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Series String Mystery


Jaco De Jongh
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Very interesting situation on my own Installation.  I started off with 12 panels (ReneSolar) on a frame, added 6 (Ja Solars) to another frame. Because the angles of the frames differ, I decided to string 6 from one frame to 3 from the other, to have 9 panels per inverter.  

Its been quite a while that I saw that one string is giving out higher volts than the other one. under load, string one will measure 230 volts and string 2 only 170 volts. I have switched of the isolater and measured the voc of all the panels in series section by section, no problems all added up , but under load condition when measured it was as if the 3 JA solar panels wasn't connected at all, the volts measured before the string and after the sting were exactly the same. Unplugged and plugged the MC4's suspecting a bad connection but no change.

Early today i started installing the frame to mount all the panels on the roof and decided to string all 12 Renesolars in series to one inverter, to free up the Ja solars. Want to get the Ja's panels on the roof first together with the new Ja's that is arriving this week. When I completed the 12 x 260 Renesolars I got the expected 400Volt voc and under load it gave me 300Volts plus. Happy. 

Decided that one of the JA's must be faulty and when the sun chased me off the roof , i decided top test them by adding one at a time to the second inverter till all six were in series, to see if i can determine witch one was creating the problem. Every time I connected one the volts increased, by the 5th one the mppt switched on and after adding the last one everything was stable. 

This is the confusing part, now with 6 panels under load i was getting 163Volt where I use to get 170Volt with 9. 

The only logical explanation to me remains a bad contact, but how the physical measurement under load make it look like the 3 JA's isnt part of the string , I cant really understand. I mean it is in series, if that section (3 panels) were having a problem, how does the rest of the string still produce? 

553187360_Seriesstrings.thumb.PNG.62edc952d88819fce32341ecbb15b9bd.PNG

 

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One other thing that I just thought of. The frame with the JA's had wooden legs and a steel top that was in contact with the panels. Sometimes if I touched the steel it shocked me. Went through the wiring and could not find a reason for power to be percent but did not make much of it. 

Have anyone heard of a panels Isolation breaking down at higher voltage? 

I am going to test that when the new panels arrive. String all of them up to the inverter before I mount them on the roof. 

Want to see if the higher voltage could have been the cause of one panel partially failing.  

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Maybe a more lengthy description is required, I suppose not everyone happening on this thread might know what I am talking about.

Eddy currents: Basically when you have an alternating current running close to another conductor (such as that steel piece), then the alternating magnetic field around the first conductor induces a current in the other one. This is why there is a rule that AC wiring must have both the live and neutral in the same conduit, especially in steel conduit: The two fields of the wires normally cancel out, but if your return cable is not in the same conduit, then the conduit itself becomes the secondary winding to a long 1-winding transformer.

So how does this happen in a DC circuit? Well, the MPPT has a switch in it that turns on and off around 40Khz or more, causing a DC ripple and therefore an alternating current in the entire length of PV cable. And steel in close proximity becomes a secondary "winding" and might show eddy currents.

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7 minutes ago, plonkster said:

Eddy currents: Basically when you have an alternating current running close to another conductor

To be expected in AC..

8 minutes ago, plonkster said:

So how does this happen in a DC circuit? Well, the MPPT has a switch in it that turns on and off around 40Khz or more,

Never experienced DC eddy currents like this before. Dealt with DC transients before, but using screened cables normally takes care of that. Also the reason that I ordered Steel spraque for each string in the final installation. With the 400volt DC strings I decided to be extra careful and run all PV wires in earthedFlexible steel conduit. 

Learned something new tonight. 

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3 minutes ago, plonkster said:

Just to  be clear... I'm making an educated guess :-)

You might be right.. ill test and report back. 

After moving all the panels to the roof and installing the spraque, I am almost sure the eddy current should be taken care off and the risk of shock reduced dramatically. Lets see. 

