Due to rapid depletion of conventional fossil fuels, various resources of renewable energy are now highly focused globally. Energy from solar and wind contribute major portion of total country’s renewable energy generation. In India, we are having about 300 sunny days in a year and therefore harnessing the solar energy through photo voltaic system becomes one of the preferred green energy options today.
A historic revolution is being witnessed by our country with the Government of India’s highly ambitious plan – The Jawaharlal Nehru National Solar mission, to reach the solar power generation to 100 GW by 2022.
To receive solar energy, number of photo voltaic arrays are required to be installed facing the sun, and photon energy in the form of DC current is carried through SOLAR CABLES to storage batteries. In our country electricity in AC form is the most popularly accepted form, and therefore using the inverter, the DC electrical power is then converted to AC electricity. Solar PV system may be a stand-alone or a grid connected system. The hassle-free performance of a typical solar PV system is generally expected to be more than 25 years and therefore optimisation of all relevant components of solar PV system becomes necessary.
In PV system, one of the important parts is the SOLAR CABLE. They are connected on DC side of the system. Cables used for connecting individual PV modules in a string to form a PV generator are called as string cables, and all strings in parallel are connected to generator junction box. The main DC cable connects the generator junction box to the inverter. The PV energy will be in the DC form with low voltage and high current and this results in principle differences for DC cables. To get the desired efficiency, selection of proper sized cable draws major attention while designing the system, because, undersized cable results into heating which may lead further to fire also. The quality and performance of solar cable plays very important role for extracting solar energy with higher efficiency in PV system. Even a small increase in cable resistance resulting into increased I2R losses is considered as a higher loss of energy and such cable will lose its acceptance. As solar cable has to function in open atmosphere over a long period, it has to withstand all environmental severities like UV radiation, rain, dust & dirt, temperature variations, humidity, insects and microbes etc. Any frequent failure / replacement of solar cable will decrease the overall project efficiency and therefore before actual installation, it becomes necessary to evaluate SOLAR CABLES to ensure its successful functionality over a desired life span through conducting series of tests.
Based on the requirements listed in manuscript of working group AK 411.2.3, Leitungen fur PV-System’ of the German committee for standardization, the specification 2 Pfg1169/08 for solar cable was published. UL 4703 is one another specification on photovoltaic wires published in USA in 2005. Recently, British Standards Institution – BSI has also published BS EN 50618 -2014 as specification for ‘Electric Cables for Photovoltaic Systems’. In this article, the specific requirements for solar cable for PV systems as per international norms, its constructional requirements like flexibility, single core with cross linked insulation and sheath; special requirements like low smoke, halogen free, stringent high voltage withstand capability; environmental requirements like ozone resistance, UV resistance, flammability requirements etc., are discussed.
Figure 1 shows the programmable ozone chamber to evaluate ozone resistance property. Significance and specific requirements of major critical tests are also discussed.
Fig. 1: Programmable ozone test chamber…
Solar cables: general requirements
Stand-alone / roof top solar PV systems are of low voltage level, in general < 100 volts. The grid connected solar PV systems are of higher voltage level and due to the added complexities and considering safety aspects, are installed at separate well protected area and allowed to operate through skilled persons only.
The voltage drop between PV array to the inverter is one of the critical parameters for selection of size of solar cable. The guidelines for size selection are specified in BS: 7671. As per NEC norms, the overall voltage drop between PV array and inverter is allowed < 3%. In order to maintain this, in addition to cable size, the most practicable shorter length of solar cable is preferred.
To suit the variable weather conditions, the solar cable is designed to meet the temperature range from -400C to + 900C. The maximum conductor temperature of 1200C is expected at a maximum ambient temperature of 900C.
Depending upon its area of application, a solar cable may be a single conductor with double insulation, which is generally routed through a suitable conduit / trunking system. For better mechanical robust solution, single conductor with single wire armouring is used. For main DC cable between a PV generator junction box and inverter, multi core single wire armoured cables are used.
The nominal DC voltage of solar cable is 1.5 kV between conductors as well as between conductor and armour, which is up to 1.8 kVmaximum.
Solar cable: constructional requirement
Solar cable should have flexible, class 5, heavy duty tin coated copper conductor. The conductor is to be provided with low smoke, halogen free cross linked insulation and over-sheath. In light of its working in open atmosphere under sun light with UV radiation, solar cables are commonly black in colour. Solar cables after installation, does not experience frequent flexing or torsion forces – and therefore these two parameters are not having major requirements. The insulation & sheath have to perform at higher temperature range with additional high mechanical stability, flame retardant and free of halogens. To meet these requirements, generally cross-linked polyolefin copolymers are preferred.
Compared to PVC type A as per IS: 694 / IS: 1554-part 1 and XLPE as per IS: 7098-part 1, the EN: 50618 specifies special requirements for solar cable insulation / jacket materials.
Few of major physical requirements are listed in Table 1.
Table 1, shows requirement of solar cable insulation / jacket material to meet atmospheric temperature variation. Solar plants are required to work under extreme weather conditions – not only for high temperature, but at low temperature also. The aging temperature, hot set temperature and cold bend temperatures are therefore stringent compared to PVC and XLPE materials. To ensure the expected performance for 25 years, standard specifies thermal endurance test, which will determine the temperature index and it should be > 1200C.
In Table 2, few more critical requirements on as a whole cable are listed out.
In view to its application in open atmosphere, solar cable has to confirm its resistance to weathering / UV exposure.Figure 2 shows the programmable weather chamber to evaluate this parameter.
Fig 2. Programmable weather chamber…
Dynamic penetration test will prove the solar cable for mechanical load. A long duration damp heat test gives idea of stability of solar cable material against any degradation in mechanical properties due to temperature and humidity. Smoke emission tests expect min. 60% light transmission, in case solar cable is under fire. Figure 3 shows the Smoke emission test set up to evaluate this property.
As solar cables are connected in vicinity of solar PV arrays, control panels, electronic controllers, inverters etc., amount of HCl content, pH & conductivity, content of bromine & fluorine requires to be controlled to meet the specified limit.
Fig. 3: Smoke emission test set up…
Conclusions
Solar cables of different sizes and construction designs are available to meet the efficiency requirements of solar PV systems. The international standard EN 50618 specifies the performance requirement of solar cables. It is necessary to evaluate solar cables before actual installation to ensure its desired life span of 25 years.
