This research attempts to quantify the complex interactions between HVDC transmission schemes in a multi-infeed configuration, particularly with regard to the voltage interactions and the commutation failure phenomena. The in-depth analysis of multi-infeed HVDC systems discussed in this research shows the application of several indices such as the MIIF, MIESCR, and CFII, that can provide researchers and planning engineers in the area of HVDC transmission with the necessary tools for their system studies. It shows that these indices are applicable in a multi-infeed system comprising HVDC schemes with different ratings. The Multi-Infeed Interaction Factor (MIIF) quantifies the level of voltage interactions between converter ac buses. The Multi-Infeed Effective Short Circuit Ratio (MIESCR) index is an indicator of ac system strengths with regard to the assessment of the transient overvoltage (TOV) and the power-voltage stability of multi-infeed HVDC systems. The Commutation Failure Immunity Index (CFII) utilizes electromagnetic transient simulation programs to evaluate the immunity of an HVDC converter to commutation failures. The CFII takes into account the ac system strength and the HVDC controls and evaluates their impact on the commutation process. The immunity of both single-infeed and multi-infeed systems to commutation failure phenomena are accurately evaluated and quantified by the CFII. Using the CFII, it is shown that the current commutation in multi-infeed HVDC schemes could fail under circumstances in which the probability of failure had been perceived to be low. The causes of, the effects of, and the remedial actions needed to deal with such anomalous commutation failures are discussed in this thesis. The capability of the new indices to provide an insight into the interactions phenomena in multi-infeed systems are clearly demonstrated by examples that show their application in the analysis of an actual multi-infeed HVDC system that is in the planning phase in the province of Alberta in Canada.

This research attempts to quantify the complex interactions between HVDC transmission schemes in a multi-infeed configuration, particularly with regard to the voltage interactions and the commutation failure phenomena. The in-depth analysis of multi-infeed HVDC systems discussed in this research shows the application of several indices such as the MIIF, MIESCR, and CFII, that can provide researchers and planning engineers in the area of HVDC transmission with the necessary tools for their system studies. It shows that these indices are applicable in a multi-infeed system comprising HVDC schemes with different ratings. The Multi-Infeed Interaction Factor (MIIF) quantifies the level of voltage interactions between converter ac buses. The Multi-Infeed Effective Short Circuit Ratio (MIESCR) index is an indicator of ac system strengths with regard to the assessment of the transient overvoltage (TOV) and the power-voltage stability of multi-infeed HVDC systems. The Commutation Failure Immunity Index (CFII) utilizes electromagnetic transient simulation programs to evaluate the immunity of an HVDC converter to commutation failures. The CFII takes into account the ac system strength and the HVDC controls and evaluates their impact on the commutation process. The immunity of both single-infeed and multi-infeed systems to commutation failure phenomena are accurately evaluated and quantified by the CFII. Using the CFII, it is shown that the current commutation in multi-infeed HVDC schemes could fail under circumstances in which the probability of failure had been perceived to be low. The causes of, the effects of, and the remedial actions needed to deal with such anomalous commutation failures are discussed in this thesis. The capability of the new indices to provide an insight into the interactions phenomena in multi-infeed systems are clearly demonstrated by examples that show their application in the analysis of an actual multi-infeed HVDC system that is in the planning phase in the province of Alberta in Canada.

Voltage source converter high voltage direct current (VSC-HVDC) technology has become increasingly cost-effective and technically feasible in recent years. It is likely to play a vital role in integrating remotely-located renewable generation and reinforcing existing power systems. Multi-terminal VSC-HVDC (MTDC) systems, with superior reliability, redundancy and flexibility over the conventional point-to-point HVDC, have attracted a great deal of attention globally. MTDC however remains an area where little standardisation has taken place, and a series of challenges need to be fully understood and tackled before moving towards more complex DC grids. This thesis investigates modelling, control and stability of MTDC systems. DC voltage, which indicates power balance and stability of DC systems, is of paramount importance in MTDC control. Further investigation is required to understand the dynamic and steady-state behaviours of various DC voltage and active power control schemes in previous literature. This work provides a detailed comparative study of modelling and control methodologies of MTDC systems, with a key focus on the control of grid side converters and DC voltage coordination. A generalised algorithm is proposed to enable MTDC power flow calculations when complex DC voltage control characteristics are employed. Analysis based upon linearised power flow equations and equivalent circuit of droop control is performed to provide further intuitive understanding of the steady-state behaviours of MTDC systems. Information of key constraints on the stability and robustness of MTDC control systems has been limited. A main focus of this thesis is to examine these potential stability limitations and to increase the understanding of MTDC dynamics. In order to perform comprehensive open-loop and closed-loop stability studies, a systematic procedure is developed for mathematical modelling of MTDC systems. The resulting analytical models and frequency domain tools are employed in this thesis to assess the stability, dynamic performance and robustness of active power and DC voltage control of VSC-HVDC. Limitations imposed by weak AC systems, DC system parameters, converter operating point, controller structure, and controller bandwidth on the closed-loop MTDC stability are identified and investigated in detail. Large DC reactors, which are required by DC breaker systems, are identified in this research to have detrimental effects on the controllability, stability and robustness of MTDC voltage control. This could impose a serious challenge for existing control designs. A DC voltage damping controller is proposed to cope with the transient performance issues caused by the DC reactors. Furthermore, two active stabilising controllers are developed to enhance the controllability and robust stability of DC voltage control in a DC grid.

