## 3 Hierarchical State-of-the-Art Control
### Power Sharing for Primary Control
#### 3.2.2 Non-Droop-Based Control  
Apart from droop-based methods, various techniques can be applied that may or may not require communication. These techniques are listed below:  
* _Master and slave:_ Similarly considered as a secondary control, it consists of a voltage controller for controlling the output voltage to provide a current reference for the other units that assigns the master-slave units. A literature review on this particular method can be found in [30; 100; 101; 102].  
The master unit can be defined through a fixed module arbitrarily or based on the maximum current. In the case of a grid-connected mode, the grid itself is presented as the master unit and no specific control is required for grid-connected and islanding operations. For instance, in [103; 104], a supplemental control algorithm is proposed to provide speedy response and sophisticated operating conditions for multiple inverters in an autonomous microgrid; thus, the active and reactive powers are shared accurately according to load demands. In [105], a voltage controller is added with a robust controller that is combined with an automatic master control for the precision of output voltage.
* _Concentrated control_: The operation of this method is based on a central controller by means of a communication link between the central control and the other units. This method consists of the central limit control of power deviation, and their reviews can be found in [106; 107; 108]. The current reference value is regulated by the voltage controller. It provides a higher degree of power quality, whether in ideal or transient conditions. However, it requires a high-bandwidth communication link, which increases capital cost.
* _Instantaneous power theory (IPT)_: In [109; 110], power sharing could be achieved among parallel DGs devoid of any communication between the devices. An enhancement of IPT is included in [111], which uses a new signal decomposition technique with a great extent of selectivity, thus making it immune to unbalanced and nonlinear power systems. A similar approach in [112] makes the necessary modifications in the fundamental computation to remedy the compensation under any condition of the voltage supply. Another method is presented in [113] for grid-connected VSI under transient conditions, employing an adaptive transformation that instantaneously makes adjustments accordingly to the dynamic voltage conditions, which allows control over constant and oscillating instantaneous power.
* _Direct Power Control (DPC)_: The DPC algorithm has become an alternative for power sharing control, particularly in grid-connected mode of operation. To provide the reference value of power, it solely relies upon the \(p\)-\(q\) theory to adjust the instantaneous active and reactive power errors and maintaining them within a constant hysteresis band [114; 115]. Due to the straightforward implementation, it offers several benefits: (1) no grid voltage information is required, as it can be realized through a binary algorithm, (2) compatibility with both sinusoidal and non-sinusoidal voltage signals, and (3) minimal commutation on converter switches.  
Following the discussions above, Table 2 addresses the advantages and shortcomings of each control method to deliver an idea that can bridge the gaps among the control approaches. The designed controller should have several features that are immune to the uncertainty model and parametric variations of the microgrid and to load variations. Hence, the advanced algorithms controller complexity can be reduced for further implementation in practical systems.