Rectifiers and voltage regulators acting as constant power loads form an important part of a
microgrid’s total load. In simplified form, they present a negative incremental resistance and beyond
that, they have control loop dynamics in a similar frequency range to the inverters that may supply a
microgrid. Either of these features may lead to a degradation of small-signal damping. It is known that
droop control constants need to be chosen with regard to damping, even with simple impedance loads.
Actively controlled rectifiers have been modelled in non-linear state-space form, linearised around an
operating point, and joined to network and inverter models. Participation analysis of the eigenvalues of
the combined system identified that the low-frequency modes are associated with the voltage controller
of the active rectifier and the droop-controllers of the inverters. The analysis also reveals that when
the active load DC-voltage controller is designed with large gains, the voltage controller of the inverter
becomes unstable. This dependency has been verified by observing the response of an experimental
microgrid to step changes in power demand. Achieving a well-damped response with a conservative
stability margin does not compromise normal active rectifier design, but notice should be taken of the
inverter-rectifier interaction identified.