Page 27 - Hybrid-Renewable Energy Systems in Microgrids

12 Hybrid-Renewable Energy Systems in Microgrids

the technical constraints. However, grid-connected mode MG, to a certain condition,
can form as an islanded mode mainly during faults in the system to avoid long-term
outage in that area. Nowadays, many researchers are studying the feasibility of the
islanded mode operation under such disturbance conditions. Intermittent nature of
RES may produce energy surplus/shortfall for a short period of time in a second
or minute scale, as well as hours in some cases. In that case, cost of energy per
unit might be low during the surplus period, but in a shortfall situation, the system
should chose to shed the load to maintain system reliability. Considering such situ-
ation, an EES is expected to provide significant support, as they can consume the
surplus energy as a motor and provide shortfall energy to the load as a generator.
Appropriate capacity of ESS can ensure power production and load demand balance
and thus guarantee the efficient use of low-cost per unit of energy and reliability, as
well as quality of the system [40,41]. ESS may support the higher-level infiltration
of renewable energy in the grid. In island mode, storage system supports the storage
of energy for later use during abundance, and in the meanwhile, it is responsible for
maintaining the reliability of the system. However, if there is a diesel generator con-
nected in the islanded hybrid system, storage system might be used only to maintain
the power quality for few seconds to several minutes, smoothing support to meet the
generation/load balance.
Energy storage can be coupled to the AC or DC side of the hybrid system, depend-
ing on the preference of the designers and/or other technical constraints. In addition,
ESSs can be either aggregated, distributed, or semi-distributed configurations as
shown in Fig. 1.5, originally drawn by the authors to demonstrate the potential BESS

Figure 1.5 Potential configurations of typical energy storage system connection.