  The output impedance matrices of three-phase grid-connected voltage source
converters (VSCs) are widely used in power system stability analysis.
Regardless of how the impedance is modeled, there always exist coupling terms
in the impedance matrix, which makes the system a multi-input- multi-output
(MIMO) system. Some approximation approaches omit the coupling terms so that a
three-phase system can be treated like a single-phase one, and the
impedance-based stability criterion for a single-input-single-output (SISO)
system is applicable. However, such handling may result in analytical errors or
even incorrect conclusions in a mirror frequency coupled system. By introducing
the concept of generalized- impedances, this letter proposes a new stability
criterion based on a virtual SISO system, which can effectively handle the
coupling terms. Further, the effects of the phase-locked-loop (PLL) parameters
on system stability are studied based on the proposed criterion. The
effectiveness of the proposed criterion is verified by a hardware-in-the-loop
(HIL) simulation based on RT-LAB.
