measuring insulation resistance on multimeter

[wpns]

Second string’s VOC 620 volt

This is a little disconcerting because I don't know what your inverter or wiring is rated for, but most inverters I've seen have a maximum MPPT input voltage of 600V, so if yours is seeing 620V it may refuse to start.

The reason it might only show the error at mid-day reboots is that as the voltage rises in the morning it'll load the panels down and they won't exceed 600V at the maximum power point input, but if you reboot it mid-day it'll wait a few minutes before connecting the panels, and then the Voc of the string will exceed 600V, and the inverter will throw an error. Why it throws can isolation fault instead of an MPPT input over voltage fault is anyone's guess, but Occam's Razor says it's software bugs. Not that it couldn't be breaking down some over voltage suppressor in the MPPT input, but that's chasing the wrong problem.

What ! Are you really sure about that ?

Well there are two ways to measure insulation resistance :

• 1.To short the positive and negative electrodes of the PV string, and measure the insulation resistance between the shorting point and earth.
• 2.Measuring the insulation resistance between the positive electrode and earth and between the negative and earth separately without shorting.

I have a digital clamp multimeter, the only thing is that my meter can’t do a shortcircuit insulation resistance measurement but measuring resistance between + or - to earth is possible

Strictly speaking you are correct, your DVM can measure insulation resistance (at one or two volts). It cannot measure insulation resistance at high applied voltages, nor can it measure breakdown voltage, for which you need a different piece of test equipment, as the electrical experts above have mentioned.

Again: You are making me nervous measuring 620Vdc strings with a meter that _SAYS_ 600V on it, I'm concerned that you aren't taking the proper safety precautions, and that you don't understand how your measurements work or what they mean.

When you say: "let’s suppose i would reboot it now, it will shows" and then post a picture of the wiring, that doesn't give me any confidence. If you are not paying attention, if you are not sure what you are doing, if you don't know what your meter readings are or how they are interpreted, you could die.

That would be bad. Start at the top, and explain it like I'm five. What panels, what strings, what wiring, which inverter, what works and what doesn't, what test equipment do you have available, etc?

 

[Zwy]

@wpns

Great find ! I am happy for you
My inverter is a 3 phase inverter but not hybrid.

I didn’t have yet issues with tripping the AC breaker and the AC wires ( on the right side ) look fine, my issue is that my string inverter shows an isolation error immediately after reboot.
let’s suppose i would reboot it now, it will shows View attachment 199475prompt after startup an isolation fault despite it has been working fine before the reboot

Where is the ground wire? PV array needs to be bonded to the service panel grounding system.

Plenty of questions such as what string inverter you are using? I also saw the 620V and wondered what you are using for an inverter as most only have a max VOC rating of 600V. Grid tie or off grid system?

I think everyone here needs more information.

 

[skyman]

Where is the ground wire? PV array needs to be bonded to the service panel grounding system.

Plenty of questions such as what string inverter you are using? I also saw the 620V and wondered what you are using for an inverter as most only have a max VOC rating of 600V. Grid tie or off grid system?

I think everyone here needs more information.

The ground wire is the yellow one.
I have a 2012 kaco inverter (made in Germany ) it has two MPP tracker, according to the documentation the range is 350-800 volt.

I have two strings : string one is connected to MPP1 ( voc 310 volt ) and string 2 is connected to the MPP2 ( voc 620 volt )

 

[Zwy]

The ground wire is the yellow one.
I have a 2012 kaco inverter (made in Germany ) it has two MPP tracker, according to the documentation the range is 350-800 volt.

I have two strings : string one is connected to MPP1 ( voc 310 volt ) and string 2 is connected to the MPP2 ( voc 620 volt )

Array and panels used?

How does string 1 work if the range of MPPT is 350V minimum and you have a VOC of 310V?

Twice as many panels in series on string 2?

 

[skyman]

This is a little disconcerting because I don't know what your inverter or wiring is rated for, but most inverters I've seen have a maximum MPPT input voltage of 600V, so if yours is seeing 620V it may refuse to start.

The reason it might only show the error at mid-day reboots is that as the voltage rises in the morning it'll load the panels down and they won't exceed 600V at the maximum power point input, but if you reboot it mid-day it'll wait a few minutes before connecting the panels, and then the Voc of the string will exceed 600V, and the inverter will throw an error. Why it throws can isolation fault instead of an MPPT input over voltage fault is anyone's guess, but Occam's Razor says it's software bugs. Not that it couldn't be breaking down some over voltage suppressor in the MPPT input, but that's chasing the wrong problem.


Strictly speaking you are correct, your DVM can measure insulation resistance (at one or two volts). It cannot measure insulation resistance at high applied voltages, nor can it measure breakdown voltage, for which you need a different piece of test equipment, as the electrical experts above have mentioned.

Again: You are making me nervous measuring 620Vdc strings with a meter that _SAYS_ 600V on it, I'm concerned that you aren't taking the proper safety precautions, and that you don't understand how your measurements work or what they mean.

When you say: "let’s suppose i would reboot it now, it will shows" and then post a picture of the wiring, that doesn't give me any confidence. If you are not paying attention, if you are not sure what you are doing, if you don't know what your meter readings are or how they are interpreted, you could die.

That would be bad. Start at the top, and explain it like I'm five. What panels, what strings, what wiring, which inverter, what works and what doesn't, what test equipment do you have available, etc?

Ok,good point!
your explanation does make sense.
However the documentation contradicts what you saying as the MPP track range is 350-800 volt ! At least that’s what‘s written in the manual !!

