Research On Mechanism And Countermeasures To SSO Caused By LCC-HVDC Transmission Systems

As the more and more wide application of the high volatge direct current transmission in chinese transmission systems,high volatge direct current transmissions relieve the transmission pressures of long distance and large capacity comparing to alternating current transmissions,which result from different distributions of the energy resources and electrical loads.However the widespread use of high volatge direct current transmission brings some threats to our power system,such as the protential risks of subsynchronous oscillation.Since 1970s,subsynchronous oscillation has been paid more and more attention.The capacitors of series capacitors compensated line,high volatge direct current transmission and power system stabilizers may cause the torsional oscillation of the turbine generators,even leading to shaft damage and other accidents.The thesis begins from the mechanism of the subsynchronous oscillation and then different varieties of analysis methods are compared to conclude that the complex torque coefficient is the optimum method for effective and convenient analysis of the system.Because the method of complex torque coefficient can detailedly analyze the problem of subsynchronous oscillation and meanwhile keep the calculation to be feasible when the system is in big scale.Therefore this thesis applies the method of complex torque coefficient to research on the problem of subsynchronous oscillation.Based on the PSCAD/EMTDC which is a simulation tool for electromagnetic transient,a example simulation model is established for valid verification of the method of complex torque coefficient applied to the subsynchronous oscillation problems.After that chosen from the countermeasures of subsynchronous oscillation,the effect of static synchronous compensator on supressing subsynchronous oscillation is focused on.The simulations of the model show that the static synchronous compensator with suitable controls of the generator shaft paramaters adding to current contrl can effectively supression subsynchronous oscillation.Last but not the least in PSCAD/EMTDC,a simulation system model is etablished according to the present power system of China,which contains high volatge direct current transmission,turbine generators and equvalent alternating current grid.Then the characteristic of subsynchronous oscillation is assessed in simulation system model.The analysis results of simulation by the method of complex torque coefficient reveal that the high volatge direct current transmission indeed may cause the torsional oscillation of generators nearby,thus leading to subsynchronous oscillation.Meanwhile static synchronous compensator mentioned can obviously lower the risk of subsynchronous oscillation when applied to the simulation system model,which proves the possibility of static synchronous compensators used for subsynchronous oscillation in our power system.