Research On Stability Mechanism Analysis And Improvement Control Method Of VSC-HVDC Connected In Weak Ac Power Grid

Facing the increasingly serious environmental pollution and energy crisis,distributed power generation technologies including renewable energy such as wind energy and solar energy have developed rapidly in recent years.At the same time,there is an objective reality of the reverse distribution between energy and load in China.The high-voltage DC power transmission project(Voltage Source Converter-HVDC,VSC-HVDC)of voltage source converters is a long-distance large-capacity power with its excellent characteristics Transmission and the Global Energy Internet Strategy provide the necessary realistic foundation.At the same time that HVDC transmission significantly improves the optimization of large-scale resource allocation,the increase in DC transmission capacity and the increase in load demand have made the AC-DC hybrid grid a “strong,straight and weak AC” phenomenon.Interaction effects have become increasingly complex,greatly affecting the safe and stable operation of the system.To this end,this article takes the VSC-HVDC system connected to the weak AC power grid as an object,analyzes the system stability mechanism from the following aspects,and conducts in-depth research on control methods to improve its stability.1)The basic structure and small-signal model of the VSC-HVDC system:a mathematical model in dq rotation coordinates is established,and a small-signal model of the global state space of the VSC-HVDC system connected to the weak AC power grid is established based on the linearized mathematical equations.At the same time,the electromagnetic transient simulation model of VSC-HVDC was established in PSCAD / EMTDC.The power step was used to simulate small disturbances,and the changes of the system related state variables were observed.The correctness of the small signal model was verified by comparing experimental results.2)The research on the stability mechanism of the VSC-HVDC system connected to the weak AC power grid:Based on the established small-signal model,the eigenvalue analysis of the system under various operating conditions has verified that the system under sufficiently weak connection conditions has the problem of instability at high power requirements.Aiming at this phenomenon,an active closed-loop control block diagram including a reactive control loop and a phase-locked loop is established.Based on the BODE diagram of the open-loop transfer function,the influence of different reactive bandwidths on the stability of the system is discussed.Based on the negative feedback stability criterion in the analysis of the steady state power flow of the AC system,it is found that the active control loop and the reactive control loop have an interactive influence in the current vector control,and the too slow response speed of the reactive control link results in active power instability major factor.3)A control method that introduces a feed-forward link in the reactive power outer loop input:Based on the mechanism that the too slow response speed of the reactive power control link causes the system to be unstable,this paper improves the control strategy and proposes a control method that introduces a feed-forward link in the reactive power outer loop input.By comparing the electromagnetic transient simulation experiments of this method with the traditional method in PSCAD / EMTDC,the correctness of the previous analysis and the improvement of the power transmission limit of the proposed method are verified.And by setting AC and DC side faults,the stability of small disturbances of the method in this paper under various faults is explored.