Partial Discharge (PD) measurements are of great importance to enable the monitoring and diagnostics of HV systems. The requirements of the Paris Agreement and climate goals have fuelled the increase in penetration and demand of HVDC for offshore wind. The HVDC Gas Insulated Switchgear (HVDC GIS) is a reliable technology to support the necessary electrical infrastructure. Nevertheless, some in-service failures may occur. These failures can occur in the insulation system and thus developing a measurement system for PD detection is essential for monitoring and diagnostics. To monitor and diagnose the HVDC GIS, a novel Magnetic Antenna (MA) is being developed to operate in the high-frequency (HF) (30-300 MHz) range. The well-established UHF method for the GIS is typically used due to its high sensitivity and resilience to electromagnetic interference. However, the UHF method is unable to calibrate to apparent charges as this information is in the low frequency (up to 30 MHz) until HF range. The knowledge of charge calibration indicates the discharge type which is important in DC as DC does not have phase-resolved information as with AC. The appropriate frequency range of the MA should enable the measurements of the apparent charge and localize the defects when monitoring and diagnosing a HVDC GIS setup. The overarching goal is to develop a measurement system to measure PDs in the HF range in a GIS setup. For this purpose, MAs are created and investigated. A workbench has been built and developed to characterize the MAs and measure its frequency characteristics. A 380 kV GIS measurement setup has been developed. This enabled the measurement and acquisition of data of the discharges using the MAs. The Threshold Peak detection (TPD), Energy Criterion (EC), and Phase Method (PM) localization methods are investigated and implemented for localization of the source of defects. The PM is unable to localize the pulse due to its sensitivity to noise and reflections. The TPD and EC are both suitable with the TPD being the preferred method due to its 95% accuracy of localizing the defect within ± 1.5 m.    ... Read more

The factory acceptance tests are extremely important for the deployment and service life of Extra-high voltage transformers (EHV) and reactors. An essential part of such tests is Lightning Impulse (LI) tests which are intended to ensure that the transformer insulation withstands the transient lightning overvoltages which may occur while in service. These tests are usually done with negative polarity to prevent air side flashovers. In recent years, there has been an increasing demand for additional positive polarity lightning impulse tests to secure the reliability and life of the transformers. The transformer, during its service life, is subjected to massive electrical stresses due to lightning strikes and switching impulses. As a consequence, this may lead to the degradation of the transformer insulation. During lightning impulse tests, ionisations might occur in the pressboard/insulating liquid insulation system, which would lead to the development of space charges. The space charges generated may have an influence on the withstand behaviour of the insulation corresponding to applied impulse voltage. There is also a possibility of field enhancements due to the effect of space charge, especially when tested consecutively with opposite polarity. During the acceptance tests on transformers at SGB-SMIT transformers, a waiting period of one hour is given between the negative polarity LI and positive polarity LI. This arbitrary waiting time is given under the assumption that this will allow any ’trapped’ charges to decay and provide relaxation time for the slower polarisation processes. So the main question is whether this waiting period allows sufficient time for the decay of charges in the transformer insulation. For this analysis, the relaxation time characteristics of the transformer insulation system were investigated by estimation of the time dependency of depolarisation currents in the oil, paper/pressboard insulation. In this thesis, the characteristic of the time-domain dielectric response and the time dependency of the currents due to depolarisation of charges under DC and impulse voltage was investigated by the polarisation-depolarisation current (PDC) measurements on the test samples. The PDC method is a non-destructive diagnostic method for evaluating transformer insulation in the time domain. A test set up was built to represent a simplified model of transformer insulation comprising of mineral oil, paper and pressboard. The measurements of discharge voltage over time were conducted on the test samples of oil, paper and pressboard to understand the time-dependency of polarisation-depolarisation processes occurring within the transformer insulation. Two dominant time constants of decay were estimated for oil, paper and pressboard samples. The results from the discharge voltage measurement were compared with the analytical solution of the output voltage of the equivalent R-C circuit of the test sample which revealed that there were more than two polarisation phenomena occurring within the composite test sample of oil, paper and pressboard. Later, to mitigate the inconsistencies with the simplified R-C model, it was extended and modified based on the linear dielectric response theory to study the dielectric response behaviour of transformer insulation under DC voltage for longer charging times (tc=10,000 seconds). The modified R-C model was envisioned and developed in PSPICE simulation environment. The model incorporates the effect of the individual polarisation processes occurring within the constituent dielectrics of transformer oil-paper-pressboard insulation. The dielectric properties like conductivity and dielectric response function f (t ) can be estimated reasonably accurately with this modified R-C model. A comparison of the maximum and minimum values of the polarisation currents of the composite test sample obtained from the simulated model and from dielectric testing was conducted. The results demonstrated that relative errors were limited to a maximum of 8 %. The time-domain polarisation and depolarisation behaviour of composite transformer insulation was analysed under DC and impulse voltages from simulations of PDC measurements using PSPICE simulation software. It was observed that for the same thickness of solid insulation, as the oil-gap increases, the magnitude of the depolarisation current at the end of the discharging period (10,000 seconds) also increases. For identical oil-gaps, as the thickness of the pressboard in the composite test sample of oil-paper-pressboard was increased, the depolarisation currents show a delayed response to decay to a minimum value at the end of discharging duration. The time dependency of depolarisation currents at the end of discharging time of one hour was realised for impulse voltages. The study of the depolarisation currents under the influence of impulse voltage revealed that the charge induced field at the end of the discharging period of one hour does not exceed the permissible threshold electric field of 2 kV/mm inside the transformer insulation. In the future, the model could be developed into a valuable diagnostic tool for studying the dielectric responses of complex transformer insulation under the influence of different parameters like moisture content, ageing products, geometrical configuration and temperature. ... Read more

