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When solar electricity production and storage are integrated into buildings, the electrical installations evolve from single-source to multi-source, from generator-based generation to inverter-based generation, and from a single operating mode to multiple operating modes.
Consequently, the way to calculate electrical installations must also change. In particular, there are two specific points to consider when making electrical installation calculations:
1. Calculations should be done for each operating mode.
One peculiarity of installations with solar production and storage is that they have multiple operating modes. This means different operating configurations become functional depending on the available power sources and connected loads.
Typically, an integration of solar production leads to at least two operating modes. In the first case, the installation is supplied by the grid and the photovoltaic system in parallel during the hours of solar production. The second case occurs when there is no PV production, for example during the night, at which time the installation is supplied only by the grid.
When storage is added to the system, the number of operating modes increases. This is primarily because
- there is a larger number of possible combinations of sources, and
- the storage must be considered as both a source and as a load.
Indeed, when the battery is charging, the electrical energy storage system is a load for the electrical installation. However, when the battery is discharging the storage provides electrical energy to the installation as a source.
Typically, a simple installation with grid connection and storage has at least three operating modes:
- Supplied by the grid only – when the electrical energy storage is in standby
- Supplied by the grid with the electrical energy storage as load (battery is charging)
- Supplied by the grid with the electrical energy storage as source (battery is discharging)
Consequently, a calculation should be done for each operating mode, because calculations can result in significantly different power and current values from one operating mode to another. Once the calculation is done for each mode, the equipment must be sized taking into account the worst constraints. The protection plan also must be set so as to be valid whatever the operating mode.
2. Solar and storage inverters must be considered as current sources.
Another peculiarity of installations with local energy production is that storage and solar inverters do not have the same characteristics and behaviors as traditional sources, such as the grid supply or generators. For example, in the case of a short circuit, solar and storage inverters limit their current output to a value not much higher than the nominal current, and so behave as constant current sources.
In practice, this means that when calculating an electrical installation by the impedance method, photovoltaic and electrical energy storage systems are not represented by impedances in the same way as all other components of the electrical installation. Regardless of the short circuit impedance, the inverters behave as a constant current source.
Therefore, when making short circuit current calculations, the solar system should not be represented by impedances, as typically is done for other components of the electrical installation. The short circuit contribution of inverters, the value of which is provided by the inverter manufacturer, must be added to the short-circuit currents in the installation.
Many electrical installation sizing tools now allow the design of electrical installations with solar and storage production. Designers must still check, however, that calculations are done for each possible operating mode and that the contribution of solar and storage inverters to the short circuit current calculations is taken into account correctly.
The video below further explains impact of solar integration to the electrical installation calculations.
An overview of the blog series and a full video playlist is also available. Learn more on electrical installation design for installations with local production in Chapter P of the Electrical Installation Guide.