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13 minutes ago, Jaco de Jongh said:

, I am almost sure the eddy current should be taken care off

If both wires (negative and positive) runs through the same metal ring, they cancel out. Where you have a piece of metal running close to only one conductor (or part of it) is when it won't cancel out. That's why I guessed that.

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53 minutes ago, plonkster said:

That's why I guessed that.

Sorry Plonk, i dont think I was clear in my description above.  The sprague is not installed yet, I am in the process of moving all the panels to the roof as the final part of my installation. Only then will the spraque go into place. I hope by installing the spraque and earthing it , the eddy currents will be taken care of. 

So your guess is still a good possibility. 

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15 minutes ago, SilverNodashi said:

It's quite a tingly feeling to be shocked by panel frames on the roof ;) Not so much fun when you're on the roof and it starts to rain. 

Someone I know was installing a SolarEdge system. Now what the SolarEdge "optimisers" do is when disconnected they put out 1V each, so you can easily measure with a DMM how many modules are in series. So while installing someone asks him... isn't there a high voltage on there? And he replies... sure... and promptly sticks the two wires into his mouth. What he didn't quite bargain on... was that there was 20 panels on the roof :-)

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2 minutes ago, plonkster said:

Someone I know was installing a SolarEdge system. Now what the SolarEdge "optimisers" do is when disconnected they put out 1V each, so you can easily measure with a DMM how many modules are in series. So while installing someone asks him... isn't there a high voltage on there? And he replies... sure... and promptly sticks the two wires into his mouth. What he didn't quite bargain on... was that there was 20 panels on the roof :-)

hahaha!

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9 hours ago, plonkster said:

and promptly sticks the two wires into his mouth.

During my First year of apprenticeship (1991), I decided to build a circuit to connect to my Component Hifi's Receiver and Amp to use them as an alarm clock. Staying in a very small hostel room with a single beb, a chair and a table with a plug that is almost inaccessible, i positioned myself on the edge of the bed, chair turned around to face me with one of the components opened on the chair, the other component open on the table with my little alarm circuit and a soldering iron. Now there was no way of reaching the plug. Decided to use my Pool cue to switch the plug on and off. Plan was, , timer circuit switches amp, aux out power from amp to power in  on Receiver. Being first year it took some time to figure out.

Basically went like this, connect wires , pool cue on, test, pool cue off, disconnect wires, reconnect wires to new place, pool cue on , test, pool cue off, disconnect wires, wires in mouth, test with fluke, ahhh that's the place, pool cue on......  That's the last part I remember before the back of my head hit the wall. Luckily the fast backwards movement pulled the wires from my mouth......

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1 hour ago, Jaco de Jongh said:

That's the last part I remember before the back of my head hit the wall

Ok, you win! Taking 230VAC to the head!

My own best was working on the 36V we had on the farm. Pretty similar situation of not having enough hands and sticking a wire into the mouth. DC stings in a very peculiar sort of way, and I do recall things turning white for a second :-)

Edited by plonkster
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9 minutes ago, plonkster said:

My own best

While we on the topic. 

In school still, walking through an abandoned factory I picked up a transformer clearly marked  230V - 12 Volt. No input or output markings. Child's way of thinking: Thick wires = 230V and thin wires = 12V. Connected 230 to thick wires, took the 12V DC Skyelectric  motor, not even knowing the real difference between DC and AC and tried to connect it to the Thin Wires. 

Held the motors wires between Thumb and index finger of each hand and then one by one connected it to the transformer wires using my fingers to clamp the wires together. 

18 minutes ago, plonkster said:

I do recall things turning white

I recall a much darker shade of white, think its called black , and the shoulder pain... ran a hole through my wardrobes door trying to get away from whatever was biting me.  

Facts about the situation. 230 / 12 = 19.166 so connecting the 230 to 12 volt side step up the voltage to 230 x 19.166 = 4408volts. I was connecting the motor to that. 

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Must say I stand in awe for those who suck on / wanna taste power cables. :-)

I limit the 220v shocks to hands, arms and once my foot. But never ever via mouth, that is just not right.

Ps. 9v batteries do not count.

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