Following deregulation in the energy sector during the 1990s, which was also triggered by the ageing of infrastructure and the increasing demands from regulators and customers, many network utilities adopted the Asset Management (AM) in the hope to earn more, have better credit ratings and gain from stock prices. In line with this fact, the emergence of the AM international standard, such as the ISO 55000 series in 2014, gained rapid acceptance among network utilities around the globe.

AM has its core in the asset decision-making process. This activity lies simultaneously at the strategic, tactical and operational level of AM, over the lifecycle of the asset. In such an environment, the asset managing department should not only focus on the reliability of the asset but also on balancing costs, risks and asset performance. Regarding maintenance, the money spent on every maintenance task should benefit the company’s business values.
This thesis focuses on the development of decision-making tools for maintenance of high voltage AC (HVAC) gas-insulated switchgear (GIS) operating under tropical conditions. GIS has been chosen because of its critical role in the transmission network. Any GIS breakdown is usually expensive and requires an extensive outage. Moreover, under tropical conditions, this study observed GIS failure rates over twice the value reported by CIGRE’s survey of 2007. The study was conducted in this research’s case study termed the Java Bali (JABA) case study. The latter consists of 631 CB-bays of 150 kV and 500 kV GISs located in Java and Bali of Indonesia.

Today’s AM decision-making tools for electrical power grids are generally based on Asset Health Index (AHI) and risk assessment (RA) models. These models assist the asset manager in answering the following questions:
1. What is the condition of each GIS in the network?
2. Which one is more likely to fail compared to the others?
3. Which one is more critical compared to the others in terms of making a possible impact on the company’s business such that the mitigating action is prioritised?
4. What optimal action(s) is/are needed to be taken?

Developing the above-mentioned models requires sufficient knowledge of the characteristics of GIS operating under tropical conditions. To that purpose, both statistical analysis and forensic investigations in the JABA case study have been undertaken to find the critical condition indicators for the AHI model. The results are as follows:
1. The tropical conditions have influenced both directly and indirectly the performance of GIS. Corrosions at the exposed GIS parts were seen to have a common direct influence of tropical conditions. They can trigger leakages, secondary, and lead to driving mechanism subsystems’ failures, which reduce the GIS’ performance. The intensive and frequent lightning in tropical conditions is a so-called Failure Susceptibility Indicator (FSI), indicating that a failure mode is expected to initiate more likely than for the same GIS in other environments, especially if the surge arrester fails to protect. Moreover, the GISs outdoor and from the older generation are more susceptible to breakdown under tropical conditions.
2. A high amount of humidity was found in the non-CB enclosures of GIS from lower voltage class (i.e. Class 2 GIS with a voltage level of 150 kV). The origin of this humidity mainly comes from the desorption of moisture from the spacer or internal GIS surfaces during operation.
3. The critical failure modes in GIS operating under tropical conditions are as follows: dielectric insulation breakdown, loss of mechanical integrity in the primary conductor and failing to perform the requested operation due to driving mechanism failure.

