## 3 Hierarchical State-of-the-Art Control
### Power Sharing for Primary Control
#### 3.2.1 Droop-Based Control
* _Robust droop (RD)_: An RD controller based on the uncertainty and disturbance estimator (UDE) for the nonaffine nonlinear system is presented in [91; 92; 93]. The UDE control algorithm relies upon the assumption that uncertainties and disturbances can be estimated through a filter with an adequate bandwidth. In such a way, the system disturbances and the model nonlinearity can be determined and compensated thoroughly. This method offers a straightforward implementation and does not require inner current control and voltage control schemes. Meanwhile, the regulation of DC-link voltage and robust power sharing in grid-connected VSC-MTDC grids is introduced in [94] by considering the impacts of the droop controller's response, instantaneous power, and DC-side uncertainties. The realization of the controller parameter tuning is through the polynomial method to fix the poles of the equation's trait under possible variations in the droop gain. As a result, it ensures rigid performance under sizeable differences in the converter's operating point to a great extent, as well as under parametric uncertainties and during the occurrence of disturbances. It can increase the system stabilization's effectiveness as well as guaranteeing a robust performance within the system. According to [95], the enhancement of the load voltage drop due to the load and the droop effect can be realized via proportional load sharing. The RD control scheme is integrated with the secondary control. Thus, the load voltage can be retained within the desired range around the nominal value, providing better stability and robustness against disturbances, feeder impedance, and component mismatches. The work in [96] concerns the robust dynamic droop's power sharing for the integration of wind turbines and fuel cells. It employs reverse and direct droop controls, where the reverse droop control is responsible for active and reactive power regulation, while the direct droop control is for the frequency and voltage outputs, which are significantly used for the fuel cells. The adjustment of frequency and voltage within the microgrid can be tracked robustly at specifically designed set points and instantaneously provides better power sharing performance. In the case of a low-inertia wind turbine, a robust control method that is invulnerable against system nonlinearities and changes in the network is reported in [97]. The control scheme exploits multivariable \(H_{\infty}\), which integrates with the centralized multi-input-multi-output (MIMO) controller (set reference of active and reactive powers) and the local measurements of active and reactive powers of the droop controllers. The application has excellent power sharing performance against significant disturbances and interconnection operation. In high-voltage microgrid applications, the work in [98] utilizes the signal detection on the high-voltage side of the coupling transformer as the feedback signal. The line impedance is the dominated part of grid impedance (coupling transformer), and hence the impact of line impedance on the power load sharing can be mitigated significantly, especially on the reactive power relationship. * _Feedforward control:_ An enhanced power flow control is demonstrated in [72] by introducing a feedforward control of the primary grid voltage amplitude and frequency to mitigate the impacts of grid fluctuation on power flow control. It offers improvement in controller stability and provides seamless transitions for grid-connected operation. A similar approach observed in [99] through an adaptive feedforward compensation provides better steady-state power sharing performance while keeping the voltage and frequency regulation. Thus, the robustness of the system stability is enhanced against droop coefficients and network uncertainties.