSCAD, it isshown that the system is self stabilizing both at steady stateand following power fluctuations of the wind farm.

This thesis deals with the analysis and control of the modular multilevel converter (M2C). The M2C is a promising converter technology for various high-voltage high-power applications. The reason to this is that low-distortion output quantities can be achieved with low average switching frequencies per switch and without output filters. With the M2C the output voltage has such a low harmonic content that high-power motors can be operated without any derating. However, the apparent large number of devices, requires more complex converter control techniques than a two-level counterpart. The internal control of an M2C must be designed so that the submodule capacitor voltages are equalized and stable independent of the loading conditions. An active submodule selection mechanism, included in the modulator, has been shown able to provide voltage sharing inside the converter arm. Apart from the individual capacitor voltage sharing, a strategy has to be designed to ensure that the total amount of energy stored inside the converter will always be controlled. Based on an analytical description of the converter, both feedback and open-loop control methods are suggested, simulated and experimentally evaluated, which will ensure stable operation in the whole operation range. The potential interaction of the internal controllers with an external motor current controller is also investigated. Both simulation and experimental results show that any interaction will not result in any problems neither for the converter nor for the motor control itself. A hardware implementation of a down-scaled 10 kVA three-phase laboratory prototype converter is performed, in order to evaluate the modeling and the controllers developed. The controller implementation is described in detail, as it exhibits remarkably fast response, and can be expanded up to an arbitrary number of levels. Therefore it can be used even by a full-scale converter implementation in the MW range.

The modular multilevel converter (M2C) is a promising converter technology for various high-voltage highpower applications. The reason to this is that low-distortion output quantities can be achieved with low average switching frequencies per switch and without output filters. With the M2C the output voltage has such a low harmonic content that highpower motors can be operated without any derating. However, the apparent large number of devices, requires more complex converter control techniques than a two-level counterpart. Even though there have been several ways suggested to control the converter itself, it is still a challenge to investigate the interaction of these controllers with an external motor current controller. It is shown in the paper that the anticipated interaction will not result in any problems neither for the converter nor for the motor control itself.

The modular multilevel converter is a promising converter technology for various high-voltage high-power applications. Despite the apparent simplicity of the circuit, the inherent dynamics of the converter and the balancing of the sub-module capacitor voltages impose high requirements on the control system, which can be implemented in quite different ways. To illustrate this, and to provide a guidance for future research on the subject, this paper presents an evaluation of four different control and modulation methods. The investigation is based on experiments on a down-scaled 10 kVA converter having 10 submodules per phase leg. The main items to be investigated are dynamics within the sub-modules, arm voltages and circulating currents. It is found that the suggested open-loop control method provides the fastest arm-voltage response and that the balancing approach based on a sorting algorithm is substantially faster and less complicated to implement than the method using a dedicated voltage controller for each sub-module.

This paper discusses the impact of modulation on stability issues of the Modular Multilevel Converter (M2C). The main idea is to describe the operation of this converter system mathematically, and suggest a control method that offers stable operation in the whole operation range. A possible approach is to assume a continuous model, where all the modules in each arm are represented by variable voltage sources. and as a result, all pulse width modulation effects are disregarded. After simulating this model and testing different control methods, useful conclusions on the operation of the M2C have been extracted. The control methods are then implemented on a model with discrete half-bridge modules, in order to compare the results and to validate continuous model approach. When assuring that this model functions as expected, the goal of this paper is to conclude into a self-stabilizing voltage controller. A controller is proposed, which eliminates circulating currents between the phase legs and balances the arm voltages regardless of the imposed alteranting current.

This paper investigates the use of transverse-flux machines (TFMs) in a free-piston generator, an integration of a combustion engine and a linear electrical machine. For hybrid vehicles (the intended application), this integrated device has benefits in efficiency, emissions, reliability, etc. Conventional TFMs are dimensioned and their shortcomings illustrated. Several surface-mounted TFM designs are suggested. These designs, initially believed to be promising, are later discovered to be. poor during the three-dimensional finite-element method (3-D FEM) analysis. These designs suffered from a large axial pole-to-pole armature-flux leakage in the stator. This problem is solved in the proposed buried-magnets TFM variants, for which patents have been filed as well. A 5-kW proof-of-concept prototype of one such design is dimensioned that allowed the near fulfilment of 3-D FEM verifiable system demands. One phase. of this dimensioned three-phase prototype is built. Measurements on this,prototype showed that the use of short pole lengths in the machine had led to manufacturing defects. Nevertheless, the electromagnetic viability of the design could still be approximately verified.