Following this study’s findings, laboratory tests in the HV Laboratory of TU Delft were conducted to investigate the influence of high humidity content on the spacer flashover in GIS. The results confirmed without condensation, humidity has no impact on the withstanding strength of the insulation system under AC, LI+/- and SI. Our model also showed that the breakdown voltage under LI+ due to condensation at the surface of a solid insulator is lower than that due to a 2 mm metallic particle attached on the identical solid insulator at 3000 ppmV.

We applied the findings from both field investigation and laboratory tests into our models in the following ways:
1. In the AHI model:
a. Statistical and JABA lab case studies were performed to assess the system’s vulnerabilities and normative levels, in particular, the humidity content in GIS the non-CB enclosure as long as the value was far from the possibility of condensation.
b. The likelihood of failure is determined by so-called condition scale codes reflecting the deterioration of the subsystems.
c. The failure susceptibility indicators (FSI) flag deviating circumstances, such as heavy environmental conditions, operation and maintenance records and the inherent/design factor of GIS. The FSI are just an expectation that is not based on evidence as in a condition indicator. Therefore, the FSI work as warning flags for the decision-maker.
2. In the RA model:
a. Risk is defined as the likelihood of failure times the consequences. The result of the AHI defines the likelihood of failure in the RA model.
b. On the other hand, the consequences consist of seven business values of a transmission utility from the JABA case study, namely, safety, extra fuel cost, energy not served, equipment cost, customer satisfaction, leadership and environment.

We have successfully implemented these models on a GIS example from the JABA case study. Evaluation of possible risk treatments was also done using multi-criteria analysis (MCA) to optimise three parameters: cost, time-to-finish treatment and residual risk.

In practice, transmission utilities face more complex situations with more types of equipment in the network. The methodology discussed in this thesis, however, can be the cornerstone for the development of decision-making tools for other assets at the tactical level of AM as well. ... Read more

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. ... Read more

The share of renewable energy sources in the electricity generation is expected to maintain a steady growth in the future driven by economic and environmental reasons. However, these renewable sources such as wind and solar have fluctuating power output. This fluctuation causes strain on the power system and can cause imbalances between generation and load which may result in frequency instability. In the current liberalized energy market, the system operator uses ancillary services market to procure frequency containment reserve (FCR) which arrests undesirable frequency excursions within the first few seconds after the occurrence of an imbalance and ensures satisfactory primary frequency control. The system operator also procures frequency restoration reserve (FRR) which helps restore the frequency to its nominal value.
Electrolyzers can manage their demand of electrical energy for production of hydrogen (i.e. power-to-gas conversion) and it is possible to store that generated hydrogen for long periods which is an advantage compared to battery storage. This hydrogen can be used for several applications (e.g. transportation), and part of it can be used by fuel cells to provide electrical power back to the power system when needed. One of the technologies used in electrolyzers and fuel cells is the proton exchange membrane (PEM). Fuel cells and electrolyzers based on PEM technology are capable of rapidly changing the power set point to increase or decrease the power demand or supply, respectively.
This thesis studies the PEM electrolyzers and fuel cells and their ability to support the frequency stability through participation in the ancillary services market. Based on DIgSILENT PowerFactory software package, this thesis develops generic dynamic models for PEM fuel cell and electrolyzer for frequency stability studies and uses these models to assess their effectiveness in providing frequency support and participation in the FCR market. Numerical simulations are performed on two dynamic test systems: The North Netherlands 380 kV transmission and its extension to include a reduced size representation of the transmission systems covering the North-West Germany and South Denmark. Both dynamic test systems are developed in PowerFactory based on the detailed model of continental Europe built in PSS®E software package.
The developed model for the fuel cell shows close resemblance to the literature data for both dynamic and static performance especially in the linear operating range. The simulation results show that PEM devices can provide frequency support in the FCR market and results in improved frequency nadir and reduced oscillations during the post-disturbance period which is considerably better than what can be achieved by using the currently in operation primary frequency control of the conventional power plants with synchronous generators.
The numerical simulations also include sensitivity analysis to changing system operating conditions such as network size, location of PEM devices and system inertia. It is found that changing the location of the PEM devices or the size of the network does not affect the performance in supporting the frequency. Also, it is found that PEM devices provide significantly improved frequency response compared with synchronous generators at lower system inertia levels. Sensitivity analysis to changing control parameters for PEM devices such as the bid size and frequency droop showed that increasing the bid size or droop results in improved frequency response in the form of lower nadir.
Some confidential information within this thesis have been removed. To request the full version please contact Dr. Ir. Jose L. Rueda Torres. ... Read more

