Electrical – Isn’t supplying three phase power to regular retail customers UNSTABLE

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Some countries, outside the US, supply 3-Phase power direct to individual apartments (aka "flats") in highrise towers. [ Especially former UK colonies and the like, and also noted on this thread in some cases in US too: Why do I have three phase electricity? ].

Isn't this a very wrong thing to do from the point of view of stability of the grid supply, because the currents in the phases will be unbalanced, compared to running proper 3-phase equipment/loads which automatically balance the phases? After all, what is the individual apartment dweller supposed to do with each phase? All his/her appliances run on single phase. Balancing the phases would be nigh impossible, precluding, of course, a wye-delta transformer for the whole apartment or house (let's ignore that possibility)!

Any thoughts?

Best Answer

  • In the USA, a small block shares a 240V split-phase transformer. The primary of that transformer is on typically 2400 volt 3-phase delta, one leg obviously. The next block over is tapped off different phases.
  • In Europe, a neighborhood gets a 415V 3-phase wye transformer, and the first house is connected to phase A and neutral, next house to B and neutral, next house to C and neutral, etc. Some houses get 2 or 3 phases, particularly if they are load-heavy. It's as simple as delivering one more wire. They can then connect appliances to 415V, but that is rare except for serious tradesmen or hobbyists with industrial tools.
  • In Manhattan, a block gets 208V 3-phase wye, house 1 gets phase A and B and neutral, house 2 gets B, C and neutral, house 3 gets A and C etc.
  • Distant rural America often sees ~10kv single-phase with a transformer at each farmhouse. However this is one side of 3-phase delta, the next farm road will be tapped on the next phase.
  • Australia sometimes distributes at high voltage with a single wire and earth as neutral, literally one solitary wire on the pole. It is 3-phase "wye" with the other 2 phases going down other distribution lines.

All distribution is 3-phase - if not to the pole, at least to the neighborhood.

So what choice do you have? Nobody is seriously going to run a big single-phase distribution system**, and do a large conversion from 3-phase to 1-phase, simply over imbalance.

It averages out

Imbalances aren't as big a deal as you might think. With a sufficiently large number of houses to average over, eventually load per phase averages out.

If for some reason it doesn't, the power company can change which phase a particular flat or block is tapping.

Now if the town's large industry decides to heavily load one phase and create an imbalance, then they might get a serious talking-to by the power company. Because they are big enough to create an imbalance too big to average out.

** Except the railroad. Look on Amtrak's Northeast Corridor, see the even number of power lines? Single phase. They have to solve the "how do you put an enormous single-phase load on 3-phase" problem.

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