Voltage result changing from D-Y YY to YY YY 12.4kv

[rdv4rjm]

3 transformers 600v to 7.2KV connected D-Y = 12.470v phase to phase; 7.2 phase to neutral.

If I were to connect them Y-Y what voltage would I get Phase phase, and phase to ground?

PRJM

 

[waross]

Divide everything by Root three.
7200 V Phase to Phase
4257 V Phase to Neutral
yours

 

[rdv4rjm]

Hi Waross,
Thank's for the answer. I was affraid I was right.
I'm preparing another question for you. I will post it sometime tonight.

PRJM

 

[rdv4rjm]

Hi Waross,
After I read what you wrote “against” delta-wye systems, I was sort of confused, though you might have described part of my problems.
Over the years I raised the voltage on my power line 2 times. Once from 2.4kv D-D to 7.2KV D-D. 3 years ago I raised it again, to 12.4KV this time. It would be too complicated to explain how come in the process, I was missing a transformer.
I found a 750 KVA Padmount 12.4KV to 600V WYE-WYE, common neutral, installed it at the load end of the power line.
At the power generation end, the 3 100 KVA transformers 600v to 7.2 KV connected delta-delta had to be connected in delta-wye to raise the voltage to 12.4KV.

I had ran a fourth wire on the power line as a neutral, installed the padmount transformer etc…
2 month of hard work. Then I closed the switch and everything seemed to work fine.

Doubts raised in my mind with time with time.
2 weeks ago: a snow storm. Melting snow, and a lot. You see the picture: branches on the line. But not many, 2 were literally laying on the wires: one on phase A the other on phase B. They were 500 feet away from each other. I cut the 2 branches and a few others, power up the line again.
But three days later, the line breaker opened again. I was away. My guys waked the 3 mile line to find the stupid branch: none. They tried and tried to power up again: no ways! They went so far to disconnect the padmount transformer from the power line and tried feeding 600v to the secondary: the diesel generator would choke.

The next day I tested the transformer with the means I had:
a) 120v H1 to neutral would give 6v from X1 to neutral. The same for H2-X2 and H3-X3.
b) Shorted X0-X1-X2-X3 and fed 120v from H1 to X0 and it drew 18 amps. The same for the 2 other phases. All three drew 18 amps.

So the transformer seemed good. Left was the oil. I sent a sample to be analyzed but I am still waiting for the results.
In the mean time we got running again, not knowing for how long.

Now, here is where I stand;
1) When powering the line from the load end with diesel generators, why does the load seem higher than when the system ran delta-delta? ( I have electric heating at the hydroelectric generation plant).
2) Why is it harder to power up the line after a power outage? The breaker trips many times before holding finally.

Do I have a ground looping problem, or a transformer problem, or a delta-wye transformer connection problem?

PRJM

 

[waross]

I'm going to answer one part and go back and read further before answering the second part.
1> Your Delta Wye arrangement is okay. It is my preferred conection because it divides the load on the supply alternator better. There should be no problem there.
2> As I understand, you have a Hydro plant with three 100 KVA transformers connected 600 Delta to 12400 Wye. No problem there.
3> At the other end you have a three phase 750 KVA transformer connected Wye Wye, with the neutral carried back to the supply neutral. No problem here with connections as long as the neutral is continous. Remember, the neutral will be carrying any third harmonic currents.
4> You are probable having inrush currents in the 750 KVA padmount that are causing problems getting energised. See if you can go to a higher instantaneous setting on your breaker.
5> Your low voltage test is not definitive. Sorry. It will indicate open windings and most shorted windings. Your windings are probably intact, but the insulation may be failing at higher voltage levels.
6> Your line breaker may be failing, but

7> Generator chokeing. I would expect the transformer to lug down and then recover when energising a transformer that is probably several times it's rating. If it slows down and does not recover its speed that is a good sign of transformer failure. The previous outages probably are not related to the present problem.

8> If you can connect the padmount to the generator when it's stopped and then start the set with the transformer already connected, but with no load, you will avoid inrush currents. The generator should come up to speed and voltage with no problem. Be ready to trip the switch if the current starts to take off. I would disconnect the primary lines in case you have a problem on the line.

9> Can you energise the line with the padmount disconnected? You may have a failed insulator or a 'coon or bird carcass on the back of one of the insulators, where it was out of sight when you were slogging through the snow.
good luck
10> The last time I had a similar problem it was a failed transformer. The time before that it was a bad lightning arrestor.

Hope this helps
yours

 

[mc5w]

You also might want to open the secondary circuit breaker of the padmount transformer. This way you are not trying to supply both transformer inrush and load inrush at the same time.

 

[rdv4rjm]

Hi Waross
1&2> The three 100 KVA or not identical, not from the same manufacture. Can it cause mismatch? I don’t have exact 600v voltage reading phase to phase on the padmount secondary when hydro plant is running. 3 to 4 volts difference.

3> Third harmonics! Tell me more about them. My system is isolated from the grid. Therefore , I was very concerned on using AC drives so not to saturate the system with harmonics. Though I have a few: could this be part of the actual transformer problem?

6> I will check the breaker to see if I can go to a higher instantaneous setting on it.

>7 A transformer failure should be detected by the amount of gaezification detected in the oil analysis. Any other means to test? Or is it sufficient?
- Supposed the insulation is damaged, how come it is been powered constantly since 25th January, without anything acting up?
- Is it possible that a transformer be arcing constantly and still be operational?
- If it isn’t what can cause it to start arcing?

