Cooperative Power Sharing Control in Multi-terminal VSC-HVDC
The Multi-terminal high voltage DC (MTDC) system is a viable solution to interconnect renewable resources into an AC grid. Using a VSC allows independent control of a reactive and an active power. There is a trend to implement MTDC into a distribution grid system in the future. Power sharing control among MTDCs is an important and critical consideration from the point of view of stability and operation. MTDC systems consist of multi-input converters(rectifiers) and single or multi-output converters (inverters), thus controlling and operating MTDC systems pose many challenges due to their complexity. Since the DC link in MTDC systems might have several connection nodes all having a common DC voltage value, using the DC voltage value as a common reference for all terminal control loops makes it possible to get a cooperative control performance.
An economical autonomous control to share active power among MTDC systems based on the availability of active power or power management policy is proposed in this thesis. Power sharing among MTDC systems has a priority or sequential procedural problem because of the use of the conventional droop strategy. On the other hand, using predefined or constant power sharing does not provide the available power that can be shared when it is not being consumed by another inverter. The proposed strategy solves these issues using different options. In this thesis, the test system consists of four simulated VSC terminals based on a detailed switching VSC model with two AC voltage levels. The MTDC system is simulated in a PSCAD/EMTDC environment. The simulation results show a significant decrease in operational costs and protection from overloading which had been an issue.