Author: Bruno De Wachter for European Copper Institute
Published July 2021
How to obtain the ambient temperature when carrying out PV string cable temperature corrections?
IEC standards include tables with correction factors for the current-carrying capacity of PV string cables, depending on the cable ambient temperature. But how is this ambient temperature determined?
In Part I, we list all the relevant IEC standards and define the white paper’s precise scope.
Part II provides an analysis of what can be found on the subject in existing standards, which is inconclusive. We pose questions which arise from a meticulous reading of these standards. In Part II we suggest a potential avenue for improving standardisation in this area.
Finally, we wrap up all the questions and summarise our recommendations. Note that voltage drop, short circuit current, and earth fault current must also be considered when determining the cross-section of PV string cables. These additional factors are not discussed here.
The following standards have been analysed:
- IEC 60228 – CONDUCTORS OF INSULATED CABLES; IEC 60364-5-52 – LOW-VOLTAGE ELECTRICAL INSTALLATIONS – PART 5-52: SELECTION AND ERECTION OF ELECTRICAL EQUIPMENT – WIRING SYSTEMS;
- IEC 60364-7-712 – LOW VOLTAGE ELECTRICAL INSTALLATIONS – PART 7-712: REQUIREMENTS FOR SPECIAL INSTALLATIONS OR LOCATIONS – SOLAR PHOTOVOLTAIC (PV) POWER SUPPLY SYSTEMS;
- IEC 61730-1 – PHOTOVOLTAIC (PV) MODULE SAFETY QUALIFICATION - PART 1: REQUIREMENTS FOR CONSTRUCTION;
- IEC 62109-3 – SAFETY OF POWER CONVERTERS FOR USE IN PHOTOVOLTAIC POWER SYSTEMS - PART 3: PARTICULAR REQUIREMENTS FOR ELECTRONIC DEVICES IN COMBINATION WITH PHOTOVOLTAIC ELEMENTS;
- IEC CD 62548 – PHOTOVOLTAIC (PV) ARRAYS - DESIGN REQUIREMENTS;
- IEC 62930 – ELECTRIC CABLES FOR PHOTOVOLTAIC SYSTEMS WITH A VOLTAGE RATING OF 1.5 kV DC;
- IEC TS 63126 – GUIDELINES FOR QUALIFYING PV MODULES, COMPONENTS AND MATERIALS FOR OPERATION AT HIGH TEMPERATURES.
It is key to make a clear difference between "environmental temperature" (official temperature obtained from the meteorological measurement station closest to the PV system’s location), "ambient temperature" (the temperature of the air surrounding the object of study, in this case the cable system) and "operational temperature" (the temperature of the object of study). Despite these clear definitions, some international standards on PV systems generate confusion about ambient temperature. This paper addresses such gaps, proposes a robust method to determine the environmental and the ambient temperature and proposes an avenue for adapting the international standards relating to PV string cable ambient temperature to make them more definitive and based on credible sources. Such proposal is based on temperature corrections for six different installation methods (going from +0ºC to +43ºC), using as a starting point the 91st percentile environmental temperature.