| In recent years,several subsynchronous oscillations of multi-machine systems with different reasons have occurred at China and abroad,which seriously threaten the normal operation of the power system.The interaction between thermal power units and between wind power and thermal power in multi-machine system brings difficulties to the research.It is urgent to analyze the interaction between units in subsynchronous oscillation and take effective measures to restrain SSO.The main contents of this paper are as follows:(1)The mathematical model of multi-machine system with PSGM and multiple thermal power units is established,the eigenvalues of multi-machine system are derived,and the modes of thermal power shafting torsional vibration and direct-drive wind SSCI are identified and verified by time domain simulation.It lays a foundation for the analysis and suppression of multi-machine synchronous oscillation.(2)A simplified complex torque coefficient method is proposed for the analysis of subsynchronous oscillation and interaction of multiple thermal power units.The expression of the simplified complex torque coefficient of a typical two thermal power unit system is derived,and the relationship between the system parameters and the damping of the machine to be studied is explicitly established.After analysis,it is found that the influence of other machines on the electrical damping of the machine is that,on the one hand,other electromechanical and magnetic parts will change the subsynchronous frequency impedance seen from the machine to be researched,on the other hand,it is the role of other shafting.When there is the same or similar torsional vibration frequency between the units,the effect of other shafts on the research machine at this frequency may affect the stability of the machine to be developed.Further analysis shows that the series compensation mainly affects the resonant frequency of the line and changes the subsynchronous frequency impedance of the system,while the effect of other machine shafts on the damping of the machine to be researched will be weakened with the increase of the electrical distance between the units and the torsional vibration frequency difference.(3)The causes of different types of subsynchronous oscillation in direct-drive wind power-thermal power system and the interaction between direct-drive wind power and thermal power units are analyzed.The SSCI of in this system is because the negative resistance effect of the grid side controller of the PSGM will cause the equivalent resistance of the system,including the internal impedance of the thermal power unit,to be less than zero.At the same time,this negative resistance effect will also reduce the electrical damping of thermal power units at subsynchronous frequency,which may lead to the thermal power shafting torsional vibration and direct-driven wind SSCI at the same time.The output change of thermal power unit has little influence on the two kinds of subsynchronous oscillation,but the parameter change of PSGM will obviously affect the SSCI.The series compensation can be adjusted to make the torsional vibration frequency of thermal power shafting far away from the system resonant frequency and reduce the influence of direct drive wind power parameters on thermal power subsynchronous oscillation.(4)A multi-machine subsynchronous oscillation suppression method based on generalized phase compensation method is proposed.The relationship between the complex torque coefficient and the real part of the eigenvalue in the multi-machine system is established,and the variation of the real part of the eigenvalue of the subsynchronous oscillation mode after adding the SVC damping controller is derived.On this basis,the generalized phase compensation method is introduced to realize the parameter tuning of the additional damping controller,which effectively avoids the interaction between the controllers.Time domain simulation shows that the designed controller has a good suppression effect. |
PDF Full Text Request