Sizing of photovoltaic (PV) generators is an important issue which might affect the PV system overall yield and performance ratio. The procedure is realized according to inverter specifications with the most important one being the operating voltage range. The parameter that is affected by this specification is the number of PV modules per string and a software tool was developed in order to study the effects of this parameter on the system performance. Moreover, the PV generator voltage is highly affected by the ambient temperature of a specific location. Thus, the optimum number of PV modules per string will differ in locations with different environmental conditions. The importance of PV string sizing is high, since improper string length can lead to significant deficiencies on the overall inverter performance. A study of the effects of PV string size on annual system energy production and yield has been conducted for several locations in Europe with significantly different weather as well as for locations with extreme climatic conditions such as high altitude and desert areas.

This paper deals with the analysis and simulation of the static series compensator (SSSC) for subsynchronous resonance (SSR) mitigation. The purpose of the paper is to derive and analyze a novel control strategy for SSSC dedicated for SSR mitigation. Objective of the proposed controller is to increase the network damping only at those frequencies that are critical for the turbine-generator shaft. By using frequency scanning analysis, the effectiveness of the proposed method for mitigation of SSR due to torsional interaction effect is presented and compared with the existing control strategy. Finally, simulation results show the performance of the proposed method in mitigating SSR due to torque amplification effect.

In this paper, the problem of online estimation of subsynchronous frequency components in the measured grid voltage is treated. Two estimation methods, one based on the use of lowpass filters and a recursive least square algorithm, are investigated and compared. In particular, due to its higher degree of freedom in the design, the lowpass filters-based estimation method is found to be the most appropriate and accordingly further analyzed. The method is improved to cope with inaccurate knowledge of the subsynchronous frequency. The simulation results prove the effectiveness of the investigated method both for the ideal and the disturbed case.

In this paper, a novel control strategy for subsynchronous resonance (SSR) mitigation using a static synchronous series compensator (SSSC) will be presented. The SSSC is constituted by three single-phase voltage source converters. SSR mitigation is obtained by increasing the network damping only at those frequencies that are critical for the turbine-generator shaft. This is achieved by controlling the subsynchronous component of the grid current to zero. Using the IEEE First Benchmark Model, the effectiveness of the proposed control algorithm when mitigating SSR due to torsional interaction and torque amplification effect will be shown.

In this paper, a novel control strategy for subsynchronous resonance (SSR) mitigation using a static synchronous series compensator will be presented. SSR mitigation is obtained by increasing the network damping only at those frequencies that are critical for the turbine-generator shaft. This is achieved by controlling the subsynchronous component of the grid current to zero. Using the IEEE First Benchmark Model, the effectiveness of the proposed control algorithm when mitigating SSR due to torsional interaction and torque amplification effect will be shown.

Voltage sags and voltage interruptions are huge power quality problems for many industries. Voltage sags cause apart from eventual tripping, large torque peaks, which may cause damage to the shaft or equipment connected to the shaft. This paper illustrates how the stator flux in a synchronous machine changes during voltage sags. These changes result in torque peaks. An analytical time dependent expression of the stator flux during voltage sags explains the behaviour of synchronous machines and why there are high torque and current peaks during voltage sags. Simulations verify the expressions, and illustrate the dependence of torque and current peaks on voltage sag duration and magnitude.

This thesis presents the results of the work carried out on the dynamic analysis and experimental performance measurements of a novel concept of electromagnetic system for hybrid vehicle powertrains.

The main focus of this work is to present how this novel electromagnetic system can be operated in a hybrid vehicle and to show how a prototype of this system has succesfully been operated in a laboratory set-up.

