Inverters in Microgrids (MGs) face significant challenges during their parallel operations; such as accurate power sharing, deviations in system voltage magnitude and frequency, imbalance between generation and load demand. To solve these techno-economic challenges, hierarchical control structures are implemented in MGs. The structure consists of three layers as primary, secondary and tertiary controls. The control approach can be either communication-based or communication-less at the various layers. The use of communication at primary and secondary layers faces problems like communication latency, data drop-up, and expense issues. On the other hand, improved decentralized control techniques being communication-less can avoid the disadvantages of using communication. This paper presents an insight into the limitations with the communication-based approach by briefing about the centralized and distributed control techniques at the secondary control layer. Subsequently, the communication-less control techniques and algorithms to achieve accurate power sharing along with restoration of MG voltage and frequency are described. A comparison among different decentralized droopbased power sharing methods in the primary control layer are done based on review and simulations. In addition to that, improved communication-less secondary restoration techniques are explained. Finally, future research directions in these areas are listed, aiming to improve the reviewed techniques.

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