2 phase transformer/diversity factor

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Mayimbe

Senior Member
Location
Horsham, UK
if the source is a 2 phase, 2 wire system and the load is 3 phase 4 wire, which type of transformer should i use on that configuration? open wye - open delta? 13.8/0.48 kV (1000kVA)

i got to design 5 feeders at 480 V (1000 KVA) for industrial purposes, one of those feeders is at 207 V for offices (3913 ft^2). im having trouble to decide what diversity factor should i use? does anybody know?

the other 4 feeders (480 V) are for welding machines, compressors, and other machinery for construction. in this case does anyone know where i can find typical values of this types of loads?:confused:
 

charlie b

Moderator
Staff member
Location
Seattle, WA
Before I could give you any assistance, I would like to ask for a clarification or two. To start with, "2 phase systems" are very rarely used these days. Are you certain you are using the correct description? Secondly, it sounds like you are trying to get three phases from a single phase source, using only a transformer. That is not physically possible. Could you give us some more information about your power source and your situation? :confused:
 

Mayimbe

Senior Member
Location
Horsham, UK
Lets see. this is what i got:

Say the company "A" tells to my company "B", design for me a termo-generation complex of aproximatedly 1000 MVA. in order to do that the company "A" gives a conecction point at 13,8 kV to build the provitional facilities (my job). when my company "B" goes to the place (very very rural), the only circuit that we see is a 2 phase with no neutral, at 13,8 kV.

i did my research and i find in "electric distribution handbook" by T. A. Short, that is posible to suply a 3 phase (4 wire) system from a 2 phase system, using an open delta open wye transformer. have you heard anything like this?

there's a lack of information in the handbook about this connection so i need somebody from there (USA) to comfirm that teory.

there are other solutions to this problems. but this one (if it works) is the least expensive.

i hope i make myself clear, sorry for my low budget english.

PD: the question about diversity factor still standing
 

Cold Fusion

Senior Member
Location
way north
May -
There are a couple of issues here.
---when my company "B" goes to the place (very very rural), the only circuit that we see is a 2 phase with no neutral, at 13,8 kV. ---
No one here has any idea of what a "2 phase, 2 wire, 13.8kv system" is. Most, including me, don't think it even exists. Take another look at your book by TA Short. Does not it show that "2 phase" has 4 wires?

13.8kv, 2 wire is most likely two wires from a 3 phase source and that is not 2 phase.

---there's a lack of information in the handbook about this connection so i need somebody from there (USA) to comfirm that teory. ---
You can't get 3 phase with only 2 wires

---, sorry for my low budget english. ---
That part is okay. Most of us will work at translating what we think you are trying to say.

---there are other solutions to this problems. but this one (if it works) is the least expensive.---
If you need a lot of three phase power, the only solution is to get the third wire from the generating plant out to the location.

---PD: the question about diversity factor still standing
No one is going to know typical loading for industrial loads. It completely depends on what the process is doing.

All I do is industrial electrical. And when I am looking at the power required, I discount all of the redundant loads and size the electrical for 125%, maybe even up to 200% of the remaining equipment. It's industrial - the owners want to make money. They will run the system as hard as they can.

cf
 

Mayimbe

Senior Member
Location
Horsham, UK
thanks for the concern.

this is what TA short says in his handbook (about the open wye open delta TRX configuration):

"The main advantage of the open wye ? open delta transformer configuration
is that it can supply three-phase load from a two-phase supply (but the
supply must have a neutral)"

so it has to be 2 phase 3 wired. the diagram is attached (i hope).


for the diversity part, what you say (Cold Fusion) isnt really what i meant, but when you said "They will run the system as hard as they can", it put the things that i was thinking in a different perspective.

Thanks again.
 

LarryFine

Master Electrician Electric Contractor Richmond VA
If your primary supply is two phases and the system neutral, you can supply 3-phase, but your secondary will be open Delta, not wye.
 

winnie

Senior Member
Mayimbe,

I believe that you are using '2-phase' in a fashion that is confusing you and your question.

