Photovoltaic for self-consumption - guidelines for sizing

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Installations with PV production for self-consumption, even operating only grid-connected, present specific requirements on load flow calculation, placement and selection of the protection devices, and sizing of equipment.

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Load flow and short-circuit currents

Installations with PV production for self-consumption are characterized by at least two operating modes (supplied by the grid, and supplied by the grid and the PV sources). The load flow and the short-circuit currents flow should be evaluated for each operating mode, and the electrical installation components and equipment should be sized accordingly, taking the worst-case constraints.

If storage is present in the electrical installation, it should be taken into account in the load flow and short-circuit currents calculation twice – once as a load (when charging) and once as a source (discharge).

Switchboard sizing

In single source installations, switchboards are sized in function of the maximal current drawn by the downstream load.

When a switchboard is fed by two or more sources, it should be sized in function of the maximal current that can transit through it, which can be:

  • the current drawn by the load
  • the current delivered by the local sources

Cable sizing

Cables’ current ampacity must be higher than the maximal expected current, which should be evaluated for each operating configuration of the electrical installation.

Transformer sizing

In single source installations, transformers are sized in function of the installed load power.

In installations with PV production for self-consumption where the injection of PV production to the grid is possible, the transformer should be sized in function of the installed local sources power capacity.

Circuit breaker location

When a single inverter is connected to a switchboard of the electrical installation, a circuit breaker should be installed downstream to the cable connecting the PV inverter to the switchboard to isolate electrical faults or overloads occurring on the PV feeder.

When several PV inverters are regrouped to a switchboard before they interface with the electrical installation, it is recommended to install protection devices at both ends of the connection between the two switchboards:

  • the role of the downstream breaker (CB2) is to isolate faults on the connection between the PV system and the building’s electrical installation (in case of fault at C2, a high short-circuit current will flow on the path from the grid to the fault location. A tripping of CB2 will isolate the fault allowing the electrical installation to continue to operate supplied by the grid). Selectivity between CB1 and CB2 should be assured. Otherwise, a fault in the PV installation may trip the main breaker and interrupt the supply from the grid.
  • the role of the upstream circuit breaker (CB3) is to isolate electrical faults on the PV switchboard, the selectivity between the downstream and the upstream breaker on the PV feeder should be met.
Fig. P30 – Circuit breaker location
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