Climate change is one of the most dangerous and simultaneously most complex threats humanity has ever faced. The key response to this threat has been an unprecedented strategic shift in the energy sector, known as the energy transition. Electricity powers the modern world and is at the very centre of the energy transition. The shift to sustainable electricity production, transmission, distribution, and consumption is therefore vital. However, such a change brings significant technical challenges that should be addressed. The objective of this research is to uncover and investigate some of the key challenges in this regard and propose solutions for their mitigation.

This thesis largely focuses on two technical aspects and related challenges: power system vulnerability and stability. The emphasis lies on modern power systems, where conventional synchronous generation is increasingly replaced by inverter-based resources (IBRs). The first research objective is to improve the understanding of both system vulnerability and stability, particularly in the context of voltage stability and system strength and their intricate relationship. Relying on this improved understanding, the second objective is to develop advanced and novel evaluation methods and algorithms.

The developed methods form a basis for advanced voltage stability and system strength evaluation of modern power systems. Such an evaluation can play an important role in the overall stability and dynamic security assessment performed by power system operators, with the goal of cutting through the complexity of numerous possible contingencies and operating scenarios. The evaluation automatically identifies the most vulnerable weak grid sections and dangerous operating scenarios that may lead to cascading faults and possible instability. Consequently, once such grid sections and scenarios are observed, more detailed simulations and analyses can be performed by power system stability experts in a much more time-efficient and targeted manner. Subsequently, proactive mitigation measures can be taken to avoid the risk of instability and blackouts.

The TSO of a power system is mainly responsible for ensuring the stability of the grid. Through continuous monitoring and control of the power system, the TSO maintains stability through emergency actions. Until now, the conventional Static State Estimation has been the Energy Management System (EMS) tool for estimating and monitoring the grid state - bus voltages, currents, and powers. However, energy transition, which involves the decommissioning of conventional generation and their replacement by renewables, leads to a more dynamic grid. In such a case, the information provided by the static state estimator is insufficient. This has, hence, led to the development of the Dynamic State Estimator (DSE), to provide insight into the dynamic properties of a power system, such as rotor angle and rotor speed. The DSE typically uses a Wide-Area Monitoring (WAMS) architecture consisting of Phasor Measurement Units (PMU), to dynamically estimate the internal states of the generators under observation. Hence, the DSE provides improved situational awareness to the TSO. However, the existing literature do not elaborate on how their proposed DSE can be implemented in an online fashion to estimate the dynamic states in near real-time. Such an online implementation is of utmost importance as it showcases how a TSO can deploy the DSE in a real world scenario. Hence, this thesis proposes an online DSE algorithm that performs batch-wise estimation of dynamic states in a near real-time setting. By collecting measurements in batches and introducing the pre-processing steps necessary for these PMU measurements, the algorithm forms the premise for the real-world application of DSE. This algorithm is validated using a cyber-physical testbed comprising a Real Time Digital Simulator and a Synchrophasor Application Development Framework. Additionally, its performance is evaluated and a sensitivity study is conducted to find deterministic relationships between the input error introduced and the estimation error. Finally, future improvements are proposed to make the implementation more suitable to real-world application.

Due to the much needed energy transition as agreed under the Paris climate agreement, it is expected that large amounts of variable renewable energy supply will be connected to the Dutch Transmission Network. Future social, economic and technological developments will determine both the amount of electrification as the central or decentral nature of the future supply of electricity. By investigating different reports with scenarios for 2030 and 2050, key uncertainties for the Extra High Voltage (EHV) network can be identified. As of now, no open source model of the Dutch EHV network is available for research. In this thesis a synthetic model of the Dutch EHV network is created from only publicly available data in DIgSILENT PowerFactory. The created synthetic model can be used as a tool to investigate future steady-state power flows considering the sensitivities of the topology, component parameters and different operational scenarios. Besides this, a method is created for generation of future scenarios in Python. Using this model and method, simulations were done to gain key insights into the Steady-State security of the future Dutch EHV Transmission Network. For the case study, it is investigated what the effect of future additions of variable renewable energy sources (VRES) and selected locations of 10 GW installed capacity of electrolysers would be on the Dutch EHV transmission network for 2030.

