This thesis investigates the effects of winding faults on the doubly fed induction generator (DFIG) operation. A DFIG is a wound rotor induction machine that is most commonly used in present day wind power applications. In combination with a back-toback converter in the rotor circuit the DFIG is capable of generating power at constant frequency while operating at variable speed. The aim of the research in this thesis is to develop analytical and experimental tools that would enable the work on establishing of DFIG condition monitoring techniques. DFIG winding faults are examined using the machine current signature analysis (MCSA) method. This method is based on exploring the machine current frequency spectra, and defining and monitoring the fault specific changes in the spectra harmonic content. For this purpose a detailed analytical machine model is developed in this work. The model is capable of representing the effects that various winding faults have on the current spectra and is based on the generalized harmonic analysis. Higher order m.m.f. ~ ·harmonics are taken into account in the model calculations. Combining the model with a connection matrix enables the machine current frequency analysis for various healthy and faulty DFIG winding configurations. The model equations are solved in a time stepping algorithm based on Euler's method. For the purpose of experimental work a DFIG laboratory test rig was built. The test rig is designed to enable the simulation of various DFIG operating regimes and winding faults. The test machine stator winding terminals are taken out to a specially constructed terminal box, where by simple reconnecting of the terminals different winding configurations are achieved. The effect that open-circuit and short-circuit faults have on the DFIG current spectra harmonic content, when compared to healthy operating conditions, was analyzed in both the analytical model and on the test rig. The comparison of the model results against the experimental results shows that they are in very good agreement, thus verifying that the developed model is capable of predicting the real life operating conditions that are present in the simulated system. Some fault specific harmonic components are defined from model predictions and corresponding test rig experiments for each of the winding fault scenarios and DFIG operating conditions analyzed. !tis shown that the frequency of the fault specific harmonic components is dependant on the DFIG operating speed.

No description available.

A future power system with a large installed capacity of intermittent renewable power sources (RE) relative to its maximum system demand, also requires large capacities of controllable thermal power plant to cover periods of low RE generation. The most prominent example of intermittency is wind power, where natural fluctuations are challenging to achieve high penetrations, especially in islanded power systems. If high wind penetrations are to be realised, two carbon emissions problems associated with managing intermittency need to be addressed. Firstly, the requirement for flexible operation of back-up fossil-fuelled power plant increases with wind penetration in order to balance the intermittent supply with the time-varying demand. This results in a carbon penalty that increases with wind penetration. Secondly, if at any time wind power plant generation exceeds that which can be safely absorbed by the power system, some of the available RE input needs to be curtailed. The curtailment of wind generation inhibits the production of low-carbon electricity and penalizes efforts to achieve high wind penetrations. The value of wind penetration at which such measures need to be taken depends on the characteristics ofthe specific power system, but an islanded power system without significant interconnections is the most challenging to manage. Solutions are therefore required for regions of high wind resource to facilitate the achievement of high wind penetrations. The solution presented in this thesis is to deploy water electrolysers as controllable loads for load management exclusively in case of 'valley filling'. In combination with hydrogen storage systems, electrolysers can thus be used for hydrogen production both in the case of a fluctuating excess supply (e.g. during prolonged and rising RE generation) and during periods of low electricity demand. The supply of electricity becomes effectively decoupled from the demand in such a way that the operation of power plant depends less on consumer demand. An analysis is carried out to assess the mass implementation and operation of a stock of .electrolysers in combination with wind power plant (WPP) and zero-carbon thermal power plant (ZPP, e.g. nuclear, CO2-sequestered). Three electrolyser implementation cases were simulated for increasing WPP and ZPP penetrations and periods of different · d availability. The key objectives are: (i) increasing the penetrations of RE in the power system (by reducing wind energy curtailment); (ii) maximizing the efficiency of utilization of FPP (by maximizing the load factor of the aggregate FPP load profile, LFTH); and (iii) creating a source of zero/low-carbon hydrogen. A generic simulation tool, namely the AELM model, has been developed for implementing and controlling a large stock of electrolysers for an islanded power system. From power plant availability, demand and RE forecast profiles, the AELM model generates utilization strategies for the electrolyser stock, ZPP, WPP and FPP. Preferred capacity levels are obtained for the required stock of electrolysers as a function of the penetration of WPP and ZPP in the power system. Other general outputs are energy balances, hydrogen yields and carbon intensities for electricity and hydrogen for the time period analyzed. Results are presented for an isolated power system based on wind generation and demand data for Eastern Denmark. It is found that load management via electrolysers is an attractive option with the view of optimizing the operation of the power system. LFTH of up to 100% can be achieved (a virtually flat FPP load profile) at wind penetrations::::: 50% of system maximum demand (SMD). For high wind penetrations the electrolyser stock must include implementations close to WPP if wind curtailment is to be avoided. Results also indicate that the deployment of ZPP in addition to WPP is a considerable benefit. In particular much greater hydrogen yields and electrolyser utilization factors can be obtained especially on days of low wind availability, thus solving the main drawbacks of a pure wind-hydrogen (or more generally renewables-hydrogen) implementation. The key parameters of the analysis presented are system-specific, and the outcomes for different energy/power systems will be different. The intention is to establish a generic methodology and the boundary conditions for the deployment of a large electrolyser stock in any given power system. The approach presented could be a valuable tool in the decision-making processes towards more sustainable energy systems and eventually towards a prospective hydrogen economy.

