## 3 Black Start Process Schemes  
As simple as it can be, a black start unit is in charge of delivering power to its step-up transformer, to the lines connecting it to the target generating unit, to the target's transformer, and to the unit itself [24]. With that said, working simultaneously through every available unit may reduce, by an appreciable amount, the duration of the interruption. The level of segmentation of the total area of the grid is limited by the availability of resources, the rigidity of the systems' interconnections, and the efficient coordination of its recovery process. Therefore, how to design an adequate partition scheme and a recovery route on each system poses a vital issue for the implementation of black start methods [35]. In general terms, to implement a black start scheme, three functional modules must be considered when planning, which are: formation, verification, and optimization of starting schemes [36]. Formation module: The power system starts an automatic black start search process through the local power distribution. Power plants which are on standby mode are identified, together with possible start routes. Every possible start scheme is generated automatically, conforming a database of initial schemes. * Verification module: With the start schemes already proposed, analysis and simulation through software tools is done for verifying every proposed scheme. This simulation also takes into account characteristics such as self-excitation of some units, commutation under no-load conditions, and low frequency check [37]. * Optimization module: Aiming for the best possible performance, an optimal startup scheme is selected from the proposed options (this is done using decision-making methods). The selection of the final startup scheme is very important, as it plays a crucial role on the system's performance. In practice, the performance of any black start scheme may be affected by a multitude of factors that are subject to constraints, some of which include: generation constraints (matching generation with active and reactive load and losses), crew constraints (availability of trained crew and operators), time constraints (time period for bringing units back online and \"minimum downtime\" for each unit) and reserve constraints (spinning reserve availability in case of any loss of units) [5]. Given the multitude of performance indicators that are relevant for evaluating proposed black start schemes, power systems take into account those indicators that best reflect the advantages and disadvantages of the startup. The general object of a system restoration is to restore as much load as possible in the shortest period. Therefore, restoration time and available generation capacity have to be selected as priority evaluation criteria. The former reflects the speed with which the generation unit restarts operations, while the latter represents the generation capacity that each unit can provide to the system [35]. Hence, considering both of the previous criteria, these five performance indicators may be chosen for evaluating black start proposed schemes:  
* Generation capacity of each unit, as it determines the amount of energy that can be supplied to the system when it starts [38]. * Actual conditions of each unit: These conditions are determined by the temperature of their turbine cylinder. As a function of said temperature, the conditions of a unit can be divided into five categories: ultra-cold, cold, warm, hot, and ultra-hot [39]. * The ramp rate of each unit: During the restoration stage, the most important loads have to be restored first. Indeed, the units with the best ramp rate should be started first, due to their ability of quickly increasing their output. An example is shown in Figure 1.
* Startup power needed for each unit: In the first stages of the restoration, only a few units can act as black start units, and their output power is limited. Consequently, units that require less startup power must be restored first [40,41].