Solar earthing re: isolation transformer

[Dbreeze]

Hi Guys

Do you know whether I could fit an external isolation transformer up to a transfomer less inverter, to save me earthing the array. Is this ok and possible. Just don't want to earth array as customer doesnt want the driveway to come up etc. Or is the only way to install a TL Inverter.

Cheers
Dan

 

[Maff]

why do you have to dig up the drive to earth the array?. If you are on TN-S earth to the MET, or if TNC-S an earth rod and 15 metres of 10mm green must be cheaper than sourcing and fitting a Tranny, or just use an inverter with a transformer in.

 

[Dbreeze]

It is a TNCS therefore having to install earth rod, just need to know as they already have TL inverter whether a isolation transformer can be used between inverter and AC isolator (to be advised)

 

[ExtremePV]

What size system is it? Can you not use a Fronius or SMA inverter one of the HF range?

 

[platty1005]

look at page 202 in Regs it shows a separate isolation transformer. So easy answer to your question is yes fit an external isolation transformer if this is the most cost effective solution. The tranny is to isolate the array from the earthed public distribution system Read section 413 page 56 in the Regs. The Inverter should be considered one item of current using equipment. 413.1.1

 

[pvguru]

Ive read through several posts and I am still not sure why you need to earth / bond the array, can anyone tell me why I should be doing this please?:dozey:

 

[edexlab]

I'm fairly certain that fitting a separate isolation transformer is not an option
this is how it was explained to me by one of SMA's engineers
one aspect of the isolation transformer is that it provides separation within the inverter in order to prevent the AC transfer on to the DC cables (electrical separation) , a transformerless inverter obviously cannot do this hence the need to earth the array ,as the above mentioned AC is carried up to the array frame via the DC cables. This is called capacitive discharge and can be over 30mA this is why some inverters require the use of an B type 100mA Rcd, this charge,(I've measured this and seen 120v) is sitting there waiting for somewhere to go, all it needs is a path to earth ,ie the customer cleans the array or is doing work on the roof off an aluminium ladder makes contact with the array and receives a shock ,which may not be enough to kill but could cause them to fall
fitting a transformer upstream of the inverter will not prevent this from happening.

 

[Worcester]

@edexlab,

The reason for the Type B RCD is DC injection into the AC, hence shifting the voltage of the AC, it's not there to protect the DC side, it's to protect the AC side from being fed with DC. In a TL inverter the built in RCD doesn't provide RCD protection for the cable, if that is needed (buried etc) you still need to provide an RCD.

Interesting comment about the AC feed into the DC, however and AC current won't create a capacitve effect - because it is an 'Alternating Current', it'll charge / discharge at 50hz, it's DC that builds the capacitve effect.

However without galvanic isolation, the DC creates a potential to earth.

Galvanically isolate it and there still could be a potential difference when measured to one of the DC Cables, howver because the units are Class II, there is no earth route.

(A bit like on a PME system, even when the systems live, ther's no PD between Neutral and earth)

 

[SolarCity]

Isn't installing a seperate isolation transformer defeating the object? Surely you'll be adding another device which will lower the efficiency of the system.

Just fit a transformered inverter and you're away.

 

[judgeelectrical]

Just to get htis straight. Are you saying that any transformerless invertor must have the mounting sytem bonded by use of an earthing rod, and it should have a type b rcd fitted to the circuit?

Also slightly off tangent, dou you conceal/protest your dc cabling on large installation? 9Runs over 50 Meters) . I am quoting for a large 5oKw system, and this will be an issue for the quote.

 

[SolarCity]

9 runs over 50m? Just make sure you do your volt drop calculations.

As it is, arrays with a transformerless inverter must have the array bonded to earth - on an earth rod if the the supply is a TN-C-S set up.

 

[pvguru]

Hi BiggsSolar

I know that is what is said in the DTI Guide, but i am wondering why this is required?

Thanks

 

[Worcester]

See my post above and my post on the Segen tech conference in Brum, why has been well covered elswehere.

50m runs shouldn't be a problem with correctly sized cable, I regularly have some which have just under 200m round trip (roof 64 long, then 30m to inverter = 94 then back again = 188m +14 panels (12m) = 200m). When you run big strings (close to 1000V, 6 Amps) and size the cable properly the voltage drops can be nominal.

 

[pvguru]

Hi Worcester

I just don't get the requirements, even the guide gets the definitions wrong, should I be earthing or bonding the array frame

Also what are the risks particular to PV arrays that needs them to be earthed or bonded?

sorry to be a pain

 

[edexlab]

Thanks Worcester , I've definitely got a bit mixed up with my facts here ,case of shoot first check later after a long week
thats one of the best things about this forum there is always someone who can steer things the right way!

I checked my notes and the SMA guy as you would expect did explain things correctly ,

however he did say that in TL models some AC is transferred to the Dc side contributing to capacitive discharge,
Also when I've tested the voltage between the array and the electrode it's been an AC voltage present
I reread the SMA pdf concerned below, ​

"2 How does a capacitive discharge current occur? During operation,the Pv module is connected to the alternating current grid via the inverter.Thus depending on the device type, a portion of the alternating voltage amplitude arrives at the Pv module.
At this point two cases must be distinguished
1.Transformerless inverter In almost all transformerless models ,half the grid amplitude is operationally passed on to the module. the set up oscillates with 115v/50Hz this applies to SunnyBoy/Sunny Mini Central devices in the product names

2.Inverter with transformer
In Devices with a transformer,the voltage within the Pv module fluctuates with a so called ripple of just a few volts"
as i read this, I'm thinking its the AC causing this effect​

​