PV system design-Conductor sizing
Conductor sizing is also very important aspect of PV system design. Once we have selected a exact series or parallel combination PV modules for desired voltage and current levels. The array needs to be connected to inverter through DC cables and selecting appropriate conductor size becomes critical .The conductor size is defined through Ampacity rating which shows that how much is the current carrying capability of a typical conductor. The Ampacity of conductor is function of various parameters such as Resistance of the wire ,Diameter of wire, Operating temperature of wire and Insulation rating of the wire. Selecting an appropriate conductor size is not only important from cost point of view also equally important from the losses point of view. Typically conductor size is decided based on the voltage drop over a given length of a conductor and voltage drop of the order of (1-3) % are selected by the system designers. If we select 1%, the conductor size will be bigger, however for 3 % voltage drop we can have thin conductor size and accordingly cost of conductor will change. It should be kept in mind that whether 1 or 3 % drop, the losses in PV plant are directly proportional to the voltage drop in the conductor. Hence even one percent saving in the conductor losses can save about 20 Lakh Rupees in case of a typical 5MW plant.
The Ampacity of DC Cable is function of temperature, while ambient temperature is 30°C, a typical DC cable operate at 60°C ,however as the temp increases the cable Ampacity reduces. For example at 50°C ambient temperature the cable Ampacity reduces to 58% (0.58 correction factor.).If number of conductors are carried together the cable Ampacity further reduces, for example, if 14 DC cables are carried together in bunch ,the cable Ampacity reduces to 50%.
DC cables in PV system are exposed to solar radiation and number of conductors are bundled together from different arrays to array junction box and subsequently to inverter .Often these conductors run in open air in bundle .There should be proper space ,physical protection and support, so that the cables do not hang too much. As hanging cables may cause damage near by vegetation which may lead to catch fire .While outside temperature is 35 °C ,it is possible that conductors bundled together have a operating temp of 75°C. Accordingly the Ampacity of conductors should be designed depending on the operating temperature of the bundled conductors. Many time the conductors selected are not copper conductors and the EPC contractor select aluminium conductors to save some money .It may noted that aluminium conductors have higher resistance and low ductile strength .Hence whenever there are turns and twists in the cable ,there are chances of breakage of cables and repairing becomes extremely difficult .The cable insulation should be such that ,it can with stand maximum operating temperature level of the conductor bundle.
It is advisable that too many single cables should not be bundled together. Developer should use cable junction box ,all these smaller conductors are terminated and the output is taken by single conductor of larger thickness which has capability to carry larger current. For example ,in a typical string of 280W(36V 8A).The string current is of the order of 8 Amp, for which 6 Sq mm conductor is enough to carry this current ,however there are about sixteen strings to join together run in parallel ,which are connected to a string combiner box ,which leads to overall current of 108 Amp. These strings are connected in parallel with single string combiner boxes in the following manner. it may be noted that these 16 strings have home run cables and these cables run for almost 100 m length together on a single conduit, which will reduce the conductor Ampacity to 50 %.It is not advisable to bring sixteen strings rather there should be array junction boxes connecting four arrays together and subsequently these array junction box can be connected to main junction box from which the output cable carry 108 amps.
Although there is no specified limit for voltage drop in any given PV circuit, but the voltage drop to be kept in the range of 1 to 2 % only by considering best judgement in terms of economics verses system loses. For example, the volt drop from string to array junction box can be 1%, from array junction box to string combiner can be 1.5% and string combiner box to inverter be 2% .In order to optimize economics and to control CAPEX for the PV system.
Generally PVC conduits are used to carry the bundle of conductors ,however it is advisable in that harsh climate conditions, steel conduit can also be used to protect the cables from the climatic conditions as well as attack of mouse. The DC cables in PV system also requires maintenance over time ,hence proper symbol should be put up to mark the location of AC and DC cables.
Although PV system is designed on based on MPP current ratings. But there should be proper protection through fuses and circuit breakers to protect cable from reverse current flow ,that can damage the wiring between individual PV modules. The fuse selected should be based on maximum short circuit current observed in a given circuit .
Dr Sanjay Vashishtha & Rishikesh Muthyal