Research on hybrid vehicle powertrains is encouraged by the increasing popularity of these vehicles. There is a large variety of solutions for hybrid vehicle powertrains. At the Royal Institute of Technology (KTH), Stockholm, Sweden, a novel machine concept intended for hybrid vehicle powertrains has been developed. It consists of two electrical machines built together in one unit. It is called the Four Quadrant energy Transducer (4QT) and is intended to link a combustion engine to the transmission in a hybrid vehicle, and by an apropriate control to improve the vehicle efficiency in comparison to a conventional vehicle of similar size and performance.

The research project on the 4QT system has been carried out by a research team, since 1995. It evolved from early machine study and vehicle simulation, to prototype design and manufacturing. The work presented in this thesis completes the previously published works on the 4QT, with dynamic simulations of a 4QT equipped vehicle and the experimental testing of the prototype machines.

An experimental setup has been built in order to test the performance of the prototype machines. The control of the machines and its experimental implementation are presented. The experimental setup is controlled to simulate drive cycle conditions, in order to measure efficiency of the prototype machines. Dynamic simulations of a vehicle equipped with a 4QT have been carried out to analyse the powertrain behaviour under transient conditions.

It is shown that it is possible to control the 4QT prototype and operate it within its full operating range. Results of the efficiency measurements show that the efficiencies are slightly lower than expected, probably due to an underestimation of the stray load losses. This gives room for further research and improvements in the design of the prototype machines. Transient simulations underline the importance of the control strategy and dimensioning of the powertrain elements to avoid excessive stress on the vehicle.

This thesis presents the design analysis of a permanent magnet synchronous motor (PMSM) for the traction application in electric forklifts. Within the scope of the study, an existing induction traction motor for electric forklifts benchmarks the expected performances of the proposed PMSM designs, as design specifications. The possibility of using the same stator geometry as the one used in the induction motor is explored for fast prototyping. The eventual prototype design is expected to be field-weakened and to have a constant power speed range (CPSR) of 2.5 to 3.

A simple analytical design approach based on the CPSR contour plot in an interior permanent magnet parameter plane is derived to obtain the possible designs that meet all the design specifications and the targeted CPSR. A prototype design with an inset permanent magnet (IPM) rotor configuration is obtained with this approach. Finite Element Method (FEM) analysis is employed to verify the expected performance. In addition, further examinations are also carried out by taking into consideration the magnetic saturation and the stator resistance. Contour plots of torque and phase voltage are applied to procure the advanced current angle required at different speeds in the field-weakening operating region.

Two prototype motors have been manufactured during this thesis work, and various experimental tests are carried out to examine and validate the expected performance. The prototype can deliver a maximum output power of 9.4 kW at the rated speed of 1500 rpm, and it has an outer diameter of 180 mm, a shaft height of 112 mm, a bore diameter of 110 mm and an active length of 165 mm. Search coil windings are implemented in the main prototype to monitor and measure the flux density waveforms in the stator tooth and the yoke back. The prototypes are naturally cooled with the cooling fins and the ventilation holes in the stator housing. The thermal analysis based on the lumped-circuit approach and the numerical method are investigated and examined by the measured results. It has been shown that an accurate loss estimation is a pre-requisite to enable both approaches to accurately analyze the heat transfer phenomenon in electric machines. The strengths and disadvantages of each method are also discussed.

An analytical approach to estimate the iron loss in permanent magnet (PM) electrical machines is also developed and extensively investigated. The proposed technique is based on the flux density waveforms predicted in the various parts of the stator, namely the tooth, tooth projection and the yoke back. The waveform in the respective region is derived from the air gap flux density that consists of a fixed PM excitation and the armature field due to the fundamental current in the stator winding. The model can be applied at any operating point with different load, including the field-weakening region. This simple approach gives a good indication on how iron loss varies at various speeds and operating points. The predicted loss shows a very satisfactory agreement (± 4%) with the measured results at no-load or open-circuit condition, but larger discrepancies are found under the load condition. In the constant torque operation region, estimated losses are approximately on average 15% lower than the measured values. Under the field-weakening operation, the model becomes inadequate due to the excess eddy current loss caused by the highly distorted tooth flux density waveform. A correction factor for the eddy current loss is therefore essential to account the harmonic effect. The rectified estimations are then within ± 21% of the measured values. This simple approach has proved to be capable of estimating and modelling the difficult phenomenon of iron loss in PM motors, and it can be easily embedded in the design process for routine use in loss estimations.

