The invention provides a virtual
admittance method for improving the stability of a grid-tied
inverter under a weak grid. The virtual
admittance method comprises the steps of S1, determining, according to a d-q
admittance model of a grid-tied
inverter, an interaction relationship between an
inverter and a weak grid, the interaction relationship being that when the impedance of the weak grid is not greater than a first
grid impedance value Zg0, a grid-tied
system can maintain stability and when the impedance of the weak grid changes to a second
grid impedance value Zg, the grid-tied
system becomes unstable, where Zg=Zg0+Z'g, Z'g being the change value of the impedance of the weak grid and the change being greater than 0; and S2, measuring the change value Z'g of the impedance of the weak grid online, and connecting a virtual admittance Yp in a control loop of the grid-tied inverter and adaptively adjusting the
grid impedance change with the virtual admittance Yp, so that a
Nyquist curve of the impedance ratio (Zg0+Z'g) (Yout_cl+Yp) of the grid-tied
system is enabled to be the same as Zg0 Yout_cl, Yout_cl being the output admittance of the inverter, the stability of the grid-tied system can be maintained and the stability of the grid-tied inverter under the weak grid is improved. Further, the realized order of the virtual admittance in the control loop can be reduced and the practicality is improved.