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Solar Power Sources: PV, Concentrated PV, and Concentrated Solar Power 37

monitoring the amount of steam and pressure that passes and turns the turbine. Any pressure imbal-
ance is mainly induced by abrupt changes in solar radiation; thus, the system’s transient stability is
affected. In some cases, a failure in governor controller may also lead to the increase of the mechani-
cal torque of the synchronous machine. A solution to mitigate abrupt changes in mechanical torque
is to equip the CSP with storage, since a storage system helps to maintain the steam-based pressure
and hence the rotor speed within nominal values.
For instance, when a large disturbance occurs, such as a three-phase fault, the rotor speed
increases giving a negative slip and starts to accelerate. Actually, there is a difference among the
electrical and mechanical power, where the former tends to be zero (decrease).
Concluding distributed generation units such as CSP do not contribute to frequency control
since they are generally equipped with power electronics interfaces that decouple electrically these
devices from the grid. Consequently, they cannot deliver inertial response for short-term frequency
stabilization.

2.4.5 CSP Large-Scale Integration: Technical Challenges and Impacts

As it has been already mentioned that CSP plants are considered as conventional thermal power
plants using standard models of synchronous generators, governors, exciters, etc. Generally, require-
ments for renewable energy system generators (including the provision of ancillary services) have
to be adapted in order to facilitate their grid integration, in line with the provisions of the respective
European network codes [44].
Moreover, in the medium to long term, additional measures have to be taken in order to increase
the flexibility of the electricity systems, including measures to further develop the flexibility of the
conventional generators, demand-side response, interconnections, and also storage options.
In brief, important challenges related to grid integration of large-scale CSP energy are the fol-
lowing [44]:

• Solar PV and CSP systems are variable in nature with no control over the energy source.
This variability needs to be balanced by controllable generation units or storage systems or
taken into account via demand management schemes.
• The optimal generation, reserve, and power exchange capacities must be redefined, taking
into account that variable renewable generation may be zero for an extended time or at
maximum value when the power demand is minimal.
• It necessitates a more dynamic operation of the power system, as a situation with variable/
predictable demand and controllable generation is replaced by a situation where also the
generation is variable and uncontrollable (but predictable). In other words, there is uncon-
trollable variability on both sides of the generation = demand equation.
• This kind of generation needs to be installed where the energy resources are good. In these
places, the grid infrastructure is not necessarily strong, putting strain on the power system.
A challenge is therefore to determine the limits and the best strategies for increasing these
limits.
• Grid infrastructure planning depends on forecasted generation capacities, whereas devel-
opment of a renewable energy plant depends on the grid infrastructure. This circular depen-
dence represents an uncertainty for both grid and plant development.
• Grid code compliance is increasingly important for renewable energy plants, and therefore,
the standardization and international harmonization of grid codes are important for simpli-
fied planning process.

PV (for modern installations) power plants have an interface to the electricity grid via power elec-
tronic converters. Thus, they interact very differently with the grid compared with conventional power
plants based on directly connected synchronous generators and necessitating new control strategies.