Numerous High Pressure Gas Cables (HPGC) had been laid decades back by TenneT, the TSO of Netherlands and parts of Germany. Now, when TenneT decided to replace or increase the loadings on such cables, the question appears how would they behave to such temperature profiles and also the progress of their aging under such conditions. Only if a robust model is developed, can we understand which cable is closer to failure. Since all the cables cannot be replaced at once, prioritizing which ones are at the very near end and in the "red zone" of lifetime should be replaced first accordingly. But making a general model for all is not that simple, especially when there has to be a perfect balance between the academic research and also making it as realistic to the main scenario so that it can be applied on the field cables. Thus, the focus of the thesis is on observation of the physics-based processes taking place due to the effect of elevated temperatures on HPGC and the electro-thermal effect resulting in aging of insulation.

The whole research was divided into three sub questions leading to solving the big problem eventually:

1) How does the insulation degrade when the field aged HPGC are elevated to higher temperatures, but lower than the maximum designed temperature?

2) Can diagnostic tests be developed or modified focusing on the material physics inside the insulation that can be performed on the cable alongside Tan Delta at 50 Hz and Partial Discharge Measurements, that is viable in field with least online time?

3) What parameters can be extracted from measurements to develop a robust model that determines the degradation of HPGC insulation with thermal and electrical aging?

Initially, the cable loading profile was obtained from TenneT, cleaned using Machine Learning Algorithms and IEC Standard was applied to evaluate its operating temperature for last decade. Then, Fast, Short-Term Ramps as well as Long-Term Step AC Stress Tests were conducted on the samples to develop an aging model and evaluate the sensitivity of aging with elevated temperature. Once, an overall model has been developed, an electronic measurement box was designed to measure the Polarization and Depolarization currents (PDC) from unaged and aged samples. This was combined with a novel model developed using Frequency Dielectric Spectroscopy (FDS) for short duration online tests, and Partial Discharge (PD) Characterization. Using all the parameters obtained from the models fitted on the measurements of the experiments, trends were observed for these parameters which indicated the dielectric degradation with thermal aging. The conclusion clearly demonstrates how these indicators can be used during field tests to predict the progress of aging for these cables.

There has been a rapid development in the growth of Wind power generation in the last decade. This has led to wind power becoming a major contributor to the modern electrical grids. For the grids that are still based on conventional sources of energy, integrating wind power causes a serious impact on their performance. While the Windfarm must ensure the quality of power it supplies to the grid, the wind farm should itself remain resistant from the small disturbances coming from the grid. The power electronic converters used in the integration process are often responsible for the disturbance. These disturbances include, among other issues, the generation of certain harmonics based on the high-frequency converter switching. This affects the power quality and performance of the entire system. Certain harmonics are often amplified by resonating impedance. This phenomenon of harmonic resonance has been identified as a major cause of several grid failures.

In this master thesis, a study is carried out to investigate the generation and propagation of harmonics in a wind farm network. A modified IEEE-13 node feeder has been considered as a network accommodating a wind farm. The suggestive wind farm consists of 10 wind turbines, with each turbine connected at a different node of the modified IEEE-13 node feeder. These wind turbines are connected with the grid using an IGBT converter and an LCL filter. The switching topology and filter tuning has been discussed in detail. All the components comprising a wind turbine including cables, Voltage Source Converters (VSCs), transformers, filters and control system have been included.

The modelling is carried out in MATLAB/Simulink with the necessary data taken from the literature survey. A control system governing the switching of the IGBT converter has been developed. It consists of power control and a current control scheme. A DQ transformation scheme has been selected for the current control of the wind farm. A Phase-Locked Loop (PLL) is used to synchronize the converter with the grid and a Pulse Width Modulator (PWM) is used to generate gate pulses, controlling the switching of the IGBT switches.

The simulations have been carried out for 5 cases: the base case, de-tuned control system, disconnection of a part of the grid, wind speed dynamics and finally, a combination of all the above contingency cases termed as "the worst-case scenario". Each case is designed to represent the impacts inside a real offshore wind farm. A quantitative and qualitative analysis has been carried out for the harmonic distortions at different buses of the wind farm network under each case.
Results from the simulations show the harmonic orders that are common across the frequency spectrum in different cases. The base case provided relatively low voltage harmonics, but with more disturbances in the network, higher harmonic content was observed. The harmonic order corresponding (or nearest) to the switching frequency of the IGBT converter and the resonant frequency of the network, contributed the most for the Total Harmonic Distortion (THD).