## 5 Hybrid AC/DC Microgrid Control Strategies
### Power-Sharing Control
A circulating current flows among multiple converters which tend to lower the power rating of equipment and can cause power-sharing deviation mainly in island mode. A unified detection method of circulating current is suggested to address this issue, e.g., virtual impedance controller in combination with droop controller to solve circulating current and power-sharing problems simultaneously. In [77], the authors propose an active power-sharing and control strategy by utilizing a synchronization machine (VSM) responsible for the inertial improvement of AC bus frequency and DC bus voltage of a hybrid microgrid. The VSM works by observing rotor dynamics of a synchronous generator, the virtual inertia, and damping control of AC bus frequency and DC bus voltage simultaneously. This damping indicates the variation of transferred power, therefore, to restore frequency and voltage at its original values. This control technique has the advantage of operation in both grid-connected and islanded modes of operation. It is quite evident that when an abrupt change or fluctuation appears in a hybrid microgrid, AC bus frequency and DC bus voltage can easily damage, which in turn severely affects power system quality. To deal with such issues, in [78], the authors proposed adjustable inertia on ICs to improve the dynamic performance of a hybrid microgrid. The proposed control method helps in improving inertia support for both AC and DC buses and protects ICs from conversion failures in a state of transition. This technique deploys an arctangent function to limit capacitor voltage so that in case of power fluctuation, it may not lower the minimum conversion voltage, and it seems to work well with the systems that utilize batteries in the dc subgrids. Similarly, in [79], the idea is based on partially decoupling the inductor current dynamics by introducing a constant virtual resistance with bounded controllable voltage for ICs, power rectifiers, and boost converters. In all these cases, the dynamics of virtual voltages guarantee desired upper bound for converter current regardless of the direction of power flow by introducing a constant virtual resistance with a bounded dynamic virtual voltage for all converters. Results show that input current follows maximum required values, ensuring exceptional power-sharing, voltage regulation, and input current limitation. In [80], the authors propose the autonomous operation of interconnected converters could also be achieved but taking into account the failure of tracking droop variation in droop control parameters, a centralized controller. It can be deduced from the comparative analysis that centralized control offers optimized power exchange and works identically during the parallel operation of converters. Many researchers proposed decentralized load sharing and power management methods for hybrid microgrids, considering the operation of photovoltaic and battery units. In [81], the authors employed a modified droop control method to control photovoltaic power, while in [82], the authors considered real-time control with both energy storage and pulsed loads. Due to intermittent generation, grid isolated systems require special attention. A comprehensive frequency scheme and active power decoupling method are used to link the AC and DC buses while simultaneously regulating the voltage and frequency of the grid. The power flow among AC and DC subgrids can be well managed by interlinking converters, and these are interconnected with the conventional DC voltage-based droop approach. This approach may cause overstressing of converters due to the line resistance effect, so employing multiple converters for AC/DC subgrids seems a viable option. In [83] and [84], the authors proposed superimposed frequency and decentralized coordination methods, respectively, in the DC subgrid. In [84], the authors coordinate the DC sources and interlinking converters by the injected ripple frequency; however, the introduction of a global variable enables accurate sharing of the power of converters as per their rating.