Study On VSC-HVDC Transmission System Characteristics And Control Strategies To AC System Faults

As an important part of the ac/dc hybrid power grid, VSC-HVDC should be able to run safely and stably for a long time, especially when the ac system occurs faults. VSC-HVDC is expected to not only ensure its safety and reliability, but also provide support for the faulted power grid, enhance the system stability and reliability, and improve the power quality. This thesis researches on the dynamic characteristics of VSC-HVDC system and control strategy to improve the ac/dc system performance under ac faults.Dynamic characteristics of VSC-HVDC system are studied under the ac faults, especially coupled with the phase angle jump. To restrain the dc-link voltage ripple or negative-sequence currents, the positive-and negative-sequence models of VSC-HVDC and two stratiges applicable to the unbalanced faults are investigated. The dynamics of VSC-HVDC with the control systems for the restraint of dc-link voltage ripple or the negative-sequence current are investigated as compared with that of the conventional strategy, furthermore the system performance considering phase angle jump is analysed.To improve the dynamic performance and mitigate the ac current deterioration presented in the conventional dual vector current control due to the sequence component decomposition, a new vector current control strategy and harmonic compensation controller in the stationary αβ reference frame are proposed. It can restrain the2nd harmonic ripple on the dc link and decrease the harmonic level in the ac currents under unbalanced ac grid conditions.Based on the unified dynamic models, a novel unified control scheme for active power/dc-link voltage and reactive power regulation is proposed. The resonant integrator and filter based scheme in the outer loop enables the simple acquision of the positive-and negative-sequence current references without harmonic component. Combined with the inner vector current control in the stationary αβ frame, besides the active power and reactive power regulation at PCC, the proposed scheme can restrain the dc-link voltage ripple or the negative currents along with the restraint of low-order harmonics without additional harmonics compensation.The unified control system based on the ac voltage regulation with droop control is designed, and its application in power supply related to power quality issue is investigated. The simulation studies with the IEEE12-bus benchmark show that it can mitigate the voltage dip, restore the system voltage quickly after the fault, retain a rather low level harmonics and then improve the system operating voltage and current quality.Considering the problems that the south Dalian grid lacks of large internal power supplies and has a higer proportion of important load, which will make reliability and stability issues, VSC-HVDC for the south Dalian grid is studied. The results show that the introduction of VSC-HVDC based on the unified control scheme can effectively ehance the stability and reliability and impove the quality of power supply in the south Dalian grid.