Keywords: Constant Power Speed Range, Electric Forklift, Finite Element Analysis, Field-weakening Capability, Iron Loss, Permanent Magnet Electrical Motor, Saliency, Thermal Analysis

This paper presents the design study of a traction motor used in electric vehicles. The design is a permanent magnet synchronous motor (PMSM) type with an inset permanent rotor configuration. With the given constant power speed range (CPSR), a design methodology is developed to reach the required specifications.

This paper presents the transient thermal analysis of a permanent magnet (PM) synchronous traction motor. The motor has magnets inset into the surface of the rotor to give a maximum field-weakening range of between 2 and 2.5. Both analytically based lumped circuit and numerical finite element methods have been used to simulate the motor. A comparison of the two methods is made showing the advantages and disadvantages of each. Simulation results are compared with practical measurements.

Induction machines convert more than 55% of electrical energy into various other forms in industrial and domestic environments. Improved performance, especially by reduction of losses in induction machines hence can significantly reduce consumption of electricity. Many design and control methods are adopted to make induction machines work more efficiently, however certain design compromises are inevitable, such as skewing the rotor to improve the magnetic noise and torque characteristics increase the cross current losses considerably in a cage rotor, degrading the efficiency of the motor. Crosscurrent losses are the dominating stray losses which are dependent on several factors among them are percentage skew and the contact resistance between the rotor bars and laminations. It is shown in this thesis that implementing a design change which has non-skewed asymmetrical distribution of rotor slots can serve the same purpose as skewing i.e., reduction of the magnetic noise, thereby avoiding the negative effects of skewing the rotor slots especially by reducing the cross-current losses.

Two design ethodologies to introduce asymmetry in rotor slots are proposed and the key performance characteristics like torque ripple, radial air gap forces are computed both numerically and analytically. Radial forces obtained from the finite element method are coupled to the analytical tool forcalculating the magnetic noise. A spectral method to calculate and separate the radial forces into vibration modes and their respective frequencies is proposed and validated for a standard 4-pole induction motor. The influence of rotor slot number, eccentricity and skew on radial forces and magnetic noise are studied using finite element method in order to understand the vibrational and acoustic behavior of the machine, especially for identifying their sources. The validated methods on standard motors are applied for investigating the asymmetrical rotor slot machines.

Radial air gap forces and magnetic noise spectra are computed for the novel dual and sinusoidal symmetrical rotors and compared with the standard symmetrical rotor. The results obtained showed reduced radial forces and magnetic noise in asymmetrical rotors, both for the eccentric and noneccentric cases. Based on the results obtained some guide lines for designing  asymmetrical rotor slots are established. Magnitudes of the harmful modes of vibration observed in the eccentric rotors, which usually occur in reality, are considerably reduced in asymmetrical rotors showing lower sound intensity levels produced by asymmetrical rotors. The noise level from mode-2 vibration in a 4-pole standard 15 kW motor running with 25% static eccentricity is decreased by about 6 dB, compared to the standard rotors. Hence improved performance can be achieved by removing skew which reduces cross current losses and by employing asymmetrical rotor slots same noise level can be maintained or can be even lowered.

Increase of stray losses in the skewed rotor is mainly due to the cross-currents flowing from the conductor into the laminations. These losses are strongly related to the contact resistance between the bar and the laminations which depends on several factors including the casting process. A substantial reduction of cross-current losses can be achieved by changing or removing skew and by employing various slot combinations. Skew removal will cause some parasitic effects such as increase in the noise level and undesirable dips in the starting torque characteristics. To minimize these effects the rotor design needs to be modified and this could be achieved by introducing modulated slots. Two such types of modulated rotor slot designs are proposed in this paper. As a first step in the performance analysis, magnetic forces, which are the major sources of noise and vibration excitations, is considered. In order to study the newly proposed modulated slot designs, a spectral method is employed and analytically calculated electromagnetic forces are validated against FEM results. The analyses shows that the proposed new designs have better force characteristics compared to the conventional design, which illustrate the fact that better performance characteristics can be achieved by removing skew and introducing modulated slots.

