## 3 Hierarchical State-of-the-Art Control
### Voltage and Frequency Restoration for Secondary Control
\begin{table}
\begin{tabular}{c c c} \hline
**Technique** & **Advantages** & **Shortcomings** \\ \hline \hline \multirow{2}{*}{Droop-based [79; 80]} & Straightforward implementation. & \multirow{2}{*}{\begin{tabular}{c} Inaccurate power sharing. \\ Performance degrades under nonlinear load and \\ unbalanced conditions. \\ Relatively slow in transient response. \\ Relatively poor active and reactive power \\ \end{tabular} } \\  & Improved current harmonics sharing. & \multirow{2}{*}{\begin{tabular}{c} Relatively poor active and reactive power \\ sharing performance. \\ If the line impedance of the parallel inverters is \\ mismatched, transformation angles will differ and \\ not be synchronized. \\ \end{tabular} } \\  & & & \\ Adaptive droop [89; 90] & Good active power sharing. & \multirow{2}{*}{\begin{tabular}{c} Sophisticated implementation. \\ High computational burden. \\ Sluggish dynamic response. \\ \end{tabular} } \\  & & & \\ Enhances power sharing accuracy and system stability. & \multirow{2}{*}{\begin{tabular}{c} Relatively poor reactive power sharing performance. \\ Involes high THD in current components. \\ Slow dynamic performance. \\ \end{tabular} } \\ Robust droop [97; 101] & & & \\ Robust dynamic performance against load variations. & \multirow{2}{*}{\
\begin{tabular}{c} Relatively poor reactive power sharing performance. \\ Involes high THD in current components. \\ Slow dynamic performance. \\ Performance degrades when communication \\ malfunctions at a single point. \\ Communication line increases capital and \\ maintenance cost. \\ \end{tabular} } \\ Non-droop-based [106; 112; 115] & Improves the dynamic stability of power sharing and quality. & \multirow{2}{*}{
\begin{tabular}{c} Slow dynamic performance. \\ Performance degrades when communication \\ malfunctions at a single point. \\ Communication line increases capital and \\ maintenance cost. \\ \end{tabular} } \\  & & \\ \end{tabular}
\end{table} 
Table 2: The advantages and shortcomings of the power sharing control. communication network. While the consensus of the voltage and frequency set reference values is accomplished over an infinite time horizon with exponential convergence, as in [104; 117], the authors apply feedback linearization methods for voltage restoration and finite-time control protocol is used to synchronize frequency to the nominal value, simultaneously achieving accurate active power sharing among the DG units. A similar approach is introduced in [118], which is capable of both restoring _V-f_ and guaranteeing reactive power sharing as well. The secondary control under switching communication topology is designed in [119], where MAS network is used for controller stability analysis. A distributed approach offers flexibility and reliability in terms of central control avoidance, which means that the failure of a single unit will not degrade the entire system. Meanwhile, in [120], the restoration of the voltage to the nominal value is realized through the dynamic consensus-based method. The enhancement of flexibility and reliability of the microgrid system is ensured without line impedance consideration. * _Event-triggered_: ETC is another approach for data sharing when a condition is fulfilled or an event is triggered, instead of continuously exchanging data among the DGs unit. In such a way, a sampled data is controlled through a designed mechanism [121]. Numerous methods have been proposed in recent years, such as the time-triggered, event-triggered, and self-triggered sampling methods; their reports can be found in [122; 123; 124]. ETC makes a computation that relies upon the measurement error and the last event of the variable states. The error is the difference between the measured and the observed values of the variable states. The benefits of ETC is that it is able to maintain superior closed-loop stability and performance while reducing the number of information exchange among the DGs.