Transformer vs transformerless inverters

[brian wright22]

Certainly with my installation they have earthed the inverter using a separate earth spike, and the Fronius IG TL appears to have the DC RCD built in. The manual on the IG TL states:

The design and function of the inverter provide a maximum level of safety
during both installation and operation.
The inverter provides operator and equipment protection through:
a) RCMU
b) monitoring the grid

RCMU = Residual Current Monitoring Unit
The inverter is equipped with an RCMU according to DIN VDE 0126-1-1.
It monitors residual currents from the solar module up to the inverter grid
connection and disconnects the inverter from the grid when an improper
residual current is detected.
Additional residual current protection may be needed depending on the
protection system of the installation or the requirements of the utility company.
In this case, use a residual current circuit breaker with a release
current of at least 100 mA.

Thanks BrummyGit for this info regarding the Fronius IG TL inverter.

The problem is that when the inverter senses any residual currents in the PV array what happens if either + ve or -VE wire feed shorts to the frame? The Fronius will disconnact the AC (Grid) side of the system from the DC side but does it totally isolate the two DC feeds or just the AC side.
This is what I have requested Eaga technical to check because if there is still a path (however high impedence) between the House earth (Inverter Frame/ Case) then the potential for an electric shock during daylight hours should a short occur between the PV DC feeds and frame are real.
As I see it there is know way that the Inverter can detect if a short occurs between one or other of the PV DC feeds to the benign PV mounting Frames.
If the mounting frames are earthed then should a PV DC short occur the Inverter could detect this and apply the necessary safety shutdown. This would require the DC feeds from the PV array being electrically isolated from the Inverter circuitry and hence no earth return.
Not ever being able to get hold of the Fronius circuit diagram so far, I am not convinced that the system is safe without a PV frame earth.

I have to say though that the TSO from Eaga has said that if I insisted on my system frame being earthed then they will fit a separate outside earth.

It all hinges on the tests in hand of Eaga as explained in my earlier thread.

 

[SolarCity]

The problem is that when the inverter senses any residual currents in the PV array what happens if either + ve or -VE wire feed shorts to the frame? The Fronius will disconnact the AC (Grid) side of the system from the DC side but does it totally isolate the two DC feeds or just the AC side.

This isn't technically possible. As long as the sun is out and the panels are functioning correctly, a DC to array fault cannot be rectified with RCD, inverter or anything else.

I'm also struggling to get my head around the DC to Main Earth voltage. As far as I can work out, if the array frame becomes 'live' due to a short from the array then there should still be no potential between the array and the main earth. I acknowledge that your test results suggest otherwise but I'm struggling to work it out.

 

[SolarCity]

I have to say though that the TSO from Eaga has said that if I insisted on my system frame being earthed then they will fit a separate outside earth.

I'm not sure if he really has any choice.

 

[brian wright22]

This isn't technically possible. As long as the sun is out and the panels are functioning correctly, a DC to array fault cannot be rectified with RCD, inverter or anything else.

I'm also struggling to get my head around the DC to Main Earth voltage. As far as I can work out, if the array frame becomes 'live' due to a short from the array then there should still be no potential between the array and the main earth. I acknowledge that your test results suggest otherwise but I'm struggling to work it out.

Without seing the Inverter circuit diagram BiggsSolar its impossible to guess what the Inverter input switching circuit is doing and the way in which it is isolated from the earth (Being Transformerless). The only way as I see itm is that I was able to measure any voltage between either of the + ve & -ve DC and earth is it could mean that there is capacitive impedance internally. I did not have the courage to see how much current could be sunk for fear of damaging the Inverter!. It has to be floating but the Fronius specification states that neither one the PV DC conductors should be earthed.
This means it probably would have a detrimantal effect to the internal electronics of the Inverter.

 

[brian wright22]

I'm not surprised, if I'm honest, that some contractors aren't bonding the array. It's out of sight and I suppose a lot of contractors see this as out of mind as well.

Regarding the figures that you have been getting, this does surprise me. I would have thought that you should be getting virtually zero voltage between the DC leads and the house earth. Or am I missing something?

Out of interest, I suppose they didn't bother with the Class B RCD either?

Hi BiggsSolar, Here is a schematic that may explain the reason I am measuring the dc valtages on my PV system with the Fronius IG 3.6TL Inverter. I don't know why the picture is small though?
AaaaaaaH ,If you double click on the image it is displayed normal readable size.

I hope this helps.

Brian

 

[BruceB]

The rule about Class B rcds comes from 712.411.3.2.1.2. They are unusual and expensive. But note the exception to the rule, "Where the pv converter is, by construction, not able to feed d.c. fault currents into the electrical installation...." rcd type B not required.

The SMA transformerless inverters appear to meet this condition so do not need an additional type B rcd. See
http://download.sma.de/smaprosa/dateien/7418/RCD-UEN103120.pdf
page 8


Regards
Bruce

 

[dansk]

Ive a question about "brummygit" install - why take an earth spike to the transformer? surley this would have been better if it was taken to the array? and the transformer would be earthed from the CPC on the A/C radial.

