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Series String Mystery


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On 2018/09/03 at 9:40 PM, SilverNodashi said:

It's quite a tingly feeling to be shocked by panel frames on the roof ;)

All the rails are on the roof now, and the 6 new panels installed. Decided to earth the rails today, so bonded all the rails together at 3 places, and put a bonding strip over all the joints. Walking backwards connecting from the top rail down to the last one when the back of my leg touched the old already earthed frame. "Tingly feeling is a bit of an understatement.

Should have seen the spark it drew when I connected the new earth to the existing earth. 

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2 hours ago, Jaco de Jongh said:

All the rails are on the roof now, and the 6 new panels installed. Decided to earth the rails today, so bonded all the rails together at 3 places, and put a bonding strip over all the joints. Walking backwards connecting from the top rail down to the last one when the back of my leg touched the old already earthed frame. "Tingly feeling is a bit of an understatement.

Should have seen the spark it drew when I connected the new earth to the existing earth. 

It shouldn't spark when you connect the earths. Are your panels' or batteries' negatives earthed as well?

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3 hours ago, SilverNodashi said:

Should have seen the spark it drew when I connected the new earth to the existing earth. 

sounds like you have a floating earth on your old  panels 

are you sure the old panels are  actually earthed  and to where

did you have shoes on when you got the tingly feeling

do you still get shocked if panels are not connected to the inverter

is your inverter earthed

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6 hours ago, maxomill said:

sounds like you have a floating earth on your old  panels

Nope, I have a good earth on the old panels and that is why I "shocked". The eddy currents as described above was in the unearthed panels/rails and while touching them and the properly earthed old panels I completed the the path to earth and felt the "Tingle"

6 hours ago, maxomill said:

are you sure the old panels are  actually earthed  and to where

Yes. 200%. It is earth to the Supply earth as well as an additional earth spike I have knocked into the garden to strengthen the existing earth. 

6 hours ago, maxomill said:

did you have shoes on when you got the tingly feeling

Yes, but on the tile roof that is irrelevant. I did not "shock" because I was touching earth with my feet. I shocked because I completed the pact to earth with the back of my legs. Shoes would not have prevented the "Shock"

6 hours ago, maxomill said:

do you still get shocked if panels are not connected to the inverter

I felt the shock while the panels was connected to the inverter, but because they were not earthed.. 

6 hours ago, maxomill said:

is your inverter earthed

Yes. 4 Earth connections from each inverter to DB board earth to the same earth as panels, so all the earths at the same potential. 

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11 hours ago, SilverNodashi said:

It shouldn't spark when you connect the earths. Are your panels' or batteries' negatives earthed as well?

My laptop has two power supply bricks, the original one it came with (that has no earth), and the new one I got from some drop-shipper via BoB from China (which has an earth, interestingly enough). When I use this new one, two things happen: 1) The trackpad doesn't work properly, and 2) when you plug in an external monitor there is a small spark just as the metal of the VGA plug touches the socket on the side of the laptop. It is so small that I can discharge it by touch and not feel it, yet it is there for some strange reason.

I found out from a colleague the other day that it is common to have a capacitor between your chassis ground and some internal supply line (negative possibly) to improve EMI. This spark I see might just be that cap discharging. Or something... my point is that it is apparently "normal" or at least not a sign of anything being "wrong" for some values of normal and wrong :-)

Edited by plonkster
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14 minutes ago, plonkster said:

my point is that it is apparently "normal" or at least not a sign of anything being "wrong" for some values of normal and wrong

My feeling as well, the spark wasn't big enough to worry me, but it was big enough to impress me... 

I must actually disconnect it again and measure the potential difference between the new frame and the existing earth.. Ill be back now with the measurement......

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1 hour ago, Jaco de Jongh said:

I must actually disconnect it again and measure the potential difference between the new frame and the existing earth.. Ill be back now with the measurement......

861427240_Earth3.thumb.PNG.73f6372d413aa964452a5f01133f33b9.PNG

Measurements as follows. 

  1. Between Earth and top panels with everything on                                                                                 = 98Volt AC
  2. Between Earth and top panels with PV isolator off                                                                               = 18Volt AC
  3. Between Earth and top panels with PV isolator off, mains AC off and Inverters off...                     = 60mV 

The 98Volts is not effective voltage. If i connect a globe rated 85-230VAC to it, it does not light up... 

Anyone else did this test before? Can we compare notes?  

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9 minutes ago, Jaco de Jongh said:

Measurements as follows. 

  1. Between Earth and top panels with everything on                                                                                 = 98Volt 
  2. Between Earth and top panels with PV isolator off                                                                               = 18Volt AC

This shows @plonkster 's theory was correct. 

On 2018/09/02 at 6:06 PM, plonkster said:

Eddy currents induced by the DC ripple caused by the  MPPT.

 

On 2018/09/02 at 6:16 PM, 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, 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.

When I disconnected the String from the MPPT the Bulk of the Eddy Currents were eliminated. 

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On 2018/09/02 at 6:16 PM, 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, causing a DC ripple and therefore an alternating current in the entire length of PV cable

Is it not rather switched DC rather than an AC current? There is a small difference.

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9 minutes ago, Chris Hobson said:

There is a difference between grounding the output of the panel and earthing the panel frame. I would earth the panel frame and NOT ground the negative of the array.

I think the real meaning gets lost in translation, and we use terms differently and understanding Chinese translation even outside of the Solar industry , proves to be a challenge. 

Edit: To be clear on this point, I do not intend removing any existing earthing.. 

 

5 minutes ago, Chris Hobson said:

Is it not rather switched DC rather than an AC current? There is a small difference.

It can only be measured as AC thus refered to as AC... imo

EDIT: Lets call it a modified sinewave... :D

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1 hour ago, Jaco de Jongh said:

Between Earth and top panels with everything on                                                                                 = 98Volt AC

Now for my next question. Does your meter measure the true RMS value? Because if this is based on the DC ripple caused by the MPPT, then the waveform is not sinusoidal and this measurement won't be accurate :-) Which is not to say it isn't useful: It does give you an idea of just how much potential there is.

1 hour ago, Jaco de Jongh said:

Between Earth and top panels with PV isolator off                                                                               = 18Volt AC

This one likely is accurate, because the PV side is off.

Edited by plonkster
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I guess I can close this for now. After a lot of test I still cant find the reason for the limited voltage in the first setup. 

On 2018/09/02 at 1:08 PM, Jaco de Jongh said:

The only logical explanation to me remains a bad contact,

On 2018/09/02 at 3:25 PM, Jaco de Jongh said:

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.  

Done all of this and tested for a few days. Even at the higher volts, no problem what so ever. This string does give about 400 watt less than the other string, but panels needs cleaning and 6 panels in these string is lying flat. 

I guess I will never know what caused the original problem 

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