Control and application of VSC-HVDC systems for grid connection

Abstract

Renewable energy sources with power generation are becoming increasingly distributed with generation facilities located far away from load centers. It is necessary to find a suitable transmission way to deliver power from power generation centers to different AC systems or between AC systems. From the technical and economical viewpoint, VSC-HVDC transmission has a number of potential advantages and applications. For example, VSCHVDC can control the active and reactive power independently and the power flow reversal happens without changing the voltage polarity; it can supply for the passive load and can operate without massive AC filters; Moreover, VSC-HVDC can be used in different locations such as large offshore or onshore wind farms and solar plants or other industrial installations. This work focuses on the control and application of VSC-HVDC system. Firstly, in order to further exploit the benefits of VSC-HVDC system and improve its dynamic performance, three different control schemes are implemented and compared under the same conditions in the VSC-HVDC transmission system. Their advantages and disadvantages are described in detail. A two-terminal VSC-HVDC system is developed in MATLAB/SPS and the dynamic performance of the system is evaluated during system startup, grid voltage sags, and power reversal. The simulation results show that the implemented control schemes for VSC-HVDC system can improve dynamic performance during the aforementioned events. Different control schemes have different responses in the system performance. Thus, we can choose different control schemes according to the system control requirements and applications and the proposed abc-axis deadbeat control scheme provides a high performance control solution to the VSC-HVDC system. As for the application of VSC-HVDC system to UHVDC transmission, currently most of converter stations for UHVDC operation in the existing projects are thyristor-based converter stations, which are usually constituted of series connection of two 12-pulse bridges of 400 kV each with a rated DC voltage of 800 kV per pole. Innovative solutions have been implemented to fully meet the extended requirements for ultra-high voltage bulk power transmission using thyristor-based converters. However, it has some disadvantages as well, such as facing the risk of commutation failure, power reversal taking place by reversal of current direction, reactive power, and active power being not able to be controlled independently, and requirements of costly DC-side and AC-side filters. In order to address those problems, VSC-based UHVDC technology is proposed. It examines the feasibility of connecting four voltage source converter (VSC) HVDC stations, each rated at 200 kV DC voltage, in series to form a pole UHVDC of 800 kV DC voltage. Different demanding tests such as step active and reactive power test and reversal of direction power flow test are implemented, and the feasibility is demonstrated by means of simulations using MATLAB/SPS.

Date	24 Nov 2016
Original language	American English
Awarding Institution	École de technologie supérieure
Supervisor	Louis-A. Dessaint (Supervisor)