AC Power Distribution

[DrivingMissLazy]

Please note that this post has been revised to remove erroneous information regarding three phase Delta connections DML

There have been several postings regarding AC voltage distribution recently, the most recent here on the other board:

http://www.busnut.com/bbs/messages/11/19999.html?1203155422#POST124052

and which contains some misleading information. Hopefully I can shed some light on this for anyone that might be interested.

GENERAL

The most standard three phase higher voltage distribution system, within a facility, is 480/277 volts in a Wye configuration. Voltage between any two of the three hot legs is 480 volts. Voltage between any one of the hot legs and neutral is 277 volts. The neutral connection is common to all three of the hot legs.

All neutral current flows thru this common neutral. This prevents installing separate neutrals for each 277 volt connection.  For example, many industrial fluorescent lights are designed to operate from the 277 voltage taken from one hot leg and the common neutral leg. These are typically utilized in an industrial environment.

For lower voltage requirements this voltage (480/277) is stepped down, by transformer to two different configurations:

1.  240/120 volts three phase delta configuration for industrial applications such as manufacturing facilities or,

2.  208/120 volts three phase Wye configuration for commercial applications such as office buildings.

120 volt single phase voltages are derived from one of these two configurations using two hot legs and a neutral.

DELTA CONFIGURATION VOLTAGES

In the 240/120 single phase connection, for the campground, two of the hot legs from the utility supply, such as phase A and phase B, are connected and the neutral is derived from the center connection between phases A and B.

240 volts is of course from the Phase A and phase B legs and 120 volts from either the phase A or phase B hot leg and the center tap neutral.


WYE CONFIGURATION VOLTAGES

The 208/120 Wye configuration is a whole totally different animal. The neutral is a single conductor taken from the center point of the Wye where the three windings are tied together at the junction of the Wye. This configuration is especially useful in a building where a lot of 120 volt outlets are required, such as a computer facility.

Single phase 208 volts is available from A and B, A and C or C and B. 120 volts is available from the common neutral and either A or B or C.  Having said that, I am not aware of any campgrounds that are configured like this, but it certainly is  possible.

Comments, suggestions or corrections to this is welcomed since I might have got a bad batch of brew last night. LOL

Richard

BTW, it is common for the Wye conductor neutral to be double the size of any of the hot conductors since the possibility exists that the neutral current may be higher than any one of the hot conductors. This is particularly true where a lot of PC type computers are installed.

 

 

[Sean]

Great post, Richard, and saves me having to do it over in that other thread.

... I am not aware of any campgrounds that are configured like this, but it certainly is  possible.

We've found several pedestals derived from 208/120 Wye, including at least one state park (but don't ask me where), a fairgrounds (I think in Stockton, California), and this fancy new system being installed at many truck stops: http://ourodyssey.blogspot.com/2008/01/more-on-shorepower.html

BTW, it is common for the Wye conductor neutral to be double the size of any of the hot conductors since the possibility exists that the neutral current may be higher than any one of the hot conductors. This is particularly true where a lot of PC type computers are installed.

Just to elaborate for a moment here, the reason for oversizing the neutral in such a Wye system, particularly in computer environments (we used to routinely double the neutrals in all our computer rooms) has to do with harmonic currents, which are common when switching-mode power supplies are in use.  Too complex a subject to delve into here, but I thought the reasons for the oversized neutral needed to at least be stated if not fully explained.

-Sean
http://ourodyssey.blogspot.com

[DrivingMissLazy]

Great post, Richard, and saves me having to do it over in that other thread.

... I am not aware of any campgrounds that are configured like this, but it certainly is  possible.

We've found several pedestals derived from 208/120 Wye, including at least one state park (but don't ask me where), a fairgrounds (I think in Stockton, California), and this fancy new system being installed at many truck stops: http://ourodyssey.blogspot.com/2008/01/more-on-shorepower.html

BTW, it is common for the Wye conductor neutral to be double the size of any of the hot conductors since the possibility exists that the neutral current may be higher than any one of the hot conductors. This is particularly true where a lot of PC type computers are installed.

Just to elaborate for a moment here, the reason for oversizing the neutral in such a Wye system, particularly in computer environments (we used to routinely double the neutrals in all our computer rooms) has to do with harmonic currents, which are common when switching-mode power supplies are in use.  Too complex a subject to delve into here, but I thought the reasons for the oversized neutral needed to at least be stated if not fully explained.

