Advantages and Disadvantages of Transformers connections 6

[mnnc]

Hi, I would appreciate it if anyone who can explain what are the advantages and disadvantages of transformer connected delta on the secondary side? connected Wye on the secondary side?

 

[waross]

PS
Thanks for the information cuky2000.
yours

 

[itsmoked]

Thanks waross.

Now for a "short" synopsis please.

Like in a couple sentences...

What is the problem with Star(pri)/Delta(sec) and why Star/Star fixes this?

I read and I read but me no understand.

 

[waross]

Thanks for the feedback; itsmoked;
Ist problem with Star/Delta;
When a Star/Delta bank loses "A" phase on the primary, "B" phase and "C" phase continue to supply the load in Open Delta configuration. The open Delta now energises the secondary of the "A" phase transformer.
If a fuse has blown on the transformer bank, no problem. Service will continue and the plant personel will probably not even be aware that they are running Open Delta. However the transformers may be overloaded.
The transformer will back feed but with the fuse at the transformer bank blown the back feed voltage is from the transformer primary terminal to the load side of the fused cutout. No current will flow.

If a fuse feeding the high voltage circuit blows then the back feed from the transformer tries to feed any other loads that are still connected to the primary phase.
If two phase fuses blow on the supply circuit then a Star/Delta bank will try to back feed 50% Voltage on the two phases. This low voltage is often responsible for equipment damage in other locations. Refrigerators and Freezers are at risk.
Transformer overloading is still an issue.

A Star/Delta transformer bank will try to balance unequal primary voltages. Unequal primary voltages will cause circulating currents in the Delta windings. This can cause fuse blowing, overheating and/or transformer burnout. Transformers with low impedance voltages are at the most risk.
Hope this helps.
yours

 

[mnnc]

Thanks waross for the fascinating post. Everyone here learned something.

 

[ccjersey]

The little transformer diagrams with wye to delta symbols don't tell you this kind of information.

We have open wye/open delta service, and the REA recently added the third primary. I was going to ask to have the open bank upgraded, don't think I will now. We have very good voltage balance (within 1 volt 240 V service) though the amperages are quite different on the phases of (usually lightly loaded)motors. Iwas hoping the amperages would be better, but that may be the motors and the light loading.

Thanks for your treatise of the subject waross.

 

[waross]

Your welcome ccjersey;
If your system is working well I would not change it.
If you have to add a third transformer to increase the capacity of the system I would recommend changing to a Wye/Wye connection. Your three phase and 240 volt motors will draw more amperage because they will now be supplied with 208 Volts.instead of 240 Volts. The motors will run a little hotter, but the cost of running will be the same.
Your single phase loads will still get 120 volts, but you will be able to distribute the single phase loads over all three phases instead of lumping them all on one phase.

Summary;
Upgrade only if you need more capacity. If you must upgrade, consider a either larger Open Delta bank, or changing over to a Wye/Wye system.
Some utilities limit the maximum capacity of an Open Delta bank.

 

[waross]

ccjersey
Another comment
I suspect that your unbalance currents may be due to a phase displacement on the unbalanced primaries. (I have encountered this on a big scale, but that's another book.) If phase displacement is causing the unbalanced currents, then your current unbalance will improve as the utility balances the loading on the primary circuit, which is the usual next step after adding the third phase.
Or, the unbalanced currents when the motors are lightly loaded may just be a characteristic of the motors.
I sometimes see this with no apparent reason and no problems.

 

[itsmoked]

Hi waross.

I understand all the conniptions with the secondary and the "back energizing" (most interesting and really completely understandable once it dawns on you that it happens.) What I'm having a problem understanding is why this wouldn't be a problem with a delta-delta transformer bank too.

