A novel dynamic model for Multiterminal HVDC systems based on self-commutated full- and half-bridge Multilevel Voltage Sourced Converters

This paper discusses a novel model for stability studies of a Multiterminal High Voltage Direct Current system (MT HVDC) as part of an overlay grid in a hybrid AC system. The dynamic model is set up for Multilevel Voltage Sourced Converter (VSC) technology. The new model provides the opportunity to analyse the impact of full- and half-bridge modules during AC and DC faults. In the present paper a radial DC system with four converter stations was built up and simulated in PSS®NETOMAC. In principle the model can be expanded to a multiterminal HVDC system with a higher number of converter stations. Generally the structure of the DC grid does not subject to any restrictions. For steady state control of a MT HVDC system the Voltage Margin Method (VMM) was implemented. The main focus of the presented model is placed on dynamic stability studies in case of DC faults and their effects on the AC grid. But due to the possibility of VSC converters to provide general system services, e.g. to supply reactive power, the effects and advantages of VSC converters during AC faults can also be analysed. In principle Insulated Gate Bipolar Transistor (IGBT) technology offers the possibility of clearing DC faults on the DC side. Depending on the type of modules (full- or half-bridge modules) used in a Multilevel VSC converter the fault clearing strategy and therefore the effects to the AC grid differ enormously. It is essential for the transient stability of a highly stressed AC grid to ensure a very low fault clearance time to keep the system stable. The proposed control design was designed as a two partition macro in PSS®NETOMAC and can be used for planning a Multiterminal DC system in any AC grid. It was applied to a small test grid in order to prove its performance. For more realistic results the model was implemented and applied in a dynamic model of the Continental Europe high voltage power transmission grid.