  Grid-forming inverter-based autonomous microgrids present new operational
challenges as the stabilizing rotational inertia of synchronous machines is
absent. The design of efficient control policies for grid-forming inverters is,
however, a non-trivial problem where multiple performance objectives need to be
satisfied, including voltage/frequency regulation, current limiting
capabilities, as well as active power sharing and a scalable operation. We
propose in this paper a novel control architecture for frequency and voltage
control which allows current limitation via an inner loop, active power sharing
via a distributed secondary control policy and scalability by satisfying a
passivity property. In particular, the frequency controller employs the
inverter output current and angle to provide an angle droop-like policy which
improves its stability properties. This also allows to incorporate a secondary
control policy for which we provide an analytical stability result which takes
line conductances into account (in contrast to the lossless line assumptions in
literature). The distinctive feature of the voltage control scheme is that it
has a double loop structure that uses the DC voltage in the feedback control
policy to implement a power balancing strategy to improve performance. The
performance of the control policy is illustrated via simulations with detailed
nonlinear models in a realistic setting.
