ELV Solar array - isolation questions

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jonescg
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ELV Solar array - isolation questions

Post by jonescg »

Hi all,
We have a very cool project happening at my employer (training org) and we are building a ground-mounted off-grid solar array which will keep a 48 V battery charged and also allow for EVs to be charged via 48 VDC to 240 VAC inverter. In order to get it happening in the first place, the project will revolve around using only ELV solar arrays (three panels in series to give <120 V DC) such that students can safely work on them, even under supervision from licensed sparkies and EC's.
This will mean putting 4 or more arrays in parallel, and then isolating them before they enter the PCE. I've been looking into the isolation and circuit protection requirements as stipulated in AS/NZS5033:2014 and according to table 4.3, it looks like using withdrawable fuse isolators as array disconnects is only a recommendation, while a master isolating switch for the combined arrays is a requirement. Also, it would appear that all disconnecting devices need to break both the positive and negative conductors. Easy for a master isolator, but starts to get expensive for fuse isolators. Given isolation is only recommended for ELV solar arrays, would it not be OK to just isolate one conductor using withdrawable fuses in addition to the double-pole disconnecting switch?
Just seems like lots of points of fail waiting to happen.
Ta,
Chris
AEVA National President, retiring WA branch chair.
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acmotor
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Re: ELV Solar array - isolation questions

Post by acmotor »

The standards seem to be moving against us e.g. the new battery standard AS 5139 ( battery >12V and energy storage >1kWh) now abolishes the ELV <120VDC for 'competent persons' category and replaces it with the Decisive Voltage Classification (DVC) system.
Under this new system, Non-electricians are now restricted to DVC-A which includes DC voltages to 60V only. Supervision is no longer a work around. The wiring has to be done by a lic.sparky.
This means that 3 PVs in series which is >120VDC Voc is now outside the DVC-A classification. (it is the Voc not Vmp that is considered)
I don't entirely agree with this new standard.

All that aside,
I think your question is about paralleling PVs.
The fault condition that sets 'max fuse rating' on the PV label usually means that no more than 2 PVs or strings can be paralleled without using diodes, fuses, MCBs to limit current direction or fault current to the max fuse rating on PVs. Otherwise a panel can potentially self destruct.
Shottky diodes to reduce wasted power are suitable but offer questionable protection as they tend to fail s/c, particularly if overheated and one is required in each + and - PV lead. They dont provide the required isolation switch though.
I isolate each string of 1, 2 or 3 PVs in series from any number of parallel mates using 2 pole MCBs for safety and convenience of diagnostics or PV performance.
I use 10 or 15A 2 pole DC MCBs above 60VDC (usually rated >200VDC though may be polarised) or 2 pole AC MCBs below 60VDC per pole are well within their typical DC voltage rating.
Yes, this consumes many MCBs but seems the safe and easy diagnostics way to do it.
You should be able to source 2 pole MCBs for less than $10 ea. e.g. https://nixon-dce.com/collections/cb-c-curve
2 x single pole MCBs (at just a few$ ea.) can potentially be used however to isolate generally means to disconnect both leads, at the same time.
Breaking both poles at the same time means a 2 pole AC MCB that can break 60VDC per pole can break nearly 120VDC.
Even though isolation is only 'recommended' at lower voltages, diagnostics and panel testing is made a lot simpler with isolation.
Once you have this bank of current limiting isolating switches, the master isolating switch seems to be the waste.
.... just my two bits worth.
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Re: ELV Solar array - isolation questions

Post by paulvk »

I used NoArk mcb's they have ones rated for AC and DC with the DC voltage 400v and greater when wired in series.
As far as needing a sparkie to wire DC systems many I have met have not a clue what is needed they do not get trained in DC systems, so many have told me "you can not use mcb's in DC systems" yet all you need to do is read the specifications.
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Re: ELV Solar array - isolation questions

Post by wattmatters »

Use a PV combiner box. It's what I did. Makes life a lot easier if you need to parallel more than two strings. This sort of thing (they come in various sizes):
https://sbpe.com.au/collections/accesso ... it-breaker

Or build your own.

The thing with the DC isolators is about ease of access at key locations. Usually one located near the inverter and one located near the array.
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jonescg
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Re: ELV Solar array - isolation questions

Post by jonescg »

That's more or less what I had in mind, except instead of DC circuit breakers, I'm just using DIN-rail mounted withdrawable fuses. And then the array isolator would isolate the inverter.
AEVA National President, retiring WA branch chair.
T1 Terry
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Re: ELV Solar array - isolation questions

Post by T1 Terry »

acmotor wrote: Mon, 08 Nov 2021, 21:53 The standards seem to be moving against us e.g. the new battery standard AS 5139 ( battery >12V and energy storage >1kWh) now abolishes the ELV <120VDC for 'competent persons' category and replaces it with the Decisive Voltage Classification (DVC) system.
Under this new system, Non-electricians are now restricted to DVC-A which includes DC voltages to 60V only. Supervision is no longer a work around. The wiring has to be done by a lic.sparky.
This means that 3 PVs in series which is >120VDC Voc is now outside the DVC-A classification. (it is the Voc not Vmp that is considered)
I don't entirely agree with this new standard.

