The Researches On Supplementary Control Strategy For HVDC

With the improvement of AC/DC transmission voltage level and the continuous expansion of China's power development scale.With the continuous development of HVDC transmission technology,the transmission mode of the power grid is growing towards a higher voltage level and a longer distance transmission mode.Many HVDC projects have been built,under construction and planned.It is also true that the interconnected power grids across regions and long distances put forward higher requirements for the stability and reliability of power grids.The current control method is not ideal for oscillation control between interconnected systems,so it is very important to search for a new control strategy with simple method and better effect.The analysis of the interaction between AC and DC interconnected systems,the design of appropriate additional controllers to improve the stability of power system,to suppress the oscillation between AC and DC systems is of great significance for the safe,stable and reliable operation of China's power system.In this thesis,the analysis and control methods of low-frequency oscillation and subsynchronous oscillation are deeply studied,combining with the drawbacks of existing analysis and control methods,as well as the hot issues in practical systems,we proposed some resolvents of the interaction between low-frequency oscillation and oscillation in AC-DC interconnected systems in existing flexible DC systems.The main research contents are as follows:(1)To suppress the low frequency oscillation and sub-synchronous oscillation in the interconnected system and improve the frequency stability of the sending end under the island operating in the HVDC.The thesis comes up with one supplementary frequency controller using the projective control theory to suppress the oscillation in the system.Taking advantage of the DC power to control the frequency of the system,firstly,the oscillation modes are identified by total least squares-estimation on signal parameters via rotational invariance techniques,then the band-pass filters are used to decompose the different oscillation frequencies into different channels,at last,a dual channel additional frequency controller are designed according to the projective control theory.The controller can reduce the mutual influence in the different oscillation modes,also the low order and good robustness have gotten.The simulations have done in the PSCAD/EMTDC to validate the availability of the designed controller.(2)A kind of design method based on projective control theorem is proposed to be used in the design of damping controller in VSC-HVDC transmission system.First,the oscillation modes and transfer function are identified by the TLS-ESPRIT algorithm.According to the pole-placement theory,the state feedback matrix of the system is identified;then retaining r order dominant eigenvalue on the basis of the projective control with p order eigenvalue retained,a modified projective controller on the basis of projective control is designed to be used in the VSC-HVDC.