Detection of partial discharge (PD) in any high voltage equipment is of great importance to improve the reliability of the device and the power system. Characteristics of PDs, such as: rate of occurrence, magnitude, location and the type; gives information of the assets that must be considered. Nowadays, there are many different methods for sensing PDs in GIS (Gas Insulated Substation), however none of them can give fully information of the PD. The most important sensor is the antenna, with great advantages over other sensors, but still, with the limitation of not giving the magnitude of the discharge. The Delft University of Technology has patented a new method for measuring PDs in HVDC GIS. It is believed that this new method has great advantages over the current method, namely charge determination and improved sensibility.

The aim of this work is to find the measuring characteristics of this new method for sensing PDs in a GIS. These characteristics include resolution, attenuation and sensitivity. The most important feature of this new sensing method is the capacity of measuring the PDs magnitude. Therefore, a calibration method for charge calculation is provided. Finally, the new method and the antenna are compared with real measured PDs in a GIS.

To analyse the calibration of the new method; electrical characteristics of the sensor and GIS were simulated and compared with measurements. The results demonstrate that a PD can be calculated using the calibration method with low error values. This error value will depend in the measuring situation. The thesis also demonstrates some measuring advantages of this new method over the antenna.
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Cable terminations are essential for connecting a cable to a piece of equipment such as a circuit breaker, transformer, motor, etc. Proper terminations are of great importance in order to provide faultless cable systems. Improper terminations could result in overheating, partial discharges, eventual breakdown, flashover and breakdown in the system. There are several defects that can take place in terminations because of environmental, ageing and imperfections caused during manufacturing and installation . A potential defect is the contamination of oil with water due to sealing failures.

Water contamination can lead to partial discharges that, when produced over the insulation result in accelerated ageing. Typically, online electrical measurements of partial discharges can be used for termination monitoring. However, online monitoring suffers from noise and interferences, and localisation of partial discharges can be a problem. To overcome the challenges of electrical measurements, optical means of detecting partial discharge is researched in this thesis. The main goal of the thesis is to study the feasibility of using the optical PD detection method in oil-filled cable termination. The aim was to detect surface discharges at the rubber-oil interface through an optical sensor.

The identification of parameters for selection and final choice of optical sensor for PD detection application were done. Various setups to produce the surface discharge at the rubber-oil interface were investigated and one of the method was finalized. The simultaneous measurements of PD with electrical and optical sensors were conducted in dry oil and oil with different moisture content. The analysis of data obtained from optical sensor in comparison with electrical sensor based on sensitivity, PD pulse, PRPD pattern and correlation factor are discussed. It is noted that most of the characteristics of optical measurements is comparable to the electrical measurements. Thus, it can be said that this thesis contributes towards initial step of using the optical PD detection for online monitoring of HV components. ... Read more

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. ... Read more

The first commercial High Voltage Direct Current (HVDC) transmission link was installed in 1954 between the mainland of Sweden and the island of Gotland. Since then, there is continuous and remarkable development in the HVDC technology making it an efficient way of transmitting bulk power over large distances. Due to the transition towards low carbon electricity, more renewable energy sources are being integrated into the grid. This trend has increased the use of Direct Current (DC) in the electrical networks at low voltage levels.

In the past few decades, space charge phenomenon in HVDC insulation have been investigated. Many techniques have been developed for studying and understanding the space charge phenomenon in the HVDC insulation. Considering the growth of DC in electrical networks, further research into these techniques and development of novel dielectric materials suitable for DC is also progressing at a quick pace.

Earlier studies on space charge phenomenon were performed mainly on Polyethylene, Epoxy, Polymethyl methacrylate and Polycarbonate insulation materials. These studies have indicated that the space charge starts accumulating beyond a threshold value of the electric field and has a strong dependency on temperature as well as the electric field.

Silicon based insulation is also known as silicones. It is a highly stable and fire-resistant fluid. It is used in cable joints, traction transformers and increasingly in compact transformers where higher than normal temperatures are expected. However, it is known that when equipment is used with DC, they are expected to suffer strongly from space charge accumulation. As a result, considerable modifications in the electric field distribution with respect to the Laplacian field occurs, especially in case of voltage polarity inversion. This may cause insulation degradation and premature breakdown.

The conductivity behaviour and the electric field threshold for space charge accumulation are two of the most important parameters for the design of insulation systems under DC conditions. By limiting the electric field in insulation system below the threshold value, space charge accumulation can be minimised.