To ensure the safe operation of high-voltage direct current grids, circuit breakers are used to disconnect a faulty link from the rest of the grid. Incorporating vacuum interrupters as a part of these circuit breakers constitutes an outstanding technology for such DC interruptions. However, testing the interrupters take a long time and can be very expensive. Hence, to reduce the time and cost of testing the interrupters, the purpose of this project was to find the most important parameters to test in a vacuum interrupter to evaluate it for use in a DC circuit breaker. This was done by modeling the particle density, before and after current-zero, and the post-arc current using a new model along with existing ones. Review of existing research was also included to support the models in order to draw conclusions regarding reignitions and restrikes. dI/dt before current-zero, dV /dt after current-zero, and contact gap length were found to be the key contributors for reignition, while temperature, contact surface condition and contact gap length were of great importance for restrikes. These breakdowns should occur around the center of the contact surface, or at surface protrusions. The following parameters should be varied when testing vacuum interrupters: dI/dt before current-zero, ranging from 10 A µs−1 to 800 A µs−1; arcing current, from 1 kA to 20 kA; arcing time, from 1 ms to 4 ms before current-injection; dV /dt after current-zero, from 0.5 kV µs−1 to 20 kV µs−1; maximum TRV from 5 kV to 25 kV, to find the threshold voltage for failed interruption; and gap length, from 1 mm to 10 mm, to find the critical gap length. Furthermore, temperature should also be measured, though measuring postarc current seems to be of lesser importance. To minimize damage to the interrupter, it was recommended to start with higher gap lengths with low values on everything else. / För att säkerställa ett stabilt kraftflöde för högspända likströmsnät används brytare för att isolera defekta delar från resten av nätet. Vakuumbrytare kan utgöra en viktig komponent i sådana likströmsbrytare. Att testa brytarna tar dock lång tid och kan bli mycket dyrt. För att minska på tiden och kostnaderna för att testa brytarna, var syftet med detta projekt att hitta de viktigaste parametrarna att testa i en vakuumbrytare för att utvärdera den för användning i en likströmsbrytare. Detta gjordes genom att modellera partikeldensiteten före och efter nollgenomgången i strömmen, och strömmen som uppstår efter den nollgenomgången genom att använda en ny modell med hjälp av äldre modeller. Granskning av befintlig forskning inkluderades också för att stödja modellerna och dra slutsatser om nytändning och återtändning. dI/dt före strömnollgenomgången, dV /dt efter strömnollgenomgången, bågströmmen och kontaktseparationen visade sig vara nyckelparametrar för nytändning. Temperaturen, kontaktytstillståndet och kontaktseparationen var av stor betydelse för återtändning. Dessa elektriska urladdningar hade en högre sannolikhet att ske närmare mitten av kontaktytan eller vid små spetsiga ojämlikheter. Följande parametrar bör varieras vid testning av vakuumbrytare: dI/dt före strömnollgenomgången, från 10 A µs−1 till 800 A µs−1 ; strömmen före strömnollgenomgången, från 1 kA till 20 kA; ljusbågstiden, från 1 ms till 4 ms; dV /dt efter strömnollgenomgången, från 0.5 kV µs−1 till 20 kV µs−1 ; maximala TRV, från 5 kV till 25 kV, för att hitta tröskelspänningen för misslyckat brytning; och kontaktseparationen, från 1 mm till 10 mm. Att mäta strömmen efter strömnollgenomgången verkade vara av mindre betydelse. Dock föreslogs det att mäta temperaturen på vakuumbrytaren. För att minimera skador på brytaren rekommenderades det att börja testandet med högre kontaktseparationer med låga värden på allt annat.