 

[zeljuga]

Upsolar seem to require earthing panels irrelevant off transformer type.

Safety class in accordance with IEC 61140

Only authorized and trained personnel should have access to PV modules. The system involves electricity, and can be dangerous if the personnel are not
familiar with the appropriate safety procedures.
A. GROUNDING
 All module frames must be properly grounded.
 Observe all local electric codes and regulations.
 A bonding or toothed washer is required to make proper and reliable
electrical grounding connection with the anodized aluminum frame.
 Devices listed and identified for grounding metallic frames of PV
modules are permitted to ground the exposed metallic frames of the
module to grounded mounting structure.
 Grounding the module properly as appropriate for the conditions t the
installation site.
a. Method #1
 Secure the stainless steel screw to the grounding hole.
 As shown in the figure 3, copper wire should be compressed by the
screw head.
 Stainless steel washer with appropriate corrosion resistant coating
should be inserted between copper wire and the frame to avoid
galvanic corrosion.
 Proper cupped washer should be inserted between copper wire and
screw head

 

[brian wright22]

I've just found this forum browsing for some information (look out for post asking for help shortly). My situation is very similar to Brian.

I also have a HomeSun & Eaga installed system using a Fronius IG TL 3.6 Inverter. During my installation a separate earth spike was installed and wired to the inverter, but the installation team explained to me that the panel frames needn't be earthed as they couldn't be reached from the only Velux in my roof. They told me that they had checked this with their supervisor.

From what I read in the Fronius manual I am not alarmed about the RCD, I'm interested in the outcome regarding earthing of the frames.

I am still pressing Eaga & Homesun regarding the earthing of my PV mounting Frame.As well as other issues.

Do you know why the Eaga installation team installed a separate earthing rod to your system ?. My system earthing of the Fronius IG 3.6TL is taken back to the main switchboard which seems the most expediant method to me.The only reason to install a separate earth rod and cable would be to earth the PV mounting Frame which your installers have not done either nor have mine.This decision would be taken regarding your local Elecricity suppliers requirements I understand?.
The main problem as I see it is that Homesun do not have the technical resorces in house and rely totally on EAGA (their contractor) to sort out most customer technical queries. I am not sure if EAGA even have a working PV system that they can carry out tests to substantiate customer technical complaints.
Homesun and Eaga are very economical in replying to any of my requests for information and the actions I have requested.

 

[brian wright22]

This is an update to my request to have my Homesun,Eaga installed PV system frame array earthed. I am insisting on this requirement because all of the relevant BS & DTI documents suggest this when a Transformerless Inverter is used.
I have had a verbal statement from Fronius that for their IG 3.6TL inverter they recommend that the PV mounting frame is earthed.To this end, I am still pressing Homesun for them to carry out this safety measure and earth the PV mounting frame.
I have communicated with all of the relevant UK committees that have been involved with this equipment and over the past two months I have got very little help because these bodies of experts only recommend what should be carried out together with common sense,then someone writes a specification around these recommendations.Unfortunately because of the ever changing technical advancement in the electronic industry and especially in this relatively new PV generation in the UK,these specifications are very quickly out of date.
I do know that the BS7671 specification is in the process of being updated and a meeting was held last month but it is not expected to be in its latest form for adoption this year!.
I have enclosed this statement from the IET which puts the responsibily on the manucturers of the specific equipment that has been installed for a particular installation and because Fronius has stated to me( but I can't seem to get in in writing from them) that their IG3.6TL inverter I am still pressing Eaga/Homesun for the PV frame to be earthed.

Hi Brian

I recall speaking to you over the phone about this and informed you that you would need to consult the manufacturer for individual design characteristics . The IET can not pass comment on a manufacturers individual design especially with regards to the emerging technology that PV exists in. The regulations are written with what ever technology is available at the time. If the technology advances it is not the fault of the wiring regulations and they cannot be updated on an almost monthly basis as new systems are designed.

If the contractor or manufacturer will not earth the array for what ever reason , the reason must be given in order for any comments to be made on it. You have stated that they (the manufacturer and the contractor) will not earth the array, they must therefore supply a reason for this. Until they supply a reason for not earthing it we cannot help you. As for individual system designs, I have already mentioned that the regulations ask/expect that the installers comply with the manufacturers installation instructions , therefore if they stipulate no earth , they are accepting any liability.



I hope these comments are usefull to you

Many Thanks
Richard

Richard Townsend
Senior Engineer
Standards and Compliance
The IET



This response came from my local electricity company.

Mr Wright,

Firstly let me apologise for taking so long to return your enquiry however I hope the information provided aides in your quest for an overall solution.

SSE Power Distribution own and operate the Distribution electricity system providing you with your power supply. As part of this installation we are required to provide an earth electrode whether this be through a Protective Multiple Earthing system or Separate Neutral Earth system. This is located at the main supply point form which you will currently have a connection to your main consumer unit.

The system you have installed should only be connected as instructed by the manufacturer as each system differs depending on manufacture. For this SSE cannot offer advice as the manufacturer is required to supply this information ensuring they comply with the Electricity Wiring Regulations.