-Sean
http://ourodyssey.blogspot.com

Sean,
Thank you. My post was getting so long that I decided to not try and go into any more details at the time. It's really hard to decide where to stop as there is so much information that could be posted. Another example is the different three phase voltages that have been used in the past 40 years. I personally have witnessed the following, and there may be more:
575, 480, 460, 440, 415, 380, 240, 220, 200 and 115. These are all three phase voltages. There also are (or used to be in the 1980's) 25 Volt DC distribution systems.

Back in the 80's when the non-linear power supplies started to proliferate, there were many cases of electrical fires, or neutral connectors and conductors running red hot. That is when the procedures changed and the neutrals started being doubled. These power supplies are contained in the PC type computers and probably many other computer devices.

Just some more useless little bits of information. LOL

Richard

[gecole]

"There are three different neutrals"
I guess you lost me here Richard. I've never seen a 240V delta transformer which had more than one center tapped neutral. The reason is that providing and additional center tap would allow the transformer to be overloaded due to the peculiar aspects of phase angles. By the way the high leg voltage is 120 X the square root of 3 which is about 208 volts.

My original post was modified to remove the "There are three different neutrals" statement. It was erroneous. DML.

[Sean]

Yes and yes.

Typical campground power is a lot like typical residential power.  It's two-wire, 240VAC with a center tap on the secondary side of the transformer.  The center tap is grounded to earth at the point of entry.  So, it is entirely possible that a neighborhood might be served by either delta or wye power (depending on the utility), with two-wire 240VAC going to each residence (or campground).  From the point of view of the subscriber, it makes no difference what the originating source is upstream, since you are using only a single phase.   (When all you have is one phase, you can't tell whether it's the side of a delta or a branch of a wye.)

-Sean
http://ourodyssey.blogspot.com

[Nick Badame Refrig/ACC]

Hi Richard,

Can you add this information to the Green Book?
http://www.busconversions.com/bbs/index.php?topic=5097.0

Nick-

[DrivingMissLazy]

Hi Richard,

Can you add this information to the Green Book?
http://www.busconversions.com/bbs/index.php?topic=5097.0

Nick-

I just checked Nick, and it is already there on the Help board.
Richard

[Len Silva]

Richard and Sean,

This is considerably OT but may be interesting to some.  Back in the 70's I was installing microwave/carrier at Bonneville Power.  One of their engineers was telling me that they had a 800 kv DC transmission line from Portland OR down to Los Angeles.  The reason was that the distance was the wave length of 60 hz and there were too many losses in the AC transmission line.

FWIW,
Len

[DrivingMissLazy]

Richard and Sean,

This is considerably OT but may be interesting to some.  Back in the 70's I was installing microwave/carrier at Bonneville Power.  One of their engineers was telling me that they had a 800 kv DC transmission line from Portland OR down to Los Angeles.  The reason was that the distance was the wave length of 60 hz and there were too many losses in the AC transmission line.

FWIW,
Len

Very interesting. I was aware that many years ago some 25 volt distribution lines had been installed in the North East and in the Chicago area, but had never heard of the high voltage DC distribution. I wonder how they reduced the DC voltage to a useable level.

Richard

[gecole]

Len I live in Portland and did not know that. There's an interesting write up in Wikipedia describing how the system works.
http://en.wikipedia.org/wiki/Pacific_DC_Intertie

[HighTechRedneck]

The Sylmar Converter Station which converts DC to AC (a process also called inverting) and phase-synchronized with the L.A. power grid.

....

In 2004, Sylmar East station was upgraded from 1,100 MW to 3,100 MW (rededicated as the Sylmar Converter Station in 2005). The controls and older converters, including the mercury arc valves, were completely replaced by a single pair of 3,100 MW 12-pulse converters built by ABB. In parallel with this project, the six-pulse mercury arc valves at the Celilo Converter Station were replaced with Siemens light-triggered thyristors (MARP replacement).

That is clearly one awesome pure sine inverter.    (presuming pure sine, since everything works fine out there, electrically that is  )

[TomC]

Awesome amount of power-and that it is earth return grounded!   I had never heard of any DC power transmission.  But obviously it works.  Good Luck, TomC

[Len Silva]

Many of the old cities of the northeast had DC power distribution.  I know from personal experience in the 60's that some Boston neighborhoods still had DC power.  Most homes had a rotary converter to make AC when the need first appeared. Back in those days, most older homes had a single 15 amp fuse for the whole house. They were usually replaced by a 30 amp (or a penny).  Burned down a lot of houses.  I grew up in such a house, power management was a constant part of our lives.  Had to turn off some lights to make toast and hope the water pump didn't come on.

Here is a story of the last system in NYC:
http://cityroom.blogs.nytimes.com/2007/11/14/off-goes-the-power-current-started-by-thomas-edison/

[DrivingMissLazy]

Great link Len. Thanks.

Richard