## 5 Hybrid AC/DC Microgrid Control Strategies
### Power-Sharing Control
This method attains load sharing without any effect on online resistances. Italso manages power-sharing among AC and DC subgrids with benefits of no circulating current, proportional power-sharing, DC link regulation, and maximum power transfer. In the presented technique, the only information that a converter requires is about the status of its neighbor. In [85], the authors suggested localized distributed controllers, which generate power references for each interlinking converter. Using this technique, each converter is allowed to exchange information in distributed communication, hence sharing power as per their own rating. Energy storage subgrid systems are very special in microgrids as they can maintain regulated voltage in case of islanding operation considering this requirement. In [86], the authors discussed the implementation of an AC dynamic local voltage compensator which is dependent on the distributed energy storage system and is responsible for active power-sharing in case of the regulatory requirement. Moreover, they utilize a voltage-controlled method that does not require frequency measurements. It shows the capability of the direct control of AC voltage with better voltage quality. In [87], the authors propose synchronverter technology that uses a controlled converter to maintain active power-sharing and power quality. The direct frequency control method is utilized in which DC-side voltage is measured and gets real-time reference frequency corresponding to the DC-side voltage variation. Then frequency is sent to a low-pass filter (LPF) to avoid high-frequency harmonics resulting in the fluctuation of frequency and active power. It aims to realize active power-sharing and improves the transient frequency stability fluctuation of frequency and active power. Model predictive methods involving predictive current and power (MPCP) and model predictive voltage and MPVP scheme are gaining attention among researchers to realize different operational modes, e.g., power generation, fluctuations, power demand, battery state of charge, and efficient energy management system. One disadvantage of the discussed method is that in comparison to other conventional controllers, these model predictive controllers need additional sensors and measurements. Hence, they require not only extra communication facilities but also high cost. Moreover, in practice, modeling such a big microgrid is a tidy task that can always offer some errors, which are yet to be further addressed in future work [88]. Reference [89] suggests series-connected converters with static VAR compensator for hybrid AC and LV DC microgrids. The authors presented _P-\(\delta\)_ and _Q-P-V_ droop control for both converter and SVC parts, presenting the phase angle of the voltage of fundamental reactive power and fundamental droop control voltage. The proposed converter also suppresses harmonic components by determining SVC impedance which in turn has two physics that are triggered in every half cycle. The authors did not discuss anything about stability and circulating current problems. In ICs containing bidirectional converters, as power flow between AC and DC subgrid and its direction changes, it largely degrades the system's stability. Reference [90] investigates the effect of power flow on the small-signal stability of hybrid microgrid by using a linearized state-space model and its eigenvalue analysis. Moreover, sensitivity analysis is also performed to study stability margins. It is concluded that power flow from DC to AC sub-grid is more stable than AC to DC sub-grid. Secondary control strategies are conventionally studied independently for each side of a hybrid microgrid, neglecting the interaction of AC and DC sides, which cannot completely analyze dynamic response and power-sharing. Reference [91] presents a novel distributed secondary control strategy for a hybrid microgrid that regulates AC/DC voltage magnitude and frequency by employing distributed consensus approach.