 

[waross]

Hi itsmoked
Good question.
You need three current carrying wires to get three phase.
On an Open Delta, that is two energised phase wires and the neutral.
On a Delta/Delta, when a fuse blows, you have only two wires left to carry current.
If you draw a diagram of a Delta/Delta system and then erase one of the primary feeds, you will see one transformer across the line and the other two transformers in series across the same line.
One phase will have full voltage and the other two will divide the voltage between them according to their respective loads. That would be half voltage if the loads are balanced.
Because the three transformers are fed from the same two conductors, all the voltages will have the same phase angle.
yours

 

[wfowfo]

Quote:
"When a Star/Delta bank loses "A" phase on the primary, "B" phase and "C" phase continue to supply the load in Open Delta configuration."

This is only true if the Star common point is bonded to the system neutral. If it is floating, as most REA connections suggest, you would have the same single phasing effect as mentioned in the Delta/Delta.
Wye (no pun intended) would it be any different?

 

[waross]

Hi wfowfo
Your right.

I am not familliar with REA standards. I welcome comments from any one who is.

I suspect that there may also be a problem with harmonic current and again I welcome comments from some one more familiar then myself.

From the point of view of simple transformer theory;
If you do run a Star connected primary with a floating neutral for any reason you may expect the following results.
The neutral carries the unequal currents that will result from unequal loads.
The Star point is a junction that is subject to Kirkoff's law. What goes in must come out, but Kirkoff said it better.
Without a neutral connection, which must be carried back to the source, the secondary phase voltages in a Star/Star connection will be in proportion to the loads. Similar to an open neutral on single phase. The phase with the heaviest load will have the least current. The phase voltage on the lightly loaded phase in a Star/Star system with a floating primary neutral will approach line to line voltage.

In a Star/Delta connection without a neutral connection it is a little more complex.
A Star/Delta system has the ability to transfer energy from one phase to another. That is what causes the problems we have been discussing.
In a Star/Delta system with a floating primary neutral, as the load on one phase increases and the voltage tends to drop, the other two phases Act as an Open Delta to transfer power to the more heavily loaded phase and tend to reduce the load on that transformer. The magnitude of the currents that result in the various transformers will be related to the impedance voltages of the transformers.
Impedance voltages of modern transformers are low enough that I would expect very usable voltages and phase relations, but I have never tried a floating primary neutral in practice nor have I seen one.
However
"I am not young enough to know everything." --Oscar Wilde

In the event of a transformer or connection failure you could experience secondary phase voltages from 87% to 173% of normal depending on mode of failure.
A Star/Star system converts power from one voltage level to another.
Star/Delta often has long answers to short questions.
Yours

 

[stevenal]

waross,

The REA is now the RUS. A lot of standards are freely downloadable at:

http://www.usda.gov/rus/electric/bulletins.htm

 

[wfowfo]

I'll have to admit that, while I find this fascinating, it is also overwhelming. I fully intend to go back over all the posts to see if I can grasp it a little more firmly.
(waross....my eyes got tired reading; your poor fingers must hurt)

However, when a lineman asks me if it's OK to bank two or more transformers together, I don't think he'll appreciate me giving him a printed handout of this.

Is there a simple rule of thumb to go by for ALL bank configurations in terms of the nameplate impedances of the transformers?
For instance, I've seen some instructors claim that the difference must be limited to 7.5% of the nameplate impedance. So a 2% impedance pot could be banked with anything ranging from 1.85% to 2.15% (IE-2% X .075 = plus or minus 0.15%)
Please bear in mind that I'm referring to a 2 or 3 pot bank, not paralleling banks together.

 

[waross]

Are you talking about the "Dark and stormy night" a transformer has failed and we don't have an exact replacement in the yard?
Star/Star system;
Use same kva or larger. A different impedance voltage will give you a slightly different secondary voltage at full load. The difference will be the difference between the impedance voltages. A 1.6% transformer will have a full load drop of 1.6%.
A 3.2% transformer will have a full load drop of 3.2%.
The voltage difference will be 3.2%-1.6%=1.6%
On a "Dark and stormy night" that's as good as perfect.

Star/Open Delta similar expectations to Star/Star.

Star/Delta
Use the same rating or larger.