No description available.

Simulations of switching transients were carried out in EMTP-RV. Overvoltages in offshore wind farms ranged from temporary over voltages to very fast front transients. Transient Recovery Voltages of the offshore circuit breakers exceeded IEC 62271 requirements in some situations. The disconnection of an array produced the most severe overvoltages, exceeding IEC 60071 requirements.

This thesis is concerned with the development of probabilistic risk assessment for substation earthing systems. A number of key parameters have been studied in detail which led to the development of a new approach that incorporates the recommendations of applicable standards and uses historical system fault data to take account of the probabilistic nature of load, protection systems and grid characteristics. In this work, an in-depth appraisal of existing standards has led to the development of safety-limit surfaces that can be used to quantify the benefits/disadvantages of particular earthing standards. The investigation has revealed that there are substantial differences between the recommended values of tolerable voltages attributed to a combination of factors: (i) difference in assumed tolerable body current (ii) differences in the parameters of the electrocution circuit (iii) differences in the predicted touch voltage and (iv) differences in the assumed worst-case shock location. The electricity industry, like other industries, is looking to reduce risk to minimum feasible while maintaining costs of mitigation within acceptable limits. Therefore, a more rigorous and comprehensive procedure of probabilistic risk assessment of the earthing systems is required. In order to achieve this, a detailed analysis of all parameters was undertaken in this work using more representative accidental circuits and parameters extracted from historical system fault data, provided by the collaborating transmission companies. These parameters were modelled and integrated into the proposed probabilistic risk assessment process. Work within this investigation on improving accuracy of calculation of heart fibrillation has led to the development of a probability surface of ventricular fibrillation and a computerised process that determines an accurate probability for a given body current and shock duration. This procedure takes into consideration the body current path and eliminates reading errors or assumptions that could result into conservative or optimistic conclusions. The above fundamental investigations on parameters affecting the overall risk were implemented in a new computerised risk assessment procedure CRAFTS suitable for transmission systems. CRAFTS allows a probabilistic risk assessment of the system under investigation. The application integrates the recent developments in the latest standard IEC 60479-1 and the developed probability surface of ventricular fibrillation. A case study performed on a typical grid has shown that the developed program is very useful when applying sensitivity analysis of the various parameters of the system and accidental circuit. The proposed full probabilistic risk assessment method incorporates the earthing system simulation results performed by specialised software, namely CDEGS, making it possible to simulate different electrocution scenarios throughout the substation instead of assuming a 'worst case scenario' and exposure to maximum possible voltages. Overall, the research in this thesis, offers an integrated solution of probabilistic risk assessment of earthing systems in aid of sound cost/benefit analysis and decision making.

