Analysis And Suppression Of Subsynchronous Oscillation In Wind Farm With Doubly Fed Induction Generator

With the rapid development of China’s wind power, large-scale wind farms havebeen built and put into operation. Since a large number of wind farms far from loadcenters, series compensation is an economical transmission technology which canimprove the transmission capacity of the AC transmission lines, control the powerdistribution between the parallel lines, and enhance power system transient stability, andwhich is also worthy of study, promotion and use. This paper focuses and make anintensive study on the subsynchronous oscillation (SSO) issue which is caused whenwind farms with doubly-fed induction generator applies series compensation to transmitlarge power, the main contents include:①Building a mathematical model which is suitable for sub-synchronousoscillation problems study, the model includes the aerodynamics, rotor shaft system, thepitch angle control system, induction generator, the rotor converter d-q decouplingcontrol system, series compensation capacitor, and discussing coordinate transformationof machine network interfaces.②Making a small signal analysis on doubly-fed induction generator wind powersystem with series compensation to derive the closed-loop model of the whole system.By solving eigenvalues and participation factors of state equation coefficient matrix, wecan draw a conclusion that the main factors affecting the system sub-synchronousoscillation are series compensation and the wind turbine speed, and the rotor sideconverter inner loop current control has a little effect on the oscillation mode, and otherfactors can be ignored. With the increase in wind speed, the system damping rise. Theincreasing of series compensation degree will result in the decreasing ofsub-synchronous oscillation frequency and weakening of damping. There will be a riskof oscillation instability with a relatively high degree of series compensation.③Making oscillation study of the research system through the complex torquecoefficient method. Combined with the time-domain simulation, we achieve complextorque coefficient method-test signal method to analyze the electrical damping curve,and know that adding series compensation will change the system electrical dampingcharacteristics. To set reasonable parameters of the controller can to a certain extentreduce the risk of sub-synchronous oscillation, but it is difficult to completely eliminatethe hidden dangers of oscillation. With the increasing of series compensation and shaft stiffness, when the torsional vibration frequency and the electrical system frequency arecomplementary, similar to conventional synchronous unit, there will also be thecrankshaft torsional interaction phenomena after disturbance.④In view of doubly-fed induction wind turbine rotor side converter, the authoradopts d-q decoupling control; the q-axis corresponds to the rotor active power controlfeatures. Adding damping control loop on power outer loop, the author makes rotor sideinjected into subsynchronous current in order to stimulate the subsynchronouselectromagnetic torque for damping oscillations. Using phase compensation so that theoscillation frequency electrical damping is positive. Since the rotor-side current closelyelectromagnetically coupled with the mechanical system, the rotor-side injection signalis sensitive toward secondary synchronization modal, at the same time, the band-passfilter is introduced, isolating effect of other frequency signal against oscillation inhibitsignal, avoiding the risk of introduction of other oscillation frequency negative damping.Results based on the PSCAD/EMTDC time domain simulation shows that adoptingSSDC program can effectively inhibit the wind farms subsynchronous oscillationtriggered by series compensation in different conditions.