DC cable layout planning in solar power projects
As we have mentioned in our earlier articles also, DC cable play very important role while system designing and array layout configurations of a solar power project. The cable planning should be in such a way that it can ensure a 25 year life expectancy for the project. While we have limited experience in implementing MW scale solar projects, DC cable management has become a critical factor to the project developers and many times the plant outages are just because of the cable breakdowns.
PV cable layout has to take care of various issues, such as optimized sizing, cable routing, grounding, voltage drop and cable cooling procedures.
Many times we have observed that cables are just hanging on the back side of the PV modules with interconnection through cable connections. Sometimes, the cables loops are very tight or very loose. Typically, wiring loop should be 5 times that cable diameter. Loose connection may become a fire hazards and tight connection mat lead to breakage of the cable. So, loops should be of the proper diameter.
Often, it is observed that cable are secures to the mounting frames/ support rails, which are typically about 25oC more than the ambient temperature and leads to increased cable operating temperature. The higher operating temperature of cables significantly increases wiring losses in the PV system. Hence, cable race ways should be properly planned in order to avoid the cable losses. Sometimes the cable clips are metal wires which may leas cutting of/ damage of DC cable and may create fault in the PV system.
Many times cable are routed along the mounting rail, however, the routing of cable perpendicular to the mounting rail should be avoided as the sharp edges itself can damage the cable. Precaution must be taken, where multiple cables enter or exit cable conduit and proper gluing should be done in order to avoid entry of mice which may cut the cable and repairing in this situation will be very difficult.
(Color coding) It is important to have positive and negative cable with different color codes and nomenclature so that incorrect polarization in combiner boxes can be avoided. This is a common mistake done by the DC cable installer and creates fire in the junction boxes due to incorrect polarization.While connecting cable thermal expansion of the conductor inside the race ways should be considered otherwise during summer due to elongation of raceway cable become too tight in junction box and may cause damage to the fittings.
Many times cable lugs are not properly tightened at the junction box and lose connection of the cable was increased losses and spark which lead to fire hazard in the plant. Solar plant work in a harsh environment, where temperature ranging is -10oC to 50 oC and wind speed 150 to 180 Km/hr. Hence, the securing of cable in a proper cable support with enough ventilation is important so that they cab sustains in harsh weather condition.
Cables are exposed to UV radiations. Hence the cable selection should be UV protection cable, which can sustain solar radiation and also it should be laid down in such a ways that the cable in not exposed to direct solar radiation.
Posted by: Dr. Sanjay Vashishtha and Gaurav K. Sharma