Reusing the existing AC cable joints under DC conditions could save considerable time as well as money in realising new DC networks. For that, conductivity behaviour and electric field threshold values of the insulation material should be investigated.

The main goal of this thesis is to investigate and characterise silicon insulation material with regards to its conductivity behaviour and the electric field threshold for space charge formation. This study is a first step towards the feasibility of using silicon fluid-based AC cable joints under DC conditions. The silicon insulation samples in liquid as well as cured(solid) state are subjected to conduction current measurements and space charge measurements using pulsed electroacoustic method.

Simulations are performed on a cable joint model in COMSOL Multiphysics. Presence of cured silicon layer results in a divergent current density. This resulted in high field concentration in cured silicon layer.

... Read more

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. ... Read more

Power outages can damage severely critical infrastructures such as telecommunication networks, financial services, water supplies and hospitals and completely shutting down production at companies. While most power blackouts usually last from some minutes to few hours, some can last days or even weeks. Furthermore, it is likely that these kinds of events would become more recurrent due to the stranded expansion of aged Electrical Power System (EPS) infrastructures in Europe for coping with the increased societal and environmental pressure for massive deployment of adopting Renewable Energy Source (RES)-based generation, which is known for having a variable characteristic. It is expected that RES will become the dominant factor in the power grid and will gradually replace Conventional Synchronous Generation Units (CGU), and it is known that Offshore Wind Farms (OffWF) are more suitable for large-scale generation applications. However, RES, including Wind Farms (WF), are not well aligned to work with EPS that were designed fifty - to - sixty years ago, due to reduced inertia, reduced short-circuit power, and limited control capabilities that imply the integration of RES. Moreover, more grids will contain more renewables, and thus more risks of outages could arise as one fault can trigger another one as a domino effect, deriving in widespread disruptions, including regional blackouts.

In this context, the contemporary control systems that regulate WF inject power to the grid via Power-Electronic Interfaces (PEI), and their schemes are designed to not interfere actively with the safe and secure regulation of a large-scaled EPS. On the contrary, they are just limited to inject a predetermined power injection setpoint with a current-injection control method that assumes all the time this power has load demand to go (grid-following control). However, this is not the case when unplanned islanding or an outage arises in an EPS, as the WF currently do not possess a control system that regulates the frequency of an island.

This MSc Thesis Project presents the design, implementation and testing of a control system attached to type-4 Wind Turbines (WT) that can manage and tightly ensure the load and generation balance during any circumstance, including a massive blackout. This control method can successfully regulate voltage, reactive power and frequency, which can be adapted automatically to the real-time conditions of the grid. The scheme takes the grid-following control approach as a starting point, which was modified in order to have Grid-Forming and Black-Start capabilities. The proposed new control approach for grid-forming and black-start design was implemented in DIgSILENT PowerFactory 2018, where a total of seventeen large-scaled type-4 WF are located into a three-area EPS. The Permanent-Magnet Synchronous Generator (PMSG) WF containing the proposed grid-forming control system are accompanied by Hydro, Nuclear and Thermal Plants, accounting CGU. Additionally, the three-area EPS also contains two HVDC Transmission Systems composed each by two Voltage-Source Converters (VSC), which also have similar grid-forming and black-start capabilities, and seven Battery-Energy Storage Systems (BESS), which give auxiliary power to the Black-Start Units and frequency support to the areas with loss of generation. The WF, BESS and the HVDC stations are interfaced via modelled Modular Multi-level Converter (MMC) controlled voltage sources.

The proposed grid-forming and black-start capabilities of the three-area EPS were tested with several EMT simulations reproducing severe short-circuit faults followed by a loss-of-generation scenario, the blocking of the HVDC converter stations and a massive relay protection program, resulting in the full outage and isolation of the area responsible for the largest power supply in the three-area EPS. After the blackout, the three-area EPS performed a Restoration plan, from the generation resetting and the reconnection of lines, transformers, etc., to the final (cold) load pick-up stage. In order to evaluate the advantage of using utility-scaled WF with grid-forming controllers to execute a conjunct Black-Start and Restoration plan, two operational scenarios were performed: one with 90% wind power share and another without any participation of wind power. As a consequence of the implementation of the proposed grid-forming control systems, the simulation results endorse that an EPS with 90% wind power share can steer a Black-Start and Restoration operation when required. ... Read more

