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Disconnects impact the total output of solar arrays

FROM: Ferraz Shawmut SAS - 1, rue Jean Novel F-69626 VILLEURBANNE
CONTACT: Hervé Le Penven - Communications Manager
AGENCY: Infocom Industrie - Amel Karim +33 472336598

Energy efficiency can be optimized for any type of fault current

Solar cells are current generators with very low short circuit characteristics (approximately 115 to 120% of the output current).
That current is proportional to solar radiation, i.e. virtually nil at night, very weak at sunrise and sunset, average in cloudy weather, 75% of its maximum (the stated peak value) between 11 a.m. and noon and again between 3 and 4 p.m., and maximum when the sun is at its highest (between noon and 3 p.m. in summer in Western Europe).

When a string of cells is short circuited, or reverse connected compared to the other strings in parallel, impedance in the circuit will be low and the short circuit current flows from the good cells into the faulted cells. No electricity will be output until the faulty string is “disconnected” from the circuit.
That can be done by:
- a diode connected in series with each string. This will avoid current flowing into the shorted string but unfortunately the diode itself can be short circuited and therefore not function. It also means sacrificing a lot of energy (a direct voltage drop of 0.6 V, i.e. 5 W) constantly.
- a breaker or a fuse placed in series with each string for safety reasons, to serve as a failsafe “disconnect”. That “disconnect” will take more or less time to trip depending on the level of the fault current.
Installation guidelines recommend that such “disconnects” be used in all arrays with at least 4 strings in parallel, generating 3 times the current in the faulty string and tripping the “disconnect” within a maximum of 3 to 4 hours. This ensures protection against losing an entire day of sunshine. The panel itself suffers no adverse consequences and can stay in this configuration practically indefinitely.

If the inverter has capacitors at the inlet (as required to have energy available as close as possible to the IGBTs), they will discharge instantaneously into the faulty string.
The “disconnects” must trip and clear such a fault with no disturbance to operation.
The advantages to using a fuse and holder or a fuse disconnector for this purpose are lower Watts losses, a smaller footprint, fast breaking on capacitor discharge, and lower price.