Dying DNO tranny?

[ericmark]

I would guess two phase fuses blown and equipment is feeding those blown fuse supply lines. Likely big three phase motor has gone bang and taken out two fuses they rarely take out all three.

Other three phase equipment is then giving you the voltage. Bet a phase rotation meter will fail to rotate?

 

[JohnW2]

I would guess two phase fuses blown and equipment is feeding those blown fuse supply lines. Likely big three phase motor has gone bang and taken out two fuses they rarely take out all three. ... Other three phase equipment is then giving you the voltage.

I rather doubt that. To the best of my knowledge, my house is the only installation getting a 3-phase supply from this transformer. It's a tiny village and apart from my house, it only supplies small residential properties (and one very small pub), so it's unlikely that there is any 'other 3-phase equipment', let alone big 3-phase motors - and I have no 3-phase loads in my house.

Kind REgards, John

 

[plugwash]

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?

Roughly speaking you can think of your three phase transformer has being three single phase transformers*

HV wiring doesn't (normally at least) have a neutral, so the primaries of the transformers are connected in delta.

Lets say you have three HV phases, lets call them A B and C and lets further say you have three LV phases lets call them X Y and Z**. Suppose phase X is fed by a transformer connected accross phases A and B, phase Y is fed by a transformer connected across phases B and C and phase Z is fed by a transformer connected across phases C and A.

Now suppose phase C goes open circuit in the feed to the three transformers. Phase X will be unaffected. The primaries of the transformers supplying phases Y and Z will now be connected in series. Therefore in this scenario one would expect the voltages on phases Y and Z to add up to 240V***. The ratio of the voltages on phases Y and Z will depend on the loads placed on those phases.

*I belive they actually use some tricks to reduce the ammount of metal in the core compared to three single phase transformers but three single phase transformers is good enough for a first approximation.
** Note: these are arbitary letters chosen for the purposes of this example.
*** The voltages you report add up to 220V which is close enough IMO for this explantion to make sense.

 

[Lectrician]

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.

The network is usually well built for redundancy, with several exceptions, and rural spurs etc. More often than not, network switching to reroute supplies can be done with no loss of power, assuming the network has been phased correctly.

 

[bernardgreen]

Reminded me of a case where a 3 phase motor was not rotating. The factory "engineer" had confirmed the supply was OK, 230 volts on each of the lives relative to neutral....

In the cabinet the three lives were connected to the same phase.

 

[JohnW2]

If the problem were on the HV side, wouldn't one expect that to affect all of the LV phases, and roughly equally?

Roughly speaking you can think of your three phase transformer has being three single phase transformers* ... Now suppose phase C goes open circuit in the feed to the three transformers. Phase X will be unaffected. The primaries of the transformers supplying phases Y and Z will now be connected in series. Therefore in this scenario one would expect the voltages on phases Y and Z to add up to 240V***. The ratio of the voltages on phases Y and Z will depend on the loads placed on those phases. *** The voltages you report add up to 220V which is close enough IMO for this explantion to make sense.

You are, of course, right. Thanks. I was not thinking straight about 3-phase transformers yesterday - as you say, one can really think of them essentially as three separate transformers, in which case, as you have illustrated, the explanation as to what probably happened here yesterday becomes very simple.

BTW, my "80V" and "140V" were just ballparks - they were actually 'eighty something' and 'one forty something'- so would add up even close to 240V than you suggest.

Kind Regards, John