Photovoltaic architectures - common characteristics
A PV array is made up of a number of modules in series or parallel, corresponding to the input characteristics of the inverter. However, since these modules are interconnected, the array is very sensitive to shade or differences in terms of the direction faced.
By following a few simple cabling rules, supply can be optimised and any operating problems may be avoided.
Position of the panels
If, when installing a PV array on a roof, panels need to face in different directions, it is essential to assemble at least one string per direction and ensure each string is facing in just one direction to ensure optimised supply. Each string must be connected to a specific inverter (or to inputs of a multi-MPPT inverter - see Inverters).
If this instruction is not observed, the array will not be damaged but supply will be reduced, thus increasing the time needed for a return on investment.
Besides the risk of destruction of shaded modules within a PV array due to the “hot spot phenomenon” as described in Photovoltaic modules for which manufacturers have devised solutions, research conducted by the Institut National des Energies Solaires (INES – France’s national institute for solar energy) suggests that shading of 10% of the surface area of a string may cause more than a 30% reduction in output!
It is therefore important to eliminate direct shading. However, in many cases this is difficult (trees, chimney, neighbouring wall, pylon, etc.).
If a PV array includes several strings:
- If possible, shaded modules should be included in a single string
- Otherwise, a technology should be chosen which responds better to diffuse light than direct light
When connecting components, the first precaution to take is to avoid loops in the cabling within strings.
Even though direct lightning strikes on arrays are relatively rare, currents induced by lightning are much more common and these currents are particularly destructive where there are large areas of looping. Figure P16 shows how to improve an array including a large loop.