Paralleling.
"It was a dark and stormy night" and that big sucker failed.
We have a yard full of transformers but nothing that big and the impedances are all different. Can we parallel something to get the plant back on line?
If the impedances are the same you can probably parallel them. Two 250 KVAs to replace a 500 KVA.
I say probably because impedance is not the only factor involved in paralleling transformers.
If the impedances are different, then the transformer with the least impedence will "Hog" the load.
Example;
Transformer #1 100 KVA, 1.6% Impedance.
Transformer #2 100 KVA. 3.2% Impedance.
When transformer #1 is at full load, transformer #2 will be at 50% load.
For different impedance voltages use the rule of thumb;
Multiply by the ratio of the impedance voltages.
If you need a 100 KVA at 1.6% and you have a yard full of 3.2% transformers, the ratio is 2;1 so multiply 100 KVA by 2 and use a 200 KVA 3.2% transformer.
If you have 2.4% transformers, The ratio is 2.4% to 1.6% or 1.5:1.
1.5 times 100 KVA is 150, use a 150 at 2.4% KVA.
The voltage will drop 1.6% at full load on a 1.6% impedance transformer.
The Voltage will drop 3.2% at full load on a 3.2% impedance transformer.
The voltage will drop 1.6% at 50% load on a 3.2% impedance transformer.

THE LONGER ANSWERS

If an exact match is not available, re-rate a larger transformer to the same impedance voltage and use it.
eg; A 3.2%, 200 KVA transformer can be rerated to 1.6% by


Transformers in the same bank. Eg; 3 transformers in one bank.

Star/Star.
At full load, the phase voltage drops on each phase will be equal to the impedance voltages of the respective transformers.
For example if you use a 1.6% transformer in the same Star bank, as two 3.2% Transformers, the voltage on one phase to neutral will be (3.2-1.6=1.6) Volts high.

Star/Delta
Always the short question with the long answer!
If your impedances are 3.2%. 3.2% 1.6%, then the 1.6% transformer will load up first.
If this is an emergency situation and you have to get back on line with whatevr you can find in the yard, then I would sugest an artificial re-rate of the transformers.
If you change the KVA rating of a transformer the impedance voltage changes.
Example; If you arbitrarily re-rate a 3.2% at 100 KVA transformer to 200 KVA, it's new impedance voltage will be 1.6% (But it will probably start to overheat at about 51% load.)
On the other hand, If you re-rate a 3.2% at 200 KVA transformer to a 100 KVA transformer, the impedance voltage will now be 1.6% and it can be safely used in parallel or in a delta bank with a 1.6% 100 KVA transformer.
Use the ratio of the impedances to re-rate the transformers to estimate the loading.

Open/Delta "A" phase "C" phase.
The percent voltage differences on "A" phase and "C" phase at full load will be the difference between the pecent impedance voltages of the two transformers. "B" phase will be the vector sum of "A" phase and "C" phase.
Considering the disparity in sizes and mixed loads that are usual on Open Deltas on some systems I suspect that normal primary voltage variations, load variations and mismatched transformer sizes will make more of a difference than mismatched impedances.

 

[stevenal]

Waross,

Any thought to what your increased kVA change-out might do to available fault current in relation to your customers' interrupting capacity?

 

[jghrist]

I think the reason for floating the wye in a wye-delta bank is to prevent the bank from being a ground source. Ground sources out on the distribution line can be a real problem in fault protection and can blow fuses on the bank for a fault on the distribution line. They may also feed unbalanced loads on the rest of the distribution circuit and overload the transformers.

If one line is open on an ungrounded wye-delta bank, then the situation is similar to an open line on a delta primary bank. The other two phases will see half the line-line voltage, in phase. They will basically be dividing the voltage between the good phases.