To fight the threat of global warming, nations worldwide are currently in the transition towards a renewable-powered energy system. One of the main technologies represented in a renewable-powered energy system is wind energy. Rapid developments in the offshore wind sector caused the rise of offshore wind farms in the North Sea and more wind farms are expected for the years to come. As this renewable-powered energy system will be heavily dependent on the availability of wind and solar energy, great challenges will arise to the security of supply in the energy system. To tackle this challenge, nine countries surrounding the North Sea (called NSEC) cooperate on researching a cost-efficient energy system design which enables the large-scale deployment of wind and solar energy. One aspect that hasn’t been researched for the NSEC so far is the potential of hydrogen for this energy system. Hydrogen can play an important role in the future renewable energy system as fuel and feedstock for different sectors and as energy carrier that allows for seasonal storage. The question of what energy system design choices bring the most cost-efficiency for the NSEC, considering the hydrogen and electricity demand in 2050, will be researched. For this research, results have been based on optimizing the future energy system in the simulation modelling program Powerfys. This model optimizes power plant utilization while taking the constraints assigned to the power plants and to the energy system infrastructure into account. Powerfys operates on a rolling planning on the intra-day and the day-ahead market. The model consists of 50 nodes, divided over 9 countries and the North Sea. By simulating scenarios focused on either renewable energy capacity expansions or transmission infrastructure design, different levels of cost-efficiency are determined. These outcomes will allow to answer the research question. This research shows that a cost-efficient energy system for both electricity and hydrogen in the NSEC can exist with the current planned electricity and natural gas transmission grid for 2050, assuming a fully retrofitted natural gas grid, exclusively utilized for hydrogen transmission. What this study does show is that extensive amounts of offshore wind (285 GW) and other renewables (245 GW of onshore wind and 434 GW of PV) must be deployed, to meet the expected electricity and hydrogen demand of 2050. For offshore wind this means that not only the potential of bottom-fixed wind turbines should be accounted for, but also the potential of the novel technology of floating wind turbines must be considered. On top of that, this study shows that only 4 to 5% of all hydrogen demand must be imported from outside NSEC. Even without the imports of any hydrogen, meeting full electricity and hydrogen demand would be technically possible, but this would lead to higher energy system costs for the deployment of higher capacities of renewables and the storage for hydrogen in salt caverns. Though, the sensitivity analysis shows that hydrogen imports can increase significantly when price levels related to hydrogen (import-, electrolyser- or hydrogen storage prices) will turn out different in 2050 than currently expected. Nevertheless, this research also shows that such a significant increase in hydrogen imports will not lead to remarkable deviations in the overall energy system design or the energy system costs.

Multiterminal dc (MTDC) network is preferred due to its reliability, security of supply and flexibility. However, MTDC network also comes with the protection challenges resulting from dc faults. Hence, the dc circuit breaker (DC CB) is imperative in such a network. In these recent years, several DC CB technologies have been proposed and demonstrated by different manufacturers. Besides, these DC CB technologies differ from each other in terms of the speed of operation, interruption capability and costs. Hence, for the optimal performance of the MTDC network, a study of the co-ordinative operation of different DC CB technologies is required. In this thesis, two typical types of DC CBs are modelled in detail and implemented in a 4-terminal MTDC network in PSCAD environment, by considering operation time, interruption capability and interruption characteristics. The obtained results are used for DC CB’s selection optimization methodology for the future MTDC networks. Similarly, a scaled model of DC CB has to be analysed in terms of its interruption capability in MTDC network considering various scenarios. Therefore, in this master thesis, technical performance of DC CB technologies is conducted for a test and multiterminal dc network in EMT based software environment.

The DC CB is the key to unlock the reliable operation of a Multi-terminal direct current network, whereas fast, effective and accurate models are frequently needed for system-level studies. Due to higher subsystem components in DC CB, a detailed DC CB model creates a bottleneck in the network analysis. This thesis also proposes and compares, an average model with a detailed model of Voltage source converter Assisted Resonant Current (VARC) and Mechanical DC CB in MTDC Network in terms of their performance and computation time for two typical simulation cases. The average and detailed model is modelled and simulated on the PSCAD/EMTDC electromagnetic transient platform. Decisively, this thesis concludes by presenting an accurate response of the average model during the fast transient event, showing additional computational advantage.

