Photovoltaic solar energy installations connected to the electricity grid are installations where the generated energy is poured directly into the electricity grid. This type of solar installation works as if it were a power plant. Electricity consumption is independent of the energy generated by solar panels. In these cases, the user continues to buy the electric energy that he consumes from the distribution company at the established price and also owns an electric power generating facility.
In the case of adapting these renewable energy installations to a building, it will incorporate a new electrical installation and will have two different electrical installations. On the one hand, the usual power supply line of consumption with its corresponding meters and protections and, on the other hand, the photovoltaic solar installation with all its own elements and own electrical equipment for control, interconnection and measurement.
The main disadvantage of solar-related systems is that solar performance is a function of the incident solar radiation at every moment of the day. Consequently, the registered electricity generation values will vary according to the time of day, the time of the year and the weather. The advantage of the photovoltaic solar energy connected to the grid is that the energy that you do not use can continue to be sold to the electricity company and that when you need more than what the solar system is capable of generating, you can obtain it from the electricity grid.
Applications of photovoltaic systems connected to the power grid
Some of the applications of these photovoltaic solar energy systems are the following:
- Solar panel installations on rooftops, terraces, etc. of homes that have connection to the electricity distribution network: The roof surface is used to install modular systems that are easy to install.
- Production plants: These photovoltaic power plants are industrial applications that can be installed in rural areas not used for other uses (solar orchards, energy cooperatives) or superimposed on large urban areas (parking lots, commercial areas, etc.)
- Building integration: It consists of the replacement of conventional architectural elements with new architectural elements that include photovoltaic elements (usually photovoltaic solar panels), and therefore are energy generators (facade cladding, curtain walls, parasols, pergolas, etc.) . Sometimes, it is possible to combine them with architectural designs to take advantage of passive solar energy or with solar thermal systems.
Working voltages of a photovoltaic system connected to the network
The most usual powers measured in watts are 2.5 and 5 kilowatts (kW) or multiples of 5 to 100 kilowatts (kW). There are larger solar installations, but they are only carried out by companies or research centers, since they are amortized over longer periods.
Photovoltaic systems of up to 5 kilowatts (kW), being low-power systems, can be connected to a low-voltage, single-phase network, at a nominal voltage of 230 volts in alternating current. On the other hand, for higher powers, they are designed with a three-phase connection.
In the solar part, of direct current voltage, there are different possible configurations in the serial-parallel connections of the modules to obtain adequate working values in direct current. Depending on the inverter chosen, the working voltages can be, from 12 volts to 600 volts in direct current. In any case, to optimize the system, there is a tendency to average voltages close to the grid connection voltages (260-420 VDC).
Electrical connectivity of a photovoltaic installation
In order to develop the interconnection of the entire photovoltaic solar energy system, special emphasis must be placed on minimizing losses arising from the connections, both in direct current and in alternating current. The steps to follow are the following: interconnection of the solar panels, connection of the modules to the current inverters, connection of the inverters in the protection panel and energy meters, grid connection point.
In any photovoltaic installation, an interconnection panel with the network will be installed. This table may include the total protection devices defined by the regulations or those established as essential. On the other hand, these equipments may include the measurement counters and voltage transformers for adaptation to the grid voltage.
We define the two types of interconnection panel:
- Complete interconnection board. Set of devices defined by the regulations specified for network interconnection. In this way, other elements that are not included by regulations, but that could be valued as important from an installation quality point of view could be included. In this case, it is logical to think that investors do not need any of their technical characteristics to be regulatory protections.
- Basic interconnection board. Table consisting essentially of two elements: automatic disconnector (contactor-magnetothermal) and differential. They must be accessible to the electricity company together with the meters, manual disconnector and fuses to enter the installation.
At this point, the connections derived from the different electrical inverters and electrical generators are gathered. It is the connection point of the entire system.
Network connection point
The electricity companies establish points of interconnection with the network. In this type of systems the connection points will normally be defined and located in parallel with the connection already installed for the consumption of the building or of the established construction. This connection to the distribution company will be verified by the relevant company, so there is the possibility of changes in the connection to other points of the same line if so determined. In this way, sometimes there may be a certain distance (hundreds of meters) from the frame to the specified point.
General rules of application to define a perfect union in the electricity grid:
- The maximum power of the photovoltaic plant cannot exceed more than 50% of the nominal power of the transformer of the electrical substation or of the capacity of the same network defined in the connection area.
- Connections from installations that produce electrical voltage drops caused by connection-disconnection that are greater than 2% will not be accepted.
Obviously, these points of connection to the commercial network will be designed so that the performance losses accumulated by the solar plant are minimized. The choice of inverter equipment, cables and connections, transformers and control in reducing the shutdown hours of the photovoltaic plant due to several factors must be well managed and informed.
The protection cabinets and / or switchboard have the function of including the instruments for measuring the energy produced and consumed, as well as the electrical protections (automatic action devices) required by current regulations. These protection elements can duplicate those that already incorporate the same electric inverters in their design, in order to avoid both the damage to the electricity grid and the disturbance of the production of solar energy and also the damages that the system itself can produce in interconnected equipment and other network users.