## 5 Hybrid AC/DC Microgrid Control Strategies
### Power-Sharing Control  
Adjustable speed drives, power supplies, and wielding machines, and many more are categorized as nonlinear loads. When the AC bus is loaded with nonlinear load, a highly distorted current enters the power system, which is usually called harmonic current. Practically, it is difficult to extract harmonics from load current directly since microgrid is very widely distributed with these types of loads [66]. In [67], the authors propose a unified control of the autonomous operation of hybrid AC/DC microgrid where slack terminal in AC/DC microgrid enables power-sharing between ICs through autonomous control, keeping in view distributing total demand among AC and DC slack terminals proportionally. In [68; 69], the authors suggest a remedy to this issue and offer a method to get interlinking converter harmonic current reference through its terminal voltage. A first-order high pass filter with a cut-off frequency of 15 rad/s is used to reduce harmonic. The voltage on AC bus common coupling point (PCC) is kept sinusoidal even in the presence of nonlinear load by converter with the injection of harmonic current. A resonant and repetitive controller is used for implementation which offers higher stability of the system. Each power converter operates independently having no communication link between them, so this is implemented easily and at a reduced cost. Many topologies of hybrid microgrids regarding conventional droop control have been employed where mostly the intermittent distributed generators operating on maximum power point tracking are employed in the scenario of power-sharing and control. A droop-based control strategy was designed with enhanced power-sharing for hybrid DC/AC microgrids. The opportunity is present to interconnect DC microgrid and AC microgrid through an interlinking converter to form a hybrid microgrid when DC and AC microgrids are available in distribution generators. Adequate frequency/voltage control and power-sharing are the essential functions of DC and AC Microgrid control systems in a standalone mode. Active power-sharing in the entire system has to be accurately attained by regulating the interlinking converter's active power input when it comes to a hybrid microgrid. Moreover, when the interlinking converter participates in a reactive power of AC microgrid, it adds some more complexity to a hybrid microgrid control system. The existing droop controller utilizes frequency deviation to calculate the reference active power for interlinking converters, and it is usually a current-controlled converter. This method has a limitation in that it only focuses on power-sharing, neglecting small frequency deviations due to which the system presents poor power quality (no support to AC subgrid and bus voltage) and stability issues; hence, it is required to have some reference for detection other than frequency [69; 70; 71; 72; 73]. In [74], the authors employ secondary voltage control, in which each distributed generator requires only its own information and that of some neighbors, to improve the system reliability. In [75], the authors discussed a decentralized power management system that considered multiple subgrids and common bus topology of hybrid microgrids and made coordination among different subgrids. The conventional droop control is utilized that focuses on AC frequency and DC voltage whose main purpose is to equally distribute all fluctuations in all subgrids as per their power ratings. It also considered islanded mode of operation that is supported by incorporating a storage subgrid that is responsible formaintaining common DC bus voltage for power balancing in islanded mode. In [76], the authors incorporate the parallel operation of a hybrid microgrid by bidirectional converters (BDC), which are responsible for power transfer from DC to AC side of hybrid microgrid and vice versa. However, as they are power electronic converters, they are operated by operating a switch at a specific frequency due to asynchronous switching of the parallel-connected converter.