Skewing which is commonly used in smaller sized induction motors for the reduction of magnetic noise causes higher cross currents in the rotor bars [1], [2]. This leads to a substantial increase of stray losses in the skewed rotor. Reduction of these losses can be achieved by eliminating skew, but at the cost of increased noise level and torque ripple in the motor. A design change is proposed and studied in this paper to minimize the increased noise level by introducing asymmetrical rotor slots. A simpler method of analysis using numerical calculations for the radial air gap forces is proposed and the vibration behavior of the motor is explained. The method is based on phase shift analysis of radial force waves in the air gap of the machine computed at certain equidistant points in the air gap with respect to the rotation of the rotor. Phase information obtained from the Fourier analysis is analyzed to identify and separate the order of the radial force waves i.e., the various modes of vibration. The final results showed improved force spectrum for the newly proposed asymmetrical rotor designs from which better performance of the newly proposed rotors is predicted. This method of analysis is compared with the previously obtained results from the noise measured on a standard motor and the results show a reasonable reliability of the proposed method to predict the noise level in the machine.

Constantly growing need for oil, all over the world, has caused oil price to rise rapidly during the last decade. High oil prices have made fuel economy as one of the most important factors when consumers are buying their cars today. Realizing this, many car manufacturers have developed or are looking for some alternative solutions in order to decrease fuel consumption. Combining two different technologies in a vehicle, the so called hybrid vehicle, can be seen as the first step toward a better and more sustainable development. There are several different solutions for hybrid vehicles today, among the best known are the Serie Electric Hybrid Vehicle (SEHV), the Parallel Electric Hybrid Vehicle (PEHV) and the Serie-Parallel Hybrid Electric Vehicle (SPEHV).

By integrating a combustion engine with a linear electric machine into one unit, a system that is called Free Piston Energy Converter (FPEC) is achieved. The FPEC is suitable for use in a SEHV. Other application areas like stand alone generator are also possible.

In this report a novel Transverse Flux Machine (TFM) with a tubular cross section of the translator has been investigated. Application of the machine in a FPEC has put tough requirement on the translator weight, specific power and force density. Different configurations of the winding arrangements as well as the magne tarrangement have been investigated. It has been concluded that the buried magnet design suffers from high leakage flux and is thus not asuitable TFM concept. Instead the surface mounted magnet design has been chosen for further investigation. An analytical model has been developed and a prototype machine has been built based on the analytical results. In order to have a better understanding of the machine characteristic a 3D-FEM analysis has been performed.

The results from the analytical model, FEM model and measurements are analyzed and compared. The comparison between the measured and FEM-simulated results shows very good agreement. Furthermore, the results from the analytical model indicates that it can be successfully developed for further analysis and optimization of the design to give a cost-effective solution of the novel generato for mass production.

This paper investigates cogging force in a linear Transverse Flux Machine, TFM. 3D-FEM and 2D-FEM simulations has been performed and also two analytical methods have been applied. The analytical methods are based on the reluctant pressure method. Once the ¯eld distribution on the side of the tooth is known, the cogging force can be calculated by integrating the ¯eld. The agreement between the 2D-FEM and Conformal Mapping, CM, analytical method is good. However the 3D-FEM simulation results indicates a lower cogging force than the other two methods.

