Evaluation of Virtual Synchronous Machines with Dynamic or Quasi-Stationary Machine Models

This paper presents a comparison of the
small-signal stability properties for Virtual Synchronous
Machines (VSMs) with dynamic and quasi-stationary
representation of the internal Synchronous Machine (SM)
model. It is shown that the dynamic electrical equations
may introduce poorly damped oscillations when realistic
stator impedance values for high power SMs are used. The
quasi-stationary implementation is less sensitive to the
impedance of the virtual machine model, but depends on
filtering of the measured d- and q- axis components of the
ac-side voltage to avoid instability or poorly damped
oscillations. It is demonstrated how both implementations
can be made stable and robust for a wide range of grid
impedances. However, the dynamic electrical model
depends on a high virtual resistance for effectively
damping internal oscillations associated with dccomponents
in the ac currents during transients. Thus,
when using SM parameters with low virtual stator
resistance for decoupling the active and reactive power
control, the quasi-stationary VSM implementation is
preferable.