Offshore Wind Power Connected to the Dutch Transmission System by VSC-HVDC Networks

Abstract

Wind power generation is predicted to increase massively within the next years. In this direction, offshore wind power is anticipated to play an important role in the future renewable energy share, especially in the North Sea countries. These trends have posed new challenges for transmission system operators to supply consumers with the required quality of service. Transmission system operators are responsible to ensure the adequate security of power system operation which implies system stability and satisfactory damping levels. High voltage direct current (HVDC) transmission system technology is the only technically feasible power transmission system solution for the connection of large offshore wind farms which are located far from the shore. Additionally, the plans for the construction of a multi-terminal dc offshore network in the North Sea region would advance the integration of offshore wind power and would further improve the European electricity markets. The VSC-based HVDC transmission is considered a mature technical solution for the development of multi-terminal dc networks. The main focus of the present thesis is the interaction of the Dutch interconnected power system with voltage source converter (VSC) based HVDC transmission systems which are facilitated for the integration of large amount of offshore wind power in the Netherlands. Special attention is given in the transient stability of critical generators in the Dutch power system in situations with large amount of onshore and offshore wind penetration. Individual and MTdc connection of offshore wind parks have been investigated both facilitated by VSC-based HVDC. A generic modeling framework for VSC-based HVDC transmission system which is compatible with the Dutch power system dynamic model of TenneT in PSSE software package is developed to carry out transient stability analysis. An averaged model of the VSC converter along with the dc cable network model is used for transient stability simulations of meshed ac-dc networks. The interaction between the Dutch power system and different VSC-based HVDC transmission systems is investigated under ac side three phase faults. In addition, different dc network power management strategies based on closed loop dc voltage control and dc droop characteristics are proposed to ensure smooth system operation and proper power sharing among the grid-side converter stations. The three basic power market dispatch schemes were studied and the basic differences in terms of dc voltage performance were discussed. A power oscillation damping (POD) controller that operates at the grid-side converter is proposed in order to introduce damping of electromechanical oscillations in the ac power system. The POD controller operates at the active power control loop of the GSVSC and performs active power modulation. It was shown for a multi-machine test system that it is possible by the proposed control strategy to introduce damping of power oscillations in the system that the grid-side converter operates. The 2010 dynamic model of TenneT has been used to represent the Dutch high voltage transmission system. Grid reinforcements which are planned for the years 2025-2030 were introduced. Additionally, certain assumptions have been made in terms of conventional generation and unit commitment. Having applied all the necessary changes, it has been achieved to build a 2025-2030 grid situation dynamic model. Seven interconnection points were considered within the former UCTE system for the connection of large amount of offshore wind power (three in the Netherlands, three in Germany and one in Belgium) according to the NSTG project. Three types of VSC-based HVDC transmission system configurations were studied. Namely the individual connection, the multi-terminal dc network connection per country and the transnational multi-terminal dc network connection which extends from Germany via the Netherlands to Belgium. A total 5200MW offshore wind power and 4700MW onshore wind power is introduced into the power flow of the Dutch power system. Furthermore, for the German part of the UCTE network, 7500MW offshore wind power is considered. Three snapshots of the Dutch power system are taken into consideration reflecting situations with different conventional generation, load and power exchange. From the simulation results it was deduced that the specific amount of wind power interconnected to the Dutch power system did not create transiently unstable situations. With regard to the performance of the grid-side converter stations in the three types of HVDC transmission system configuration, it was found that the main difference in the dynamic response is the active power overshoot of particular grid-side converters. All grid-side converters have illustrated capability to support the ac voltage with reactive power and contribute limited short circuit current in all three HVDC configurations. Additionally, the connection of the converter stations in the existing 380kV substations of the Dutch power system did not introduce considerably negative interaction between the generators and the converters.