'2-phase' is an archaic polyphase system where the voltage vectors are 90 degrees out of phase.

As others have noted above, you probably have two wires of a _3_ phase system.

Now a 3 phase distribution system will actually consist of _four_ wires; 3 'phase' conductors and a grounded neutral conductor.

Can you confirm that you have 2 'phase' conductors and the grounded neutral?

If you have 2 'phase' conductors and the grounded neutral, then you can connect open-wye/open-delta as per your diagram to derive a complete 3 phase system. If you have 1 'phase' conductor and the grounded neutral, then you cannot use this connection. The diagram that you posted shows the whole connection; but you need _three_ wires on the primary side; 2 phase conductors and the neutral.

Note the loading described in the diagram. Your primary neutral has to carry far more current than the phase conductors, and the VA rating of the transformer bank is degraded by the inherent power factor associated with open banks.

-Jon
 

winnie

Senior Member
Oh, with a suitably connected transformer bank, you can go from 'open-wye' to 'full-wye'. The high neutral loading is the same as per your diagram, and conceptually it is your diagram feeding a standard closed-delta to full-wye transformer.

This requires a bank consisting of 4 separate single phase transformer secondaries, 2 per supply phase (these can share a single primary coil and core). Two phases are supplied directly, the third phase is supplied by the other two secondaries in series. The 'zig-zag' connected phase will have poorer voltage regulation and higher impedance than the directly connected phases.

-Jon
 

Cold Fusion

Senior Member
Location
way north
---this is what TA short says in his handbook (about the open wye open delta TRX configuration):

"The main advantage of the open wye ? open delta transformer configuration
is that it can supply three-phase load from a two-phase supply (but the
supply must have a neutral)" ---.
I haven't read TA Short's book. However, someone should slap him for using sloppy terminology in a published text. It's not your fault, it is TAS's.

Your picture is not showing "2 phase". It is "2 phases of a 3 phase Wye". The terms used really matter. "2 phase" is something completely different.

---so it has to be 2 phase 3 wired. the diagram is attached (i hope). ---
Well, I would not use the term "2 phase". That will only confuse anyone you talk to. As to getting 3 phase power, you still have to have three wires: Either two phases and a neutral from a three phase supply, or all three phases.

cf
 

Mayimbe

Senior Member
Location
Horsham, UK
I haven't read TA Short's book. However, someone should slap him for using sloppy terminology in a published text. It's not your fault, it is TAS's.
its all confusing, the reference that TA Short uses is the IEEE 105-78. This is what the standard says about the open wye open delta configuration:

"Open Y Primary to 4 wire 208 V Service. if a 4 wire 3 phase 208 V service must be installed and only an open Y primary is available, and if the expense of adding the third phase conductor is considerable (my case), resort may be had to a connection of two single phase transformer with dual voltage secondaries. At least one transformer must have four secondary bushings, that is, the secondary voltage rating must be 120/240 V. The other transformer may be a 3 bushing transformar rated 240/120 V :)-?). Each single phase transformer must be sized for 2/3 of the balanced 3 phase load."

im still confused, but i guess i will forget the whole matter and ask for the 3 phase usually connection to the company. anyhow the standard also says that "the combined effect of unbalanced current and unsymmetrical banks is too complex for any but the most general remarks". :cool:

PD: does anyone knows where i can find the ampacity calculation for cables/XLPE in horizontal conduit in air, 600 V, unshielded, double circuit, single conductor? tables work too. im having trouble with the IEEE 835-94.

sorry for all the confusion
 

Cold Fusion

Senior Member
Location
way north
--- the reference that TA Short uses is the IEEE 105-78. This is what the standard says about the open wye open delta configuration:

"Open Y Primary to 4 wire 208 V Service. if a 4 wire 3 phase 208 V service must be installed and only an open Y primary is available, and if the expense of adding the third phase conductor is considerable (my case), resort may be had to a connection of two single phase transformer with dual voltage secondaries. At least one transformer must have four secondary bushings, that is, the secondary voltage rating must be 120/240 V. The other transformer may be a 3 bushing transformar rated 240/120 V :)-?). Each single phase transformer must be sized for 2/3 of the balanced 3 phase load."---
The guide is C57.105 and it is an IEEE guide. It is a pretty good one. The C57.105 diagrams show how it is connected - pretty much as Winnie described. You may wish to get a copy of the C57.105.

