An Investigation Study On The Role Of DFIG's Pro-active Power Control Techniques In Power System Stability Performance

In order to realize the grid-friendly frequency support,the virtual inertia control(VIC)has attracted extensive attention in doubly-fed induction generators(DFIGs)for emulating the P-f characteristics of synchronous generators(SGs).However,the VIC of the DFIGs will couple the SGs and has an adverse impact on the electromechanical oscillation characteristics in power networks,thus posing new problems and challenges to the system stability.To investigate the effect of the pro-active power control on system damping enhancement,the role of the active and reactive power outer loops in DFIG's VIC strategy is systematically analyzed.Specifically,the main works of this thesis can be summarized as follows:1)A small-signal model of the power system with DFIG integrated is firstly established,including the wind turbine,transmission system,doubly-fed generators,rotor side and grid side converter controls.A single-DFIG-infinite-bus system is used to validate the effectiveness of this small-signal model.Meanwhile,the effect of the network strength on the system stability is preliminarily analyzed.2)A four-machine-two-area system with DFIG and synchronous machines is used to illustrated and deduced the function of DFIG using modal analysis and transient simulation.It is revealed that increasing the droop coefficient and the filter time constant can improve the system damping.Setting the bandwidth of the PLL and power outer loop too small will delay the active control and provide a negative damping torque,thus resulting in a reduction in system damping.3)To make full use of the reactive power injection ability of DFIG,a further research on additional reactive power damping control is made.The function of this additional damping control is deduced based on a two-area system with DFIG and SGs.It is revealed that the damping of the system is enhanced by dynamically changing the reactive output and increasing the proportional coefficient can improve the dominant oscillation mode of the system,providing a useful reference for parameter design.