Introduction

Protecting a photovoltaic system is a DC protection problem, and DC is different from the AC protection that most engineers meet first. An array produces a fault current that has no natural zero crossing, so the fuse must be rated for the DC voltage and for the low-overload, high-ambient conditions of a solar installation. As utility-scale plants move to 1500 Vdc, the fuse rating and the coordination become more demanding. This application note explains string-current sizing, ambient derating and holder selection for Bussmann photovoltaic fuses.

Sizing the String Fuse

The string fuse protects the string cable against a reverse current fault, which can occur when one string is faulty and the others feed current into it. The rating is set above the array short-circuit current, corrected by the factors in the module datasheet and the fuse standard, and below the cable and module rating so the cable is still protected. The result is a narrow window, which is why the derating must be done carefully rather than estimated.

Ambient Derating

A fuse carries less current as the ambient temperature rises. A solar installation in a hot climate, inside a combiner enclosure, can be well above the laboratory temperature at which the fuse was rated. If the rating is not corrected, the fuse may nuisance-open on a hot afternoon; if it is oversized to compensate, the cable may be under-protected. The rule is to use the site maximum temperature and the manufacturer derating curve, and to check the fuse and the holder together, because the holder can be the limiting part.

Choosing the Fuse and the Holder

The fuse body follows the current and the mounting. A small ferrule fuse such as the PV-A10 series covers a string, an NH fuse such as the PV-ANH series covers a combiner, and an XL fuse covers the largest currents. Each family has a matched Bussmann holder, such as the CHPV finger-safe holder for the small links and the SD blade blocks for the NH links, and the holder must be rated for the DC voltage of the array, not just the fuse. Choosing the fuse and the holder as a pair keeps the voltage rating and the touch protection correct and simplifies the assembly.

Inverter and Storage Protection

On the inverter side, a high-speed fuse protects the power semiconductors, coordinated with the DC link. In a battery storage system, DC-rated fuses protect the battery strings and the power conversion system, and because the system is bidirectional the fuse must clear a fault in either direction. The same discipline applies everywhere: DC rating, ambient derating and coordination with the rest of the chain.

Diagnosing a Blown PV Fuse

When a PV fuse operates, find the fault before replacing it. A string fuse that opens may indicate a reverse current from a faulty string, a cable fault or an insulation problem, and replacing the fuse without finding the cause usually leads to a repeat. The BeiLuo FAE team helps with the sizing, the derating and the diagnosis so the array returns to service safely.