Transformers - question about number of phases used

1. Mar 13, 2013

vinothgerson

transformers -- question about number of phases used

why often single phase and three phase transformers are widely used rather than other phases like four,five phases

2. Mar 13, 2013

SteamKing

Staff Emeritus
Because AC power is generated in either single phase or three phase only.

3. Mar 13, 2013

Averagesupernova

It has nothing to do with the transformer. It is the source that determines number of phases. So is your question why we use 3 phase instead of 4 or 5 phase?

4. Mar 13, 2013

sophiecentaur

3 Phase is great for power generation and for big motors, which is why it's used for power distribution. Any more phases for that purpose would just add complication and expense with little, if any, advantage in motors.

5. Mar 13, 2013

rbj

there are six-phase power systems. just type it into google. there's a nice looking paper and a youtube.
apparently there is some advantage (over 3-phase) for ultrabig AC transmission.

check it out.

6. Mar 13, 2013

sophiecentaur

I couldn't find any reference to a working system in either of those two links. That guy in the You tube movie implied it was more useful for educators than for practical engineers. It seems like a lot of trouble to go to - plus there's the compatibility factor.

7. Mar 13, 2013

cabraham

With 3-phase power generation/transmission/distribution, the amount of conductor material needed is only 75% that of 1-phase, or 2-phase power. I don't remember the exact requirement for 4, 5, 6, etc. phases, but there is no advantage over 3-phase, maybe not even as good. The 6-phase & 12-phase systems do offer lower ripple for applications requiring such.

Also, w/ 3-ph, motors produce constant torque, unlike that of 1-ph/2-ph. Rectified 3-ph is used in aircraft power, and has much less ripple than 1-ph, 4.2% vs. 48%. The advantages of 3-ph over 1-ph & even 2-ph are abundant. As I said, there are special needs for more phases, such as rectification w/ very low ripple, where 6-ph/12-ph systems are employed. Have I helped?

Claude

8. Mar 13, 2013

rbj

ever see a tower like depicted in the attached?

pick out 3 alternate phases (not adjacent phases) and you have 3Φ.

the student paper shows a simple transformer circuit to get 6Φ outa 3Φ (just like getting 2Φ, 220V, with center-tap outa 1Φ like in your house, but there are 3 of them).

the advantage is, i believe (i'm a DSP guy, not a power guy), that the voltage between adjacent lines is reduced relative to 3Φ. meaning less insulation or spacing. dunno exactly how that helps when the insulators are connected to the steel frame which is ground, the same neutral that the phase voltages are referenced to and the phase voltage is not reduced.

but, nonetheless, i think that 6Φ is a reality in practice for some of the super high-voltage transmission lines. but it would be nice if someone in the power industry could confirm.

Attached Files:

• tower.pdf
File size:
61.8 KB
Views:
38
9. Mar 14, 2013

sophiecentaur

I have seen plenty of towers like this, carrying two separate (3 Ph) circuits in the UK. I think the picture was of 'what it could look like'.
I think that could introduce serious problems with Imbalance unless you could arrange two equal loads, one on each triad.
I'd just like a reference to somewhere that it's actually been done on a large scale.
I have no problem with this idea at low power- the sums all seem quite reasonable.

10. Mar 14, 2013

rbj

when i fiddle with the phasors, if both 3Φ systems are themselves balanced, but one has a big load and the other a small load (or no load), the return current in the neutral line (which you want to minimize) is zero.

if either triad is unbalanced, there is return current in the neutral line.

but there are a lot of IEEE papers referring to fault analyses of 6Φ systems. it would seem odd that all this research is done on something that doesn't exist anywhere.

11. Mar 15, 2013

sophiecentaur

I think the best indication of the feasibility or even any advantage would be whether or not such a system has actually been implemented and I haven't found a mention of a working system, yet. That paper doesn't have references or mention of a practical, large scale implementation - which give me reservations.
I think you could be right about the result of an imbalance.
BTW, dual 3Ph circuits have a very real advantage of redundancy in the case of damage.

12. Mar 15, 2013

Staff: Mentor

The advantage of high phase order for AC power transmission is increased power capacity for the same right-of-way footprint. Utilities have a hard time getting permission to build new lines, so increased capacity using the same right of way is attractive. Higher voltages (and higher towers) are one way. Higher phase order or double circuit lines are another.

Six phase transmission has an impedance advantage over two three-phase double-circuit arrangements. Same 6 conductors, same right-of-way footprint for both schemes. However at the substation, six-phase transmission causes extra complexity and cost, so it is not used. However, double circuit lines are commonplace. Look for towers with six conductors.

Six phase arrangement

2 3
1 4
6 5

Double circuit arrangement, circuit 1-2-3 and circuit 4-5-6
1 4
2 5
3 6

For non AC transmission applications where right-of-way footprint doesn't matter, one would have to think of some other advantages for higher phase orders.

13. Mar 15, 2013

sophiecentaur

So you're saying that it's no more than a theoretical exercise, at the moment?
I strongly suspected it was.

14. Mar 16, 2013