What causes unequal phase-to-neutral voltages in three-phase distribution systems?

Question

A four-wires cable runs into an apartment building to supply it with three-phase 380/220V alternating current. Each apartment is connected to one phase, different apartments are connected to different phases with interleaving so that phases are hopefully loaded equally.

Now it turns out that voltage between phase 1 and the neutral is something like 215 volts and the voltage between the other two phases and the neutral is about 203 volts each.

One of the tenants is connected to a phase which is 203 volts relative to the neutral and has problems with appliances malfunctioning because of undervoltage. He calls a serviceman and the serviceman claims the substation transformer feeds such voltages and the only thing he can do is to disconnect the tenant from his phase and connect him to the phase with 215 volts.

What causes such unequal voltages? Is it unequal load on different phases or anything else? Also has the serviceman done the right thing or did he just put extra load on the higher voltage phase and induce risk of distribution failure?

Answer 1

Each apartment is connected to one phase, different apartments are connected to different phases with interleaving so that phases are hopefully loaded equally.

Yes.

He calls a serviceman and the serviceman claims the substation transformer feeds such voltages and the only thing he can do is to disconnect the tenant from his phase and connect him to the phase with 215 volts.

Yes.

Is it unequal load on different phases or anything else? Also has the serviceman done the right thing or did he just put extra load on the higher voltage phase and induce risk of distribution failure?

Serviceman did the right thing.*

I think you've answered most of your own question!

The substation is presumably supposed to have some kind of voltage regulator (after all, the transformer has a nonzero output impedance, so voltages sag with load). I assume it's theoretically possible that this could be done on a per-phase basis, but my guess is that they only do it as a 3-phase set, in which case the imbalance is caused by unequal loading.

*caveat: I suppose technically someone should measure the currents out of the transformer, and make sure that the imbalance in currents matches the sag in the voltages. If the currents are about the same but the voltages are not, then it could be a higher-than-normal impedance somewhere in the distribution network.

Answer 2

Bad design.
Overloading.
Shouldn't happen.
Does happen.
Very close to "in spec" in UK.
Not at all uncommon in many countries.

Voltage on lowest phase under worst case loadings should be not worse than guaranteed minimum in theory and perhaps somewhat worse in practice.

Your cited 203V on nominal 220 V system is 203/220 = 92.3%.

WIKIPEDIA - ELECTRICITY BY COUNTRY says for

• UK: Voltage tolerance of 230 V +10%/−6% (216.2 V to 253 V),
  widened to 230 V ±10% (207 V to 253 V) in 2008.
  The system supply voltage remains centred on 240 V

• N Z: Electricity (Safety) Regulations 2010 state supply voltage is 230 V ±6%
  ~= 216 - 243.

So in the UK your example 203V is so close to the minimum allowed 207V that the appliance is essentially "non compliant". No satifaction for the user.

Choosing a less lightly loaded phase would help BUT there is no guarantee that this is a consistent condition and if it is then somebody should be rebalancing things.

Appliances that are affected will be non-heat ones - or not the heating part. eg probably eg television and similar. So one "solution" is a variac or auto transformer, with the realisation that if mains goes high the output will go very high. A "safer" alternative is a UPS or inverter run permanently - assuming it is rated to do so. You can also get mains conditioning units which automatically adjust output voltage - usually specialist and expensive.

I've found that using an isolating transformer tends to improve waveshape and reduce noise and may be a "fix" of sorts in marginal cases.

The excessively enthused could use an eg 12 VAC output transformer to add a voltage to mains. So a say 10A 12V transformer = 120 VA, could be placed in series with mains and allow loads up to about 2300 Watts to "see" a 12V increase in mains voltage. This would probably be frowned on by the regulatory authorities.