Despite the uncertainties on the generation end and the consumption end, the transmission network thrives to be reliable. This includes its effort to predict the power flows in its lines and maintain the flows under a safety margin. Due to the said uncertainties and approximations, predicted power flows in the lines can exceed the acceptable limits. Under such a scenario, the responsible TSO has to take actions to ensure that this predicted violation won’t occur in the reality. It needs to be noted that the flow in the line that mandated these actions could be caused partly by power exchanges outside the borders of the TSO. The responsible TSO bears the costs of these actions initially. The incurred expenses can be split in a fair manner by partitioning the power flow in the line into power flows caused by the power exchanges of all zones. Full Line Decomposition(FLD) is an application developed by the FB4INV team at TenneT that calculates the power flow partitions. To compute the power flow partitions, it requires all power exchanges occurring in the system. The existing method of computing power exchanges(called the Bialek method) models the power network as a directed graph based on power flows in the branches. It traces the flow of power from the generator to the load(or vice-versa) along the paths available. If a power flow path is not available due to the directed modelling, an error of zero power exchange is introduced. There exists multiple ways of computing the power exchanges. This paper attempts to find an unquestionable method. A method of computing power exchanges based on electrical distance reflects the behaviour of the power system in distributing flows. Through the literature review, the superposition method was found that makes use of circuit theory to find generator-to-load power contribution. In the course of the thesis, the superposition method was found to have a flaw in the equations. A new approach was developed in this thesis where power exchange is computed by defining it as an optimisation problem. The objective function of the optimisation problem reflects the excepted behaviour of the power system. The optimisation process was implemented using a genetic algorithm. The existing and developed methods were tested on an IEEE 30 bus system.

During the last years, electric power generation was based mainly on large centralized fossil fuelled power plants. However, nowadays and in the future, efforts are made to increase the share from renewable sources, as a result of increasing environmental concern. Indeed, in some networks areas, wind generators gradually start to replace conventional synchronous generators, while the European Union has set a “20 - 20 - 20” target plan, which aims to achieve a 20% reduction in green house gas emissions compared to 1990 levels, have 20% of the energy on the basis of consumption coming from renewable sources and a 20% increase in energy efficiency by 2020. Due to the increment of wind generator share, power system stability and reliability may be affected. This is attributed to the fact that the characteristics of wind turbines are different from the ones of conventional plants. Therefore different frequency, voltage and transient stability system performances arise which require different measures to be tackled. This report, underlines the importance of rotor angle stability studies in power systems with high share of power electronic interfaced generation units. These studies are related to the ability of the synchronous generators to remain in synchronism when subjected to large disturbances (e.g. faults). A supplementary damping control as a mitigation solution against rotor angle stability threats is studied in power systems dominated by full-scale power electronics wind generation units. This solution, as it is shown in this report, is cost-effective and is adopted for modification of the outer control of fully decoupled wind generators. Concerning related work reported in existing literature, several researchers have used the frequency deviation as input for power system stabilizer added to the grid side converter of a fully decoupled wind generator, whereas other researchers have used the terminal voltage. However, rotor angle remote signals for damping the Synchronous machines oscillations can also be used. Therefore, a supplementary damping control for fully decoupled wind generators, considering rotor angle deviations as input signal, is adopted in this report. The thesis report also provides a detailed overview of the adopted structure of supplementary damping control. Besides, a discussion, based on EMT and RMS simulations performed by using RSCAD and DIgSILENT PowerFactory tools respectively, is presented to address research gaps in existing literature, namely, identifying the most suitable wind generator locations for addition of supplementary damping control, the degree of improvement in damping performance due to the use of rotor angle deviation as input signal, the effectiveness of wind generator with the damping controller as affected by distance to reference generator, and the effectiveness of the controller when the distance between synchronous generator and wind generator is taken into account. Different indicators related to transient stability studies are used so as to evaluate the performance of the controllers, whereas optimization techniques are also utilized to find their appropriate settings. For the rotor angle stability studies, different power systems were studied. In particular, faults were examined in a modified IEEE 9 Bus benchmark system as well as the test system of the Great Britain (GB) transmission power system. The aforementioned systems are used to examine the performance of wind generators with damping controller in systems with different dynamic behavior. For each system, different scenarios were applied, by simulating different fault locations (modified IEEE 9 Bus system/GB system), or different wind share level (GB system), different time delays due to data latencies (GB system) and different input signals in the proposed damping controllers (GB system). The selected cases include critical operational scenarios, i.e. fault locations in lines with high pre-fault apparent power dispatch, faults in prone to instability synchronous machines’ terminals, high latency delays of 200ms. Faults with relatively high clearing time of 120ms are applied in the system, so the systems are examined close to their limits.

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.

