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?

 

[R-fur]

The rule of thumb is when the combined value is below one ohm they can be connected.
HV has to be below 40 ohms, LV has to be below 20 ohms, 8 meters separation between them.
Measurements taken before any cables are connected.

 

[Pete999]

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?

HV,LV whats MV? thought MV was old hat.

 

[Julie.]

HV,LV whats MV? thought MV was old hat.

Nope

ELV, LV, MV, HV, EHV etc all still very well defined.

it is only the wiring regs that simplifies everything above LV as "HV"

 

[Pete999]

Nope

ELV, LV, MV, HV, EHV etc all still very well defined.

it is only the wiring regs that simplifies everything above LV as "HV"

wheres does MV Start and HV kick in? and where does it state the parameters ?

 

[pc1966]

wheres does MV Start and HV kick in? and where does it state the parameters ?

I thought MV was 1-33kV

But for many it is just HV >= 1kV and LV < 1kV

 

[Julie.]

wheres does MV Start and HV kick in? and where does it state the parameters ?

Iec 60071 defines MV as 1kV to 36kV

Most others take the value as 35kV (such as iec60038)

HV is 35kV to 230kV
EHV >230kV

So the usual distribution in the UK is MV (6.6/11/22kV)

Primaries are usually 33kV but could be 66kV, some grid is 66 or 132kV - all HV.

but the bulk transmission is 232kV and 400kV (EHV), although some was 132kV (HV) up to a few years ago - not sure now.

Some transmission towers carried both 132 and 232 in the North West, and some carried 232 and 400kV - for example galloping gertie between Darwin and Bradford.


Edit : apparently gerty is spelt gertie - but neither looks right to me!

 

[Cookie]

Interesting- thanks!

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?

 

[Moley]

Most power distibution is at 132kv (highest voltage for a DNO), 275kv and 400kv. I've never seen or heard of 232kv.

 

[pc1966]

Does any one know why the voltage thresholds were chosen?

I'm assuming they are phase-neutral (i.e. earth) since the main risk of shock is touching a line conductor while in contact with the Earth (or attached parts), and I am guessing that the 1kV LV/MV(HV) threshold is related to dry skin breakdown / arc risk where the shock current is much more then 1000/230 sort of scale factor.

But is there some aspect to HV engineering that makes ~35kV a point where different techniques, etc, have to be used to justify the MV/HV category?

 

[Julie.]

Most power distibution is at 132kv (highest voltage for a DNO), 275kv and 400kv. I've never seen or heard of 232kv.

Oops, yes 275, got mixed up with some other work - must pay better attention.

 

[Moley]

I'm assuming they are phase-neutral (i.e. earth) since the main risk of shock is touching a line conductor while in contact with the Earth (or attached parts)

At EHV you don't have to worry about touching a live conductor. You would be dead well before you touched anything. NG safety distances are 1.4m at 132kv, 2.4m at 275 and 3.1m at 400kv but in reality it's 'as far away as f'in possible'.

 

[Moley]

Most power distibution is at 132kv (highest voltage for a DNO), 275kv and 400kv. I've never seen or heard of 232kv.

A reasonably quick way to tell the voltage of a pylon. If it's just got 1 cable per phase it's 132kv. 2 cables is 275kv and 3 or 4 is 400kv.

 

[marcuswareham]

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??

 

[Soi disant]

Iec 60071 defines MV as 1kV to 36kV

Most others take the value as 35kV (such as iec60038)

HV is 35kV to 230kV
EHV >230kV

And next comes UHV.

 

[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.