The present work was performed at HVDC ABB as an initial study on how to adopt probabilistic concepts into the VCSHVDC insulation coordination. Due to large voltage levels in HVDC applications the corresponding insulation need to be properly addressed to ensure a safe, economical and reliable operation. Traditionally, only the maximum overvoltage is considered, where no adoption to the shape of the overvoltage distribution is regarded. Use of probabilistic concepts in the insulation coordination procedure can ideally reduce insulation margins with a maintained low risk of flashover. Analysis and understanding of probabilistic concepts of AC systems is needed in order to implement the concepts into VSC-HVDC. With use of advanced VSC-HVDC models, faults are simulated with varied fault insertion time in PSCAD. The resulting overvoltages from the simulation is gathered using different statistical methods in order to obtain the approximated overvoltage distribution. It was found from the simulation results that use of a Gaussian distribution is inappropriate due to shape variety in the overvoltage distributions. Instead, Kernel Density Estimate can serve as a flexible tool to approximate overvoltage distributions with a variety in number of modes and shape. The retrieved approximated overvoltage distributions are compared with the insulation strength in order to calculate the risk of flashover. The comparison shows that the insulation can be tuned in order to match set requirements. The thesis work should be seen as pilot study, where key problems have been pointed out and recommended further studies are proposed.

This thesis proposes a novel consolidated approach for substantial compaction of HVDC lines that includes both new tower geometries as well as novel control concepts. This is based on a thorough discussion on the basic overhead line design parameters and their impact on the right of way width and tower height. Then the electrical aspects of the new approach such as dc overvoltage assessment and lightning performance are investigated. The required horizontal clearances between pole conductors and tower members, as a component of the right of way width, depend on the maximum expected overvoltages. Detailed electromagnetic transient models for the point to point MMC HVDC with different transmission configurations, all including the proposed dc overhead line, are developed for this thesis. The models are used to assess fault contingencies that result in the most significant overvoltage stresses on the HVDC transmission line for finding minimum air clearances and for the design of overvoltage limiting devices, such as surge arresters. New control approaches are proposed that significantly reduce the dc side overvoltage and consequently minimize the required air clearances for maximum compaction of the HVDC overhead lines and also reduce the required surge arrester size for line insulation. Because power transmission lines are the most exposed component within a power system, they are subject to lightning strikes which, in turn, are the main cause of disruption to power flows. This thesis will include an analysis of lightning occurrence on the proposed compact transmission line in order to assess the risk of pole faults. The focus of this analysis is mainly on evaluation of the critical lightning currents that cause fast front overvoltage stresses that may result in insulation failure. / May 2017

Com o aumento de uso de fontes renováveis para geração de energia e o aprimoramento da tecnologia de conversores fonte de tensão (VSC – Voltage Source Converter), sistemas baseados em corrente contínua não só se tornaram viáveis como também se tornaram uma alternativa vantajosa em diversas situações. O uso de diversos conversores conectados em uma rede, isto é uma rede VSC-HVDC multiterminal, se apresenta como um caminho para a conexão de diversas fontes, como geradores eólicos e solares de maneira eficiente e economicamente vantajosa. No entanto, este tipo de rede possui algumas limitações, por exemplo, é mais sensível a faltas que um sistema de corrente contínua baseado em conversores do tipofonte de corrente (CSC – Current Source Converter). Desta forma, o estudo de faltas em sistemas VSC-HVDC é necessário para o desenvolvimento de mecanismos de proteção para estes sistemas. Neste contexto é importante o desenvolvimento de metodologias para a localização de faltas em sistemas MTDC, uma vez que este tipo de informação pode ajudar as equipes de manutenção a encontrarem as falhas e resolverem o problema da maneira mais rápida possível. Embora existam diversos estudos e metodologias desenvolvidas para sistemas de corrente alternada, ou para sistemas de corrente contínua de dois terminais, os estudos em redes de corrente contínua multiterminais ainda são poucos e em geral atendem a apenas um tipo de rede, mostrando assim a necessidade de mais estudos na área. Desta forma, o objetivo desta dissertação de mestrado é o desenvolvimento de uma metodologia localização de faltas em um sistema HVDC multiterminal, que contará também com técnicas já propostas na literatura para a detecção das falhas.Neste documento são mostrados o embasamento teórico, os estudos realizados para o desenvolvimento do tema, bem como os resultados obtidos para a localização de faltas em um sistema teste de simulação. / The increase of the use of renewable sources for power generation and the enhancement of Voltage Source Converter (VSC) technology, DC-based systems have become viable and an advantageous alternative in many situations. The use of several converters connected in a network, ie a multi-terminal VSC-HVDC network, is presented as a way to connect several sources, such as wind and solar generators in an efficient and economically advantageous way. However, this type of network has some limitations, for example, it is more sensitive to faults than a DC current system based on Current Source Converter (CSC). Therefore, the study of faults in VSC-HVDC systems is necessary for the development of protections for these systems. In this context it is important to develop fault location methodologies in MTDC systems since this type of information can help maintenance teams to find fault and solve the problem as quickly as possible. Although there are several studies and methodologies developed for alternating current systems or for two-terminal DC systems, there are few studies on multiterminal DC networks and this studies generally address only one type of network, thus showing the need for more studies in the area. Thus, the objective of this master\'s thesis is the development of a fault localization methodology in a multi-terminal HVDC system, which will also have techniques already proposed in the literature for the detection of faults. In this document are shown the theoretical background, the studies carried out for the development of the theme, as well as the results obtained for fault localization in a simulation test system.