TenneT’s vision of the North Sea Infrastructure (NSI) creates a basis for a joint European approach up to 2050 which focuses specifically on developing the North Sea as a source and distribution hub for Europe’s energy transition. High wind speed, central location and shallow water qualify the Dogger Bank as the location of the central hub.
For further development of the NSI, more detailed research is needed.
The research gap is recognized that there is not sufficient research on proposing new HVDC grids in the North Sea considering optimization (e.g. following a cost-related objective) of different topology structures within a scope of six surrounding countries (BE, DE, DK, GB, NL and NO), with sensitivity tests regarding uncertainties (e.g. in meteorological condition, load, or industry development).
Therefore, the main research objective of this thesis project is to evaluate CAPEX, overall OPEX of participating countries and other operational performances (e.g. energy mix, nodal price, EENS) of possible topologies of HVDC network in the North Sea, including NSI (with a central North Sea Wind Power Hub at the Dogger Bank) and other two competitive topologies, considering uncertainties in green energy technology development, European coordination, load and meteorological condition (e.g. wind speed, solar radiation and hydrology).
Three specific research questions were studied in order to achieve the aforementioned research objective: How to define 3 topologies of the North Sea HVDC network with different feasible structures? What are the criteria to optimize each topology and to compare the topologies? What are the implications of each topology, when evaluated against a wide range of uncertainties, on the overall CAPEX and OPEX of countries involved?
The simulation considers the scenario in the year 2030. Software PowerGAMA and PowerGIM were used for operation simulation and topology optimization. When calculating OPEX throughout the lifetime of the equipment (assuming 30 years), the year 2030 is taken as a representative year and the 30-year OPEX is obtained by multiplying OPEX in 2030 by an annuity factor.
In short, 3 topologies of the North Sea HVDC grid, with hub-and-spoke structure (for the NSI), point-to-point structure and meshed (without central energy hub) structure, respectively, are defined. They are then optimized towards and compared for the lowest overall cost (i.e. the sum of CAPEX and OPEX including CO2 prices) throughout the lifetime.
Simulating under different uncertainties/selected critical scenarios (4 Visions from ENTSO-E’s Ten Year Network Development Plan which reflect different RES share target and European coordination level, and extreme RES inflow and load conditions), the optimized NSI design stays most socio-economically preferable (with lowest overall cost) topology.
It is also recognized that NSI is able to realize its expected functions, namely transmission of renewable energy, enhancement of system security and price convergence. On the other hand, launching of NSI brings in challenges such as grid congestion and benefit asymmetry.
Main contributions of this thesis include:
• Creation of the baseline model/dataset for European power system in 2030 as a background/environment for the North Sea HVDC grid planning;
• Design and optimization of the North Sea HVDC grid topologies in three different feasible structures;
• Verification of NSI’s advantage in cost saving, compared to two competitors, in 4 Visions reflecting different green energy transition and European coordination level, and under extreme RES inflow and load conditions;
• Verification of NSI’s function in improving energy sustainability, affordability and security;
• Realization of Non-Homogeneous Markov Chain algorithm in Excel to generate wind power inflow time series.

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.

The increase of new power generation in Europe using renewable energy sources
leads to the application of HVDC for high efficiency transmission. The growth of energy market and the development of the HVDC technology cause the multi-terminal HVDC system based on half-bridge MMC is one of the best option of transmission system, especially for offshore power grids. The aim of designing transmission systems is to obtain a network with high reliability. In this master thesis the reliability modelling of multi-terminal HVDC system based on half-bridge MMC is analysed as an important step to designing transmission networks. Firstly, the reliability of all subsystems related to the HVDC connection is modelled. The Modular Multi-Level Converter - Voltage Source Converter
(MMC-VSC) is discussed from the lowest stage. After that, the converter unit and other components such as transformer, converter reactor, control system and DC switchyard in the connection are described by their reliability and is combined into a reliability model of a converter station. Two types of converter stations (onshore and offshore) and the cables assemble the system based on the case study. Two systems are modelled: Point-to-point and three-terminal configuration. In the three-terminal configuration, two scenarios are performed: with and without possibilities. The reliability modelled is performed using two approaches: analytical approach and Monte Carlo simulation. The reliability model of the subsystems and systems involve the availability, unavailability, outage duration, energy not transmitted, failure frequency, average duration per interruption, and the range of time between failures. After having the model and the reliability parameter, the reliability analysis is performed.
From the analysis, it is found that the highest unavailability and thus the outage
duration in onshore and offshore converter station are the transformer and the converter reactor, respectively. The offshore converter station is found to have the highest unavailability. The failure frequency, the average duration per interruptions, and the range of time between failures, which is produced by Monte Carlo simulation, can be applied for further action regarding the asset management of the system. The energy not transmitted through the point-to-point and three-terminal system is useful as a further research to choose which scenario is the most profitable.

