Dynamic Modeling and Control of Multi-Terminal HVDC Grids

Overview & Synopsis

This thesis deals with control and stability of Multi terminal HVDC (MTDC) grid used to connect wind farms to several onshore injection points. This work discusses both the primary control methods to provide DC grid power balance and coordinated control methods to dispatch power as scheduled by TSOs. The literature review on primary control methods allows choosing the droop voltage method as the best candidate to control the DC grid. An in-depth analysis highlights the influence of the droop parameter and the DC storage elements on the DC grid dynamics and this leads to a methodology to size the droop parameter. Next the stability of DC grid alone is assessed by small signal stability analyses. Also, interaction origins between AC and DC systems are traced using modal analysis. Since, primary control act as converter level using local measurements, a master controller is proposed to manage the DC grid power flows. Based on steady study state considerations, this controller computes references to send to converter stations in order to restore the system at nominal value and to satisfy TSOs wishes: interconnections schedule power transits and wind power sharing. Finally, methods theoretically developed are assessed on a multi terminal mock up. This is an hardware in the loop mock-up where AC systems are simulated in real time and cables and some converters are real at low scale. The low scale mock-up is homothetic to a full scale system: electrical elements are the same in per unit, DC storage is also homothetic and converter’s controllers are tuned to achieve identical time responses. The mock-up is monitored by a SCADA system in which the coordinated control is implemented.