delta-delta, wye-wye, delta-wye, wye-delta transformer connections, which?

I have a number of question why I hope electrical engineers can shed light on:

  1. What is generally the ideal 3-phase transformer connection for a
2400 primary and 240 secondary?
  1. I am told that the advantage of the wye connections is its grounded neutral. However, is it really necessary to have system grounding?
  2. If loads are unbalanced, which connection is generally more advantageous?
  3. Safety-wise, which connection is more advantageous?
Reply to
johnn
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There are lots of primary voltages and secondary voltages it depends on the area of the world you in. 12470-120/208, 13200-240 3 phase, 13800- 120/208,

4160-240 3 phase to name a few that I have worked on. The only place I have seen 2400 is in the mining industry and residential distribution. I am speaking from my experiences only

Delta-wye connections in transformers help reduce harmonics to the lower voltages.

Depends on what your doing. There are lots of ways do have electrical distribution. If I was in a factory and did not need an neutral. I might consider an delta system. Non grounded systems are used in certain industries.

I like grounded systems for most situations. Less training for the users and less of an issue with insurance companies.

Reply to
SQLit

On 9 Sep 2004 01:57:56 -0700 johnn wrote: | I have a number of question why I hope electrical engineers can shed | light on:

I'm not an EE, but I've been reading a bunch of stuff here and other places, and maybe my different perspective could help make a better explanation.

| 1. What is generally the ideal 3-phase transformer connection for a | 2400 primary and 240 secondary?

It depends.

Is your power coming in 2400 delta, or 4160/2400 wye (it matters)?

| 2. I am told that the advantage of the wye connections is its grounded | neutral. However, is it really necessary to have system grounding?

A grounded neutral is definitely a benefit, at least on the last transformer. It's not essential, and there are tradeoffs that can in many cases favor not having one. Only qualified personnel should ever work on ungrounded or resistive grounded systems. They can be made safe, but customary practices for grounded systems can put people at risk on ungrounded systems.

| 3. If loads are unbalanced, which connection is generally more | advantageous?

I'm not aware that this would make that much of a difference. But a delta secondary would let the other phases supply some power to an overloaded phase, at the tradeoff of a poor power factor on those phases. But I don't know that this is a good thing. If your loads are (nearly) all single phase, then a third solution (see below) might be better.

| 4. Safety-wise, which connection is more advantageous?

I'd go for the wye secondary. You can always connect a delta load to a wye secondary if the voltages are right. For a 240 volt delta load, you'd have to have a 240Y/139 volt system to do it, which is probably why the

240 delta has not been phased out as rapidly as 480 delta.

When you have a wye-delta, you can also get backfed power when one phase goes out. The 2 live secondary windings will energize the 3rd, which in turn energizes the coupled primary phase at primary voltage, feeding the primary lines. Then anyone else who has a delta-wye transformer on that same primary circuit will be drawing that power from your transformer. If there's enough current in that, your transformer primary fuse will blow, and you'll have to wait for additional power company service before your power is restored. If your choice of wye-delta is overriding the power company choice of delta-wye, you may be obligated to pay for the fuses and labor to replace them at a few hundred dollars a pop (the fuses are not cheap, nor are the people).

If you can better describe your power load and usage patterns, I think we all can give you a better idea of your choices, and make a better recommendation. There are scenarios where just about everything could be a best choice.

If all your loads are SINGLE phase, and you just need to supply it all from a three phase service, then my recommendation would be 3 separate SINGLE phase transformers producing 120/240 with their neutrals all solidly tied together and grounded at one point just before the 3 separate single phase panels. This will handle extreme imbalances due to crazy single phase usage patterns better. Tranformer costs go up while panel costs go down.

If you also have some three phase needs, you can also get them from that bank of 3 transformers. You'll have 2 separate 208Y/120 systems there (actually a 6-point star system in total).

If you have 240 volt motors that cannot be rewired or reconfigured to 208 volts, you'll need some other solution. A separate smaller delta-delta transformer just for that would be one way to go (since 240Y/139 transformers are next to impossible to find).

Reply to
phil-news-nospam

delta delta- possible and works well- no ground can mean that the neutral on the secondary floats. That is not good from a safety basis. This is also true of a wye delta. Single phase voltages will be line to line so if you have a 240V line to line secondary you can handle 240V single phase loads. If you center tap one winding to ground you can handle 120V loads but suh loads are limited to one leg.

Wye-wye is not used as there is no path for triplen harmonics in the magnetising current and there will be 3rd and multiples of 3rd harmonics in the voltage waveform. Not wanted. (where wye-wye is used, there is usually a tertiary winding which is in delta- this takes care of this problem)

Delta-wye with the delta on the high side provides for a grounded neutral and elimination of floating voltages Single phase loads can be taken off any leg to neutral and can be balanced between phases- this is also an advantage. Thus you get 208/120V service. You can use either 3 single phase transformers or a single 3 phase transformer for each connection. At this voltage level single phase units are everywhere so that it is often advantageous to use 3 single phase transformers.

Overall, except in very specific situations, a grounded secondary provides safety benefits. Unless there is a good reason to do otherwise- the delta HV/Wye LV is generally the best choice for this level of distribution.

Backfeed has been mentioned (wye/delta transformer with one HV leg open ) -possible but probably a non -issue in general. There will be an associated inbalance due to impedance differences and, while there may be some loading on the transformers, it will not appear on the energy bill.

For high voltage lines the HV is usually wye and the LV of the transformers are delta -for several advantages which result -on a delta HV, atmospheric charges can cause flashovers, ground faults can produce severe damage without detectable changes in line currents (fault fed through line capacitance), cost advantage to transformers as the insulation can be graded from hot end to ground rather than be the same throughout.

Don Kelly snipped-for-privacy@peeshaw.ca remove the urine to answer

Reply to
Don Kelly

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