Westie - HV fault

[Lectrician]

What is the configuration of HV to LV transformers? The LV side is obviously star, but what about the HV? Is it star or delta? As there is no neutral on HV, I had kind of assumed it would be delta, derived from a star at the 33KV substation.

If a single HV phase goes down, leaving two working, what's the effect of the LV?

Quite often your lights go dim rather than off during a power failure, and I assumed this was one HV phase going down on a DELTA-STAR TX. One one winding is receiving full volts, the other two ending up in series because of the lost phase, in series cause lower volts on two of the LV phases.

The other day though, a site fed from its own pole mounted TX lost an LV phase completely, and it turned out the farmer had pulled down one HV phase quite close to the TX. I assume that must be a STAR-STAR TX? Can't see how loosing one HV phase would cause one LV phase to drop otherwise?

Farmer hadn't even realised he had taken out the line.

 

[plugwash]

My understanding from things i've read is that delta primaries are the norm but there can be exceptions.

What was the loading like?

If the transformer has a delta primary and a star secondary then losing a phase on the input side will put two of the output phases effectively in series.

If one of those two phases has a much larger load than the other then the loaded phase would see very little voltage while the unloaded phase would see nearly full voltage.

 

[ericmark]

Pole mounted I would expect is single phase. Often the output is 230 - 0 - 230 or split phase so losing one phase on incomer would mean full power cut.

 

[JohnW2]

If a single HV phase goes down, leaving two working, what's the effect of the LV? ... Quite often your lights go dim rather than off during a power failure, and I assumed this was one HV phase going down on a DELTA-STAR TX. One one winding is receiving full volts, the other two ending up in series because of the lost phase, in series cause lower volts on two of the LV phases.

You might be interested in this thread which I started about 18 months ago.

A fault resulted in one of my phases falling to about 80V (relative to neutral and TT earth) and one phase fell to about 140V, but the third phase remained normal (~240V). As you will see, on that occasion, westie said that those figures were compatible with loss of one of the HV phases.

Kind Regards, John

 

[JohnW2]

If the transformer has a delta primary and a star secondary then losing a phase on the input side will put two of the output phases effectively in series.

Indeed. You will see in the thread of mine that I've just linked to that (presumed) loss of one HV phase resulted in my seeing secondary voltages of about 80V, 140V and 240V - and 80V+140V is not far off 240V. I presume that the difference between 80V and 140V was probably the consequence of unequal loading of those two phases

Kind Regards, John

 

[westie101]

All three phase feeding the networks are delta/star

Pole mounted I would expect is single phase.

General sizes in use (well here anyway)

Single phase kVA: - 5 (Installed in the 1960's and only a few left), 15 (16), 25, 50.
Split single kVA: - 25, 50, 100
Note : - where the above sizes are the same they can be connected in either format
Three phase kVA: - 25, 50, 100, 200

Yes the loss of a single phase can give the voltages quoted, but often the lower (80V) can seem to be 0 volts depending on what is connected and the reason for the loss of the HV phase.

Pole mounted I would expect is single phase.

 

[Lectrician]

Pole mounted I would expect is single phase. Often the output is 230 - 0 - 230 or split phase so losing one phase on incomer would mean full power cut.

I am talking purely three phase HV.

 

[Lectrician]

If a single HV phase goes down, leaving two working, what's the effect of the LV? ... Quite often your lights go dim rather than off during a power failure, and I assumed this was one HV phase going down on a DELTA-STAR TX. One one winding is receiving full volts, the other two ending up in series because of the lost phase, in series cause lower volts on two of the LV phases.

You might be interested in this thread which I started about 18 months ago.

A fault resulted in one of my phases falling to about 80V (relative to neutral and TT earth) and one phase fell to about 140V, but the third phase remained normal (~240V). As you will see, on that occasion, westie said that those figures were compatible with loss of one of the HV phases.

Kind Regards, John

I did reply to that thread, and it has always been my understanding a single lost HV phase would result in volts being odd, rather than down. That's what got me thinking with this one the other day, as two phases remained normal, one dead. The loading was very light, andI am not too sure on the balancing, although some load would of been on the lost phase. It's a 315amp TPN supply on a caravan park, but the park was very empty.

 

[JohnW2]

I did reply to that thread, and it has always been my understanding a single lost HV phase would result in volts being odd, rather than down. That's what got me thinking with this one the other day, as two phases remained normal, one dead. The loading was very light, andI am not too sure on the balancing, although some load would of been on the lost phase. It's a 315amp TPN supply on a caravan park, but the park was very empty.

Fair enough. Westie seems to be saying that either start or delta primaries are possible - and what you observed is presumably what one would expect if it was a start-star tranny, isn't it? AIUI, in effect, one would then have three single-phase transformers - and if one does not have a pd across it's primary ....!!

Kind Regards, John

 

[Lectrician]

That's the way I saw it, yes.

 

[Spark123]

The thing to remember is the output is a mixture of the inputs, i.e. L1 on the LV side of things is a mix of L1 and L2 of the HV side. This is where the normal 30º phase shift comes from with a delta star transformer.
If you drop a HV phase then there will still be 2 phases left, two of the primaries will be effectively in series and one will still have the full voltage across it so we'd expect to see 230v on one LV secondary and somewhere in the region of half the voltage on the other two (albeit as it will has been designed to operate 3 phase the output voltage will probably be a bit less i.e. 80v).

 

[Lectrician]

The thing to remember is the output is a mixture of the inputs, i.e. L1 on the LV side of things is a mix of L1 and L2 of the HV side. This is where the normal 30º phase shift comes from with a delta star transformer.
If you drop a HV phase then there will still be 2 phases left, two of the primaries will be effectively in series and one will still have the full voltage across it so we'd expect to see 230v on one LV secondary and somewhere in the region of half the voltage on the other two (albeit as it will has been designed to operate 3 phase the output voltage will probably be a bit less i.e. 80v).

That's the way I was expecting it to be, but I think this TX was a STAR-STAR.

 

[westie101]

Westie didn't make it clear but I have never come across a star primary transformer.
Such a design would make a mess of our protection systems and could risk an interesting failure if fault current got into it

 

[JohnW2]

Westie didn't make it clear but I have never come across a star primary transformer. Such a design would make a mess of our protection systems and could risk an interesting failure if fault current got into it

Fair enough - but if it wasn't a star primary, how could you explain what Lectrician observed?

Kind Regards, John

 

[westie101]

Too many variables to predict what will actually occur.

Certainly despite the theory the usual outcome is 1 x full volts
1 x roughly half volts & 1 x nearly zero volts.

It was tested in ideal conditions many years ago and that was the outcome. It can also change if a conductor is on the ground, the effect of nearby parallel on the same circuit (on overhead networks in is more likely that there will be a number of transformers controlled by the same switchgear/fuses) transformers, connected load etc.