Westie - HV fault

[westie101]

One I've never figured is where we have a 2 wire HV overhead line (single phase) and one fuse operates.
We still get reports of low volts as some customers have some power!

Never figured that

 

[JohnW2]

One I've never figured is where we have a 2 wire HV overhead line (single phase) and one fuse operates. We still get reports of low volts as some customers have some power! Never figured that

There could be a Nobel Prize in that for you - it sounds as if you may have re-invented at least some of the laws of physics

Is the capacitance across an open fuse enough to let a little bit of power through? (I would have thought only a tiny amount - not enough for any customers to be aware of!).

Kind Regards, John

 

[AdrianUK]

One I've never figured is where we have a 2 wire HV overhead line (single phase) and one fuse operates.
We still get reports of low volts as some customers have some power!

Never figured that

Earth Fault?

ie. The faulted line is grounded after the (now open) fuse so the 11kV primary is seeing 11000/sqrt(3) = 6358V hence the Secondary drops to around 130-0-130V if split-phase or around 130V is single phase?

 

[bernardgreen]

One I've never figured is where we have a 2 wire HV overhead line (single phase) and one fuse operates.t

The leg that blew its fuse has shorted to ground which took out the fuse. The short remains conductive so the 11kv / 230 v transformer's primary is now between one phase and ground. The impedance of the return path from the transformer's can via the ground to the earth rod at the source of the 11kV will drop quite a lot of voltage but not enough to lose all the 11kV

Potential gradients around the transformer's earth rod are likely to be steep and could present a hazard to people approaching the site.

 

[JohnW2]

Earth Fault? ie. The faulted line is grounded after the (now open) fuse so the 11kV primary is seeing 11000/sqrt(3) = 6358V hence the Secondary drops to around 130-0-130V if split-phase or around 130V is single phase?

One I've never figured is where we have a 2 wire HV overhead line (single phase) and one fuse operates.t

The leg that blew its fuse has shorted to ground which took out the fuse. The short remains conductive so the 11kv / 230 v transformer's primary is now between one phase and ground.

Adrian and bernard have given a simple and very credible explanation for what westie described. When I commented, I think I was focussing on a different aspect of what westie had said, and probably taking it too literally. He said that following operation of the fuse protecting one HV phase of a 2-wire feed to a single-phase transformer, some (which I took to mean 'not all') customers still had some power (at reduced voltage). My thinking was that, in the absence of at least on further fault, it would be impossible for some (but not all) people to have some power, if all customers were supplied by the same single-phase supply from the secondary of the tranny.

Kind Regards, John

 

[bernardgreen]

some customers

The potential gradients in the earth around the transformer would mean ground rods in the area would be at different potentials. Those rods and other grounded items ( service pipes ) being at different potentials and in various ways connected to the Neutral would affect the potential of and current in the Neutral such that different customers could have different voltages between Live and Neutral

 

[JohnW2]

some customers

The potential gradients in the earth around the transformer would mean ground rods in the area would be at different potentials. Those rods and other grounded items ( service pipes ) being at different potentials and in various ways connected to the Neutral would affect the potential of and current in the Neutral such that different customers could have different voltages between Live and Neutral

Interesting theory, but I would very much doubt that would have a significant effect - given that, in the absence of an additional fault in the LV network, all customers (and their local paths to earth) are connected to a common neutral conductor.

I'm sure the only problem was that I over-interpreted westie's comment when I read the "some customers".

Kind Regards, John

 

[SimonH2]

Bear in mind that "some customers reported" could simply mean "the call centre got some reports of low voltage" - with the others either not noticing (not at home at the time) or just didn't report anything or reported it as "not working" (as in the lights work properly or it's "not working").

So I wouldn't read into it that some customers still had 240V or some had nothing.

Some people might actually not notice (at least immediately) if the mains went down to (say) 100V. With the prevalence of wide input voltage range switch mode supplies these days, they may well "just carry on working" as normal. SO the computer keeps going, the TV keeps going, the LED lights keep on going - so customer doesn't notice. They will as soon as they come to use something that doesn't work - but if some stuff is working, they may not realise what's going on.


Another factor to consider is that if there are any 3 phase motors running directly on-line (ie not via a variable speed drive etc) then they will act as induction generators and partially control the phase balances. Depending on the mix of customers involved, and time of day, this could be a significant factor.
[/quote]

 

[AdrianUK]

One I've never figured is where we have a 2 wire HV overhead line (single phase) and one fuse operates.
We still get reports of low volts as some customers have some power!

Never figured that

Another factor to consider is that if there are any 3 phase motors running directly on-line (ie not via a variable speed drive etc) then they will act as induction generators and partially control the phase balances

Whilst I agree that it is possible for a DOL induction motor to regenerate a missing phase, the scenario here is a single phase spur line so the only motors present would be single phase: that can't be the mechanism at play here.

 

[JohnW2]

Bear in mind that "some customers reported" could simply mean "the call centre got some reports of low voltage" - with the others either not noticing (not at home at the time) or just didn't report anything or reported it as "not working" (as in the lights work properly or it's "not working"). ... So I wouldn't read into it that some customers still had 240V or some had nothing.

Quite - as I wrote ....

I'm sure the only problem was that I over-interpreted westie's comment when I read the "some customers".

Kind Regards, John

 

[SimonH2]

Another factor to consider is that if there are any 3 phase motors running directly on-line (ie not via a variable speed drive etc) then they will act as induction generators and partially control the phase balances

Whilst I agree that it is possible for a DOL induction motor to regenerate a missing phase, the scenario here is a single phase spur line so the only motors present would be single phase: that can't be the mechanism at play here.

Yes. I should have been clearer as that was more addressed to the earlier part of the thread about losing one leg of the 3 phase HV system, and the effects that has on the downstream 3 phase LV network.

 

[ericmark]

It was said HV side is never star well not quite HV but I did come across a step down transformer where the supply side was star.

A Robins TBM supply to machine was 10KV to a transformer which stepped it down to 660 volt for the main motors. I would guess both delta windings but never checked.

The a further transformer star 660 and delta 220 transformed it down further for the control circuits and lighting. It was really hard to work on the 220 volt supply as there were no reference points. I suppose it is wrong but I have for years clipped onto an earth point to test for a phase down. I suppose from working a lot with three phase 110. But earth was floating and seemed to alternate between the three phases. So you have no reference point.

Not a clue why it used the IT earth system it was a real pain which people always getting shocks with the iron laden water of Hong Kong.

I really do think some one had fitted a step up transformer instead of a step down I just could see no point in star at 660 with star not connected to anything.