Control of multi-terminal VSC-HVDC systems for combined AC/DC systems to improve power system dynamic performance
Renewable energy resources are being integrated into power systems around the world and replacing conventional generators to reduce carbon emission. Voltage source converter based multiterminal HVDC (VSC-MTDC) is identified to be a promising technology to facilitate the integration and utilization of large amounts of renewable energy resources across large geographic areas. Many challenges exist in operating such a combined AC-MTDC power system under a high renewable future. This dissertation addresses three challenging problems in terms of improving dynamic performance of an AC-MTDC system, which are AC system frequency control, DC voltage control, and implementation of VSC-MTDC controls on practical large scale systems.
The first part of this dissertation focuses on designing frequency control for VSC-MTDC to provide frequency support among asynchronous AC systems for AC side events. A global frequency control scheme is designed and is shown to have superior performance to the traditional local frequency droop scheme with improved frequency nadir and reduced impact to the DC voltage profile. Consequently, the amount of load shedding and the need of total online spinning reserves are reduced. In the second part of this dissertation, an adaptive DC voltage droop control strategy is developed for converter outages taking both DC voltage deviation and power sharing into consideration. The proposed control enables accurate power sharing based on converter operating conditions, with the capability to differentiate the outage of a rectifier from that of an inverter. As a result, converter overloading is avoided and transient DC voltage profiles are improved. Moreover, the proposed control has the flexibility to adjust the strength of DC voltage regulation. In the last part of our work, in order to investigate the effectiveness of the designed frequency and DC voltage controls on practical large scale systems, a six terminal VSC-MTDC system is modeled into a 100k bus North American power system with an HVDC overlay, with which the advantages of the developed controls are demonstrated. With the proposed control strategies for VSC-MTDC systems, the reliability of the overall AC-MTDC system is improved in terms of both AC and DC side contingencies.