This thesis treats the subject of a complete steady state and dynamic model of the VSC-HVDC covering both the AC and DC system-side of the converter. The topology of the model is recreated after the scheduled transmission line in the south of Sweden, called SydVästlänken. The topology covers both a simple two terminal connection as well as a multi-terminal one. This model is to be implemented in the power system simulation program ARISTO. The main directive of the model is operation planning and education during real-time scenarios. The model is deliberately designed as a complete and complex model but with methods of reducing it's complexity to suit the users needs at the time for implementation. Further more the author have made sure that it is a complete generic model to suit the application of the program as the technology to the day of this study are unknown. Lastly, a method of controlling the converters are presented where Droop-control take a prominent roll as the AC and DC voltage regulator.

SaskPower has two separate systems, namely the North and the South systems. The South system contains SaskPower major generation and system load. The North system load is located relatively far from its generation (200 to 300 km). The North system is considered, therefore, to be electrically weaker than the South system. Recently there has been an interest in connecting the two systems to improve the security, stability and reliability of the integrated system. Grid interconnections, however, especially between weak and strong systems, often result in the arising of low-frequency oscillations between the newly connected areas. These oscillations that are termed “inter-area oscillations” exhibit, generally poor damping and can severely restrict system operations by requiring the curtailment of electric power transfers level as an operational measure. There are two options for SaskPower North and South systems interconnection, namely HVAC and HVDC interconnections (tie-lines). This thesis reports the results of digital time-domain simulation studies that are carried out to investigate the dynamic performance of a proposed 260 km, ± 110 kV, 50 MW Voltage-Sourced Converter HVDC tie-line that would connect SaskPower North and South systems. The potential problems that might arise due to such an interconnection, namely power flow control and low-frequency oscillations are studied and quantified and a proposed feasible solution is presented. In this context, the effectiveness of the HVDC and a Power Oscillations Damping (POD) controller in damping power system oscillations in the tie-line is investigated. Time-domain simulations are conducted on the benchmark model using the ElectroMagnetic Transients program (EMTP-RV). The results of the investigations have demonstrated that the presented HVDC link and its POD controller are effective in mitigating the low-frequency oscillations between the North and South systems at different system contingencies and operating conditions.

Offshore wind energy is seen as the most promising source of electricity generation for achieving the European renewable energy targets. A number of wind farms are planned and under installation to collect the huge potential of wind energy at farther distances in the North Sea. The number of HVDC links in the North Sea is expected to increase with the development of offshore installations in Round 3 of the UK offshore windfarm programme. The increasing number of HVDC links and high power transfer control requirements leads to the formation of Multi-Terminal HVDC (MTDC) grid systems, which have become possible due to the technical advancements of VSC based HVDC systems. Additionally, a meshed MTDC grid structure can also provide interconnections for power trade across the Europe, which can help in better utilisation of power from offshore installations and can also support the AC network in tackling wind power variation issues. However, the integration of the meshed MTDC grid with the existing AC grid has more challenges to overcome alongside the added advantages. One of the major challenge is to ensure the secure and optimal operation of the combined AC/MTDC grid considering stability requirements of the AC and DC grids in different operating conditions. The behaviour of the DC grid is governed by the fast acting controllers due to the high number of power electronic equipment unlike AC grid. In combined operation the response to a disturbance of two integrated grids can be different. The power balancing, co-ordination and dispatch requirements need to be identified, to implement appropriate controls and formulate a control structure for combined operation of two grids with different characteristics under normal and disturbance conditions. In this thesis, the basic principles of well-established three-layered AC grid control is employed to identify the power balancing, coordination and dispatch requirements of the DC grid. Appropriate control methods are proposed for primary, secondary and tertiary control layers in order to accomplish the identified requirements for the secure and optimal operation of combined AC/MTDC grids. Firstly, a comparison study is performed on different power balancing controls to find the most suitable control method for the primary control of the meshed DC grid. Secondly, the combined AC/DC grid power flow method is proposed to provide updated references of the VSC station in order to maintain coordinated power flow control under secondary control layers. Finally, security constraint optimization method for combined AC/DC grid is proposed for economic dispatch under the tertiary control layer of the three-layered hierarchal control. A number of case studies are performed to implement the proposed control methods on a combined AC/DC test case network. The performance of the proposed control methods is validated in a hierarchical control structure for secure and optimal operation integrated AC/MTDC grids.