Electronics Forums > Re: Three Phase/Single Phase Voltage Fluctuations

# Re: Three Phase/Single Phase Voltage Fluctuations

Paul Hovnanian P.E.
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 08-11-2005, 01:35 AM
Perrin wrote:
>
> Hi everyone.
>
> I hope this is the right group for this post of mine - hoping someone
> will have some insight to this one.
>
> Our house power supply is three phase (i.e. 230v/400v).

I'm assuming that this is 3 phase 4 wire, i.e. a wye connected system
(since you refer to a neutral farther on).

> Since I live in the southern hemisphere, it is our winter at the
> moment, so high load demand at peak times. The voltage on red phase
> dropped several times between 6pm and 7pm (also once or twice in the
> morning till around 10:30am), going as low as 200v, while the other two
> remain around 215v to 225v.

This could be due to imbalanced loads.

It could be loads of some of your neighbors (if any). Based on the
secondary distances and transformer sizing, I'm guessing that you share
the secondary system and transformer with a number of other services.

> With this problem, I contacted City Power, and eventually they stepped
> up the transformer tappings to 240v (420 roughly for 3 phase).

Did they send someone out with a clamp-on ammeter to check the
transformer imbalance?

> Things have been much better, but I am still not entirely happy.
> I have noticed that adding a load of 3KW (geyser) drops a phase to
> neutral voltage by 7v, while the next phase in the sequence picks up 2v
> or 3v at the same time.

What's a geyser? A single phase load, I assume.

The voltage drop and rises you are seeing are characteristic of a single
phase load on a three phase system. The load causes a drop on both the
line and the neutral. The neutral's voltage drop is a voltage vector
which will appear to be a rise with respect to the two unloaded phases.

> This to me sounds like a problem with the neutral return path
> somewhere. This is a bit worrying when I start getting readings of
> 247v phase to neutral!
> Electrical standards allow for 207v to 253v, so it is still within the
> limits.
> The transformer is 250m away from the overhead pole where our
> Overhead cable size is a mix of copper=? and aluminium=185mm˛ and our
> cable feed is around 60m to the meter board using copper 16mm˛ cable.
> No stray voltages are present in the neutral or earth conductors.

I don't have my metric wire tables handy. At any rate, the '?' copper
size makes any calculations only a guess anyway.

> I am not actually an electrician, but have picked up my knowledge over
> many years. What I need to ask is - does this sound plausible, or are
> these voltage drops indicative of a fault somewhere?

At first glance, it sounds OK, although comparing your voltage drop
measurements due to the 3 kW load to a calculation should indicate
whether these levels are out of line.

> Thanks so much for the help
>
> P.S. The transformer is (according to City Power) a 500kVA transformer.

--
Paul Hovnanian (E-Mail Removed)
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Time is the best teacher; Unfortunately it kills all its students.

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