The voltage-reactive power (volt-var) control can effectively mitigate the overvoltage issues caused by large-scale photovoltaic (PV) grid integration

but improper design may lead to inverter oscillation instability. Existing stability studies on volt-var control fail to consider the interaction between the volt-var control and the traditional grid-following control

resulting in inaccurate stability boundaries. To address this issue

this paper clarifies the coupling characteristics between volt-var control and other control loops in grid-connected PV inverters under weak grid conditions. A simplified 5th-order single-input single-output model is developed

and based on the Nyquist stability criterion

the instability mechanism of volt-var control under weak grid conditions is revealed. The research shows that

under weak grid conditions

the weaker and slower the voltage-reactive power response

the shorter the control delay is

and the stronger the network is

the more stable the PV grid-connected system will be. Additionally

when the line resistance-inductance ratio is large

a lower DC voltage control bandwidth is more conducive to system stability

whereas when the line resistance-inductance ratio is small

a higher DC voltage control bandwidth is more favorable for stability. Based on these findings

the stability boundary for volt-var control is derived. Moreover

the accuracy of the analysis and the proposed boundary is verified through simulations and hardware-in-the-loop (HIL) experiments across multiple scenarios.