Interconnection of MV Earth with LV Earth in Public Supply

[Cookie]

What factors determine the interconnection of the MV neutral/earth with the LV neutral at the local substation? When are they connected together and when would it be appropriate not to connect them together?

 

[Soi disant]

At EHV you don't have to worry about touching a live conductor. You would be dead well before you touched anything.

Unless you're not on the ground...

 

[Moley]

I wouldn't do that job for any money! I wouldn't climb the pylons either. I'm happy when I'm just climbing about all over the transformers ?

 

[Julie.]

Interesting stuff,

I thought HV power distribution was delta, so no neutral (star point)

I have no basis for this, other than my own conclusion from looking at 3 wire overhead lines, going to what i presumed were delta to star transformers to step voltage down to 230v (415v) and provide a neutral / earth for consumers??

Transmission and distribution is 3 wire, however the network is still earthed, the star earthing point is generated either by the "upstream" transformer or more commonly a seperate earthing transformer - it has a special winding called zigzag, and no secondary.

The connection to earth is often through a resistance, or reactance to limit current, it also reduces the possibility of circulating current through the earth if the network ends up earthed at multiple locations (generally avoided).

 

[Julie.]

I wouldn't do that job for any money! I wouldn't climb the pylons either. I'm happy when I'm just climbing about all over the transformers ?

Nope, the same, I have had little to do with transmission stuff in a practical sense.

Too high, too many volts, too scary...

Only been involved in the protection side of it, so far far away...


Edit:

I remember one of my university friends was at CEGB and did a lot of work up transmission towers - didn't seem bothered about it at all, perhaps it was bravado as when he told us we were all absolutely terrified just hearing about the work!

 

[R-fur]

Does the grounding at the HV-MV substation play any role? I've heard that MV systems in the UK are resistance or peterson coil earthed. Is this true?

LV networks are always solidly earthed and may be TT TNS or TNC-S.
The main distribution voltage is 11kV although other voltages are used. The 11kV network can be solidly earthed or resistance earthed through liquid or dry resistors. Peterson or arc suppression coils are used occasionally in some areas where the network is mainly overhead.
33kV and 132kV can be either resistance earthed or use an earthing transformer. 275kV and 400kV usually have earthing transformers.
There is no neutral conductor in HV but the starpoint of a star winding is usually earthed either solidly or through a resistance as above.
Earthing transformers have a zigzag primary which creates a starpoint for earthing through a resistor. The secondary of the earthing transformer is 400/230V and is used to supply power for the substation ancillary's.
This is a very general view of earthing arrangements, various voltages and earthing arrangements are used throughout the UK.

 

[Moley]

Only been involved in the protection side of it, so far far away...

Protection as in relays or on TX protection like Bucholz, PRV's WTI/OTIs etc?

 

[Julie.]

Protection as in relays or on TX protection like Bucholz, PRV's WTI/OTIs etc?

Relay, at one (two) time I worked for a couple of protection equipment manufacturers and their associated consultants.

Quite a lot in motor, and generator protection for one (although they also did most everything else) - very involved in the design of new relays at one point although I was mainly in applications and managed on-site guys for the testing/proving, so a great balance of techy, on-site and office stuff; and more general stuff at the other company - from unit, through over current to distance etc, this was almost all office design stuff just the occasional site visit so boring as...

Quite a varied history really; I started working for a MV/HV switchgear manufacturer who was really good at getting trainees into the utilities and LV contractors for extended periods, I worked throughout the departments, primarily in design but was responsable for all the testing of switchgear, including working at the switchgear testing station trafford park; then stafford at HV. (I hated this so left!!) So I did a spell at installation design/contracting at both LV and MV, then moved into protection equipment at a couple of places (as above), then a company building power stations - some CHP (UK USA and throughout the world) but some large stuff - mainly in India - I was responsible for electrical designs and protection systems. I worked for a few consultancies doing power system and protection analysis etc. and gradually moved into LV design/consulting as there was more work in this area.

