Absence on neutral & earth conductor on 11kV & 33kV systems
Absence on neutral & earth conductor on 11kV & 33kV systems
(OP)
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?
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?






RE: Absence on neutral & earth conductor on 11kV & 33kV systems
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!
RE: Absence on neutral & earth conductor on 11kV & 33kV systems
Some 11kV systems have concentric neutral, when a single phase supply is needed.
All systems usually include a ground wire.
RE: Absence on neutral & earth conductor on 11kV & 33kV systems
RE: Absence on neutral & earth conductor on 11kV & 33kV systems
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.
RE: Absence on neutral & earth conductor on 11kV & 33kV systems
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.
RE: Absence on neutral & earth conductor on 11kV & 33kV systems
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!
RE: Absence on neutral & earth conductor on 11kV & 33kV systems
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
RE: Absence on neutral & earth conductor on 11kV & 33kV systems
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).
RE: Absence on neutral & earth conductor on 11kV & 33kV systems
Is this the same as an unearthed system ?
How does it differ from direct earthing and impedance earthing?
RE: Absence on neutral & earth conductor on 11kV & 33kV systems
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.
RE: Absence on neutral & earth conductor on 11kV & 33kV systems
Alan