Absence on neutral & earth conductor on 11kV & 33kV systems 2

[Tangy]

Why is there no neutral or earth conductor on 11kV & 33kV systems and that of higher voltages (110kV, 275kV).
But there is such a conductor(s) on LV systems?
If there is no neutral conductor each phase needs to be perfectly balanced is this not impossible? The role of the neutral conductor is to carry the current imbalance of the three phases. For example at LV you would not provide a three phase supply with no neutral conductor, but would do at 11kV

In systems with no earth conductor how does that fault current (three phase or earth fault) return to the transformer neutral point on earthed systems ?

If neutral and earth conductors are not a requirement on 11kV systems why are that a requirement on LV systems?

 

[ScottyUK]

The neutral point of the 3-wire system you describe is typically earthed at the origin of the supply, often through the use of star connected winding with the star point earthed either solidly or through a resistance or reactance. The other loads are delta windings which have no requirement for a neutral, so although the line currents may be imbalanced they have a zero vector sum.

Earth current pases through the mass of earth, which is why substations have an earthing mat or grid. Earth fault current returns to the source through the mass of earth to the neutral of the source.

It is possible to operate ungrounded 3-wire systems although from the voltages this sounds like a British-influenced and such a system is unlikely if that is the case.


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If we learn from our mistakes I'm getting a great education!

 

[xxjohnh]

Neutrals are needed if you have phase to neutral load, nothing to do with unbalance. You can have 3 WIRE 400 volt system without neutral. common when you have high fault levels, then with 400/ 400/220 transformers to supply the 220 volt loads.
Some 11kV systems have concentric neutral, when a single phase supply is needed.
All systems usually include a ground wire.

 

[jwparker]

The role of the neutral isnt exaclty to carry the imbalance, but more to add a reference voltage to be able to obtain a lower voltage. There are plenty of LV systems with no neutral. Specifally look at 3ph motors or 1ph heaters most have no neutral. So lets say you have a 600V 3ph system, if you only need 600V and not 347V you dont need to use the neutral. Simiarly, a 240V 1ph, if you dont need 120V you do not require a neutral either.

 

[Hoxton]

I remember some utilty systems in the uk were unearthed - I cannot remember if 33kv or 132kv, but were overhead lines.

By an accident of history, they were fed from the delta side of transformers at all substations.

Earth faults were detected by capacitor vts connected in star, with the star point connected to a voltage sensitive relay. Any earth fault would move the vector point of the star and energise the relay.

 

[PHovnanian]

I remember some utilty systems in the uk were unearthed - I cannot remember if 33kv or 132kv, but were overhead lines.

They still are. I just returned from England. Many of the rural OH lines are three wire and all the single phase loads are connected line to line.

I'm not sure what sort of secondary or ground connections were employed at the substation, or what the protection philosophy is. A voltage sensing system would suffer from not being able to detect which circuit on the station's secondary bus was faulted and should be tripped.

 

[ScottyUK]

Hi Hoxton and PHovnanian,

Are there still ungrounded systems in the UK? I thought they had faded into history long ago, although I think there is still a bit of 2-phase network left out in the remote parts of Northumberland so I guess other legacy stuff is probably still in service.



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If we learn from our mistakes I'm getting a great education!

 

[Marmite]

I hope not. Ungrounded systems are illegal in the UK under the ESQC Regs, Section 8.
2) A generator or distributor shall, in respect of any high voltage network which he owns or operates, ensure that -

(a) the network is connected with earth at, or as near as is reasonably practicable to, the source of voltage but where there is more than one source of voltage in that network, the connection with earth need only be made at one such point;

Don't confuse lack of a separate overhead line earth conductor with an unearthed system. As Scotty said, the systems are single point earthed at the source of supply using the transformer star point if there is one, or earthing transformer if not.

Regards
Marmite

 

[PHovnanian]

Are there still ungrounded systems in the UK?

Ungrounded? Three wire distribution, no neutral. Probably earth grounded. I just finished the Hadrian's Wall pub crawl, er, I mean hike. And most of the rural overhead systems in that area are three wire. Single phase transformers are connected line to line, so there's no neutral return (earth or wire).

I tried to make a side trip to get a closer look at a substation and grounding bank details, but my hiking partners were not interested (no EEs among them).

 

[Tangy]

What is an isolated neutral system?

Is this the same as an unearthed system ?
How does it differ from direct earthing and impedance earthing?

 

[m3ntosan]

Usually medium voltage grids are supplied from transformers with delta secondary winding, so there's no access to neutral (HV/MV transformers are usually wye-grounded/delta and MV/LV transformers are usually delta/wye - grounded, so you have neutral access for LV network).

A special transformer/coil (usually zigzag connection with 3 phase input - neutral output - and resistance between neutral and ground) connected after the HV/MV transformer will transform the isolated operation of the system in grounded through resistance (calculated to limit the earth fault magnitude), in order to clear the earth fault by earth fault protection. The earth fault will return through the ground to the resistance grounded link. In an isolated operation of the system for single phase fault you will end up with 1.73 x phase to ground voltage on the remaining (healthy) phases. That's the advantage of isolated operation, you can go on functioning with 2 phases for limited time (but you may jeopardize equipment isolation), it is still a common arrangement in rural areas.
Another mode of MW network operation is with Petersen coil.

The reason for earth conductor absence on medium voltage OHL is due to impracticality (I guess), the distances between phases are small and earth conductor could not protect from lightning strikes very well.

 

[racobb]

And that is the very reason we always recommend operating ungrounded wye - delta in those instances. No need to burn up what's left of the bank.

Alan