Transformers subjected to PWM voltage excitation are becoming more and more common in industrial applications and renewable energy supply systems. Therefore, the assessment and improvement in the performance of transformer cores under PWM voltage excitation have become prominent. This project intends to characterise the no-load performance, total power loss and acoustic noise level, of a model three-phase, three-limb laminated transformer core, operating under sinusoidal and PWM voltage excitation. Measurements of total power losses and localised flux density in the joint regions of the core under sinusoidal and PWM voltage excitation for assigned modulation index ma with switching frequency fs varied from 1 kHz to 3 kHz, has been carried out. The analysis highlights the form factor Kf of the secondary induced voltage as a key parameter in controlling the performance of eddy-current component loss in the core. The estimate localised rotational losses due to rotating flux and planar eddy-current losses due to normal flux density in the joints have been analysed, also the analysis of measurement results contributes towards a better understanding of the influence of ma and fs. Moreover, the acoustic noise level of the core and the corresponding vibration at investigated points on the core surface has been measured under sinusoidal and PWM voltage excitation. Measurement results show that the values of acoustic noise and core vibration under PWM voltage excitation were much higher than under corresponding sinusoidal voltage condition. Also, the magneto-mechanical resonant phenomenon of the core under PWM voltage excitation has been observed, which was due to switching frequency fs close to the resonance vibration frequency of the core laminations that is a possible cause of increasing acoustic noise. The measurement results inferred that the resonant phenomenon could possibly occur in cores with different length laminations leading to variability of noise output according to how close the magnetising frequency or predominant harmonics are to the resonant frequency.

The main focus of this thesis was to assess the performance of a full scale Horizontal Axis Tidal Turbine (HATT), using the CFD package, FluentrM, and measured high shear tidal profiles. Two sites are considered: the Anglesey Skerries and a site in the Severn Estuary, both off the Welsh coast. In order to achieve this aim a number of key steps were performed including the use of an existing laboratory scale prototype HATT to establishing the optimum blade pitch angle and provide an experimental data set. Once established the HATT CFD model was used to scale up from the laboratory scale to 30 m diameter. By the use of non-dimensionalised characteristics of power, thrust and torque coefficients, it was shown that the HATT was scaleable and independent of Reynolds number. Using these findings a suitable turbine diameter was determined for site specific analysis. Velocity profiles from the two sites were obtained via vessel mounted Acoustic Doppler Current Profiler (ADCP) surveys. These data were used to define a high velocity shear environment. When non-dimensionalised these data were found to also collapse onto the scaling curves provided a true average for the velocity, across the swept area, is used. In addition, when the HATT was 'positioned' at varying depths down the water column the power extraction was shown to reduce considerably with depth. When positioned close to the seabed, the cyclic torque, power and axial thrust loads were studied with and without a stanchion positioned downstream of the turbine. The presence of a stanchion was also shown to significantly increase the amplitude of the cyclic torque, power and axial thrust during rotation. The findings of this thesis suggest that the complexity of the dynamic torque, power and axial thrust, along with the wake profile, are influenced by the HATT's interaction with the ocean seabed. These complexities are therefore of prime importance when considering a deep water application which encompasses all or part of a high velocity shear regime. The work presented in the thesis shows that it is possible to predict a turbine's performance (for a given geometry) for any scale and velocity profile, from a single diameter. When positioned lower in the water column, the downstream wake also showed a high level of asymmetry which was also shown to influence the upstream flow field.

Integrated Operation is a strategy to operate the Energy MicroGrid as an integrated system. In this strategy, a CHP plant, an electric water heater, and thermal storage were operated to support both electricity and heat networks of the Energy MicroGrid. An Integrated Optimal Power Flow was formulated and was used to investigate the advantages of this Integrated Operation. Simulations indicated that Integrated Operation minimised generation curtailment and load shedding. It also reduced electricity imported from the main grid and the operating cost.

The UK government’s policy to achieve a 20% renewable energy generation target by 2020, will require significant amounts of SSEG (Small-Scale Embedded Generation) to be connected. In addition to the expected economic and environmental benefits, the anticipated growth in SSEG brings with it numerous challenges for the operation of low voltage and medium voltage distribution networks. At present, there are a number of competing active network management concepts being considered to overcome these challenges and at Durham University a concept defined as the Small Scale Energy Zone (SSEZ) has been proposed and is investigated as part of this research. To further this, a bespoke active low voltage distribution network emulator known as the Experimental SSEZ has been developed by the author. Controllable emulated SSEG, controllable energy storage and controllable emulated load are incorporated into this laboratory. A transformation system has been developed to relate the operation of this system to that of low voltage distribution networks. Centralised and distributed network control systems have been developed for the Experimental SSEZ. These systems were used to evaluate, in conjunction with the relevant literature, the implementation of similar systems on future low voltage distribution networks. Both centralised and distributed control system architectures were found to have their merits. This research should therefore be useful in informing design decisions when developing and implementing active distribution network management systems on LV networks.