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2. Intermittent and Stochastic Renewable Energy Production 81

electrical machines, the security system stops the electrical machine leading to a po-
wer failure in the network.
As demonstrated, the power system is a dynamic system, continuously subject to
changing conditions, some variations can be anticipated, while others cannot be pre-
dicted [6]. A power system has to meet a continuously varying consumer load. In
terms of energy control, the load represents the primary independent variable e
the driver whose controllable elements within the power system should be posi-
tioned and should respond. It is continuously changing in amplitude at annual, sea-
sonal, daily, minute-to-minute, and second-to-second time steps. The reliability of
the system becomes dependent on its ability to accommodate expected and unex-
pected changes and disturbances while maintaining quality and continuity of service
for the customers.

2.2 INTERMITTENCE OF RENEWABLE PRODUCTION AND IMPACT ON
THE ELECTRICAL GRID MANAGEMENT
Wind and sun are variable and unforeseeable generating resources. They impact, at
all time, the planning and operation processes. The wind and solar variability have
their own characteristics and time frames (Fig. 3.2). The variability and unpredict-
ability of wind and solar sources are important as the penetration levels are high
[7e9].
The uncertainty and variability of wind and solar resources pose problems for
grid operators. This variability requires additional and complex actions to balance
the system. The integration of ISRES into an electrical network intensifies the
complexity of the grid management [10e12]. A greater flexibility in the system is
necessary to accommodate supply-side variability and the relationship between gen-
eration levels and loads.
When the renewable electrical power is lower or higher than the power demand,
other generation units must compensate the difference, which implies that the global
production system should operate with a sufficient reserve margin (electrical sys-
tems already in operation but in part-load operation) because most traditional gen-
eration units require a considerable start-up time. This allows the system to cover up
the loss of generation when needed [13,14].
The introduction of solar or wind systems in the electrical network induces two
kinds of constraints [13,15e17]:
• Some constraints related to the source: The power is not guaranteed because of
the intermittent and stochastic behavior of the source, leading to the necessity to
continuously provide other means in reserve able to compensate the variations
of power and to react immediately (in increasing or decreasing their production
in a short time). Solar and wind sources are not always available when they are
needed and sometimes are present, whereas the network does not require. In this
case, their production is in excess. The conventional energy system (as fuel
motor) works in part-load mode for the purpose of being able to react rapidly to