Components Of Photovoltaic Installations Connected To The Network
A photovoltaic solar installation connected to the network has only three basic elements:
- A group of photovoltaic solar panels. These solar panels are usually located on the roof of a building or integrated into any structural element of the same building. The photovoltaic panels can also be arranged directly on any land near the electricity grid.
- Ondulator or electronic inverter-converter. This device transforms the energy in the form of direct current provided by the solar panels, in alternating current of the same type and value as the one transported by the electric network
- Table of interconnection with the commercial electric network.
These photovoltaic solar energy installations considered as small power plants. In the case of adapting these facilities to a building, it will incorporate a new electrical installation and will have two different electrical installations. On the one hand, the usual electricity supply line for consumption with its corresponding meters and protections and, on the other hand, the photovoltaic solar installation with all its own elements and electrical equipment for control, interconnection and measurement.
The solar circuit produces electrical energy according to the incident solar radiation at each moment of the day; therefore, the registered electricity generation values will vary according to the time of day, the time of year and the weather. All this energy is accounted for and injected into the commercial network so that any customer of the electricity company consumes it. Most likely, they are the closest, including the subscribers of the same building.
Therefore, in this case, the owner of the facility becomes a consumer of electricity and a power plant, all at once.
Working voltages of a photovoltaic installation connected to the network
Systems up to 5 kilowatts (kW), because they are low-power systems, can be connected to a network in low voltage, single-phase, to 230 volts in alternating current and, for higher powers, they are designed with a three-phase connection.
In the solar part, from voltage to direct current, there are different possible configurations in the serial-parallel connections of the modules to obtain adequate working values in direct current. Depending on the chosen inverter, the working voltages can be, from 12 volts up to 600 volts in direct current. In any case, to optimize the system, it tends to average voltages close to the voltages of connection to the network (260-420 VDC).
In any project of photovoltaic solar installation of connection to the network, the inverter is the heart of the system. It is very important to have clear technical characteristics: power, working ranges, direct current- alternating current, frequency and maximum power reached. The power accumulated by the number of inverters will determine the nominal power of the photovoltaic solar plant in any photovoltaic system connected to the grid.
For each photovoltaic system connected to the network, we can find a whole range of equipment (in nominal powers) for its use. In systems where more than 100 kW are already installed, the equipment can be of nominal power of 10 kW and upwards. Basically, equipment with the technical characteristics of the maximum of the protections established by the current regulations is chosen, so that derive in an increase of the security of the system and reduce installation costs in general. These protections are:
- Galvanic separation. This condition can be achieved outside the converter itself, basically because, due to the technical needs of connection to the network, the installation of a transformer is designed to do this function at the same time.
- Network voltage control relay.
- Network frequency control relay.
- Timing at reconnection.
- If these protections are verified, the manufacturer's certificate specifying that the values determined in regulations can not be modified by any person, equipment software.
- Certificate of island non-operation.
- Certificate of emission of harmonics and electromagnetic compatibility.
- The power factor of the supplied power should be as close as possible to the value 1.
- Signaling system on / off.
- A fegit the points reflected above, the electric companies demand, depending on the company or the zone or region, other technical considerations. This is the case, for example, of "dielectric strength".
Also, and although the regulations do not specify it, equipment that includes visualization, monitoring and control of the data and operating parameters of the entire system are highly valued. The display of the equipment and the software for monitoring on the computer are increasingly important.
The location of the installation or physical situation of the investors can be of the forms already commented previously:
- Installed in the same solar field, outdoors and with IP65 waterproof box.
- Installed in the solar field inside an IP65 enclosure or enclosure suitable to include the equipment.
- Installed in a specific room very adequate in temperature, environment and space, accessible for maintenance technicians and user / owner.
- Installed in the booth room or in the covered place within the specifications of the previous point, but with inclusion near the counters and protection panel.
To develop the interconnection of the whole system, measures will be taken to the installations already expressed in previous sections, with special emphasis on the minimization of the losses derived from the connections, both in direct current and in alternating current.
- Module interconnection.
- Connection of the modules to the inverters.
- Connection of the inverters in the protection box and energy meters.
- Network connection point.
In any photovoltaic installation, an interconnection grid will be installed. This table may include the total of the protection devices defined by the regulations or those established as essential. On the other hand, these equipment may include measuring counters and voltage transformers for adaptation to mains voltage.
We define the two types of interconnection table:
- Complete interconnection table. Set of devices defined by the regulations specified for network interconnection. In this way, other elements could be included that are not included by regulation, but that could be considered important from a point of view of installation quality. In this case, it is logical to think that investors do not need one of their technical characteristics to be regulatory protections.
- Basic interconnection chart. Frame composed essentially of two elements: automatic (contactor-magnetothermal) and differential disconnector. They must be accessible to the electricity company in conjunction with the meters, manual disconnector and fuses to the installation.
At this point, the connections derived from the different generator inverters are brought together. It is the point of connection of the entire system.
Network connection point
The electric companies establish points of interconnection with the network. In this type of systems, with power strength, they will normally be defined and located in parallel with the connection already installed for the consumption of the building or the established construction -where consumption counters are normally already- and / or for a point located by the same company. 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 it is determined. In this way, there can sometimes be a certain distance (hundreds of meters) from the box to the point specified by the company.
General rules of application to define a perfect union in the network:
- The maximum power of the plant can not exceed more than 50% of the nominal power of the transformer of the electric substation or of the capacity of the same network defined in the area of the connection.
- Connections from installations that produce voltage drops caused by the connection-disconnection that are higher than 2% will not be accepted.
Obviously, these points of connection to the commercial network will be designed in such a way that the losses of performance accumulated by the solar plant throughout the journey that can occur from the generation of a kilowatt-hour until it is injected into the electrical network are minimized. The choice of inverter equipment, cables and connections, transformers and control in the reduction of plant downtime due to several factors must be well managed and informed.
The protective cabinets and / or connection panel 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 elements of protection can duplicate those already incorporated by the same investors in their design, in order to avoid both damage to the electricity grid and the disruption of solar production and also the damages that the system itself can produce in the interconnected equipment. and the rest of the users of the network.
Last review: May 22, 2018