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3. Need for Solar and Wind Forecast: Forecast Horizon and Time Step 83

a quick variation of intermittent renewable power (then, with a low efficiency
and an increase of the fuel consumption per kWh produced and of the kWh
production cost) [11,18].
• Some technical constraints: Wind and PV systems have no inertia and then are
sensible to voltage and frequency drops, which cut the production and aggravate
the incidents. Moreover they do not participate in the “system service,” to the
electrical network stability and quality (voltage and frequency regulation,
switch-on in autonomous mode or black-start) [19e21]. These systems behave
like passive generators from an electrical point of view. Then, some conven-
tional production means with inertia must work permanently to maintain a
voltage and frequency regulation provided by conventional electrical plants.
The penetration rate of ISRES, i.e., the power generated by ISRES compared
with the total electrical consumption must be limited to guarantee an electrical
grid stability and a security of supply. Some feedbacks in Denmark show that for
a penetration rate up to 20% or 30% some stability problems can occur. A French
directive limits the active power from these plants at 30% of the total active power.
The same maximum penetration rate of random renewable electrical systems is used
in other countries such as Canary Archipelago (Spain). This ISRES power limitation
imposes to develop methodologies for determining the renewable energy absorption
capability [22] and need to have a good knowledge of produced and consumed en-
ergy fluxes at various temporal horizons.

3. NEED FOR SOLAR AND WIND FORECAST: FORECAST
HORIZON AND TIME STEP
Forecasting the ISRES output power is a requirement for a good operation of the po-
wer grid and for an optimal management of the energy flows occurring into the
ISRES [23]. It is necessary to estimate the reserves, scheduling the power system,
and congestion management, for optimally managing the storage and for trading
in the electricity market [3,4,21,24e27].
As underlined in paragraph 2.1, even if no ISRES is integrated in the network,
energy and power reserves are needed and can be divided in two categories: contin-
gency reserve, used in case of specific event (such as power plant switch-on), and
“no-event” reserves used continuously (for instance, because of unreliable load pre-
diction) [28]. These reserves (contingency and no-event ones) are started at various
time scales: within 1 min (primary reserve) using spinning generators, from 1 min to
1 h (secondary/tertiary reserves), and more than 1 h [29]. ISRES introduction in an
electrical network only affects the nonevent reserve particularly because of the
imperfect forecast of their production [28].
Already, it appears that a predicted and anticipated event is easier to manage. The
electrical energy operator needs to anticipate the future of the electrical production
and consumption at various temporal horizons (Fig. 3.3) [12,30].