Re: Three Phase/Single Phase Voltage Fluctuations

Perrin wrote:
>
> Hi SQLit and Paul Hovnaian P.E.
>
> Thanks to both of you for the speedy answers.
> I hope you don't mind me answering both of your questions in one reply.
>
> The distance of 250m I hate to say is not to the end of the transformer
> run - it still goes on a bit.
> We are not the only household feeding off this transformer - it feeds
> more than eleven houses - not sure how many down a side street. All
> were given 3 phase connections since they needed the higher voltage for
> boreholes long ago before there was a piped water supply (boreholes now
> for garden irrigation).
>
> Where do I live - in good old Johannesburg, South Africa (normally
> sunny, but not this morning ;-)
>
> A geyser I think is called a hot water cylinder in the States (a tank
> of water heated by an electric element - for the household hot water
> supply). It is a single phase load of 3KW, so around 13 amp.
>
> We have a multiple earth neutral system in place - an earth connection
> is made at the service connection, and the neutral and earth of the
> house are connected together at this point. There are also several
> connections to the metal water pipes and an additional earth rod near
> the borehole.
> The neutral from outside to the service box is functioning, and has a
> parallel earth helping to carry current. The clamp of the neutral to
> the overhead cable outside has been replaced last month by City Power
> when I found out that there was no current flowing in the neutral, but
> all flowing back through the parallel earth (checked with clamp
> ammeter). All seems in order.
>
> Load imbalance - it is definitely the neighbours causing this
> imbalance. I have been adjusting the balancing between phases - so far
> we only run pretty much at around 25 to 30 Amp per phase maximum
> (obviously we may go slightly over at times). The service breaker per
> phase is rated 60A (5KA).
> Unfortunately, City Power has not checked to see how unbalanced the
> load is from the transformer.
>
> By the way, I have been looking for a formula to calculate the voltage
> drop that I should be expecting - do either of you have one?
>
> Thanks again for your help - it is much appreciated.
> Regards,
>
> Pierre

I ran a quick calculation from your original post of the drop caused by
a 3 Kw single phase load. The calculation for balanced 3 phase balanced
loads are similar, but there is no current (and no drop) on the neutral.

185mm² Aluminum is close to our 350 kcmil Aluminum, which has R = 0.20
ohms/Km
16mm² Copper is close to our #6 Cu, which has R = 1.7 ohms/Km

For a 250 m run of 185mm² Al plus 60 m of 16mm² Cu, R = (0.25)*(0.20) +
(0.06)*(1.7) = 0.152 ohms
per conductor (there are two for single phase, the phase plus the
neutral). A 3 Kw, 230 volt load draws 13 amps.

Vdrop = 2 * (0.152 ohms) * 13 A = 3.95 volts

A guestimate of the transformer bank resistance per phase is about 5 to
10 milliohms, so a 3 Kw (13 A) load adds negligible additional drop.

This is close to what you measured. It wasn't clear if your voltages
were measured at the service entrance or at the water heater, which will
add the drop of the branch circuit to the above figures.

It appears that your 7 volt drop figure may not be too far off from
calculations.

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Perrin wrote:
>
> The reading I took where there was a drop of 7v was at the DB that
> feeds the geyser - I have just checked at the service connection, and
> the readings went from 233v to 227v, so not much of a difference.
>
> As to why I am so concerned about this, its not so much the drop in
> volts (as long as we remain within the limits that appliances can work
> within), but rather the fact that there is an increase in the adjacent
> phase voltage. Measurements (also taken now) indicate that there is an
> increase in the blue phase from 236v to 239v when the geyser is once
> again energised, the geyser being on white phase. (Phase colouring in
> South Africa is the same as Europe = Red, White (or yellow), Blue).
> Is this quite normal? I ask this because we had a situation a couple
> of years ago where the feeder neutral was faulty, and we ended up with
> roughly 280v on red and blue phase, with roughly 150v on white.
> I found out afterwards that we were only connecting to 4 other houses
> via the neutral at that time.
> Needless to say, all that was destroyed by the overvoltage was an old
> sprinkler controller, but it could have been far worse.

Some voltage rise is OK on the unloaded phases when a single phase load
is applied.

A broken neutral will result in excessive voltage swings on the neutral.
A really rough rule of thumb is that if the voltage rise on an unloaded
phase is around half of the voltage drop on the loaded phase, you are
OK. This indicates that the phase and neutral voltage drops due to the
load are roughly equal, which is normal and due to a drop on the entire
circuit. If the voltage rise on an unloaded phase is nearly equal to the
drop on the loaded phase, it indicates that most of the drop is due to a
high resistance on the neutral connection somewhere. Its a little
trickier to visualize on a three phase system than on our simple single
phase stuff.

> Thanks for all the advice and the links/calculations. I really
> appreciate you guys giving your time to help out!

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