Choosing and installing the right kind of overcurrent protection is becoming more important as projects increase in size and power, says Tony Garlinge-Warren, applications engineer at Eaton's Bussmann business.
News that cumulative PV demand in the UK reached 2.5GW recently is a reflection of the increasing size and complexity of systems now being installed across the land. The corollary of that is system design and build needs to factor in appropriate overcurrent protection in order to minimise damage, downtime and the financial cost of replacing failed components, which on larger sites may be considerable.
Unlike typical AC power generation systems, the available short-circuit current within micro generation PV systems is very limited and the overcurrent protective devices need to operate effectively on very low levels of fault current. The International Electrotechnical Commissions (IEC) recognise the protection of PV systems is different to standard electrical installations. This is reflected in IEC 60269-6 (gPV) which defines specific characteristics that a fuse link should meet for protecting PV systems.
It is worth reminding ourselves that in faulted circuits PV modules may be damaged by reverse overcurrents exceeding the reverse current withstand of the modules IMOD_REVERSE. Typically the effects of fault currents may range from permanent damage to PV modules and reduced efficiency to broken conductors resulting in electric arcs and fire. Dangerous fault currents can originate from external sources, ie. from modules or strings of modules that are connected in parallel to the faulted string, from storage batteries in the system or from backfeeding through grid-interactive inverters. However, correctly rated PV fuses are able to protect PV modules or strings and internal wiring against these dangerous reverse overcurrents.
So where should fuse links be installed? Depending on the desired capacity of the PV system, there may be several PV strings connected in parallel to achieve higher currents and subsequently more power.
PV systems that have three or more strings connected in parallel need to have each string protected. Systems that have less than three strings will not generate enough fault current to damage the conductors, equipment or modules. Therefore they do not present a safety hazard, provided the conductor is sized correctly, based on local codes and installations requirements.
Where three or more strings are connected in parallel, a fuse link on each string will protect the conductors and modules from overcurrent faults and help minimise any safety hazards. It will also isolate the faulted string so that the rest of the PV system can continue to generate electricity. An important factor on large scale installations.
It should be remembered that PV module output changes with the amount of sunlight the module is exposed to along with the ambient temperature. The exposure is dependant on irradiance level, incline as well as shading effect from trees/buildings or clouds. The total power output of the PV array is greatly affected by such external factors. Fuse selection should always be based on the maximum power output from each PV string and relate to conductor size. In operation, fuse links, as thermal devices, are influenced by ambient temperature. The current capability of the PV fuse links should be de-rated according to the fuse manufacturer's guidelines to prevent nuisance fuse link operation.
The procedure for PV string fuse selection should be based on manufacturer data and system design. The basic method requires the minimum conductor size to be calculated and selected. Array short circuit current should be calculated and if found to be higher than the conductor maximum current rating then string fuses are required. Conductor size can be calculated using the formula 1.56 x Isc (of PV module from manufacturer data). The array max short circuit current can be calculated using (Np -1) x 125% x Isc where Np is the number of strings in parallel. If array short circuit is higher than the maximum conductor current carrying rating then the next highest rating above the conductor max current should be selected. Since PV fuse links are designed to operate at over-currents of ≥ 1.35 x the fuse rating protection is achieved.
Further downstream, depending on the desired capacity of the PV system, there may be several PV sub-arrays (each sub- array consists of multiple strings) connected in parallel to achieve higher currents and subsequently more power. A fuse link on each sub-array will protect the conductors from fault current and help minimise any safety hazards. It will also isolate the faulted sub-array so that the rest of the PV system can continue to generate electricity.
A fuse link positioned in the cable that carries the combined output of a number of strings should be protected by sub-array fuse links. If a number of sub-arrays are subsequently combined then a further fuse link should be incorporated. This would be termed the array fuse link as shown in diagram A.
It should be remembered that the characteristics of PV modules vary with module temperature as well as irradiance level. In operation fuse links are influenced by ambient temperature. Since PV arrays are positioned in direct sunlight the fuses are often subject to similar conditions. The effects of sunlight will increase the ambient temperature where the fuse link is positioned. It is vital to take this into consideration when selecting a fuse rating. A de-rating curve is available to ensure the selected rating will provide protection and be rated for maximum PV power output.
The requirement for PV string and array protection is a vital component in the micro generation industry. The cost of equipment, repairs and down time is significant and the investment in equipment protection vital. Selection of the correct fuse link must be accurate and correct to ensure system security and minimize risk.
What is certain is that as PV systems evolve with new equipment, new wiring procedures and methods of installation, the development of the PV fuse will continue in tandem. As the asset value of PV energy systems increases, in terms of the revenue that can be generated, it would be wholly prudent and commercially sensible to protect that asset with a product that meets the necessary performance requirements and standards.
