Non-Minimum Phase Behavior Analysis of the Weak Grid-Tied VSC's Q-Axis Current Control Dynamics

In this letter, the phenomenon is found that there are non-minimum phase and “undershoot” behavior in grid-tied VSC, even if the outer loop is not applied. Based on the small-signal model, the mechanism that q-axis-related non-minimum phase behavior reduces VSC's stability and the “undershoot” behavior are analyzed in the letter. It is found that the q-axis current control dynamics of weak grid-tied VSC introduce an open-loop unstable zero when the reactive power is supplied from the AC system, which is the key factor for the non-minimum phase behavior.