This paper investigates the electrical machine, which is one of the important parts in the FreePiston Energy Converter (FPEC). In the previous work it was found that one of the best candidate suitablefor the FPEC is a Transverse Flux Machine (TFM) due to the very tough requirements on the electrical ma-chine such as a low weight of the translator (movable part in the machine).This paper presents a new conceptof a linear TFM in which strong emphasis has been put to achieve a design that is simple to manufacture.Different types of the magnet confgurations on the translator have been investigated. It has been found thatthe buried magnet design suffers from a very high leakage to the nearby poles and is thus not suitable forthis type of machine. Another confguration, which has been investigated, is the surface mounted magnetconfguration. Analytical models for this confguration have been established. It has been found that thetough requirements, 4kN from a 6kg movable mass, can be achieved at the required speed, specifc powerand effciency

The electrical machine is one of the key parts in the Free Piston Energy Converter (FPEC), and as such the electrical machine faces very tough requirements such as low weight of the translator and high power density. During the design process great emphasis has been put not only on the machine performance but also on the manufacturing process. This paper investigates a novel Transverse Flux Linear machine for the FPEC application. At first, the analytical calculations are performed based on some estimation of the leakage flux in the machine. However, during the FEM analysis and measurements it was obvious that a more detailed analysis was required in order to achieve a better agreement in results. A more detailed reluctance network is developed and used for the analysis. The results are compared with both FEM simulations and the measurements.

In general Transverse Flux Machines (TFM) are considered to be complex and difficult to manufacture. However, there are some exceptions. In the linear TFM the flux plane is the same as for the conventional rotating electrical machine. Therefore, iron laminations from conventional rotating machines can be used to manufacture the stator. The manufacturing process of the stator becomes then more simple compared to the manufacturing process of the conventional linear machine. This paper deals with a novel design of a linear TFM machine for the free piston energy converter application. It describes earlier designed and manufactured prototypes and a new manufacturing approach in order to avoid the loss in flux and thereby also loss in the force production.

This thesis investigates simulation of synchronous machines using a novel Magnetic Equivalent Circuit (MEC) model. The proposed model offers sufficient detail richness for design calculations, while still keeping the simulation time acceptably short.

Different modeling methods and circuit alternatives are considered. The selected approach is a combination of several previous methods added with some new features. A detailed description of the new model is given. The flux derivative is chosen as the magnetic flow variable which enables a description with standard circuit elements. The model is implemented in dq-coordinates to reduce complexity and simulation time. A new method to reflect winding harmonics is introduced.

Extensive measurements have been made to estimate the traditional dq-model parameters. These in combination with analytical calculations are used to determine the parameters for the new MEC model.

The model is implemented using the Dymola simulation program. The results are evaluated by comparison with measurements and FEM simulations. Three different operation cases are investigated; synchronous operation, asynchronous start and inverter fed operation. The agreement with measurements and FEM simulations varies, but it is believed that it can be improved by more work on the parameter determination.

The overall conclusion is that the MEC method is a useful approach for detailed simulation of synchronous machines. It enables proper modeling of magnetic saturation, and promises sufficiently detailed results to enable accurate loss calculations. However, the experience is that the complexity of the circuits should be kept at a reasonable low level. It is believed that the practical problems with model structure, parameter determination and the simulation itself will otherwise be difficult to master.

This paper investigates simulation models for salient pole solid rotor synchronous machines through experimental model identification. Two different methods are used, the classical Stand-still frequency response test method and the more recent Prediction error method. The results are compared and analyzed. It is shown that a third order model is required to describe the machine dynamics accurately. Further, the impact of extra copper pole screen is investigated, and it is concluded that these could have a beneficial effect with inverter supply.

There are serious concerns relating to a Multi- Infeed HIVDC system when feeding a weak AC network. Typical issues concerning multi-infeed configurations are: need for coordination of the recovery control, need for different DC modulation strategies to stabilize the system, possibility of voltage instability of the area receiving large amount of power from multiple HVDC links and the risk of mutual commutation failures. In contrast, if the area receiving electrical power from multiple HVDC transmission links is relatively strong due to the presence of large amount of generation units nearby there are still some questions that need to be investigated such as the issues underlining the operation of such a multi-infeed system, the proper design of the controls of the HVDC systems and the system dynamic performance under extreme contingencies. This paper investigates into an example of such a multi-infeed HVDC system. The authors have performed small signal analysis of the system to assess instability associated with the control modes. Electromechanical and voltage stability analysis were performed for harmful contingencies. Dynamic performance analysis was also carried out to analyze the interaction amongst various HVDC inverters during disturbances.

