  Inverter-dominated microgrids are quickly becoming a key building block of
future power systems. They rely on centralized controllers that can provide
reliability and resiliency in extreme events. Nonetheless, communication
failures due to cyber-physical attacks or natural disasters can make autonomous
operation of islanded microgrids challenging. This paper examines a unified
decentralized secondary control scheme that is robust to inverter clock
synchronization errors and can be seamlessly applied to grid-following or
grid-forming control architectures. The proposed scheme overcomes the
well-known stability problem that arises from parallel operation of local
integral controllers. Theoretical guarantees for stability are provided along
with criteria to appropriately tune the secondary control gains to achieve good
frequency regulation performance while ensuring fair power sharing. The
efficacy of our approach in eliminating the steady-state frequency deviation is
demonstrated through simulations on a 5-bus microgrid with four grid-forming
inverters.
