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Effects of electrical infrastructures in grid with high penetration Chapter | 8 215

FIGURE 8.10 Microgrid frequency variation.

the microgrid. After the primary control response the distribution system
starts to operate in the transient region, 60.5 60.1/59.9 59.5 Hz [31]. The
operation in these levels is allowed for 30 seconds, being necessary its cor-
rection to steady-state limits, 60.1 59.9 Hz [31]. In this sense, secondary
control strategies are required to correct the frequency deviation and guaran-
tee satisfactory operating condition within limits established by the system
operator. A detailed representation of the microgrid frequency variation dur-
ing islanded operation is shown in Table 8.3. It should be noted that the pre-
sented values refer to the frequency levels after the transient response of the
controllers.
Furthermore, the three-phase representation of microgrids’ supplying is
described in Fig. 8.11. It is possible to observe that when operating in
islanded mode, local controllers must provide the required unbalance contri-
bution. This process is divided into two scenarios; first, if the distributed and
renewable generators are capable of providing the necessary supply to the
grid demand, these units are responsible to perform primary and secondary
controls, as well as phase balancing. Still, as the period of islanding evolves,
distributed and renewable generations are not sufficient to supply the
demand of the microgrid. In this sense, flexible resources assistance is
requested to provide the remaining demand supply. During these moments,