Analysis And Control Of VSG Based On Generalized PSS

Rotating reserve and moment of inertia in power system are reduced with the increase of the penetration rate of renewable energy featuring low inertia and weak damping which is connected to grid through grid-connected inverters,making traditional power system tends to power electronic power system.The randomness and fluctuation characteristics of renewable energy,such as wind energy and solar energy,make the stable operation and control of power electronic power system more difficult.A control strategy of grid-connected inverter is proposed to compensate the voltage and frequency support ability of renewable energy by implementing the frequency and voltage regulation characteristics of synchronous generators(SG),which is called virtual synchronous generators(VSG).However,VSG introduces the problem of power angle oscillation to grid-inverters.Consequently,this paper focus on the characters of power angle oscillation in VSG.This paper analyzed the characteristics of VSG output power based on the mathematical model.The coupling degree between active output power and reactive output power are different under different grid and work conditions.A VSG power outer loop and voltagecurrent inner loop scheme are designed when the active power and reactive power can be completely decoupled,then the small signal model of VSG is established and the stability of main system parameters is analyzed by root locus method.This paper focus on inertia and damping characteristics of VSG active control link under the condition that the active power and reactive power can be completely decoupled,founding the traditional inertia damping control scheme cannot satisfied the indexes of dynamic response and static control accuracy at the same time.Afterwards,the concept of virtual power is established in this paper,and the expressions of inertia support power and damping power is derived.A new virtual power control scheme including inertia control and damping control is proposed in this paper based on the analysis of amplitude and frequency characteristics of virtual power,which is named as generalized PSS control structure.Subsequently,a small signal model of the new VSG was established,and its control parameters are analyzed.We can draw the conclusion that the generalized PSS can adjust the system gain flexibly,meeting the indexes of dynamic response and static control accuracy at the same time.Finally,the parameters design principles of new VSG are given in this part.In order to expand the application range of VSG based on generalized PSS and reduce the parameter design complexity,A generalized VSG small signal model was established when the active power and reactive power cannot be completely decoupled.Then,this paper introduced torque analysis theory to define the index of virtual torque of VSG.Otherwise,the generalized PSS is extended to the reactive power link.Afterwards,this paper derived the expressions of virtual torque index,and introduced the characteristic root which represent the system oscillation mode into this expression.Finally,system characteristic root analysis verifies the correctness of virtual damping torque method in determining the stability of system.