PV System and Installation Rules
Off grid installation
Historically, these were the first places in which photovoltaic systems were used, supplying telecommunication relay stations or remote settlements which were difficult to access and could not be connected to the network.
They remain one of the only means of supplying electricity to 2 billion people who currently do not have access to it.
In order to size these installations correctly, it is first necessary to identify the load curve required and the number of days where the installation will not be exposed to sunlight in order to identify how much energy needs to be stored in the batteries. This information is used to determine the size and type of batteries required.
Then, the surface area of the photovoltaic sensors must be calculated to ensure that the batteries can be recharged in the worst case scenario (shortest day of the year).
Specific issues
This method entails over-sizing the system to ensure continuity once or twice a year. As a result, this type of installation is very expensive!
It should be noted that according to the EPIA (European Photovoltaic Industry Association) this type of installation will account for 20% of the photovoltaic market in 2012 and 40% in 2030.
Storage
Storage is crucial to this type of installation.
Several types of batteries are available:
- Lead batteries
These batteries operate in cycles (charge/discharge). Open batteries are recommended to prevent inflating which may occur due to excessively rapid charging and large emissions of hydrogen.
Their purchase price is certainly their main advantage although they have short service lives. This is influenced by the depth of discharging but they last no more than 2 or 3 years at a discharging rate of 50% and above. Furthermore, deep discharging may “kill” the battery. Therefore, when operating such equipment at a remote site, the batteries should be changed on a regular basis to maintain their charging performance.
- Ni-Cd or Nickel Cadmium batteries
These batteries have the advantage of being much less sensitive to extreme temperature conditions and deep charging or discharging. They have a much longer service life (5 to 8 years) but are more expensive to purchase. However, the cost of the Wh stored over the service life of the installation is lower than that of lead batteries.
- Li-ion batteries
These are the batteries of the future for these types of operations. They are insensitive to deep discharging and have a service life of up to 20 years. At present, they are prohibitively expensive but prices are set to fall by 2012 with the start of mass production. They will therefore become the most economic variety for this type of usage.
Connected to the public network
Owners of power generation systems connected to the network have 2 options:
- Sell all the power they produce (option known as “total sale”). For this option, a separate connection must be established to the network, apart from the connection for consumption. This also requires an administrative declaration.
- Use the power they produce locally as required and only sell the excess (option known as “sale of excess”) which has two benefits:
- The difference in the rates payable by the producer (purchase) and the consumer (sale)
- It is not necessary to establish a new connection which may be expensive and requires an administrative declaration.
Since different rates are charged, a profitability analysis should be carried out to choose the best option.
Installations connected to the network – 3 important points
The following points are important to note with regard to installations connected to the network:
- In contrast to independent installations, no correlation is required between consumption for the building and output.
For the “total sale” option, the two elements are completely independent.
For the “sale of excess” option, the network will compensate when production does not cover consumption.
- The network must be present in order to supply and sell energy. Furthermore, energy distributors require automatic disconnection systems to be in place in case of incidents on the network. When activated, these stop supply and therefore sales. Reconnection occurs automatically when the network returns to its nominal operating conditions.
- As a general rule, no provision is made for local storage using batteries or other means. This is true for mainland France where there is a high quality network with the capacity to absorb all the energy produced.
However, the system does have one fault. If the network fails, owners of installations who are also generally consumers are left with a power generation facility which they cannot use (see previous point). In countries or towns with frequent network incidents, systems are being developed which include batteries. Xantrex, a subsidiary of Schneider Electric, is the leading provider of these systems worldwide.
Safety devices
Protecting people and property against electrical hazards
- In terms of direct current, a DC isolator is compulsory as, even though a connector can be disconnected when live, an electric arc may occur and damage the connectors if photovoltaic modules are exposed to light. There are currently two methods for installing these DC isolators. They can either be integrated into the PV inverter or placed in an external enclosure.
If more than three strings need to be paralleled for the same inverter input when installing a PV array, the statutory safety devices are much more complex. Indeed, current reversal may occur in a string which would be destroyed under the combined power of all the other strings.
Schneider Electric also supplies paralleling enclosures for strings as well as protection units which include a general load break switch enabling work to be carried out safely upstream of this unit even in daylight.
- In terms of alternating current, a more standard range of safety devices is available. The cable between the inverter and the network must be protected since any fault in this connection would be exposed to the short circuit power of the network. A safety device protecting against short circuits should therefore be positioned close to the network connection, and the inverter should disconnect automatically in the absence of voltage in the authorised range. Schneider Electric supplies enclosures including upstream and downstream safety devices.
Protecting PV installations against the effects of lightning
Overvoltage may occur in electrical installations for various reasons. This may be caused by:
- The distribution network as a result of lightning or any work carried out
- Lightning bolts (nearby/on buildings and PV installations, or on lightning conductors)
- Variations in the electrical field due to lightning.
Like all outdoor structures, photovoltaic installations are exposed to the risk of lightning which varies from region to region.
- Equipotentiality
Equipotentiality is the first safeguard to put in place and entails connecting all conductive elements and metal conductive parts in the photovoltaic installation using an equipotential conductor.
The minimum section for this conductor is:
- 4 mm2 in the absence of a lightning conductor or if a lightning conductor is in place but not connected to the installation
- 10 mm2 if the installation is connected to the building’s lightning conductor (this must be connected by a cable of 10 mm2 if the lightning conductor is less than 2.5m from the installation)
- Lightning arresters
The positions of lightning arresters are explained by Figures P10 and P11.
Fig. P10: Positions of lightning arresters in the DC part stipulated in Guide UTE C 15-712
Fig. P11: Positions of lightning arresters in the AC part stipulated in Guide UTE C 15-712