The aid I can supply is that the metal box in which the inverter is contained must be earthed and is likely to be completed through the twin and earth cable connected to the unit. The metal work on which the Solar Panels are mounted could be earthed via the incoming supply but depending on how the installation is complete this may not be required.

In short you will need to liaise with the manufacturer to gain full knowledge on how the system should be connected. I would expect your installer to verify this before making a connection. It is vital no connection is made to the main incoming supply point without reference to the manufacturer as any such connection may contravene the afore mentioned regulations.

Regards

Antony
Antony Urquhart
Performance Manager
Slough Depot
PO Box 123
Slough
Berks SL1 2NQ

My Direct Dial Tel: 01753-695701
My Mobile Tel: 07767 850156
My Internal Tel Ext: 23701
Slough Depot Tel: 01753-695700
Slough Depot Fax: 01753-695751
E-mail:[email protected]
SSE Power Systems Web Site: www.ssepd.co.uk

From: [email protected] To: [email protected] Date: 24/03/2011 23:02 Subject: Message posted from SSE.com



Any further info I will report

 

[Graeme Harrold]

Not got my G83 handy, but Im sure thats a requirement of a certified transformer that it cannot inject DC into the AC export. Same goes for array earthing, manufacturers instructions are to be followed and most of the SMA range of TL inverters dont need array earthing.

 

[SolarCity]

Are you sure that the SMA range of TL inverters require no array bonding? Surely it will require a bond because there is no galvanic seperation and therefore a dangerous potential can exist on the array frame. If SMA have worked around this problem then I'd love to know how.

 

[brian wright22]

Not got my G83 handy, but Im sure thats a requirement of a certified transformer that it cannot inject DC into the AC export. Same goes for array earthing, manufacturers instructions are to be followed and most of the SMA range of TL inverters dont need array earthing.

The Fronius IG 3.6TL Inverter has several hundred volts measureable between real earth and the +ve & -ve feeds from the pv array when it is generating several kw.Should either one of these conductors short to the pv array mounting frame during its intended 25 year life span, then therefore there is a risk that anyone coming into contact with the pv mounting frame whilst on a ladder or in my case the TV aerial mounting assembly,an electric shock could be a health hazard.
If the pv mounting frame is earthed then the system being earthed will at least prevent any risk of electric shock.

 

[brian wright22]

Are you sure that the SMA range of TL inverters require no array bonding? Surely it will require a bond because there is no galvanic seperation and therefore a dangerous potential can exist on the array frame. If SMA have worked around this problem then I'd love to know how.

I said I would give an update about Homesun/Eaga and their reluctance to make a decision to earth my PV array mounting frame.

I have had this statement from Fronius backing up their previous verbal statement on the need to earth the PV array mounting frame when using Fronius Transformerless Inverters (including mine IG 3.6 TL)
The Fronius IG 3.6 TL is Class I


Please find fronius statement,
>
> Dear Mr. Wright,
> First of all I'd like to thank your trust in Fronius and in its products.
> according to the DTI Earthing and Lightning Protection only if the modules
> are Class II and the cables, connectors/ions & Junction Boxes are Class II
> and the Inverter has an isolated transformer you are permitted to leave
> the array frame floating.
> Otherwise the array frame has to have a connection to the earth.
>
> Should you require any further information or assistance do not hesitate
> to contact us again at [email protected] or 01908 512 316.
> Come and see us at Ecobuild between 1st-3rd March 2011 on stand N1758.
> Best Regards
> _______________________________________________________________________________________________
> Techsupport
> Fronius UK Ltd
> Solar Electronics Division
> Technical Support
>
>
> Maidstone Road
> Kingston
> Milton Keynes MK10 0BD
> United Kingdom
> mailto[email protected]://www.fronius.com


I am still awaiting Eaga/Homesun response after gathering all of this information and contacting the relevant bodies responsible for the safety of these microgeneration Solar systems. They will not commit an answer to this safety requirement.

 

[brian wright22]

I'm not sure if he really has any choice.

This is the reply "at last" from Homesun/Eaga regarding the earthing of my PV array mounting frames when the installation includes the FRONIUS IG 3.6 TL Inverter.
This response from Homesun/Eaga is at odds with the statement from Fronius technical who have stated that the PV array mounting frame must be earthed when using there Class I Transformerless Inverter.. I have already included their reply previously in this thread & so won't repeat it here.

Any comments from you guys please,is it worth taking the issue up with the Health & Safety.




Dear Mr Wright,

I am sorry that I have not come back to you earlier. However I have been speaking with Ben and other technical engineers within Eaga about the earthing of your mounting frame.

Our first concern with any installation is the safety and security of the homeowners, as such I’m sure you can appreciate that we do not make these decisions lightly. After extensive conversations with Eaga’s Technical Engineers for quite some time now, It is their considered and professional view that your array mounting frame does not require earthing and the installation of the Solar PV system meets all relevant regulations.

Once again I’m sorry that it has taken us a long time to get to this stage.

Kind Regards,

Krish Patel
HomeSun Customer Team
0844 824 0000