Page 47 - Renewable Energy Devices and System with Simulations in MATLAB and ANSYS

34 Renewable Energy Devices and Systems with Simulations in MATLAB and ANSYS ®
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[36, 37]. As the name already implies, in passive storage systems the heat storage medium is in a
passive state and does not circulate. An additional heat storage concept successfully tested is the
thermochemical energy storage. This system relies on endothermic and exothermic chemical reac-
tions that are reversible. However, this type of thermal-energy storage is at the moment at a very
early stage of development and still subject of research.

2.4.3 Grid Integration and Stability Considerations

In order to analyze a power grid from an electrical point of view, examine its stability and define the
potential integration of distributed generation, that is, CSP into the grid, static, and dynamic analy-
sis needs to be carried out [38]. Stability considerations can be divided into two case studies: the
steady-state and dynamic stability analysis. Steady-state analysis involves the calculation of power
flows on the network lines, transformers, and the voltage profiles of system bus bars. This study is
important for the planning and design of the connection of any distribution generation system to the
transmission and distribution grid.
Contingency and particularly N − 1 analysis is also essential in order to ensure the security and
reliability of power supply, where N is the number of generation units. The N − 1 criterion is that
the system must withstand on its own a loss of any single element. That means that all the line flows
must be below their limits not only for a given normal operating state but also when any of the lines
is disconnected [39]. Short-circuit calculations introduce the proper selection of high-voltage equip-
ment and protection relays.
A transient event occurs undesirably and instantly in a power system and can either be an oscil-
latory or impulsive disturbance. For instance, branches and bus faults, lines, loads, and generators
tripping may induce frequency, change rotor angle, and cause voltage oscillations and can lead to
the failure of the distribution system where the CSP is planned to be connected to or even can cause
damage to the power electronics equipment. Therefore, a transient stability study is necessary to
investigate the power system response to disturbances and check over its fault ride through capabil-
ity to see if the network elements have adequate stability margins [40, 41].

2.4.4 CSP Modeling for Transient Stability Analysis
Thermal and solar plants work on identical principles as fossil fuel–based power plants, where the
only difference is the primary source like shown in Figure 1.11. Moreover, the dynamic modeling of
conventional generators requires the definition of the standard generator model, the automatic voltage
regulator modeling for voltage regulation, and the governor model (e.g., speed control). The steady-
state voltage, power, and speed outputs need to be regulated and retain their equilibrium during normal
conditions or obtain a new one even after the occurrence of critical disturbances in the system [42, 43].
CSP plants are considered as conventional thermal power plants using standard models of syn-
chronous generators, governors, exciters, etc. In CSPs, sunlight produces high-pressure steam,
which is used to generate electrical power by a turbine and an electrical generator that is a conven-
tional synchronous machine. Table 2.2 concentrates the parameters utilized for the dynamic model-
ing of synchronous machines in order to study and analyze the transient stability of power systems.
The single-machine infinite-bus model illustrated in Figure 2.19 is the basis for transient stability
analysis for systems driven by synchronous machines, where P is the generator’s electrical power
m
output. The grid is represented by an infinite bus like a constant voltage source. The key assumptions
considered here are the following [43]:

• A three-phase balanced system is considered.
• The network is assumed to be in a steady-state equilibrium prior the fault incident.
• The resistive part of the transmission line is neglected.
• The synchronous machine is modeled only with the positive sequence components.