Burnt out Cable Connection on 100kW inverter

[RCherry]

Hoping someone can give some input on what could be possible causes of what happened here. I am going to investigate the situation next week but for the time being I am trying to get as much input as possible to possible causes .

The install consists of 7x 100kW Huawei Grid Tied Inverters ,the system has been running smoothly for the past 9 months, faults came through on the monitoring platform and when an technician was sent out this is what was found .

My first and quite frankly only reasonable explanation is a loose connection ,but ,to the best of my knowledge a loose connection would have burnt out locally (at the point of termination where cable was loose ) . But in these pictures the cable is burnt out both at the inverter and then all the way back to the circuit breaker in the distribution board .

Any other ideas would be greatly appreciated and if my thinking about "localised" damage is incorrect if someone could explain otherwise that would also be greatly appreciated.

 

[RCherry]

The mccb did trip. I will take some pictures of the other inverters connections and post them

 

[westward10]

That pic in #9 does look like overheating from the spread down the yellow conductor but there also looks like a short. Someone tried to tighten it and ....................

 

[RCherry]

If there was negligence on one of my guys behalf when it was initially installed last year and the terminal was not torqued ( unlikely because I check and then then the installation undergoes quality checks by a 3rd party) but if it was loose connection would there not at least been a minimum been a considerable voltage drop ? In which case the inverter would have pushed warnings a long time ago

 

[westward10]

I'm no expert on solar inverters so I can't really comment.

 

[plugsandsparks]

If these are grid tied inverters then you have the other issue of grid generated fault current if there has been a breakdown of the phase barriers in the inverter. We have had some great weather recently and it possible the inverter could have initially generated thermal damage through solar action and the grid has finished it off.

Total guess of course, what really matters is who pays for all that damage.... eek

 

[James]

If these are grid tied inverters then you have the other issue of grid generated fault current if there has been a breakdown of the phase barriers in the inverter. We have had some great weather recently and it possible the inverter could have initially generated thermal damage through solar action and the grid has finished it off.

Total guess of course, what really matters is who pays for all that damage.... eek

Even if they were not grid ted and working in their own island, there are several of them so the others could gang up on a faulty one and finish it off in much the same way as the grid would.

 

[marconi]

Correctly connecting al and cu | Klauke - https://www.klauke.com/gb/en/correctly-connecting-al-and-cu

https://solar.huawei.com/en/download?p=/-/media/Solar/attachment/pdf/au/service/Supporting/SUN2000/SUN2000-100-125KTL-Series-UserManual.pdf

I would thoroughly check the way the aluminium cored power cable has been terminated to the inverter copper connections OT/DT. See section 5.6 of the second reference which is the installation manual for the inverter. The first reference is a brief introduction to the problem of galvanic corrosion between aluminium and copper surfaces as in a bolted connection and the employment of transition terminations to ensure corrosion does not happen.

I think the yellow line connections at the inverter has over time failed through galvanic corrosion and become more resistive leading to significant ohmic heating, carbonisation of surrounding plastic and surface tracking between yellow and blue lines (and some visible also between red and yellow). Surface tracking has caused more ohmic heating and R to Y and Y to B line to line currents additional to the normal peak 100kW circa 150A inverter output line currents especially in the yellow line.

Eventually an arc has occurred between yellow and blue lines which has allowed such a high current to flow that the mccb has tripped. What you see as heat stress at the mccb yellow and blue terminals is because they have operated beyond their rated current and become too hot again through ohmic heating. Given more time I expect there would have soon been an arc fault between red and yellow well as between yellow and blue at the inverter terminals.

 

[RCherry]

@westward10 I actually accidentally posted the video thinking it was the picture ,it is the termination points at the inverter itselfView attachment 98791

Correctly connecting al and cu | Klauke - https://www.klauke.com/gb/en/correctly-connecting-al-and-cu

https://solar.huawei.com/en/download?p=/-/media/Solar/attachment/pdf/au/service/Supporting/SUN2000/SUN2000-100-125KTL-Series-UserManual.pdf

I would thoroughly check the way the aluminium cored power cable has been terminated to the inverter copper connections OT/DT. See section 5.6 of the second reference which is the installation manual for the inverter. The first reference is a brief introduction to the problem of galvanic corrosion between aluminium and copper surfaces as in a bolted connection and the employment of transition terminations to ensure corrosion does not happen.