 

[skyman]

Array and panels used?

How does string 1 work if the range of MPPT is 350V minimum and you have a VOC of 310V?

Twice as many panels in series on string 2?

Exactly, string one has 10 panels ( voc 31 volt per string ) while string 2 has 20 panels ( voc 31 volt per string )

The panels are the same!

My understanding is that the inverter could still work even below the minimal MPP track voltage

 

[Zwy]

How does string 1 work if the range of MPPT is 350V minimum and you have a VOC of 310V?

 

[skyman]

How does string 1 work if the range of MPPT is 350V minimum and you have a VOC of 310V?

Well according to the manufacturer if the MPP voltage is not reached, current is generated but not too much

 

[wpns]

Ok,good point!
your explanation does make sense.
However the documentation contradicts what you saying as the MPP track range is 350-800 volt ! At least that’s what‘s written in the manual !!

OK, that's one positive step. Now show the specs on the wire you used.

 

[Zwy]

Well according to the manufacturer if the MPP voltage is not reached, current is generated but not too much

I'm certain there is a reason you didn't use 15S on panels?

If you disconnect that string, does the fault still occur?

 

[Zwy]

OK, that's one positive step. Now show the specs on the wire you used.

Yes, THWN is rated at 600V .

 

[skyman]

I'm certain there is a reason you didn't use 15S on panels?

If you disconnect that string, does the fault still occur?

May i ask you what is 15s?

If I connect only one string ( no matter which one ) the inverter connects to grid and starts working without any issue

 

[skyman]

OK, that's one positive step. Now show the specs on the wire you used.

AC side or DC side ?
For the DC side I am using 4 mm2 DC cable up to 1000 volt

 

[Zwy]

The ground wire is the yellow one.
I have a 2012 kaco inverter (made in Germany ) it has two MPP tracker, according to the documentation the range is 350-800 volt.

What model?

I have two strings : string one is connected to MPP1 ( voc 310 volt ) and string 2 is connected to the MPP2 ( voc 620 volt )

Panel specs? Brand and size always help too.

 

[Zwy]

May i ask you what is 15s?

Did you install this yourself?

15S means you have 15 panels in series, this increases the string voltage.

If you went 15S, with identical panels on each string, the string VOC would be the same on both strings.

If I connect only one string ( no matter which one ) the inverter connects to grid and starts working without any issue

And it never faults like you describe?

 

[skyman]

Did you install this yourself?

15S means you have 15 panels in series, this increases the string voltage.

If you went 15S, with identical panels on each string, the string VOC would be the same on both strings.



And it never faults like you describe?

- No ,i didn’t install it

- String 1 ( 20 panels ) is on the house‘s roof
While string 2 (10 panels ) is over the garage ( so only 10 panels could fit there )

- yes if strings are connected separately, there is no issue and inverter starts working smoothly after reboot

 

[wpns]

Hmm, curiouser and curiouser. What's the exact error you get from the inverter again? Last time you posted a picture of the inverter wiring instead of the error message.

 

[Zwy]

Hmm, curiouser and curiouser. What's the exact error you get from the inverter again? Last time you posted a picture of the inverter wiring instead of the error message.

Without the model of inverter and possible link to the manual it is a wild guess.

I did read in another Kaco model manual the following.

Although there is sufficient insolation, the inverter terminates the power delivery process shortly after starting-up

Defective mains isolation relay in the inverter.

Although there is enough insolation, the inverter delivers power for a few seconds only and switches off again.During this short time of power delivery it shows a power delivery between 0 – 5 W. If insufficient generator power can be ruled out in this case, the mains isolation relay is presumably defective and prevents the inverter from being activated.

I also ran across this general test for isolation faults. https://www.libra.energy/en/faq/how-does-an-isolation-fault-occur-and-how-do-you-resolve-it/

A few things come to mind, one is the low VOC of the one string could cause this fault to occur. If there is any resistance in the circuit, the voltage drop when the MPPT pulls power could be enough to cause the isolation fault.

There could possibly be an isolation fault due to a ground fault on PV wires.

There could be a fault with the MPPT itself.

 

[skyman]

Hmm, curiouser and curiouser. What's the exact error you get from the inverter again? Last time you posted a picture of the inverter wiring instead of the error message.

The error is a typical isolation fault

 

[skyman]

Without the model of inverter and possible link to the manual it is a wild guess.

I did read in another Kaco model manual the following.

Although there is sufficient insolation, the inverter terminates the power delivery process shortly after starting-up

Defective mains isolation relay in the inverter.

Although there is enough insolation, the inverter delivers power for a few seconds only and switches off again.During this short time of power delivery it shows a power delivery between 0 – 5 W. If insufficient generator power can be ruled out in this case, the mains isolation relay is presumably defective and prevents the inverter from being activated.

I also ran across this general test for isolation faults. https://www.libra.energy/en/faq/how-does-an-isolation-fault-occur-and-how-do-you-resolve-it/

A few things come to mind, one is the low VOC of the one string could cause this fault to occur. If there is any resistance in the circuit, the voltage drop when the MPPT pulls power could be enough to cause the isolation fault.

There could possibly be an isolation fault due to a ground fault on PV wires.

There could be a fault with the MPPT itself.

@Zwy

Thank you very much for this.

A faulty isolation relay is a potential cause.

if there is a sustained earth fault on the DC side, why the inverter would pass the morning check and starts producing power ?