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Green | October 2011

Solar cables

Though they are a critical component connecting and ensuring efficient transmission of energy, their usage as a vital part of solar panels is rarely discussed, says Santhosh Kumar.

Thanks to the solar industry, India is being recognised and on the global map of renewable energy (RE). In the past five years, the country's renewable capacity has increased at a compounded annual growth rate (CAGR) of 24.18 percent, which is much faster than any other fuel source.Riding on this robust growth the country has acquired a standing among the top five countries in the world in terms of renewable energy capacity with an installed base of nearly 20 gigawatts (GW). This is being further pumped up with an addition of over 2 GW of renewable power annually. Despite these striking statistics, the contribution of RE is only about 3-4 per cent of the total to the real power generated.

To balance the skewed electricity mix the government has been actively rolling out plans to increase the RE capacity. Prominent ones being the announcement made by the Ministry for New and Renewable Energy (MNRE) in December 2010 to increase the RE capacity to 72,400 MW by 2022 - four times the present capacity and 18 percent of the estimated total capacity in 2022. Here the real generation from renewable sources was expected to be around 7-8 per cent of the total electricity mix

At the end of the 10th Plan, the grid interactive renewable energy capacity was around 10,257 MW and capacity addition planned for the 11th Plan (ending March 2012) by MNRE is 14,000 MW. Out of this, only 50 MW was envisaged from grid-interactive solar. By the end of March 2011 the actual standing is 19,973 MW.

With the launch of the Jawaharlal Nehru National Solar Mission (JNNSM), the 50 MW solar target for the 11th Plan has become largely irrelevant as nearly 250 MW or more is expected to be commissioned by March 2012.

In the 12th Plan (2012-2017), capacity addition will be largely driven by JNNSM which envisages adding 20,000 MW grid-connected solar power by 2022 (by the end of the 13th Plan period) and at least 5,000 MW by 2017. In this, the solar thermal to solar PV proportion is expected to be 60:40.

Spurred by JNNSM and various state government schemes, the industry is set to witness comprehensive growth in all spheres. This will be especially dominated by equipment and solar PV module manufacturers. Lots of companies have already ramped up their module manufacturing capacities.

The JNNSM guidelines stipulate that all grid-connected solar PV plants in India under the guidelines of the scheme will have to mandatorily use solar PV modules that are made in India. That translates to the solar module demand as follows.

•Phase 1 (2010-2013): 500 MW
•Phase 2 (2014-2017): 2,000 MW
•Phase 3 (2018-2022): 7,500 MW

As a ripple effect, the engineering, procurement and construction (EPC) segment is also witnessing some good action and industry consolidation is also expected. Keeping in tandem with this growth trajectory and to further explore the potential market, equipment manufacturers such as those into inverters are seriously considering setting up businesses in India.

Needless to say, cables, the life connecting veins of the manufacturing sector especially the customised cables used in solar open-field installations are set to rein the industry by catering to these mammoth stipulations. Though they are a critical component connecting and ensuring efficient transmission of energy, their usage as a vital part of solar panels is rarely discussed.

In a typical moment magnitude scale solar PV power plant, cables are required to interconnect the solar modules to form a string array, then further between string arrays to array junction boxes up to the inverter. This in total forms the direct current circuit. Armoured starts from the inverter output to the transformer and beyond. Other than this, depending on the design, MODBUS cables or fibre OPTIC connectivity is used for string monitoring purposes.

A lot depends on the selection of type and size of the cable. Though copper is preferred due to its inherent advantages of flexibility, reliability and lower lifecycle costs, aluminium is also considered as an alternative at times citing the cost factor. Other than the conductor, the cable should be ultraviolet and Ozone-resistant and also be suitable for wet and dry installations across a wide temperature range.

Apart from cables required for solar system integration, another key driver is the PV module manufacturing sector which has a huge demand for approved cables, typically with TUV approval for European markets.

Considering the average power rating of the module as 135 WP and 2 metres length of cable per module, this amounts to over 10 million metres of high quality PV wire per annum at the existing module manufacturing capacity of about 700 MW. This is estimated to double in the next 2 -3 years' time-frame.

