Dying DNO tranny?

[JohnW2]

Something has just gone wrong with the (3-phase) supply to my house. One phase to neutral (and to TT earth) has dropped to ~80V, the second one to about 140V, but third phase (at ~240V to neutral) remains 'normal'.

I wonder what the most likely cause of this is - a dying tranny?

Kind Regards, John

 

[radioredcat]

Something has just gone wrong with the (3-phase) supply to my house. One phase to neutral (and to TT earth) has dropped to ~80V, the second one to about 140V, but third phase (at ~240V to neutral) remains 'normal'.

I wonder what the most likely cause of this is - a dying tranny?

Kind Regards, John

I would say a fault on the high Voltage side we have had this problem in the past here.

Andy

 

[JohnW2]

I would say a fault on the high Voltage side we have had this problem in the past here.

If the problem were on the HV side, wouldn't one expect that to affect all of the LV phases, and roughly equally? That's why I suspected it had to be the tranny itself. A single-phase problem could have been 'wiring', but with two phases (but not the third) to be affected, to differing extents, the tranny seemed to be the only likely 'common factor'.

Kind Regards, John

 

[radioredcat]

I would say a fault on the high Voltage side we have had this problem in the past here.

If the problem were on the HV side, wouldn't one expect that to affect all of the LV phases, and roughly equally? That's why I suspected it had to be the tranny itself. A single-phase problem could have been 'wiring', but with two phases (but not the third) to be affected, to differing extents, the tranny seemed to be the only likely 'common factor'.

Kind Regards, John

Well no as we are fed via 11 kv overhead lines these kind of faults (brownouts) have been caused by bad connection on the joints as that's what a linesman told me when we had this problem.

Andy

 

[JohnW2]

Well no as we are fed via 11 kv overhead lines these kind of faults (brownouts) have been caused by bad connection on the joints as that's what a linesman told me when we had this problem.

Well, as I said, it makes little sense to me that a fault on (or upstream of) the primary side of the transformer could have a differential effect on the three phases of the secondary - but maybe I'm missing something!

Of course, I'll probably never know the answer.

Update: problem (whatever it was due to) now resolved. All phases went totally off, simultaneously, and then about 30 seconds later, 'normal service was resumed', with all three phases ~240V to neutral. It almost sounds as if someone pressed a 'reset' button! Any ideas?

Kind Regards, John

 

[radioredcat]

Well no as we are fed via 11 kv overhead lines these kind of faults (brownouts) have been caused by bad connection on the joints as that's what a linesman told me when we had this problem.

Well, as I said, it makes little sense to me that a fault on (or upstream of) the primary side of the transformer could have a differential effect on the three phases of the secondary - but maybe I'm missing something!

Of course, I'll probably never know the answer.

Update: problem (whatever it was due to) now resolved. All phases went totally off, simultaneously, and then about 30 seconds later, 'normal service was resumed', with all three phases ~240V to neutral. It almost sounds as if someone pressed a 'reset' button! Any ideas?

Kind Regards, John

Hmm that's strange when we have had that problem yes power goes off but for hours while they do the repair.

I am hoping someone will shed some light on it.

Andy

 

[JohnW2]

Hmm that's strange when we have had that problem yes power goes off but for hours while they do the repair.

Yes, that's what one would expect with a fault. The power was off for literally no more than 30 secs at most (I didn't have time to 'reach for my torch'), so no time for anyone do do anything much other than to 're-boot' the switchgear!

I am hoping someone will shed some light on it.

Me too!

Kind Regards, John

 

[westie101]

Loss of one phase on the HV side of a 3 phase transformer will cause those voltages (work it out on a diagram for a delta/star connected transformer).

Either a blown un-ganged fuse on one phase of the HV o/h line or a connection broken on one phase

 

[JohnW2]

Loss of one phase on the HV side of a 3 phase transformer will cause those voltages (work it out on a diagram for a delta/star connected transformer). ... Either a blown un-ganged fuse on one phase of the HV o/h line or a connection broken on one phase

Thanks. I need to think about that - and maybe revisit my understanding (or otherwise!) of how 3-phase tranny's are actually arranged, and work!

One interesting aside. When the phase supplying my ground floor dropped to ~80V relative to neutral, 3 out of 8 CFLs still worked and produced reasonable amounts of light - which certainly makes one wonder about the 3% maximum VD on lighting circuits in the BS7671 guidance!

On the other hand, my 'phase failure relays', with nominal 240V coils, didn't like 80V at all - it was their buzzing and 'chattering' which gave me the first clue as to what was going on.

Kind Regards, John

 

[Lectrician]

Sounds like a loss of one of the HV phases. The sudden restoration would likely be due to some network switching to feed the affected part of the network from elsewhere. (or the restoration of the lost phase).

 

[westie101]

Interestingly the early Pace Sky receivers did not react well to this situation!

Once the voltage dropped they would happily operate at the reduced voltage, but did not like it going suddenly from 140 or 80 to 240, so reacted by letting the smoke escape!
Later versions corrected this problem!

 

[RF Lighting]

While we've got you westie, in an 11kV distribution system, is this the phase to phase or phase to earth voltage?

 

[westie101]

in an 11kV distribution system, is this the phase to phase or phase to earth voltage?

Phase to Phase. All HV voltages on UK distribution and transmission refer to the Phase to Phase voltage.

Here's a nice Australian video of trying to replace an HV pole mounted fuse!

http://www.youtube.com/watch?v=ATEWXsMawMg

 

[radioredcat]

It was strange seeing some items work which had SMPSU's and other items Transformer fed not I have measured 90 v on one occasion.

The last on we had some years ago our phase was ok but I noticed 2 streetlamps kept going out shortly after the power went off for the repair.

Hopefully we don't have any more lol.

Andy

 

[mfarrow]

Yeah it'll be network switching which has got you going.

Wait for a few nights and they'll do it again.

We had a power cut some time ago at midnight and I rang up and asked when it would be switched back. They said it was all fixed now, well they lied and 4 nights later we had another cut. I even asked "when are you switching it back?" to which they said "we don't need to it's all fixed".

No problem you'd think but as it screws up my clock radio overnight I don't take kindly to it.