The COBRAcable project is one of the major interconnectors being constructed in the North Sea, connecting the Netherlands with Denmark. It is a 325 km submarine cable which will have the ability to transfer 700 MW. The importance of the interconnector is manifold: to facilitate the exchange of renewable energy coming from the onshore Danish power system. Since electricity demand and supply of wind energy is geographically spread an adequate transmission capacity is essential for the growth of renewable energy. The link will also provide a strong connection that will enhance the security of supply in the Northwestern European region. That is in line with the ambitions of the EU for a stronger interconnected European electricity transmission grid and it will be a backup in case of breakdowns.
Moreover, it will enhance the internal European electricity market. COBRAcable will contribute to the development of the internal European electricity market and specifically contribute to the further investigation of the Northwest European electricity market. Lastly, COBRAcable has been granted a European subsidy for researching and developing activities necessary for the connection of wind farms to the cable.
The main focus of the present thesis is to create a RMS model of the COBRA cable project in PSSE software. The starting point of the master thesis project was a basic model of a VSC station which consisted of the basic VSC converter controllers: the active power controller, the reactive power controller, the AC voltage controller and the DC voltage controller. Gradually, the active and reactive power controllers were upgraded in order to support the special functions of the COBRAcable project. Also, an equation regarding the injection of reactive current was added to implement the fault ride through capability of the converter. In the end, the responses of the final model created throughout this master thesis project were compared against the responses of the model built in Powerfactory, which was created in more detail and was already compared against the results of the EMT model in PSCAD.
The modelling framework for VSC – based HVDC transmission system was initially developed and tested in a benchmark system consisted of two areas. Both areas had three buses connected in a meshed configuration and the performance of the controllers mentioned above was evaluated there. Following the benchmark system, the modelling framework was then tested with a reduced model of the Dutch power system around the Eemshaven region.
Different tests were defined in order to evaluate the suitability of the model and in more details to evaluate the performance of the controllers. To evaluate the performance of the active power controller several cases were introduced associated with the special functions of the COBRAcable regarding the regulation of active power. The reference of the reactive power was changed and the performance of the reactive power controller was tested while using different ramping rates. Moreover, the voltage at the point of common coupling was changed in order to assess the performance of the AC voltage controller. Finally, the equations regarding the injection of reactive current were used to evaluate the fault ride through capability of the converter.
From the simulation results, it was observed that the active and reactive power controllers were able to follow the changes of the reference power quite stably regardless the different ramping rates that were used. The reference changes created a dynamic behavior regarding the performance of the AC voltages of the buses in the Dutch power system. The investigation regarding the AC voltage controller led to the conclusion that there is a limit on how much you can increase and decrease the voltage at the PCC. The equations associated with the fault ride through capability have shown that the converter is actually trying to inject reactive power when the voltage has dropped below certain levels. In the end, the creation of a user – written model of the COBRA cable project in PSSE added an increased level of complexity. The absence of block diagrams with their respective signals as well as the required knowledge of the software made the translation of the control structure of the VSC station from the Powerfactory model difficult.
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The phenomenon of partial discharge has been long studied in various power components and in recent times, it has been agreed that the occurrence of such discharges can be considered as a vital parameter of component health. Partial discharges in power cables is a pervasive phenomenon that over time may lead to failure. For this reason, their monitoring has become increasingly sought after by asset owners. Nevertheless, the symmetry in the structure of a coaxial cable makes the isolation of the discharges in power cable more complicated when compared to other power components. In addition to this, there are added limitations to a given system with regard to the sensitivity of measurements and the maximum length of cable monitored, due to the increased attenuation towards high frequencies. Therefore, this thesis investigates the possible methods that could be used for online-monitoring of sensitive partial discharges (10 pC) in long cable systems ( >10 km), by means of new techniques or new applications of existing techniques. In this thesis a comprehensive feasibility study is followed by a thorough evaluation of the most promising methods and a demonstration of proof of concept of the parts thereof. The prime focus of this thesis is to investigate distributed measurement over a long cable system with the help of built-in sensor units. A built-in capacitive sensor type is modelled analytically, and simulated to check its performance before being tested experimentally in the last phase of this project. The final results indicate the feasibility of its implementation. Furthermore, the possibility of communication between such distributed sensor nodes is explored. One such method of communication proposed in this thesis is the wireless communication inside a cable using antennas. This thesis proves and quantifes to a certain extent the performance of an antenna (transceivers) inside a coaxial power cable paving the way for other tailor-made applications using `RFID-like' (Radio Frequency Identication) technology in the future. ... Read more