This study presents a frequency-domain model of an high voltage direct current (HVDC) transmission link with a line-commutated current-source converter. Using space-vector transfer functions between superimposed oscillations in the control signal and the AC and DC sides, expressions for voltages and currents have been derived. The dynamic properties of the HVDC link, taking the characteristics of the networks on both the AC and the DC sides into consideration, can be studied by applying classical Bode/Nyquist/Nichols control methods. The resulting model was validated by time-domain studies in PSCAD/EMTDC. The model is described in two papers. In this paper (Part I), it has been assumed that the overlap angle during commutation remains constant. It is shown in the validation that this assumption introduces resonances that cause severe errors at certain network conditions. In the second paper (Part II), the model is extended so as to cope with the varying overlap angle in order to bring the frequency-domain model into agreement with the results obtained from time-domain simulations.

The second part of the description of a frequency-domain model of a high-voltage direct current (HVDC) transmission link with line-commutated current-source converters is presented in this study. The model is based on transfer functions between superimposed oscillations in the control signal and the AC- and DC-side voltages and currents. In 'Part I' of the description the overlap angle was assumed to be constant. At certain resonance frequencies severe deviations were obtained between the calculated transfer functions and the ones extracted from time-domain simulations. In this study (Part II), the model has been extended to deal with varying overlap angle. It is shown that the agreement between transfer functions calculated in frequency- and time-domain has improved substantially.

High-frequency dc-dc converters are nowadays widely used in a diversity of power electronic applications. High operating frequencies entail a reduction in size of the passive components, such as inductors, capacitors and power transformers. By operating the converter at higher frequencies with conventional hard-switching topologies, the transistor switching losses increase at both turn-on and turn-off. High-voltage converters in the power range of 1-10MW will therefore have excessive switching losses if the switching frequency is higher than 4 kHz. In order to achieve a high-frequency operation with moderate switching losses a number of soft-switched topologies have been studied in [Dem1]. The favourable DC-DC converter was found to be the Dual-Active Bridge when a bi-directional power flow is demanded. Additionally, the Single-Active Bridge (SAB) topology was introduced for the first time.

In this thesis the two topologies are thoroughly studied. The dynamic small-signal models are presented and the dynamic behaviour of the converters is discussed in deep. Different control strategies are presented concerning the two converters and the advantages and the disadvantages of the different control strategies are stated.

Critical issues as efficiency and stability are presented separately for the two converters.

The half-bridge SAB topology is the simplest soft-switched topology known to the authors and is firstly presented in (G.D. Demetriades). In the present paper the SAB topology is thoroughly studied and the aspects as stability and efficiency are addressed as well. High-frequency oscillations occurring during the discontinuous-time interval are reported.

In the present paper the Dual-Active Bridgetopology employing a duty-cycle modulation is studied. Thebehavior of the converter at steady-state is examined.Additionally, the small-signal model of the topology ispresented and the dynamic behavior is examined.Simulations and experimental results are presented. It isworth noticing that the converter is operated under softswitchingconditions in a wide load range.

In the present paper the staedy-state and thedynamic behavior of the Dual-Active Bridge topology havebeen studied. The small-signal model of the converter hasbeen derived and theoretical and experimental results arepresented.The Dual-Active Bridge is an attractive solution for highpowerapplications where a bi-directional operation is needed.

In the present paper the small-signal model ofthe half-bridge soft-switched Single Active Bridge is derived.In order to derive the small-signal model of the specifictopology an extended state-space averaging method basedon half-cycle symmetry has been used and is brieflydescribed in the paper. The results obtained have beenverified experimentally.

The higher specific torque and power density of atransverse flux permanent magnet (TFPM) machine in comparisonto a conventional machine makes it a promising direct-drivenwind turbine generator. The new topology presented in this paperis believed to have a better utilization of the available in thenacelle space. This would favor a reduced inactive weight of thegenerator and thus remove one of the major obstacles for largewind power units. The new topology is presented together withthe analytical model used in the design procedure. The analyticalmodel is used in a parametric study and the first conclusionsabout the performance of the new TFPM topology are drawn.

