## 3 Hierarchical State-of-the-Art Control
### Power Sharing for Primary Control
#### 3.2.1 Droop-Based Control
Thus, VI compensates the impacts of the mismatched in the physical feeder impedances and simultaneously achieves precise adjustment of the DG unit's corresponding impedance at both fundamental and selected harmonic frequencies. An adaptive VI proposed in [81] relies upon an extra small injection of AC signal in the output voltage of the parallel inverter to significantly highlight the unbalanced and harmonic power sharing problems caused by the feeder impedance mismatch. Whereas in [82], an adaptive VI is used for accurate reactive power sharing in an islanded microgrid. The compensation of voltage drop mismatch across the feeders is counteracted by employing communication to facilitate the tuning. In [83], the distributed adaptive VI is employed to suppress large circulating currents caused by the slight differences in both magnitudes and phases in the output voltage of the DG units. * _Adaptive droop (AD)_: The current paradigm in autonomous power sharing is by utilizing the voltage deviation in multi-terminal DC (MTDC) grids. The regulation of voltage deviation, as well as frequency deviation, can be realized through the outage stage of the voltage source converter (VSC) and instantaneously performs power sharing. This method is becoming an alternative approach for system planners and operators. In MTDC grids, the AD approach is applied to offer a better performance of the VSC, which enables the sharing of the burden of power mismatch. For instance, [84; 85] include frequency support and ensure sufficient power sharing by considering the available headroom of each converter station into the control action. This means that when the VSC relatively achieves the operating point (close to limits), it has the capability to constrain the burden of loss sharing to a great extent to the neighboring converters with higher headroom and spare capacity. The relationship of voltage-current-frequency (_V-I-f_) characteristics is derived to establish the correlation between frequency and DC voltage. It can adjust its DC voltage reference autonomously according to grid frequency deviation. Hence, it offers the avoidance of overloading and burden in a desirable way. In [86], it is reported that a better dynamic security assessment is performed and autonomous power sharing is provided following the outage of the VSC. It offers stability-constrained adaptive droop gains through the trajectory sensitivity analysis (TSA)-based approach. Meanwhile, the enhancement of power load sharing and the minimization of circulating currents in low-voltage DC microgrid are presented in [87; 88]. It can be achieved by applying AD resistance and instantaneous virtual resistance shifting to compensate the trade-off between the difference in current sharing and the adaptive voltage positioning. Fast transient response is shown through the right-half-plane zero analysis method, where the stability of the system is guaranteed when the series resistance zero is above the equivalent to right-half-plane zero. The integration of wind farm converters in MTDC grids can be found in [89]. Power sharing improvement and voltage deviation minimization under the presence of any faults and power imbalances in the system can be achieved by employing the voltage droop control with the help of the derated operation of the wind farm converters. It offers control freedom over the active power adjustment and provides ancillary services to the primary grid. In [90], the study integrates a wind farm converter with fuzzy logic-based control. The method can update the droop coefficient through the availability of power capacity of the converters, which revamps the conventional fixed droop coefficient. Through this method, the scheme continuously tracks the dynamic behaviors of the converters and realizes desirable responses under different outage scenarios. As found by the authors, the AD scheme ensuring good transient response within proper power sharing, its still experience high computational burden and slow dynamic response.