9> Yes I did power the line with the padmount transformer’s high voltage disconnected, and it was OK.

10> Any quick way to locate and test a bad lighting arrestor?


Thank’s for the feedback.

PRJM

 

[waross]

Hi prjm
1&2> Probably not a problem.
3> There are other friends on the forum much more conversant with harmonics than I am. I don't think they will be a problem with your system but let's wait and hope someone else handles this question.
7> Internal arcing will pretty soon contaminate the oil. Contaminated oil will kill a transformer eventually.
10> The lightning arrestors that I am familiar with are all fused. Most of them are sharing a fuse with a distribution transformer.
The usual test here is to disconnect the arrestor or the transformer and refuse the cutout.
If the arrestor is visibly damaged or blows the fuse, change it out. If the transformer blows the fuse change it out.
I think our most probable failure is the transformer.
We may have a problem with the Feeder breaker.
If there were lightning arrestors connected to the transformer when it was checked with the generator one of them may be shorting.
If testing the transformer with the generator is not feasible, I can suggest some other tests. I don't recommend "Fuse" testing because of the relative size of your system and transformers. Do you have any spare distribution transformers of any size? If so, tell me what's on hand.
Sorry, I'm out of time right now. I’ll try to get back to you this evening.
yours

 

[rdv4rjm]

Thank's. I have to go too. I'll come back some time tonight.
PRJM

 

[waross]

Good evening PRJM
Back to the problem at hand. If the lightning arrestors were in circuit when the line was energized without the transformer, they are probably okay.
If not, it may be wise to energize just the lightning arrestors. Check the cheap and easy stuff first.
Next, set up the trips in the breaker and try once more.
Until I get some more feed back from you on the action of the generator, I am tending to suspect either the breaker or the generator.
If it is possible to close all the supply breakers and connect both transformers, (But without load) to the hydro alternator and then start it up and bring the voltage up slowly then do it.
WATCH THE AMMETERS FOR A SUDDEN SURGE.
Slowly is compared to closing a switch. Bringing the Hydro set up to speed at the normal rate would qualify as slowly.
Please give me a good description of the generator "Chokeing" Some chokeing is normal, some is not.
Checking the transformer.
How did the oil look? Clear like water or weak tea is good. Black like a GM diesel is bad.
Checking a transformer with less than rated voltage will find a lot of faults but it won't find them all.
I'm going to assume that we have almost no equipment to test with other than a multimeter and possibly a Clamp-on Ameter and suggest some workarounds.
If it is possible to bring the voltage up with the generator, that would be the best test. If you can't do it with the generator, read the following, and have another thought about how you can use the generator.
In the long run it may be the easiest and quickest.
If there is some reason that the generator is out of the question, read on.
As the voltage goes up, so does the hazard level, so I'm going to give this in steps. of increasing voltage and hazard.
If it fails a test, it's bad. If it passes a test, we go to a higher voltage.
You don't have to do every step. Basically we are going to put progressively higher voltages on the transformer primary until it either holds at full voltage or we prove conclusively that it's faulty.
Look for some electric heaters to connect in series with your supply voltagfe to limit the current.
I suggest 600 volts to the primary before hooking up a distribution transformer for higher voltages. Check the current that each winding draws. Wrap the supply lead around the jaw of the ammeter to multiply the reading. Be suspicious of unequal currents.

First energise the high voltage windings one at a time with 600 volts. Limit the current with baseboard heaters or something similar in series with the supply. This is a repeat of the test that you originally did with 120 volts.
If the transformer is passing these tests, the next is a higher voltage test.
You need a spare distribution transformer.
You are going to use it to produce a high voltage to test the other transformer. If all else fails, you may be able to borrow one of the 100 KVA units from the Hydro plant.
Connect the high voltage terminals together. connect the high voltage neutrals together.
GROUND THE CASES OF THE TRANSFORMERS. ESPECIALLY THE TRANSFORMER UNDER TEST.
THERE IS A VERY REAL POSSIBILITY THAT THE TRANSFORMER IS FAULTED TO GROUND INTERNALLY.
IF SO, YOU CAN GET A LETHAL VOLTAGE ON THE CASE UNLESS IT IS WELL GROUNDED.
I MEAN TAKE A FEW MINUTES TO FIND CONNECTORS AND BOLT YOUR CONNECTIONS.
A FEW TURNS OF WIRE TWISTED AROUND A RUSTY BOLT WON'T CUT IT.
I'm sure you know this, but it doesn't hurt to say it again.
With a resistance bank in the circuit, you can get a check on the current by measuring the voltage drop across the resistor.
I would start with 120 or 240 volts and I would look for a couple of baseboard heaters or something similar to limit the current. Check the actual voltage at the low voltage terminals of both transformers. With 600 volt windings and 7200 volt windings on both transformers, the low voltage readings should be similar. If you have different voltages, do the arithmetic.
When and if you hit a voltage level that causes a breakdown, you may hear arcing in the transformer and the current may increase more than it should.
Keep stepping the voltages up 120 Volts, 240 volts, 347 Volts if you have it, and the full 600 volts.
I haven't forgotten your questions about different loading but let's get you back on line first.

 

[rdv4rjm]

Hi Waross,

Thank's for your interest.

PRJM