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.

The use of esters as transformer insulation has been gaining increasing interest over the past few years because of their better environmental performance, higher resistance to the influence of moisture and for some particular liquids, higher flash point as well. However, in order to completely replace the traditional mineral oil and be considered as a viable insulating fluid for high voltage equipments, these liquids must have comparable responses to mineral oil under DC,AC, lightning impulse as well as switching impulse waveforms. The study of pre-breakdown phenomenon(streamer activity) under different stresses has proven to be crucial in determining the breakdown process taking place in transformer insulations. To study the streamer characteristics in ester (MIDEL 7131) and mineral oil (Shell Diala Transformer oil), both electrical and optical techniques have been used in this thesis. Significant research was conducted in identifying the parameters for the selection of the optical sensor suitable for this application. Various diagnostic techniques were employed for accurate detection of streamers electrically as well as optically. Streamers were initially studied in the highly non uniform needle-plane geometry under DC and lightning impulse stress. This configuration represents the situation of a defect in the transformer. Pressboard was introduced at the needle tip to simulate the effect of triple point as a source of discharge. Experiments conducted in Lodz University of Technology proved to be crucial in understanding the optical detection of streamers under this configuration. The main goal of this thesis was to detect streamers originating from transformer winding. For the first time, electrode arrangements involving transformer windings were tested in ester and mineral oil under lightning impulse for streamer activity. The thesis presents the various streamer signals obtained under different configurations and provides important analysis regarding the characteristics of streamers in transformer insulation. The possible effect of spacers between transformer windings and wooden blocks used in transformer coils in also analyzed. The tests performed in this thesis can contribute to the ultimate goal of understanding the difference in streamer propagation in real life electrode arrangements between ester and mineral oil.

Most ocean-going ships are fitted with an impressed current cathodic protection (ICCP) system. The currents impressed by this system onto the seawater find their way to corroding parts of the ship’s hull, and prevent the hull from corroding. The electric field resulting from the currents around the ship is called the electric signature. This signature is a threat in seas with mines, as mines explode when they detect this signature. Therefore controlling the signature is important, especially to naval ships. In order to be able to control the signature the location and size of corrosion damage needs to be known. Today no method exists for corrosion damage estimation on a sailing ship. This study aims to develop a method for corrosion damage estimation using only measurements of an ICCP system. The research question is: To what accuracy and with what measurements can the damage distribution on a ship’s hull be reconstructed? The quality of the damage reconstruction is evaluated by how well the signature is estimated. First an analytical model is used to explore the theoretical limits of damage reconstruction. This shows damage reconstruction is limited by the number of measurement electrodes. Then damage reconstruction by numeric model inversion is attempted, but is shown to be unattainable. The proposed solution is damage reconstruction by repeatedly calculating for different damage configurations, comparing the calculated measurements to the true ones, and iterating until an optimum is found. This method works well on a ship geometry if 140 measurement sensors are used. In order to verify the damage reconstruction algorithm a scale model facility is developed. The produced field sensors show shifting offsets that are not explained, but good linearity with the electric field. Simulations and measurements with a calibration source show good similarity, but a factor of 1.6 difference that is not explained. Test with sacrificial anodes show large variations over time and a large dependency on the sailing speed. The results show that corrosion damage reconstruction using only on-ship measurements can be achieved. The resolution of damage reconstruction is limited by the number of measurement electrodes, at least one hundred are needed for the signature to be resolved. Current ships have between one and eight measurement electrodes; it is recommended future ships will have at least one hundred. These results furthermore indicate that time-dependent effects should be included in the physical model used for the simulations.

The CIGRE B1 – B3.49 JWG defined a standardised 145 kV inner-cone GIS cable termination design. This standardisation allows the creation of new common interface insulators. This would eliminate the planning hurdles due to the fact that the cable system is not usually defined at the time of switchgear manufacture. The new design also requires a detailed study to find the relation between interfacial pressure and electrical performance of the epoxy/ silicon rubber interface.

The first step is to design and build a test setup to study the epoxy/ silicon rubber interface. Next, AC breakdown and lightning impulse tests are carried out. Additional AC breakdown testing with oil at the interface, defects on epoxy and heated samples are also carried out. The relation between interfacial pressure and electric field strength of the interface is found and documented. The effects of lubricant, defects and heat is used to further characterise the interface. Simultaneously, the silicon rubber is modelled using hyperelastic material modelling techniques.

The results from the tests and FEM models are used to propose two new designs of the 145 kV inner-cone GIS cable termination. The high repeatability of breakdown values and distinct features of this test setup have prompted the sharing of the experimental setup and results through an IEEE publication.

Managing electrical distribution assets entails making the decision on how to proceed with an ageing asset. The three options that asset managers have are run to fail, perform maintenance or repairs and lastly to replace. This decision making process is becoming incrementally more important due to the threat of replacement waves. This a phenomenon that comes forth from the fact that a large portion of the assets were installed around the 1970's, the so called installation wave, combined with the fact that these assets have a life expectancy of 40 to 60 years. It is up to the asset managers to accurately asses the condition of these assets and prioritize investments due to budget restrictions. Asset management has been evolving from time based maintenance (TBM) to condition based maintenance (CBM) and risk based maintenance (RCM). To aid in the decision making process for the latter two methods, accurate condition assessment and failure probability methods are needed. At Stedin, one of the three largest network operators in The Netherlands, the asset managers require a better condition assessment method for transformers, as the one being used now lacks certain capabilities; trend analyses and proper prioritization is difficult and time consuming. Furthermore, there lies the question whether the condition of the transformer can be used to improve the failure or survival probability model. The assumption is that the condition indicators of an individual asset can be used to adjust the population based probability models, which are then comprehensive and more accurate. In this thesis, an improvement to the transformer condition assessment is implemented and the effect of the condition indicators on the survival probability of the transformer is studied. Several condition assessment methods were reviewed, with the chosen method being the so called health index (HI). The results from this method is accurately reflected in the transformers which are to be replaced by Stedin. However, it has become clear that using one single number for decision making is not recommended, as the subsystems that are in moderate or bad condition might be masked by those that aren't. This can be compared to the analogy that a chain is only as strong as its weakest link, and thus the condition of that link should not be masked by the condition of the rest. For the survival model, machine learning and classic statistical methods were reviewed. The chosen method was Cox's Proportional Hazard Model, which has the advantage of being applicable to situations in which the underlying probability distribution of the events is unknown. Due to missing failure data, the survival model was used to model the effect of the condition indicators on the probability of corrective maintenances. Further constraints in the data quality lead to the confidence bands of the model's parameters to be relatively large. However, the univariate models do prove the significance of the effect of these condition indicators. The conclusion is that there is a relationship between the condition and the observed failures, but that proper documentation of failures is necessary to increase the accuracy of the model.