I have inherited an ongoing project which is a multi-engine generation station. The generators output at 11kV which is stepped up to 33kV for export through the DNO connection.
Unfortunately the site is classified as hot, but the 33kV and 11kV switchrooms are part of the same structure. This means that when the DNO designed their earthing system, they have included the 11kV in the sphere of influence, so the 11kV switchgear will be housed in a "hot" area - not a good move.
Our 11kV and LV systems are sufficiently restricted to not cause a problem, but I still have an issue with the switchroom itself.
The DNO is fairly unhelpful and just states that it is the customer's responsibility for the earthing system, and whilst that is true, I still need to find a solution.
I believe the best option is to try to get the site reclassified as cold, by increasing the earthing. What sort of information am I going to need to be able to get the earthing design modified?
It sounds like the DNO's earthing specialist is a bit miffed that he was not approached to do the rest of the earthing design, so I can't guarantee co-operation from that source.

Unfortunately this is another area in which I'm not a specialist, so some unbiased advice would be very welcome.

Thanks

Pete

How hot?

Regards,

Alan.

Looking at some previous posts on this subject
http://www.theiet.org/forums/f...R_FORUMVIEWTMP=Linear

I think the general consensus was to bring in a specialist to conduct some measurements/ a survey of the substation to establish just what the status of the general earthing network is.

I don't have personal experience of substation earthing surveys, but ERA http://www.era.co.uk/ or http://www.ees-group.co.uk/
www.cablejoints.co.uk/upload/Earthing___Substation_Earthing_Guide___ERA.pdf

May be able to help.... also, is there anything wrong with having a 'hot' substation, provided it's not too hot?

Reading the various guides/ IEEE 80 etc.. is it more about insulation breakdown (telecoms cables) and general safety when in the substation with regards to footwear or other PPE when working on or adjacent to equipment that could have an earth fault?

The earthing design from the DNO showed at 33kV worst case the EPR was 2275V, with touch potential 128V and step 405V. They indicate that the 1 sec clearance was <430V and the 0.2 sec clearance <650V.
The DNO had a specialist to carry out the survey.

Reading the thread suggested by MickeyB, the remedies seem to be either to segregate the 33kV and 11kV systems by at least 3m - not feasible in this case - or to include a substantial earth on the customer side.

We have an HV specialist on board, but since another specialist carried out the DNO design, I like to make sure I'm asking the right questions.

Regards

Pete

Your best option is probably to ask what recommendations the author of the original report has to reduce the EPR. If you go to another company, it is highly likely that they will have to repeat all the measurements and calculations to come up with the conclusions. You also need to be careful whether there any 3rd party pipes / cables or properties within either the 430v contour or 650v contour (depending on the grade of protection you are using). This can lead to having to carry out additional works to move this voltage contour closer towards (or preferrably within) your own boundary fence.

Regards,

Alan.

Thanks. I've got a request for the recommendations in hand with the DNO.
Do you know if the classification is based purely on the local earthing arrangements, or is it a historical one that takes a number of months to change, if it can be changed at all?
A previous site earthing arrangement resulted in an EPR of 1012V.
Since the DNO have advised a maximum earth fault of 1750A, I calculate that to achieve below the hot threshold of 430V, we would need 0.24 Ohms, which I think would be achievable, given that the previous site earth that was abandoned, reached 0.58.

Regards

Pete

From memory, the difference between whether it is 450v or 650v depends on the speed of operation of the protection. If you can upgrade the line to use a fast differential protection, possibly with distance as a backup (or persuade the DNO to do so by waving sufficient pound notes) you would be able to use the 650v limit instead. The use of earthing resistors on the star points of the transformers can also help, as it is usually a phase to earth fault that is going to produce the worst case ROEP.

As long as the "hot" contour doesn't extend beyond your boundary, there is nothing actually wrong with having a hot site, as long as it is designed accordingly.

Regards,

Alan.

I believe the 650V limit is for 0.2s operation. The site is a former colliery, so, assuming the DNO's project engineer actually understands the questions, I can check the speed of disconnect, but I'm having to be very specific about the questions I ask - information is not provided freely!

The transformer on site is 33kV to 11kV and we do have a neutral earth resistor, so the earth fault current is limited on that side. We don't know whether the 33kV comes on overground or underground lines - just that it's buried when coming onto site.

Regards

Pete

A 33/11kV transformer is usually connected star-star as far as the windings are concerned. It would be worth checking if the star point on the 33kV side is solidly earthed, and whether it could be earthed through a suitable resistor to limit the earth fault current on the HV side.

Regards,

Alan.

According to the single line diagram, the 33/11kV transformer is delta/star, and the dimensional drawing only shows 3 terminations on the primary, but all four on the secondary. The client had already got this transformer and it was refurbished for this project, so it will be worth me checking, but I suspect not star-star.
The armouring of the 33kV cables to the transformer primary will be isolated from the DNO earth end.

Regards

Pete

I have checked the transformer on site and it is definitely delta-star, so a star point resistor is not an option on the primary and the disconnect at the DNO end is 1 sec, which means that we are dealing with the 430V earth rise potential.
I've asked the DNO if reclassification is possible, but he is non-committal, and their original designer will not make a comment without us placing an order with him!
The DNO's (external) designer would be appointed by the DNO to verify any design we came up with, so I am reluctant to appoint the guy myself, as I feel it would be a conflict of interests for him to both design and verify through two different clients.
I'm trying to go ahead and get the earthing design improved so at least we can be safe.

Regards

Pete

You could try Trevor Charlton at Earthing Solutions for advice and a design. I have no personal connection with Earthing Solutions.

Regards,

Alan.

Thanks for the contact details.

Regards

Pete

I have another concern - the 33kV switchroom is adjacent to the boundary with another premises and is 1.1 to 1.5m away from public access. The plot of the EPR across the zone shows that this falls within the 22-33% contours. Who is responsible for mitigating this? Us or the DNO?

Regards

Pete

I've now had advice from the DNO that they will not energise our site until the earthing arrangements are acceptable and we have carried out any mitigation necessary. They will only address the issue around the boundary once their substation is energised, if that is still a problem.

What I find unhelpful is that it is their design which has introduced the problem, as we already have 0.81 Ohms on our side, and we are still expected to take whatever steps necessary to overcome their own high impedance. If I had been on board and knew what I now know, I would have separated the 33kV and 11kV switchrooms so the problem would remain firmly with the DNO.

Is the DNO entitled to wash their hands of their problem in this way?

The best earthing design they will probably get from me is the actual measured value of the earth nest at the site earth terminal. Reality is better than a set of calculations

Regards

Pete

Hi Pete,
We carry out earthing calculations and fault simulations for the electricity industry throughout the UK.
If I can be of help, perhaps you would like to email me to discuss further ?
Regards.

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Matthew Taylor B.Eng C.Eng MIET PhD