Standards to consider during purchase of solar cables
Coming back to the topic of cables that are fundamental to the solar industry, MNRE guidelines stipulate the usage of cables as per Indian Standard (IS) 694/IS 1554 with ultraviolet protection. This however, in reality, seems to be grossly inadequate as it does not imbibe the good practices of solar open field installations in developed markets like Europe and Germany.

Taking a cue from mature and developed markets, both the United States based agency Underwriters Laboratory (UL) and German based agency Technischer Überwachungs-Verein (TÜV) have approvals specifically for wires used in photovoltaic (Capable of producing a voltage, usually through photo-emission, when exposed to radiant energy) applications.

UL has two types of approvals for photovoltaic applications: USE-2 and type PV. Originally, the standard approval for photovoltaic applications was USE-2. However, once there was a greater demand for wires for solar power, UL designed the UL type PV approval to meet the needs of such applications better.

USE-2 and type PV approvals are similar. However, there are a few differences. PV wires can be used in both grounded and ungrounded PV arrays and is rated +90°C wet and +150°C dry with approvals for 600 V, 1000 V and 2,000 V. Although USE-2 is only suitable for grounded PV arrays per NEC, it has a +90°C wet and dry rating and is only rated for 600 V.

In addition to the better temperature and voltage range, type PV wires have better sunlight resistance, lowtemperature flexibility, flame resistance and thicker insulation for additional element protection.

TÜV's certification Pfg 1169 approval is based on International Organisation for Standardisation (ISO)  4892, European Standards (EN)50395 and EN 50396 standards and tested according to International Electrotechnical Commission (IEC) 60811 standards. The TÜV approved wire is also generally low smoke and zero halogen, making it more environmentally-friendly. This approval is written specifically for the European market and is the preferred approval for solar panel manufacturers.

Another important aspect is that the conductor used in these cables is usually made out of tinned copper, making it more corrosion resistant.

Consequences of using inappropriate cables

Safety and efficient usage of energy being imperative, usage of uncertified or wrong selection of cables, which may not be the right option in terms of application, can result in a host of problems.  Most important of them being increased losses and failure of insulation, both of which impacts the generation yield and defeats the very purpose of adapting to renewable energy sources.
The most severe aftermath is that even a single failure in any of the links will make the system largely redundant, considering that the system is largely a series connected schematic. Another parameter to be kept in mind while making purchase decisions is that solar energy generation systems should be maintenance-free for at least 20-25 years.
In the Indian scenario it is nice to note that at least up to the string level most players are unwilling to compromise on the demands of cabling and the right products are often used.

However, there is a tendency among many players to use the wrong grade of cables when it comes to insulation materials and conductors, especially when the cable size goes up.

Conventional cross linked polyethylene (XLPE) insulation with PVC ST2 outer sheath with either copper or aluminium conductors are being widely considered as an alternative. The cables are generally armoured in nature. This is a marked deviation from the actual proven international norms and it compromises the overall integrity of the system as well. Though cost is cited as a factor here, it is interesting to note that using proper cabling will account for merely 2-3 per cent of the total project costs.

Issues and challenges for equipment manufacturing

On a macro level understanding and building on these industry bottlenecks can take the solar equipment manufacturing industry to the next level.


•Lack of uniform norms/guidelines for cable selection and usage. Unvetted application needs and reliability requirements except for requirement  of UV protection
•Confusion prevailing among EPC contractors, consultants and clients when it comes to approvals required/mandatory requirements
•Lack of sufficient professional technical consultants

•To devise an appropriate standard/certification, which captures all elements of safety along with specific application          needs.The existing international norms could form a basis for this
•Availability of appropriate indigenous  technology
•Cost control since the existing accepted technology (electron beam cross linked cables) is comparatively expensive than conventional wires and  cables.

The way ahead
Along with the government's strong support to drive the growth of solar industry and the industry's effort to harness it, it is all about creating a state of informed consensus among the mindset of the masses at large.

So using solar technology is really about the usage of appropriate solar equipment with the right components in the true sense of 'going green in a sustainable style'.

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