All that aside,
I think your question is about paralleling PVs.
The fault condition that sets 'max fuse rating' on the PV label usually means that no more than 2 PVs or strings can be paralleled without using diodes, fuses, MCBs to limit current direction or fault current to the max fuse rating on PVs. Otherwise a panel can potentially self destruct.
Shottky diodes to reduce wasted power are suitable but offer questionable protection as they tend to fail s/c, particularly if overheated and one is required in each + and - PV lead. They dont provide the required isolation switch though.
I isolate each string of 1, 2 or 3 PVs in series from any number of parallel mates using 2 pole MCBs for safety and convenience of diagnostics or PV performance.
I use 10 or 15A 2 pole DC MCBs above 60VDC (usually rated >200VDC though may be polarised) or 2 pole AC MCBs below 60VDC per pole are well within their typical DC voltage rating.
Yes, this consumes many MCBs but seems the safe and easy diagnostics way to do it.
You should be able to source 2 pole MCBs for less than $10 ea. e.g. https://nixon-dce.com/collections/cb-c-curve
2 x single pole MCBs (at just a few$ ea.) can potentially be used however to isolate generally means to disconnect both leads, at the same time.
Breaking both poles at the same time means a 2 pole AC MCB that can break 60VDC per pole can break nearly 120VDC.
Even though isolation is only 'recommended' at lower voltages, diagnostics and panel testing is made a lot simpler with isolation.
Once you have this bank of current limiting isolating switches, the master isolating switch seems to be the waste.
.... just my two bits worth.
The interesting part of this nonsense is, it can only be policed on back to grid systems. Anything stand alone is outside these crazy rules.
The knowledge the licenced sparkie has when it comes to DC current is clearly evident in all the roof top fires at the isolation switch. Anyone who actually knows what they are doing knows the contact size and the arc quenching required for DC switching makes the isolation switches DC specific, yet it appears all the ones that went up in smoke were AC rated .......

T1 Terry
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wattmatters
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Re: ELV Solar array - isolation questions

Post by wattmatters »

jonescg wrote: Sat, 13 Nov 2021, 17:04 That's more or less what I had in mind, except instead of DC circuit breakers, I'm just using DIN-rail mounted withdrawable fuses. And then the array isolator would isolate the inverter.
My combiner box uses those fuses.

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T1 Terry
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Re: ELV Solar array - isolation questions

Post by T1 Terry »

The down side of using fuses to cut the circuit, DC current at higher voltages will pull quite an arc and not only scare the cr*p out of the apprentice, but also destroy the contacts. I remember seeing a big DC motor go into run away and when the sparkie pulled the knife switch it arced both the contacts and the blades of the knife switch and burnt the sparkie with the flash and the molten bits that spewed out.

After having a 300 amp Mega fuse rupture while I was installing it, I make sure the contactor is actually open before attempting anything like that now days. The contactor was not energised, but the contacts had welded closed ...

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Re: ELV Solar array - isolation questions

Post by wattmatters »

T1 Terry wrote: Fri, 19 Nov 2021, 11:13 The down side of using fuses to cut the circuit, DC current at higher voltages will pull quite an arc and not only scare the cr*p out of the apprentice, but also destroy the contacts. I remember seeing a big DC motor go into run away and when the sparkie pulled the knife switch it arced both the contacts and the blades of the knife switch and burnt the sparkie with the flash and the molten bits that spewed out.
It's why an inverter/charge controller/load should first be shut down before any DC disconnect. That way you are not breaking the circuit while under load. Or do it at night.

That cheapie combiner box has a double pole breaker in the middle plus the individual fuses, and I have a separate DC isolator box near the inverter. I would choose NONE of them to disconnect while under load unless I really really had to.
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Re: ELV Solar array - isolation questions

Post by T1 Terry »

In the case of off grid systems, the isolation under load is because a fault situation has caused the system disconnect. 120vdc @ 7 amps pulls quite an arc, 240v @ 7 amps pulls a very long arc that requires the magnet at the contacts to pull the arc away to allow the required gap to be established where the arc can no longer initiate because the ionised air is no longer present to sustain the arc ...... probably said that all wrong but that is the way I understand it and why very expensive circuit breakers need to be used for high voltage DC circuit cut and isolation. If there is a short circuit in the solar array, you can't wait for the sun to go down, it needs to be isolated from the rest of the solar array and the battery immediately.
I still maintain that a length of fuseable link wire or a HRC fuse provides a far better protection than a circuit breaker in the case of a short circuit up stream. An isolation switch should be close to the power box so a diversionary load can be applied to the solar input side of the isolator before it is tripped and before it is reconnected to prevent the arc occurring at all, but this would require something more than just a simple rotary isolation switch and more a circuit that determines there is no load across the contacts before they are opened. Sort of a pre-charge circuit in reverse ....

T1 Terry
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