Installing long DC Cable homeruns fromd string to inveerter

Bendesa1962 · November 2020

My question is not so much about amperes and voltages but about the most efficient method for connecting a high volume of DC PV Strings cables.

It concerns a project of 3.5 MW in total divided over 3 phases. The first phase consists of:

• 2,766 solar panels

• 156 Strings

• 13 x 60KW Inverter / inverter

It is a large factory hall. The panels are mounted on the roof in a String Layout of 156 Strings. The roof is inclined approximately 6 degrees.

Now I am looking for the best way to run the DC String cables between the PV Strings and the Inverter. The cable reels are large, each 1000 meters. A cable tray is mounted on the roof to guide the DC cabling. The cable trays meet on the roof where there is a feed-through to the room with all inverters.

Roughly speaking, I've actually seen, heard of two different methodologies:

1. String by string

2. Combine multiple strings

1 String for string

The method is as follows. The cable reel is placed in a metal frame so that it can be unwound freely. On the roof is someone who takes the cable by the hand and sound it through the DC cable tray until you reach the right string. Downstairs he is helped by someone who ensures that the cable is sufficiently unwound. On the roof, the cable is labeled with inverter number and string number. At the bottom, the cable is unrolled to the correct inverter and there it is cut and also labeled with inverter number and string number. This operation is repeated until all DC cabling has been installed / pulled.

1. Combine multiple strings

The method is as follows. You measure with a tape measure string 1 to 12. (Each inverter has 12 strings). So these lengths are measured manually. The cable reel is placed in a metal frame so that it can be unwound freely. Then I roll off the 12 measured distances for string 1 to 12. Each string is labeled on the ground at the end and at the beginning with the correct inverter number and string number. Fixed on the ground with also the MC4 connectors to the cables for connection to the string on the roof. These 12 cables are bundled together and these 12 cables are pulled from the inverter room to the strings on the roof in 1 go.

Which of you has experience with such large installations and would like to share knowledge about the best way to implement them?

mike95490 · November 2020

way out of my league, sorry.

mcgivor · November 2020

When pulling multiple cables in a cable tray with different length drop off points, the method I've used is to pull the longest first with 2 poly ropes tied to the cables, use one to pull the second longest run leaving the second for possible future use. Attach a new poly rope to each pull ending with the shortest, I've used for multi conductor in security and access control systems as well as industrial electrical environments feeding loads from a motor control center, but the theroy would be the same as PV.

Use cable lube, avoid twisting, identify the conductors or use different colors for DC, this method eliminates the time consuming measurement of multiple runs pulled together and would be consistent with the string by string method.

Bendesa1962 · November 2020

Hi McGivor, Thanks for your reply I do appriciate that. If you pull multiple cables die you measure the length of each seperate cable and then cut them on the ground. Bind them together and pull? How much extra cable you take as a safety factor so you don't cut to short? I'm afraid I will have a lot of waste if I do that for in total fase 3,4 and 5 588 strings. What is your experience?

Many thanks in advance

Kind regards

mcgivor · November 2020

Pull each run individually, it may seem like more work but it actually isn't, poly rope (twine) is inexpensive, always cut with excess, the scraps can be recycled, cutting short is a pain in the ass, requiring splices. From experience allow about 1 meter excess on both ends to allow for any alterations from the original design. Factor in the waste/recycling value in the initial bid for the contract, your estimates will be more refined after the first install, lessons learned in the first phase will translate to more accuracy in the upcoming phases.

Bendesa1962 · November 2020

Hi McGivor, Thanks a lot for yoour reply. I do appriciate it. Thanks! Take care

Kind regards

mcgivor · November 2020

Made mistakes myself in the past, which is where experience teaches one to preemptively consider shortcomings, I learned much from an employee who I supervised, we worked in a symbiotic manner each feeding on past experiences resulting in positive outcomes. Collaboration is the key, my belief is is there is always someone out there who has better ideas, listen and learn, by no means do I consider myself as the best, far from that, I'm all ears to better solutions, sharing knowledge and or experience won't matter in the end, but may benefit those in transition to enlightenment.

Installing long DC Cable homeruns fromd string to inveerter

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