The accuracy of modelling distribution networks plays a very important role in analysing the stability of transmission systems. In recent years, due to the surge in the integration of renewable energy resources via power electronic interfaces, distribution networks are evolving from passive networks to modern active networks. This means that their characteristics are time varying following the operating condition of renewable energy sources. Real Time Digital Simulator (RTDS) is a specially designed hardware and software integrated computer system used to study Electromagnetic Transient (EMT) Phenomena in power systems. As the name implies, it can perform power system simulations at computational speeds equal to real-time operation. However, modelling of detailed distribution networks in RTDS would require many hardware resources. The goal of this thesis is to develop an equivalent dynamic load model for the stability analysis of transmission networks in RTDS, which should not only enhance RTDS’s capability of simulating large power systems, but more importantly, improve the accuracy of model distribution networks. Most studies about transmission systems do not require a full representation of distribution networks. However, the dynamic behaviour of distribution networks still needs to be preserved. Therefore, the dynamic equivalent parameters of distribution networks must be sufficient to ensure an accurate representation for the analysis of transmission networks. In this thesis, the identification and optimization of parameters are done using Mean Variance Mapping Optimisation (MVMO), a unique heuristic optimisation technique. The IEEE 34-Bus distribution system is used as a reference model for data collection and is developed in RSCAD, the RTDS software. An external grid is used to represent the transmission system wherein, several disturbances are simulated in order to compare the responses of the reference detailed model and the dynamic equivalent model. Furthermore, the validity of the developed DE load model is confirmed by comparing its behaviour to disturbances that were not
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At the end of a power transmission line, the cable should be terminated carefully, otherwise it will create problems during the operation. A special equipment called termination is designed to tackle down the problems from cable ends. The main functional part of a termination is a piece of rubber with a cone shape. This rubber cone has a semi-conductive layer which is connected with ground potential. By doing this, the cone smoothens the concentric equipotential lines at the cable end, so the electric field stress there can be decreased. The cone in the termination is called stress cone. However, insulation degradation on the termination stress cone due to water contamination is often found in its operation. This, in the end, will cause operation failures in the power network. This research aims to investigate the influences brought by water contamination on the stress cone, and discuss possibilities of detecting the presence of water content by using electrical methods. By using 3D FEM simulations, the changes of electric field distribution, especially the filed along the oil-rubber interface, were studied. Based on the simulation results, we built up a small scale test setup to observe how water contamination behaves on the oil-rubber interface, and measured the changes of partial discharge signals and tanδ values. As conclusions, we discussed the electrical ways of detecting the presence of water contamination on the stress cone, and gave suggestions for the future work. ... Read more

To enable the next generation subsea boosting and processing facilities, high power electrical connectors are strongly needed and considered one of the most critical components of the system.
Electrical tree growth is a precursor to electrical breakdown in high voltage insulation materials. Therefore, the study of the tree growth dependency with hydrostatic pressure is needed to understand the behaviour of the insulation material used in subsea connectors. Silicone rubber (SiR) is used as an insulation material for these applications thanks to its higher viscosity characteristic in comparison with other solid insulation materials used in subsea cables. This property is the main factor that allows the water to be swiped off the connector when a receptacle is mated into the plug of a subsea connector. In addition, the silicone rubber must provide similar electric field control as other insulation materials used in cable terminations and connectors.
The characteristics of partial discharges generated during the electrical tree growth and the light emission from the partial discharge pulses, have been studied under different pressure conditions. SiR samples, with a needle to plate electrode configuration, have been put into a pressure vessel to grow the electrical tree in the material under high hydrostatic pressure conditions. The electrical tree growth has been divided in three stages (initiation, intermediate and final or pre-breakdown stage) and tests have been performed at 1, 20 and 60 bar. A digital NIKON camera and a CCD camera have been used, both attached to a long-distance microscope, to observe in real time the tree growth and light emission, respectively.
Pictures showed a higher growth speed for the electric tree as voltage and pressure were increased. The length of electrical trees pre-grown at lower pressures collapsed faster as the pressure increased, than those pre-grown at higher pressures under the same pressure increasing conditions. As the pressure increased, Pulse Sequence Analysis performed to the partial discharges measured confirmed the partial discharge inception and extinction voltage increase and showed a polarity dependency to space charge generation in addition to other patterns regarding the charge magnitude and phase of occurrence characteristics. Pressure vessel internal reflections have suggested changes to be done in future studies for the light emission measurement. Finally, partial discharge patterns from the electrical tree growth process have been identified to be characteristics from void faults in the dielectric with a spherical void shape.
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The electrical properties of a widely used stress control tube for medium voltage heat-shrink joints are examined in this master thesis by using non-destructive laboratory methods. They were determined by use of time domain dielectric response test while changing the DC electric field, temperature and level of humidity.

