Three phase in the street?

[Robbox]

Hi all leccy experts. The network guys are putting in electricity to a small development next to me. Digging up the street and laying a huge cable in the trench (maybe +50mm diameter). So my question is first, is this three phase in the big cable? I know you take separate legs to get single phase from three phase, so if it is three phase, and each house is getting single phase, how do they make sure it's all balanced....or does it even need to be balanced?

 

[sparkwright]

I don't work on electricity board cables, but as far as I know, generally, every third house is on the same phase, which balances things out roughly.

 

[Nozzle]

Balanced roughly is correct, it doesn't need to be perfect (but a heavy imbalance will cause heavy vibration at the generator end), Or if really bad will cause negative phase sequence faults, which call for a deload or trip.

Nozzle

 

[JohnW2]

Balanced roughly is correct, it doesn't need to be perfect (but a heavy imbalance will cause heavy vibration at the generator end)

It presumably would be next-to-impossible that there would ever be a 'heavy' phase imbalance at the generator (i.e over much/most of the entire grid)?

Kind Regards, John

 

[AdrianUK]

......Or if really bad will cause negative phase sequence faults, which call for a deload or trip.

Nozzle

I think you mean negative sequence overcurrent - a form of protection used to pick up an excessive phase imbalance in a distribution system.

All this stuff about balancing a three phase system on the micro level is a storm in a tea cup. There is no metering penalty (3 phase meters DO NOT charge the highest phase current x3) and the distribution system will cope just fine - the out-of-balance currents will flow in the neutral just as they should. The only time it becomes an issue is if a large (relative to the size of the machine) single phase load is supplied from a 3 phase genset - even so this still won't exceed 1/3 the capacity of the machine.

Rough balancing of the loads across the phases is necessary to get the most power out of a given system but distribution boards etc will not fail even if severely out of balance.

 

[Nozzle]

It presumably would be next-to-impossible that there would ever be a 'heavy' phase imbalance at the generator (i.e over much/most of the entire grid)?

Kind Regards, John

The grid is so large that all points on it are NOT equal. The conductors can be both an inductive and capacitive and it varies with how heavily loaded the conductor is, and also whether it is above or below ground. Consider the grid as a network of R, L and C and you can then see how it's possible for a lightning strike on a single phase in Somerset to have no measureable impact in Kent.

Nozzle

 

[Nozzle]

The only time it becomes an issue is if a large (relative to the size of the machine) single phase load is supplied from a 3 phase genset - even so this still won't exceed 1/3 the capacity of the machine..

I don't buy this. The principle is sound, but I think you start to see high vibrations at a much lower threshold. It's like the equivalent of having a perfectly balanced motor with a flywheel on. If you start to add masses, it vibrates very heavily. Think the 10 or 15g masses tacked to your car wheels and how much the wheels vibrate if you loose one.

I suggest to you 10-15% imbalance is more the limit for taking remedial action.

Nozzle

 

[JohnW2]

The grid is so large that all points on it are NOT equal. The conductors can be both an inductive and capacitive and it varies with how heavily loaded the conductor is, and also whether it is above or below ground. Consider the grid as a network of R, L and C and you can then see how it's possible for a lightning strike on a single phase in Somerset to have no measureable impact in Kent.

Yes, I accept that, so I suppose I shouldn't have talked about 'the whole grid'. However, I would still think that, fairly random variation being what it is, the loads on a segment of the network which was sufficiently small that 'all points' were roughly equal, would almost certainly always be pretty well balanced across phases, wouldn't they? After all, I would have thought that the balance is likely to always be pretty good even at the small scale of a single LV tranny - even if there are only a few dozen consumers being surprised, it would be incredibly unlikely that 'every third one' would, simultaneously, be creating a demand substantially different from other consumers.

Kind Regards, John

 

[Nozzle]

^^^Exactly

But what about large single phase loads, like electric locomotives
What about circuit faults that leave only two phases in service - 3 phase motors will still run (and more importantly, the processes driven by the motors) they just wont start.

 

[AdrianUK]

^^^Exactly

But what about large single phase loads, like electric locomotives
What about circuit faults that leave only two phases in service - 3 phase motors will still run (and more importantly, the processes driven by the motors) they just wont start.

But aren't locos driven from traction transformers which tend to be fed Line-line from two phases on the primary.... its only the secondary which is single phase with one side grounded??

 

[Nozzle]

I don't know, my knowledge ends there. I was wondering how a locomotive can swap from one track circuit to the next without (a) shorting out two phases or (b) loosing supply altogether. What you suggest above though leaves a pair of phases more heavily loaded that the one left over.

 

[AdrianUK]

There is the Scott connection which can be utilised to get two single phase supplies (eg. the upline & the downline) from a three phase supply but even this can't balance the load evenly across the phases. Another alternative is the LeBlanc connection but its still not possible to supply a large single phase load EVENLY from a three phase source.

 

[JohnW2]

^^^Exactly .... But what about large single phase loads, like electric locomotives

Well, in the context of this thread, I doubt that one finds many of them in a small housing development

Kind Regards, John

 

[SimonH2]

I was at a talk a little while ago about HS2. They will be changing from using the local DNO at 11kV or 33kV (as is currently done) to taking their power from the grid at 132kV (or higher ?). The trains will be ¼ of a mile long and (IIRC) take something in the order of 9MW* when pulling away from the station. Can't remember if they are going to be using regenerative braking.
* Not sure of the number, but it was certainly in the multi-MW range

 

[AdrianUK]

I was at a talk a little while ago about HS2. They will be changing from using the local DNO at 11kV or 33kV (as is currently done) to taking their power from the grid at 132kV (or higher ?). The trains will be ¼ of a mile long and (IIRC) take something in the order of 9MW* when pulling away from the station. Can't remember if they are going to be using regenerative braking.
* Not sure of the number, but it was certainly in the multi-MW range

Hmmmm ... HS2 is a touchy subject..... its gonna take my house!