 

[Moley]

On the relays eh? So you're on the high paid jobs and the nice warm/ air con switchrooms tapping keys on a laptop while us real workers are crawling all over the transformers getting soaked in transformer oil with the rain running down the necks of our overalls ? Okay, we do spend time in the switchrooms too, doing the maintenance/timing tests etc. on the breakers. We normally use our own guys for protection or if we are fully booked then we call in guys from Oberon Engineering.

 

[Julie.]

On the relays eh? So you're on the high paid jobs and the nice warm/ air con switchrooms tapping keys on a laptop while us real workers are crawling all over the transformers getting soaked in transformer oil with the rain running down the necks of our overalls ? Okay, we do spend time in the switchrooms too, doing the maintenance/timing tests etc. on the breakers. We normally use our own guys for protection or if we are fully booked then we call in guys from Oberon Engineering.

You ought to see some of the switchrooms, certainly not comfortable, pretty much the worst conditions you can find, once there was one that didn't even have a coffee machine!!!

 

[Moley]

No coffee machine??? I hope you had a word with management and got that sorted.

 

[pc1966]

The main distribution voltage is 11kV although other voltages are used. The 11kV network can be solidly earthed or resistance earthed through liquid or dry resistors.

I had wondered about this with the small (0.5MVA) substation that feeds our stuff. The map of buried cables shows it fed from an 11kV 95mm 3-core aluminium cable.

I had always assumed it would be a simple delta-star transformer with just the star point of the LV (230V) side earthed and the HV side earthed at the source end, but is it likely to have an HV side earthing transformer as well?

 

[Julie.]

I had wondered about this with the small (0.5MVA) substation that feeds our stuff. The map of buried cables shows it fed from an 11kV 95mm 3-core aluminium cable.

I had always assumed it would be a simple delta-star transformer with just the star point of the LV (230V) side earthed and the HV side earthed at the source end, but is it likely to have an HV side earthing transformer as well?

Not locally, no.

The 11kv network would most likely be supplied via Dyn transformers in the primary substation (eg 33/11kV), the star there forming the earthing point, solid, resistance or reactance (peterson).

The three phases plus earth would be distributed about the local 11kv network where your local Dyn transformer produces your 400/230v

Just the same as you might supply a three phase motor with just 3 phases, you would also provide an earth (cpc), it's pretty much the same for the 11/0.4kV transformer.

 

[R-fur]

As Julie says there wont be a HV earth point in your local substation to earth the HV system, but there will be an HV earth connection to earth the transformer tank the LV cabinet and any HV switchgear. There should also be an earth mat in the ground in front of the switchgear so that the operators hands and feet are 'bonded' incase of a fault while switching causes the HV earth to rise in potential.
The HV steelwork rising in potential is the reason why there is a 8 meter separation between the HV and LV earth systems, so that any rise in potential of the HV steelwork is not transferred to the LV system.
Text books often show the LV neutral earth bond as being at the star point of the transformer but in reality this is not the case it is usually in the LV switchgear.
There is often a third earth as well for the fence, this is not connected to the HV or LV earths but has its own earth connection to tie it to earth potential so members of the public standing next to the fence are not affected by rises in potential of the HV or LV earth mats.

 

[pc1966]

Thanks, I was going to ask about the transformer, etc, earthing but as explained it would be consider the HV side.

In our case the "sub-station" is a transformer and some switchgear surrounded by a feeble wood/mesh fence about 1.2m high and covering an area of around 2m x 2m in total.

However, it is an area of good soil conductivity, as I got around 10-15 ohms for a couple ord 2m x 16mm rods driven in to the forever damp clay soil there, so I can imagine it might just have a mesh mat that is < 1 ohm and so does both HV & LV earths, as well as serving to keep any personnel away from a high hand-ground potential if there were a fault.

 

[pc1966]

However, it is an area of good soil conductivity, as I got around 10-15 ohms for a couple ord 2m x 16mm rods driven in to the forever damp clay soil there

I mean each rod was 10-15 ohms.