I think the yellow line connections at the inverter has over time failed through galvanic corrosion and become more resistive leading to significant ohmic heating, carbonisation of surrounding plastic and surface tracking between yellow and blue lines (and some visible also between red and yellow). Surface tracking has caused more ohmic heating and R to Y and Y to B line to line currents additional to the normal peak 100kW circa 150A inverter output line currents especially in the yellow line.

Eventually an arc has occurred between yellow and blue lines which has allowed such a high current to flow that the mccb has tripped. What you see as heat stress at the mccb yellow and blue terminals is because they have operated beyond their rated current and become too hot again through ohmic heating. Given more time I expect there would have soon been an arc fault between red and yellow well as between yellow and blue at the inverter terminals.

I appreciate your input but the conductor and the lugs used are copper

 

[marconi]

Undaunted……

I suspect then the output power stage, the output and EMI filters, the contactor or the ac surge protection device all of which connect across the lines, neutral and to earth. See section 2.5.1

What I would do is turn off and isolate two good inverters and after allowing time for capacitors to discharge take a low voltage ohmmeter and measure the resistances between all possible pairs of lines, neutral and earth. It takes 15 minutes before a de-energised inverter is safe to touch.

Then do the same for the damaged inverter and compare with the two good inverters. If the inverter is damaged internally you will see differences in resistance measurements. Armed with this information you can contact the supplier/manufacturer and discuss what has failed and warranty/ guarantee replacement.

Please resist the temptation to open up the damaged inverter because you will likely void the warranty/guarantee. (This is what happened to a French farmer I helped with his solar system problem. If he had not opened up the inverter it would have been replaced gratis but he was too curious not to have a gander to see if he could repair it himself).

ps: when doing r measurements do both polarities.

 

[marconi]

I recommend you check the software settings for grid connection. See Section 7.1.5.1 Grid Parameters and 7.1.5.2 Grid Protection Parameters.

Did you log any fault messages? Or did the inverter continue to provide power to the grid until the mcb tripped?

Did the inverter trip during a period of solar electricity generation or at night when it was either in standby or if selected generating and exporting reactive power?

I’d measure the line to line, line to neutral, line to earth and neutral to earth voltages.

what is the earthing system please?

is the internal cooling fan functioning well?

 

[marconi]

RCherry - Greetings from London.

I wondered what you discovered when you visited the site to investigate the burned out inverter connections.

 

[Lucien Nunes]

Yes it's an interesting one, I thought I had posted in this thread but it seems not.

There are multiple aspects to the damage but the main one seems to have been a parallel power arc at the inverter terminals, evidenced by the loss of metal. So let's start with the question of what initiated it.

1) Voltage transient. If this were high enough to flash over those barriers, it would probably have destroyed the inverter, or more likely been absorbed by it. Examination of the innards might give some clues as to whether it's likely.
2) Momentary short by conductive object, unlikely if the covers were in place.
3) Carbonised insulation due to prolonged creepage over moisture. Unlikely due to covers and heat dissipation.
4) Carbonised insulation due to high-resistance connection overheating. This would tally with the cable insulation thermal damage, which is sufficiently different between yellow and blue phases and shows a sufficiently shallow temperature gradient along the conductor, that it seems unlikely to have resulted only from the heat dissipated in the arc.
5) Series arc caused by mechanically loose connection, inferences as per 4).

From just the single picture and no further evidence, I would say the most likely root cause is high resistance at the yellow and/or blue phase terminals, either the lug cimps or bolts or even the hidden internal connections to the inverter itself. This caused carbonisation of the terminal insulation, self heating and arcing resulting in a full parallel arc between the studs.

But then we have to consider the situation at the MCCB. We can imagine that it might spit out some vapour when clearing this fault, but we see insulation shrink-back on yellow and blue cables as though prolonged moderate overheating had taken place. So what caused this thermal damage?

1) Voltage transient. Could have caused a flashover both at the breaker and the inverter, but would have been equally likely to break down at any point in the panel, not specifically the breaker associated with the damaged inverter.
2) Failure of the MCCB to clear the fault promptly. Seeing that the fault is double-fed from the grid and the inverter, could the fault current have been low enough to endure long enough to cause thermal damage at both ends?
3) High resistance connections at the breaker, unlikely coincidence as above.

I'm struggling to make a coherent inference from this. More evidence needed!

 

[DPG]

OP was last seen on here June 26th. I expect we will never know