HV/LV transformer on Jetty/Berth (fed from shore) - Earthing 2

[DrDrreeeaaa]

Hey All

I am trying to understand the requirements for earthing of a 11kV/400V transformer which is installed on a jetty.

The transformer is probably 1km from the shore and is fed from an 11kV switchboard located onshore.

The transformer LV and HV earths are bonded together and are bonded to the jetty metalwork. There is a earth conductor (maybe, 1 core 50mm2) that runs from the HV earth onshore out to the transformer earth bar.

The jetty is constructed of steel and wood, I am not sure of the exact composition

The transformer is being replaced and the question has been asked: what kind of testing should the contractor do and is there a chance that the contractor will have to do extra work to augment/modify/improve the earthing

Any idea? Thanks.

 

[ScottyUK]

This arrangement is often utilised to segregate the shore earth and jetty local earth to allow cathodic protection to be applied to the jetty structure. Might this be the case with your jetty? If so then the transformer LV winding neutral is bonded to the jetty structure, and the transformer tank is isolated from the structure but connected to the HV earth. Obviously the transformer tank must be supported on some form of insulator. If a touch potential could be introduced during fault conditions then additional measures will need to be taken, for example keeping the transformer in a locked pen and out of reach.

 

[DrDrreeeaaa]

G'Day Scotty, thanks very much for your reply. I had not considered the cathodic protection. There is definitely Cathodic Protection operating here.

So just speaking aloud, in order to prevent the area of cathodic protection from including the metalwork onshore, the earthing system on the jetty needs to be separate from the earthing onshore - which includes the onshore HV earthing system.

So the compromise that is taken is to connect the transformer case to the 'onshore' HV earth, incase of an insulation failure for example that could lead to an HV fault - but the case of an HV fault to the local earth on the jetty isn't catered for (there wouldn't be an effective return path in this case)?

How would one determine 'if touch potential could be introduced during fault conditions'? I would expect there is always going to be some potential for this to exist - for example - if there was a HV fault onto the transformer frame, the return path would be through the earthing conductor back to shore - but there may always be some portion of return current through a person and through the jetty structure and seawater for example?

 

[ScottyUK]

The 11kV cables we use in the UK would have an earthed screen or shield around each phase conductor, and overall steel wire armour connected to earth, so the chances of an HV fault to the local earth occurring without having first faulted to the HV earth are minimal. At 11kV I'm pretty sure you'd find some earth fault return via the mass of earth even if it did occur.

I said 'touch potential' but I should really have said 'transfer potential' because any rise of earth potential at the feeder substation will appear on the equipment connected to the HV earth, e.g. the transformer tank. Whether this is a safety hazard or not depends on the magnitude of the earth fault current and how good the earth electrode is at the feeder substation.

 

[DrDrreeeaaa]

I see, thanks.

Is it absolutely essential for the jetty earth to be isolated from the shore earth?

 

[ScottyUK]

Your CP system may struggle to achieve adequate protection levels, leaving the structure susceptible to corrosion. It's probably worth discussing with your CP contractor. On land-based systems where I have a little more experience, remote earths like substation earth electrodes can drain a surprising amount of current out of a CP-protected area such as an oil tank and the T/R has to be cranked up to compensate for the shortfall. That will shorten the life of the anodes / groundbed and could lead to hydrogen formation if they're worked too hard.

If you can maintain separation between shore and jetty then I'd suggest that's a good thing, but you do need to be mindful of the transfer potential. If the distribution system is resistance earthed then there's a fair chance the rise of earth potential won't be high enough to worry about, but if you're fed from a solidly-earthed transmission / primary distribution system then you could have a significant ROEP / GPR.

 

[DrDrreeeaaa]

From looking at this thing, I think it’s unlikely that the shore earth and the jetty earth are isolated. There is a covered conveyor that runs out to the jetty which is full of equipotentially bonded structures and equipment. While it’s possible that there’s a point at which there’s a physical separation in the conveyor and structure I think it’s unlikely.

So when it comes to HV earthing for this particular case it’s all about the path back to the shore via the bonded structures and the earthing conductor.

If the transformer was being replaced would you expect the earth resistance provided by this system to be validated and tested?

Thanks.

 

[itsmoked]

I'm a bit surprised as in my understanding a jetty is usually dirt perhaps boxed in with concrete or riprap. I don't see how that's much different from regular dirt designs. Seems like Scotty is thinking this encompasses piers that are structures of support etc.

Maybe my perception is wrong.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[DrDrreeeaaa]

The 'jetty' I am talking about is about 1 km long - large bulk freighters berth at the jetty and there is a conveyor that runs from shore to the end of the jetty which is used to transport bulk commodities from the shore to the ship.

There's significant structure involved - steel trestles support the jetty which in turn supports the conveyor which is covered along it's entire length in what turns out to be a 1km long tunnel that goes out to sea.

There is also significant infrastructure on the jetty platform - switchboards, other equipment, etc.

The new transformer will sit on the jetty platform which is supported by the steel trestle structure and is about 800m out from the shore. There is an 11kV mains cable that runs parallel with the conveyor from the shore out to the transformer.

I suppose I should have been using the word 'pier' - that's more appropriate.

 

[itsmoked]

OK. Thanks for the clarification! I just went and looked at "jetties" and it does seem that about half of them meet my definition of a pier. So I guess my definition was too specific.

You seem to have quite the tussle ahead, good luck.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[ScottyUK]

Sounds like you might have already lost the electrical separation battle. As a quick and dirty test to see what further detrimental effect would occur if shore earth and local earth were bonded, you could connect a heavy cable from the 11kV cable armour or gland to the local steelwork. The voltages involved are low - much lower than land-based CP systems - so it needs to be a bolted metallic connection, not just laid against the steel.

The problem isn't so much the return path for the 11kV supply because that's relatively easy to arrange; it's the loss of CP current to onshore structures. It can cause accelerated corrosion outside the CP-protected zone where the current flow reinforces the corrosion cell rather than opposes it. The lower voltages found in marine CP applications is a factor in your favour in terms of current bleed to the remote earth as there's less drive voltage to push current around the loop. If there is a DC current flow in the cable armour then you should get some help from the CP contractor because DC current flows can cause accelerated corrosion at the remote (source) end.

 

[stevenal]

[URL unfurl="true"]https://www.dairyland.com/applications/decoupling-from-utility-grounding-systems[/url]
Makes more sense to me to isolate the DC rather than attempting to isolate two AC grounds. Less chance of an unfortunate error.

 

[ScottyUK]

It arguably doesn't matter if the conveyor effectively bridges the shore and jetty earth anyway.

I've never been 100% convinced on the Dairyland couplers / decouplers. Earth conductors need to be beyond doubt, particularly if it's a main bonding conductor, and those things introduce doubt in my mind - perhaps unfairly so. Has anyone got practical experience of them, and how do they behave under fault conditions? In the UK they're very much an oddity from over the pond, and I've never seen one 'in the wild'. I can read the website blurb as well as anyone, but real world operating experience is hard to come by over here.