## 2 Literature Review
### Constitution of Photovoltaic Systems  
Photovoltaic systems are formed by several elements to be defined [34], firstly the Photovoltaic Cell, which is the smallest semiconductor element capable of converting sunlight into electrical energy via direct current; secondly a Photovoltaic Array (PVA) consisting of a circuit formed by several branches of photovoltaic modules connected in parallel. With these elements of energy source the construction of a Photovoltaic Generator (PVG) is possible, defined as a generating unit capable of converting incident solar radiation directly into electrical energy in the form of direct current. It consists of the electrical and mechanical integration of the following components, as shown in (Figure 4).  
Indeed, the photovoltaic principle is obtained from solar cells, through a set of cells that form a module; the union of several modules forms a photovoltaic panel and the connection of several panels forms a photovoltaic array, which in turn is transformed into a PVG. It should be clarified that, up to this point, the energy produced is in direct current (DC), so the element called the inverter is the power electronic device whose main function is to convert the DC signal of a PVG in AC signal (Alternating Current), synchronized with the CFE network. It is the central element of the interface between the PVG and the power grid, and the AC output can be single-phase or three-phase. In addition, it performs other protection and control functions for the efficient and safe operation of the On Grid-PV [33; 34].  
Additionally, there is the possibility of building autonomous or interconnected systems to the grid; an autonomous system is a generator system that converts sunlight directly into electrical energy, with the appropriate characteristics to be used by the intended electrical load [42], i.e., the user accesses this energy by connecting directly from the system to the selected load, which is used in remote locations where there is no access to electrical service for interconnection to the grid.  
On Grid-PV systems are photovoltaic electricity provider systems in which direct current power from the PVG is converted into alternating current power, with the voltage and frequency specified by the grid and synchronized with it. By connecting in parallel with the grid, On Grid-PV contributes to the supply of the demanded power to the grid. If there is a local load on the property, it must be supplied by one or both sources simultaneously, depending on the instantaneous values of the load and the output power of the On Grid-PV.  
Figure 4: Components of a photovoltaic system based on [34].  
Surplus power from the On Grid-PV is injected into the grid and deficits are demanded from the grid.  
Figure 5 shows the block diagram of an On Grid-PV system; the arrows indicate the power flow [34; 43]. The PV system consists of (1) PV modules to convert solar radiation into DC electricity, (2) a power inverter to convert the electricity from the panels into AC electricity and (3) a bi-directional meter that measures both the amount of electricity consumed from the CFE and also a \"discount\" of the electricity that is generated in the system and delivered to the CFE; this is also known as a Distributed Clean Generation (DCG) scheme.  
This type of interconnection scheme in Mexico is known as an interconnection contract with net metering of energy (Net Metering), where the energy produced is discounted from consumption, in the same supply contract with its energy company [33; 44].