Converting three phase power transmission to single phase power

[Tsiolkovsky]

Hi all, I'm not an electrical engineer but a question is itching my brain.

How does one convert three phase power to single phase power? (preferably with a diagram attached?)

The reason I ask is that the three phase power transmission system is the de facto power transmission system.
Also, concerning domestic households, single phase domestic power consumption is the de facto domestic power consumption method.

Given that these systems are ubiquitous in every corner of the world, I would expect an explanation of this on the internet yet not a single hit on google search. In fact, all I get are articles explaining the opposite process and claiming phase converters are much more used in converting single phase to three phase (the opposite of what Im looking for). REALLY?? Much more used in converting single to three phase??? Well maybe Im on the wrong planet here, but I in my world, three to 1 phase conversion is WAY more common as per my reasons above.

 

[jmbelectrical]

See Figures 4-17 and 4-18 in [URL unfurl="true"]http://constructionmanuals.tpub.com/14027/css/14027_75.htm[/url].

Depending on the transmission line system configuration (That is, three wire or four wire), a single-phase service can be derived from a single-phase transformer whose primary windings are connected in a phase-to-neutral or phase-to-phase configuration.

 

[LionelHutz]

There is no conversion, if you only use part of the 3-phase system then you get single phase. Connect between 2 phases or between 1 phase and ground. You also connect the different loads/sites onto different phases to keep the overall load on all 3-phases somewhat balanced.

 

[Tsiolkovsky]

Lets for simplicity sake ignore the transformer in the diagram and say we dont want to step up/down, we just want to branch off.

Please check the attachment picture below is what I'm talking about.

The top part of the pic is the transmission line (with no neutral line) and the bottom part is trying to tap off into a single phase for domestic use. Is it really this simple?

 

[stevenal]

If there is no neutral, what's your reference for the voltages displayed? If you allow one of the taps to be your reference, there is only a single sinusoid instead of two. Yes, it's that simple.

 

[waross]

The red trace is derived from the red line to the blue line.
The blue trace is derived from the blue line to the green line.
The green trace is derived from the green line to the red line.
Take any pair and you have single phase.
Take all three and you have three phase.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[waross]

Three phase generators may be reconnected in true single phase with a zig-zag connection or a double delta connection.
A ten lead generator may be connected for single phase on the low voltage only with the bar-diamond or Collins connection.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[LionelHutz]

Yes, it's really that simple. If it still isn't clear then try plotting blue - green which is the voltage applied to the load.

 

[stevenal]

If Bill is correct, the lower graph should only display the blue trace to represent the voltage applied to the single phase load.

 

[waross]

Good comment stevenal.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[prc]

One of the major applications of three phase to single phase AC power conversion is in AC rail traction systems. Power is taken across the two lines of 33 to 400 kV 3 phase lines and a single phase transformer( called traction supply transformer or track side transformer- rating 10 to 60 MVA)converts this to singe phase 25 KV, one terminal of transformer is going to overhead traction line and other end to rail or a return cable running parallel to rail. Some times a Scott connected transformer is used which is energized from 3 phase supply and two single phase outputs are got on secondary which are 90 degrees out of phase. To avoid shorting of these a small neutral link will be made in the overhead traction line to avoid shorting between secondaries.

 

[waross]

Something that is never mentioned in regards to the Scott connection and other phase transformation connection is the effect on the line power factor and the resulting line voltage drops.
An example that I know off the top of my head from generator conversions is the zig-zag connection (not the same as a zig-zag grounding transformer).
The zig-zag connection, with a unity power factor load, will have a power factor of unity in one supply phase, 60 degrees lagging in one phase and 60 degrees leading in the third phase. The KVA capacity of the generator or transformer bank will be reduced to 2/3 of the nameplate ratings. eg: When a 60 KVA three phase generator is reconnected in single phase, the single phase capacity will be 40 KVA.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[aruskuat]

actually you can use scott-T connection in order to make the balanced when you use 1 phase from 3 phase

 

[waross]

Define "balanced".
Current balanced?
Wattage balanced?
KVA balanced?
Power Factor balanced?


Bill
--------------------
"Why not the best?"
Jimmy Carter