---im still confused, but i guess i will forget the whole matter and ask for the 3 phase usually connection to the company. anyhow the standard also says that "the combined effect of unbalanced current and unsymmetrical banks is too complex for any but the most general remarks". ---
Asking for the three phase is definitely a good choice. However, as you said, getting the third conductor is going to be expensive. So the question we have asked is: Is there a neutral at the site along with the two phases? Because if there is, then the zig-zag wye described in C57.105 (and by winnie) is an viable alternative - and may well be cost effective.

---PD: does anyone knows where i can find the ampacity calculation for cables/XLPE in horizontal conduit in air, 600 V, unshielded, double circuit, single conductor? tables work too. im having trouble with the IEEE 835-94. ---
IEEE835 is pretty formidable - that thing is over 3000 pages. I stay out of it. Have you considered using the tables in the NEC, T310.16? As I recall, XLPE insulation is types USE-2 or XHHW-2. I am not sure what "double circuit, single conductor" means. Is this two parallel conductors per phase, total six conductors in the conduit?

cf
 

winnie

Senior Member
This is the phase diagram for the 'open wye' to 'full wye' that we've been describing.

Each primary phase needs _two_ secondary coils, and each coil needs to provide the full 'line-neutral' secondary voltage. As drawn, the 'A' coils are connected end to end, so they can be provided by a 'three bushing' center tapped coil. The 'B' coils are connected separately, thus requiring a 'four bushing' secondary with two separate coils.

-Jon
 

Mayimbe

Senior Member
Location
Horsham, UK
Asking for the three phase is definitely a good choice. However, as you said, getting the third conductor is going to be expensive. So the question we have asked is: Is there a neutral at the site along with the two phases? Because if there is, then the zig-zag wye described in C57.105 (and by winnie) is an viable alternative - and may well be cost effective.
i didnt see any neutral along with the two phases. picture is attached. theres a ilumination circuitry under the 2 phases, but it wont do anything to the problem? does it?

IEEE835 is pretty formidable - that thing is over 3000 pages. I stay out of it. Have you considered using the tables in the NEC, T310.16? As I recall, XLPE insulation is types USE-2 or XHHW-2. I am not sure what "double circuit, single conductor" means. Is this two parallel conductors per phase, total six conductors in the conduit?
cf
yes two parallel conductors per phase, six cables in total. i dont have the NEC. i just did a calc this morning to see the ampacity level, but i feel something is not right. I obtain a current allowable of 838 A for conductor with this data: 1000 MCM, copper class B, PVC conduit, 38 ?C enviroment temp, conductor temp at 60 ?C, XLPE (thickness 4.45 mm) , jacket (thickness 2 mm). i did follow the procedure from the IEEE835, but the problem is, why they dont consider the solar radiation over the conduit? neither the wind velocity or the height of the conduit?
 
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LarryFine

Master Electrician Electric Contractor Richmond VA
At least one transformer must have four secondary bushings, that is, the secondary voltage rating must be 120/240 V. The other transformer may be a 3 bushing transformar rated 240/120 V :)-?).
Allow me to explain.

The former refers to two 120v windings, which can be wired either in series (for 240v with what is effectively a center tap) or in parallel (for 120v only).

The latter is a single winding with a permanent center tap, giving you only 120/240v, the same as the series connection with a dual-secondary unit.
 

Mayimbe

Senior Member
Location
Horsham, UK
in the lower crossarm? that photo of the pole was taken with large zoom because it was to far away, and due to the conditions of the site (rained the day before) nobody was able to be near it. i dont know how to say this in proper english, but i will try. the lower crossarm its a configuration that you use when the angle between the two lines adyacents to the pole its bigger than 10 degrees.
 
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