Photovoltaic Systems String Fuse Selection

Guide to the Installation of Photovoltaic Systems

String Fuses 

The short circuit current of a module is little more than the operating current, so in a single string system, a circuit fuse would simply not detect or operate to clear a short circuit fault. In systems with multiple strings some fault scenarios can result in the current from several adjacent strings flowing through a single string and the prospective fault current may be such that overcurrent protective devices are required. Hence, the selection of overcurrent protective measures depends upon the system design and the number of strings. 

Photovoltaic Array

While string cable sizes can be increased as the number of parallel connected strings (and the potential fault current) increases, the ability of a module to withstand the reverse current must also be considered. Where currents exceed the modules’ maximum reverse current rating, there is the potential for damage to the affected modules’ and also a fire risk. 

Reverse Current Test

IEC61730-2 Photovoltaic (PV) module safety qualification – Part 2: Requirements for testing , includes a reverse current overload test. This reverse current test is part of the process that enables the manufacturer to provide the maximum overcurrent protection rating or maximum series fuse. Fault currents above the maximum series fuse rating present a safety risk and must be addressed within the system design. 

For a system of N parallel connected strings, the maximum module reverse current (IR ) to be experienced under fault conditions is:

I R = (N – 1) × Isc 

Hence, overcurrent protection is required where (N – 1) × Isc is greater than the module maximum series fuse rating. 

While some fault combinations are less likely than others, in order to provide full protection of all cables and modules – string fuses are required in both the positive and negative legs of the string cabling. 

Note: For small systems where it is determined that fault currents do not present a risk to the modules, only the string cables & connectors need to be considered. A common approach in this case relies on oversizing string cables & connectors - such that they may safely carry the maximum possible fault current. Such a method does not clear the fault but simply prevents a fire risk from overloaded cables. 

Where the inverter is of such a design that it has multiple MPPT inputs and the design does not allow fault currents to flow between these inputs, each MPPT input may be treated as a wholly separate sub-array for the purposes of deciding whether string fuses are required. 

Fuses should not be mounted in such a position where their rating may be compromised by the build-up of heat from solar gains. The use of MCBs (miniature circuit breakers) is permissible provided they meet the string fuse criteria and are rated for use in an inductive circuit and will operate for currents flowing in either direction through the device. A system fitted with suitable removable string fuses provides a means to achieve the requirements for string isolation 

String Fuse Selection 

The following requirements apply where the PV array provides the only source of fault current, such as in a typical grid connected system with no battery. 

For a system of N parallel connected strings, with each formed of M series connected modules: 

  • String fuses must be provided for all arrays where: 

          (N – 1) × Isc > module maximum series fuse rating 

  • Where fitted, fuses must be installed in both positive and negative string cables for all strings. 

  • The string fuse must be of a type gPV - according to IEC60269-6 

  • The string fuse must be rated for operation at Voc(stc) x M x 1.15 

  • The string fuse must be selected with an operating current In such that:                                                                                                                                                                                           Ø I n > 1.5 x Isc stc Ø I n ≤ 2.4 x Isc stc Ø I n ≤ Maximum series fuse value

This article has been extracted from the "Guide to the Installation of Photovoltaic Systems" by Microgeneration Certification Scheme the full full guide can be downloaded free  at the below link. 

Download the Free Guide Here

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