This paper presents analysis of a novel transverseflux direct-driven wind generator. The analytical model for thecalculation of different parts of the inductance is developed andapplied for the evaluation of machine performance with respectto its geometry. Generators rated for 3, 5, 7, and 10 MW outputpower are investigated. The possible ranges of design parametersare discussed and conclusions are drawn.

Development and manufacturing of rotating electrical machines have been essential for the Swedish society and industry for more than a century. The dominating manufacturer has been Asea/ABB, even if a more diversified structure has emerged in recent years. The thesis deals primarily with Asea/ABB’s development of four kinds of electrical machines, although reflected against a wider national and international background. The purpose of the research has been to study the development of these machines and the related industrial processes, focusing on factors with a major influence on the development.

The thesis contains introductory chapters presenting the research, the public importance of electrical machines, their initial history, as well as a technical introduction of the machines and the development process. The main chapters start with standard induction motors which are manufactured in large numbers and can be seen as a commodity with little product differentiation. The development focus has been on rational production and increased use for frequency controlled variable speed drives. Large directly water-cooled turbogenerators were developed for nuclear power plants, in the 1970’s, and created initially many difficulties. Advanced technologies and strategic matters have been strongly interlinked in this development, which is described comprehensively in the thesis. Electrical machines for automotive drivelines have, in recent years, been subject to intensive international development and several new concepts have been introduced. The thesis analyzes the Swedish attempts that have been technically satisfactory but have not led to commercial products. ABB launched synchronous machines for very high voltages as a revolutionary product ten years ago, but without commercial success. This controversial development and the difficult business situation are subject for discussion in the fourth main chapter

The study presents conclusions, concerning the development, individually for each machine type, but also comparisons based on divisions in large and small machines and in mature and new technologies. A common result is that the development, in retrospect, has been more successful from technical point of view than from commercial, independently whether the development has been market or technology driven. An important contribution of the thesis is that it presents the first comprehensive Swedish study of electrical machine development and which factors have been most influential. The thesis ends with a discussion of future prospects for the Swedish electrical machine industry and the possibilities and threats it is facing.

The Swedish electro-technical company ASEA reached very early a leading position in development and manufacturing of 3-phase generators and motors. Russia was one of ASEA's most important markets and it planned to start local production in the country. The revolution delayed these plans but finally an ASEA motor factory started in Yaroslavl in 1928. It was then one of the world's most modern induction motor factories. The factory was sold to the USSR in 1932 and it still remains an electric motor factory, nowadays named ELDIN.

Sweden is highly electrified and the electrical power engineers have had an important role in this development. Fundamental for these engineers is their education, their working experience, and their professional network. The Swedish development over time of these factors is highlighted. Organizations for engineers are essential parts of their network and the country has a long tradition in this respect. It started with local and national assemblies, but international organizations have successively grown in importance.

Large steam turbine-driven generators rated at a few hundred MW and higher constitute, in many respects, a big engineering challenge. The Swedish manufacturer of heavy electrical equipment, Asea was faced with this challenge in the late sixties, when the company started to develop such generators for nuclear power plants. Due to the company's background, it was necessary to choose new design concepts, and Asea decided on a very unique concept, turbogenerators with directly water-cooled rotors. The development led to difficult teething problems before the generators could be delivered and operate satisfactory; a process, which took around a decade to complete. Since then, the operation records have been very good. These turbogenerators constitute the only existing group with a significant number of two-pole, directly water-cooled rotors. The background, the development and results are summarized in this paper.

The Swedish government and the automotive industry cooperate in HEV related research programs, which primarily are carried out as postgraduate university projects. The aim is both to achieve technical results and to educate PhD graduated researchers for the industry. Two coordinated programs have resulted in an extensive network of specialists from academia and industry. A number of courses have also been developed for undergraduate students with focus on HEV related subjects. These are common for five technical universities. Special attention is paid to the Hybrid Vehicle Drives course and how the students step by step learn to use more accurate models for simulation of HEV performance.