In the Netherlands the main sources of electricity are currently coal and gas fired power plants. Due to the increasing share of electricity that is produced from renewable energy sources, the operational hours of these conventional power plants are decreasing. However, these power plants are also the main source of frequency reserves, which are required to guarantee the stability of the electricity grid. Stability is maintained if there is a real time balance between the electricity production and consumption. Due to the decreasing availability of conventional power plants, the possibility to offer frequency reserves with other power sources must be explored. A possible provider is the Car Park as Power Plant (CPPP). This is an aggregation of Fuel Cell Electric Vehicles (FCEV) parked in a car park and operating in Vehicle to Grid (V2G) mode. This thesis contains a technical and economic feasibility assessment of a Car Park as Power Plant offering frequency reserves in a future power system with a low share of conventional power plants.

The dynamics of the frequency was analysed considering the reduction of the operational hours of the conventional power plants. This causes the inertia in the system to decrease. As a consequence, this increases the rate of change of frequency (RoCoF). A high RoCoF makes the frequency react faster which will make it more feasible that the maximum instantaneous frequency deviation will be reached. When this maximum deviation is reached, the system enters the alert state, which could endanger the global security of the system. To prevent the frequency from reaching the maximum instantaneous frequency deviation under conditions of a high RoCoF, the full activation time (FAT) of the frequency reserves could be reduced. The FAT is the maximum time between the moment that the signal for a change in power output is given and the actual moment that the required power output is reached. Reserves with a reduced FAT are referred to as fast frequency reserves. By measuring the FAT of the FCEV in V2G mode with an experimental setup, it was tested if the FCEV could offer the fast frequency reserves. The FCEV appeared to be a suitable power source to offer fast frequency reserves. When the power output of the battery and the fuel cell are combined, an even higher power gradient and thus a shorter full activation time, can be obtained. However, improvements must be implemented in the V2G discharge unit and the energy management system of the FCEV to optimise the operation in V2G mode. The economic feasibility of the CPPP as provider of frequency reserves was then analysed. The factor that has the highest impact, is the position of the CPPP on the bid ladder. Only when the price for frequency reserves is relatively high, the reserves of the CPPP will be activated. This is caused by the high price for hydrogen, which is the dominant factor in the marginal costs of the CPPP. Most of the time the prices for frequency reserves and for electricity sold on the spot market are lower than the marginal costs of the CPPP.

From a technical point of view the CPPP is a suitable power source to offer fast frequency reserve. The profitability of the CPPP is, however, strongly related to the occupation pattern of the car park and the price for frequency reserves. The occupation pattern can be influenced by the aggregator by giving incentives to the car owners, combining different car parks with deviating occupation patterns or by operating company car parks with, for example, autonomous driving cars. The price for frequency reserves is dependent on the quickly evolving market and can not be influenced by the aggregator of the CPPP. The aggregator can influence the marginal costs of the CPPP. By adding relevant components to the system, like hydrogen production units or storage facilities, the aggregator can offer reserves with different power sources, which will have different marginal costs. From the point of view of the transmission system operator, it could be possible to add an extra product for fast frequency reserves instead of changing the requirements of the existing reserves. This could have a positive effect on the profitability of the CPPP. The specific requirements such as the minimum bid size, validity period and payment mechanism for this product should be evaluated in further studies.

The transition towards a sustainable society calls for the massive deployment of renewable energy sources such as large wind parks located far offshore. High-voltage direct current transmission based on voltage sourced converter technology (VSC-HVDC) offers a wide range of technological benefits that foster the grid integration of offshore wind parks. Coupling AC and HVDC grids comes with significant challenges. Control and system functions, which were formerly separated, interact, especially during faults in the transmission system. Classical (transient stability) modelling and simulation does not suffice and must be made ready for VSC-HVDC.

This Ph.D. thesis answers two questions to master these challenges. First, what is the impact of the operation and control of a, possibly multi-terminal, offshore grid based on VSC-HVDC on the transient stability of the onshore power system? Second, how can we model and simulate these impacts while maintaining the desired simulation accuracy and speed? The results of this thesis facilitate fast and accurate assessment of stability impacts of large transmission systems with a significant proportion of converter-interfaced generation.