Wide-area Control For Improving The Stability Of AC/DC Hybrid Power Grid

With the increasing HVDC projects,the coupling between AC and DC systems is becoming tight.The impacts of faults on the power grid chang from local to global.The secure and stable operation of AC/DC hybrid grid faces severe challenges.Applying appropriate wide-area con-trol to the DC system has become an effective measure to improve the stability of AC/DC hy-brid grid.Stabilization measures can be divied into two categories: generator control on the source side and HVDC supplementary control on the grid side.On the source side,the transient stability is improved by optimizing the forced excitation function of generators.On the grid side,HVDC supplementary control is used to improve the dynamic stability and transient stability.The main research work carried out and results achieved are listed as follows:(1)To make the best of the excitation function of generators and improve the transient stability of AC/DC hybrid power grid,a wide-area forced excitation emergency control considering the differences among generator performances is proposed.In this method,the sensitivity of the system transient stability margin to the excitation system ceiling voltage of each generator is calculated to measure the different adjustment performance of critical generators.Then,the ex-citation system ceiling voltages of critical generators are configured respectively according to the obtained sensitivity value.Simulation results verifies that the proposed controller can effec-tively improve the transient stability of the power grid.(2)Aiming at the problem of time-delay stability in wide-area supplementary damping control of HVDC,a method for calculating practical delay margin(PDM)of time-delay power systems is proposed.And then PDM is used as the performance index for designing the widearea supplementary damping controller(WSDC)considering time delays.Taking the New England system as an example,the analysis results verify the accuracy of the proposed PDM calculation method and the effectiveness of the designed WSDC.(3)To improve the transient stability of the AC/DC hybrid power grid,a wide-area supplementary controller for HVDC based on nonlinear adaptive control(NAC)is proposed.This method uses an observer to estimate and then compensate the nonlinear disturbances without the need of an accurate system model.The wide-area equivalent power angle is used as output feedback to control the transient stability directly.Simulation results verifies the proposed widearea supplementary controller can effectively improve the transient stability of the AC/DC hybrid power grid under different operating conditions and parameter uncertainties.