 

[waross]

jghrist;
I mostly agree with you.
However while I was researching the RUS standards I noticed that they require four grounds per mile on the neutral and also require everything except the Star/Delta banks to have the neutral grounded and connected to the system neutral.
That is Single phase, Open Delta and Star/Star.
That takes care of the first sentence. The rest of your post I agree with. One of the points I was trying to make is that Star/Delta banks with the neutral connected to the system neutral do feed unbalanced loads on the rest of the circuit and sometimes burn out in the attempt.
An overloaded phase implies a greater voltage drop on that phase. If there is an unbalanced voltage the Star/Delta bank tries to correct it. And yes as you correctly stated, a fault on the system (Which may be considered a really big overload) does blow the fuses on a Star/Delta bank. All over the system.
I think the voltages on single phase conditions will be different then on a Delta/Delta sytem be but I'm not completely sure. Give me your thoughts on this folks.
Even though the primary neutral is floating the Delta secondary is doing a good job of keeping it where it should be. On a 19920/34500 Volt system each primary will have 19920 volts across it.
When a primary fuse blows on "A" phase the "B" phase and "C" phase transformers go in series across "B" phase and "C" phase which is 34500 Volts or 1.73 times normal voltage. The voltage across each will now be 1/2 x 1.73, or 17250 volts. For a 240 volt system that would be 240 x .5 x 1.73 = 208 volts.

Normally, the 240 volt windings would have a 120 degree phase displacement and would add vectorilly to 240 volts which is equal to "C" phase. Now the voltage across each winding is less (208 Volts instead of 240 Volts) but the voltages are now in phase because of the single phase condition. When the voltages add vectorily the result is 416 volts. Don't forget, this is a delta secondary and "A" phase secondary is solidly connected from the start of "B" phase to the finish of "C" phase. But we have the voltages on "B" phase and "C" phase adding up to 416 volts on a 240 volt circuit. As I visualize the circuit and calculate it, I see a 19920 Volt transformer back feeding 34500 volts. It must be able to take it without saturating, because, if it saturated and as a result blew a second fuse it would be difficult to energise without a three phase load break switch. (The RUS drawings show individual fused cutouts)
Consider; If you are energiseing with a hot stick; When you install the second fuse you have a single phase condition and that third transformer is going to be seeing 1.73 times normal voltage until you get that third fuse installed.
I never knew that a distribution transformer was that robust.
Someone please check my figures.

 

[jghrist]

waross,

I think the voltages on the delta secondary windings will be 180° out of phase, not in phase. They will add to zero between B and C. This makes sense because the B to C phase winding is related to the voltage to neutral of the open primary phase, which is zero (no current in the winding, so no voltage).

 

[Cooplineman]

Hello all.

New to the forum. I'm a lineman of 21 years, not an engineer. I'll come in to your discussions humbly.

I was just reading this thread and intrigued. I've worked with and around many wye/delta banks. I have never seen anybody tie the floating wye bus (common point of star) to neutral because of the backfeed and damage to transformers that can happen (except for once which I will explain later). It is an INDUSTRY standard to float it. On 19.9/34.5 KV, due to ferroresonance, it is tied until all transformers are connected, then it is untied. Therefore, I have changed out about equally transformers on wye/wye banks and wye/delta banks due to failure. We do not like them on our system because of only partial backfeed if one phase is lost - just enough to make it unsafe to work on the one phase. The backfeed runs about 1/2 the regular primary voltage. We have a lot of single phase protection on our three phase lines, so this is a very real issue to us. Other than that, they have been no problem at all. Medium sized cooperative.

The only time it happened was when one of the crews didn't know what they were doing and tied H2 to neutral. This is when major problems happened just like you explained happened. When the line operated, it would blow a fuse. Well, when we lost one phase, we had a meltdown at the transformer bank.

 

[waross]

Hi Cooplineman
Thanks for your feed back.
I have never worked under RUS standards. Where I am the standard practice is to tie the H2 to neutral.
Different strokes.
When the practice started here, the major load on the systems were large industrial installations. The Wye/Delta banks were large enough to balance the primary voltages and withstand the circulating currents, but residential refrigerator burnouts at the time of power outages were common. Residential loads have increased as a percentage of the total load, and the utility has upgraded from 4160 V to 34500 V. Now the small Delta connected distribution transformers are burning out instead of the refrigerators. I saw three almost new transformers destroyed (50 KVA distribution) in one month. It seems the practice now is to remove one fuse and run Open Delta with a spare transformer warmed up and ready to go in the event of a transformer failure. The learning curve is sort of an ampersand. You end up back where you started but facing in a diferent dirrection.