## 2 Conventional and Non-Conventional Generation Start-Up Strategies  
NCRES have significantly increased their presence in electric power systems; therefore, such technologies are more frequently integrated into power system planning and operation studies [28]. This section presents the main guidelines that must be considered when integrating NCRES into a restoration process. * The objective of the methodology is to maximize the generation capacity of the system and minimize the non supplied energy in a blackout. * Before starting the restoration process, a preliminary analysis should be carried out to identify the cause of the event as well as available and unavailable resources. Knowing this before starting the system restoration will make the process more effective. * BS units are the first ones to enter the system. For these units, a start-up time equal to zero is considered in the model. * BESS will serve as BS units and will aim at bringing cranking power to NBS units to accelerate their start-up. They will also contribute to the normalization of priority loads. * A priority order must be considered when performing the service restoration. There are priority loads that must be attended first such as control centers, aquducts, hospitals, and substations. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Reference** & **Year** & **Methodology** & **CG** & **NCRES** & **BESS** \\ \hline
[16] & 2011 & Fuzzy logic & X & & \\ \hline
[14] & 2012 & Lexicographic optimization & X & & \\ \hline
[10] & 2014 & Pareto search through NSGA-II & X & & \\ \hline
[26] & 2014 & Heuristic strategies & X & X & \\ \hline
[27] & 2014 & Spanning tree search & X & X & \\ \hline
[12] & 2015 & Firefly algorithm & X & X & \\ \hline
[22] & 2015 & Lagrangian relaxation & X & X & \\ \hline
[20] & 2017 & Dynamic programming & X & X & \\ \hline
[18] & 2018 & MILP & X & X & \\ \hline
[17] & 2019 & Branch-and-bound & X & & \\ \hline
[19] & 2021 & MIQP & X & X & \\ \hline This paper & 2021 & MILP & X & X & X \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Optimal generation start-up methodologies. * NCRES are considered in this methodology as NBS units; however, due to the benefits of their control systems; their power rise time is considered to be much less than that of conventional generators. With this fact, it will be assumed that the start-up of renewable resources are of the step type. * Minimum starting power. - Frequency stability to events in the restoration process. The starting of NCRES takes place when there is minimal inertia in the system. This inertia value is determined by the system operator through planning studies such as TMR (transmission must run). TMR is an indicator that determines the minimum generation required that must be online and operating at specific levels. This generation compensates for the lack of transmission networks in relation to the demand that is being restored. - Minimum firm generation. To comply with this condition, it is necessary to have adjusted historical and forecast data for both wind and solar generation production and the primary resource associated with these generation sources. These data serve as input to the proposed methodology to calculate average values and generation probabilities from the data series. * PV and wind NCRES will have as their main function within the restoration process to accelerate the demand meeting process and compensate the load-generation imbalances while gaining stability in the system. A MILP model is built based on the proposed methodology as illustrated in Figure 1. At first glance, it might be inferred that NCRES units should be the first ones to be started, prioritizing the fastest units to speed up the restoration process; however, there are more factors that affect the start-up process such as the stochastic characteristics of solar and wind resources and also the lack of inertia of the system. In consequence, an optimization process is required to find the best solution that ensures a fast but secure restoration.