In order to determine the physical and mechanical characteristics of the material at different aging stages and conditions, both differential scanning calorimetry and tensile test measurements were performed on stress control tube samples.

Two types of test samples were prepared. For dielectric response tests, Raychem JSCR 42/16 stress control tube was shrunk on a cylindrical rod, consisting of two metal electrodes, separated by a PTFE (Teflon) insulating rod.
For differential scanning calorimetry and tensile tests, the stress control tube was shrunk on a cylindrical PTFE rod only, with the same diameter as for the dielectric response tests without the presence of metal electrodes.

Tensile test measurements were performed in order to characterize and compare the mechanical properties of the stress control tube material at different aging stages.

Differential scanning calorimetry measurements were used to estimate aging degree by assessment of thermal resistivity of the material and antioxidants consumption when exposed to high temperatures for a certain period of time.

To calculate the conductivity, dielectric response measurements were performed in time domain at different voltage levels up to 20 kV and charging and discharging time duration of 10800s. They were conducted by placing the test object in a climate chamber in order to examine the influence of temperature and humidity. The measurements were performed on unaged and thermally aged heat shrinkable stress control tubes. The aging process was obtained in heat cabinets at 98°C dry air, 98°C wet air and 150°C dry air conditions.

Results from the analysis show that conductivity has not significant dependency of the electric field up to 0.2 kV/mm at different temperatures, aging degree and humidity.
It was revealed that conductivity is more temperature dependent, as a significant increase was observed when the temperature was elevated at higher values. Conductivity was found out to be strongly dependent on humidity, especially when combined with high aging degree.
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The load of the electric grid in increased by the connection of more renewable energy sources and less controlled power stations. The transmission operator has to decide where and when new overhead lines are required. However the operator can also chose to utilize existing lines more by increasing the transport capacity. This can be accomplished by applying dynamic line rating. In this case the conductor temperature and sag will increase but should not not violate the safety limits. Therefore a device can be installed on over-head lines to monitor the temperature and sag in real-time. Several devices are already manufactured and tested. However these devices require a reliable communication. In this thesis the feasibility and reliability of communication with Low Power Wide area Network (LoRaWAN) protocol in an overhead line environment is studied. The first objective is to determine the consequences of DLR on overhead lines. Second shielding of the electric and magnetic field in an overhead line is evaluated. Finally an antenna is chosen and a prototype is designed to evaluated its thermal behavior, low frequency magnetic shielding capabilities and communication feasibility and reliability. The first objective is to understand how the conductor temperature, the sag and magnetic field increases in respect to the transport capacity. The change in conductor temperature is examine by considering several heating gains and losses mechanisms. The sag is approached by a sagging and parabola curve to evaluate the decrease of distance to the ground. The maximum magnetic field is determined for close distances from the conductor.Two methods are examine to shield form the electric and magnetic field from the overhead lines. There-fore the materials copper, aluminum, steel and mu-metal are evaluated on their conductivity and permeability. The antenna is determined by comparing a dipole, loop and slot antenna on their complexity and expected radio noise interference. The prototype is exposed to a test setup in which the surface temperature, and magnetic field, is measured. The reliability of communication is evaluated by analyzing the amount of send and received packages.In case of an increased transport capacity the conductor temperature of an overhead line mostly increase by Joule heating. For a conductor temperature increase and concentrated weight the sag increases with negligible tents of centimeters. The maximum magnetic field was calculated and resulted in a required shielding effectiveness. The thickness of shield was determined for each material to reduce the magnetic field inside an enclosure. A slot antenna was designed and made in the prototype and compared with simulations in respect to the reflection coefficient. The surface temperature of the prototype did barely gain heat by the generated induced currents. However the prototype does reduce the magnetic field inside and confirms the theory of low frequency magnetic shielding. Furthermore the communication is tested and resulted in reliable data transfer in the case of a 0.5 cm distant from the conductor through which a current of 800 A flow. Furthermore corona discharges were generated up to 130 kV in which data still was transferred successfully ... Read more