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Old 04-05-2009, 04:24 PM   #1
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3-Phase transformer question.


When is an open wye-open delta setup with two xformers, lighting and power used and when is an ungrounded wye-delta with 3 xformers, two powers and a lighting used?

Whats the difference? It looks like open wye-open delta are fed with two phases and ungrounded wye-delta is fed with three phases. Output voltages are the same.

Yeah, I know I'm stupid. But I want to learn more about power distrubution.

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Old 04-05-2009, 05:26 PM   #2
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3-Phase transformer question.


An "open wye" and open delta are electrically the same thing, except an open wye is only used on the primary side of some distribution systems. This systems isn't used much, but it allows the utility to save some money by not using two transformers, one on the primary and one on the secondary.

The open wye is just two legs of a three phase system plus the neutral. You have the necessary 120 degree phase shift to produce 3-phase power. On the open delta secondary, you can get 3-phase, but your available power is reduced to 57.7% of what you would have with a full delta.

This is NOT an ideal system. It is purely a money saving scheme. Almost all distribution at the consumer level will either be delta-wye or delta-delta. Ungrounded delta secondaries are ususally for special purposes in industrial plants. Other deltas will usually be grounded between A and C phase to produce a neutral for loads that require it. Other deltas will be "corner grounded" in order to give a ground reference for overcurrent devices.

More info available upon request....

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Old 04-05-2009, 06:04 PM   #3
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3-Phase transformer question.


Thank you very much I'm beginning to understand 3-phase more.

What I have wondered is how electrical is usually distributed in a large commercial building. Are the poco kv lines usually stepped down to 480 and then distributed to points in the building where the 480 is brought down to 120, 208, etc.
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Old 04-05-2009, 06:10 PM   #4
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3-Phase transformer question.


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Originally Posted by rgsgww View Post
Thank you very much I'm beginning to understand 3-phase more.

What I have wondered is how electrical is usually distributed in a large commercial building. Are the poco kv lines usually stepped down to 480 and then distributed to points in the building where the 480 is brought down to 120, 208, etc.
In our plant, the poco comes into a transformer at some higher voltage from 2400 up and the secondary side is 480 wye.
This feeds into the switch gear, which then feeds the buss duct through out the building. From the buss, 480 is taken to a panel for lighting, and then to a transformer to feed a 208/120 volt panel.
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Old 04-05-2009, 06:34 PM   #5
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3-Phase transformer question.


http://en.wikipedia.org/wiki/Electricity_distribution
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Old 04-05-2009, 06:56 PM   #6
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3-Phase transformer question.


Thanks guys.

JBfan, thats how I thought it might be done, but I was not sure.
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Old 04-05-2009, 07:07 PM   #7
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3-Phase transformer question.


Some of the huge buildings around here have 4160 3 phase as the service.

There are transformers (substations) that go from 4160 to 277/480, and 4160 to 120/208. If the 120/208 load is not much, there'll be a transformer from 4160 to 480, then another from 480 to 120/208.

A lot of these buildings have large A/C units (chillers) and the motors can be several hundred HP. These big motors are usually 4160.

There are a couple of buildings that have 7200/12470 or 14400/24940 services. This is normal POCO distribution voltage around here, they simply meter is as it comes in. Inside the buildings there are transformers with 277/480 or 120/208 secondaries.

Several of the power plants I've done have 34,500, 60,000, or 120,000 coming into the property, there is a large transformer with either a 4160 or 13,800 secondary.

The reason for the high voltage is because the higher the voltage, the smaller the wire. Here's an example; The last power plant I did operated at 13,800 volts. It had a 2000 KVA diesel generator that was used to start the rest of the plant. This generator operated at 13,800 volts, just like the rest of the plant.

The current of the generator was 251 amps. The wire size was 4/0 copper. This was 15KV wire, so it is a bit harder to terminate than 600 volt wire. (I know...4/0 cu is only good for 230 amps, this is 15KV, so table 310.77 applies.)

Had the generator been 480 volts, the current would be 2406 amps. The wire here would be 6 sets of 750 MCM! I've done both, it's MUCH easier to terminate one set of 4/0 15 KV than 6 sets of 750. Also, while 4/0 15KV wire isn't exactly cheap, it costs far less than 6 sets of 750. Further, 3-4/0 15KV wires will fit in 1-5" conduit, the 750s would need 6-4" conduits. A huge difference in labor.

A bit different than a little ol' house service, huh?

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Old 04-05-2009, 07:20 PM   #8
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3-Phase transformer question.


And some people think doing electrical work is easy and us electricians are over payed.
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Old 04-05-2009, 07:59 PM   #9
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3-Phase transformer question.


Yeah, wire size kind of reminds me of the ac dc war
Dc needed such large wire to compensate for VD.

2406 amps? I'd think Id go for 13kv instead of 480
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Old 04-05-2009, 08:01 PM   #10
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3-Phase transformer question.


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And some people think doing electrical work is easy and us electricians are over payed.

I'm not an electrician of course (hence the question I'm asking) but I do understand that it isn't an easy job. Does look like an awesome job to have
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Old 04-05-2009, 09:52 PM   #11
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3-Phase transformer question.


Don't know if this helps, but here are a few pictures of a service/distribution job I did a few months back

The utility transformers are behind the gear to my left. No 480 in this one, just 120/ 288. Incoming was 12,000 volts on the primary. To my left is the service disconnects for both sections of gear that fed distribution panels, which in turn fed load centers (panels) all over the building.


The next photo is basically some of the pipes that are in the distribution system. Some of them are parallel feeds, but most are not, one pipe per panel.



I have some pics of a utility vault I can post once I dig them up if you are interested.
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Old 04-06-2009, 07:21 AM   #12
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3-Phase transformer question.


Sure, those are pretty neat pics.

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