| In recent years,voltage source converter-high voltage direct current(VSC-HVDC)has developed rapidly.Compared to conventional line commutated converter-high voltage direct current transmission,voltage source converter-high voltage direct current(VSC-HVDC)transmission has unparalleled advantages in the field of AC/DC transmission,which controls active and reactive power transmission independently.However,it will affect the operation state and dynamic response of power grid when large-capacity DC power is connected to the grid.With decoupling coordinated control of active and reactive power as well as millisecond dynamic response,VSC-HVDC transmission system can assist AC system to achieve safe and stable operation through additional online control strategy.In fact,the power system has strong nonlinearity,whereas the additional controller designed by the traditional linearization method is based on the approximate linear model near the equilibrium point.As a result,it has inevitable limitations when the operating point of system is far from the equilibrium point under large disturbances.Therefore,it is of theoretical and engineering practical significance to study nonlinear control so as to improve the transient stability of power system after interference.This paper establishes a detailed mathematical model of VSC-HVDC transmission system in the abc coordinate system,and then derives its mathematical model in the dq0 rotating coordinate system,which acts as the basis of mathematical modeling of VSC-HVDC transmission system.For multi-generator AC/DC power grid with VSC-HVDC,the influence of DC power change on the dynamic power balance of synchronous generators is analyzed with a power factor defined.Furthermore,the power factor has an analytical expression and specific characteristics to be analyzed together.Aimed at the stability of rotor angles in AC/DC systems,an effective VSC-HVDC active modulation control strategy is proposed based on the active disturbance rejection control theory.The synchronous generators are chosen according to the abilities of the VSC-HVDC to control them,which determines the aggregated weights.After that,the synchronous generators are weighted into an equivalent controlled system and a transient stability controller aimed at stability of the equivalent controlled system is designed based on the linear active disturbance rejection control theory.The controller modulates the rotational speed deviations as well as the unbalanced power of the relevant synchronous units,maintaining the stability of the system by fully exerting its ability to adjust.In terms of other advantages,the control strategies do not rely on the detailed model of the synchronous unit and the HVDC system,which is well adapted to changes in the operation modes of the system.The effectiveness and robustness of the controller are both verified in the modified IEEE 39-bus system with a VSC-HVDC system.A VSC-HVDC active-reactive power combined control strategy based on active disturbance rejection control theory is designed according to the characteristics that the control of reactive power is independent of that of active power in VSC-HVDC system.In view of transient stability of AC-DC system after large disturbances,additional control strategies of the active and reactive power for VSC-HVDC are designed based on the active disturbance rejection control theory.According to the strategies,there is a joint switching process for different control characteristics between two strategies,which enable the controller to fully utilize the control capabilities as well as perform coordinated control of active and reactive power in the VSC-HVDC system during the disturbance based on the real-time state information of the system,for the purpose of reducing fluctuations of transmission power of the system after the disturbance.Therefore,it can improve the transient stability of an AC/DC system.Furthermore,not only can the strategies adequately exert the VSC-HVDC active power modulation to maintain the ability of transient stability of a system,but it can also decrease the time of the DC active power fluctuation caused by continuous active power modulation.In this paper,the effectiveness of the control strategies is verified in a four-machine two-area system and a modified IEEE 39-bus system with VSC-HVDC system.The research as well as results of this paper have important reference for promoting the research of the transient stability control in VSC-HVDC system and improving the stability level of AC/DC power grids. |
