Power systems nowadays are experiencing a significant energy transition characterized by the progressive increment of power electronic interfaced units. Voltage source converters are extensively utilized to interconnect renewable energy sources at various transmission levels, facilitate the asynchronous interconnection of system areas through HVDC links and enable the installment of responsive demand units into the grid. The latter increase of power electronic interfaced units in the power system has significantly altered its dynamic behavior as the system is characterized by low inertia conditions, fast and more frequent dynamics, lack of inertial response, limited short circuit capability and uncertain generation and demand mixes. Under these new operating conditions one of the main concerns of TSOs is the ability of the system to withstand active power-frequency imbalances. To cope with the frequency stability issues encountered, power electronic interfaced elements introduced, can be effectively utilized to offer ancillary services such as fast frequency support to the grid due to the high controllability levels and the rapid response capability they exhibit. This thesis project deals with the optimization of the frequency response of a multi-area, multi-energy hybrid HVAC-HVDC power system when common active power-frequency disturbances occur. The system analyzed consists of 3 electromagnetically decoupled areas through MMC-HVDC interconnection links with different generation and demand mixes characterized by different inertia levels due to the integration of fully decoupled wind turbines type IV and proton exchange membrane electrolyzers. The controllers of the power electronic interfaced elements have been modified with the active power gradient control scheme to provide fast frequency support in case of commonly occurred active power-frequency imbalances. However, to avoid insufficient or over regulation of the systems’ frequency when multiple elements target the same variable, a coordinated tuning strategy is more than necessary. For this reason, in this study two different problem formulations have been proposed aiming at a coordinated tuning of the parameters of the frequency controllers of the synthetic inertia elements participating in the frequency regulation. To effectively solve the optimization problem and enhance the frequency stability of the system a powerful metaheuristic optimization algorithm, the mean variance mapping optimization algorithm has been used. The optimization results can effectively highlight the tuning strategy that achieves the best frequency response of the system under various commonly occurred active power-frequency disturbances and provide further insight on the proper utilization of various synthetic inertia sources with respect to their response capabilities and location.

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.