Harmonics in transformers 3

[Zogzog]

Anyone have an idea of the percentage of harmonics that would be seen on the primary side of a transformer that are caused from loads on the secondary. I dont need an exact answer, just a ballpark figure.

Bascially we have a customer that we believe has a harmonic problem causing overheating in thier transformer, the utillity has monitered the primary and claims there are no problems (Of course) on the primary. So could there be a harminic problem in the plant and not be evident on the primary, or is the utillity getting (or providing) poor data?

 

[jghrist]

Harmonic currents in the secondary would show up in primary current harmonic measurements. You should confirm that the utility is measuring harmonic currents, not voltages and that the measurement includes only the transformer primary current, not a combination of loads that include the transformer primary current.

Are you able to measure harmonic currents in the secondary? If not, why do you believe there is a problem causing overheating?

 

[Zogzog]

The customer has gone through 4 primary transformers in 4 years, all due to excessive gasses caused by overheating, loading is only about 70% with a 62C rise. They wont let us monitor the secondary because the utility has the contract for PQ, the utility only monitored the primary side.

I suspect this is a PQ issue due to process of elimination of other possible issues.

Am I understanding your post correctly that harmonic voltages would not be seen on the primary but harmonic currents would be seen? (I am not a PQ guru, my PQ guy is on vacation this week)

 

[ScottyUK]

Harmonic voltages are often very small, especially if they result from a relatively small harmonic source connected in to a relatively stiff HV supply network. That doesn't mean that the harmonics are insignificant to the transformer serving the load, just that the harmonic source is unable to seriously affect the primary voltage. The harmonic current would still flow in both the primary and secondary windings.

What loads do you have which are likely to be bad sources of harmonics?

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I don't suffer from insanity. I enjoy it...

 

[Zogzog]

I dont have a list of all the loads yet, but they make plastic cups, sporks, etc. and have alot of conveyers and forming machines so I assume there are a lot of drives.

 

[jghrist]

It isn't that the harmonic voltages would not be seen on the primary, but that it is currents, not voltages, that cause transformer heating.

You can determine the capability of transformers to supply harmonic currents with ANSI/IEEE C57.110, IEEE Recommended Practice for Establishing Transsformer Capability When Supplying Nonsinusoidal Load Currents.

 

[stevenal]

It does not sound like a PQ issue at all. Harmonic currents can distort the voltage waveform and create a problem for the utility's other customers. But the utility has checked and determined there is no PQ issue. But that does not mean that this particular customer doesn't have a problem. Customer loads causing overheating of a customer owned transformer is a problem for the customer and his consultants to deal with. Whether or not a particular harmonic current appears on the primary is meaningless. If it's in the transformer anywhere it contributes to the heating. The utility cannot prevent you from monitoring on the load side of the meter.

 

[Zogzog]

jghirst,

"It isn't that the harmonic voltages would not be seen on the primary, but that it is currents, not voltages, that cause transformer heating."

That I do understand, but thanks.

stevenal,

The PQ report and quality of the data gathered is in question here. I am not saying this is a utility problem, it is the customers problem. So if you think it is not a PQ problem, what do you think is causing the transformer failures?

 

[stevenal]

Overloading the transformer with fundamental or harmonic currents or both can cause overheating and transformer failures. PQ issues deal with the voltage waveform, which if distorted can cause data loss, flicker in lights, equipment shut down, and so on. When checking into PQ problems, current is usually monitored. This is in order to find the cause of a PQ problem, but it's the voltage that determines if a PQ problem exists.

 

[stevenal]

From IEEE 519:

10.2 Development of Current Distortion Limits
The philosophy of developing harmonic limits in this recommended practice is to
1) Limit the harmonic injection from individual customers so that they will not cause unacceptable voltage
distortion levels for normal system characteristics
2) Limit the overall harmonic distortion of the system voltage supplied by the utility

In other words in the PQ world current distortion is the cause and voltage distortion is the problem. But in your case you are suggesting that current is the problem. This is called overload. The fix is to adjust your loading or your capacity. See jghrist's reference above for determining the load-ability of your transformer in the presence of harmonic currents.

 

[Zogzog]

Well they cant adjust the loading, this is a main transformer 3000 kVA feeding the plant, so that leaves replacing the transformer with a larger (or k rated) transformer? They are looking for a better (Or should I say cheaper) solution.

BTW Steve, you get a star for the reference.

 

[jghrist]

If they are looking for a cheaper solution, why won't they start by paying you to monitor the secondary current? Your customer is being penny wise and pound foolish IMHO. You can't be expected to find the most economical solution without adequate data. Maybe you could convince them that this is a loading issue, not a PQ issue, so it is not covered by the utility's PQ contract. I would agree with stevenal that it is basically not a PQ issue.

If there isn't a high harmonic current, maybe the transformer load can be reduced by correcting power factor. Be careful with adding capacitors, though, if there are harmonic currents. You may create a resonant condition and really make things bad.

Is the transformer in an adequately ventilated area? Is the ambient temperature particularly high?

 

[waross]

You havn't told us anything about the transformer. If the transformer has a grounded wye primary winding and a delta secondary or tertiary winding it will be sensitive to primary voltage unbalances and will try to "Balance" the primary voltages. For example, a primary voltage unbalance of 3% will cause serious heating in a transformer with an impedance voltage of 2.8% and a delta secondary winding. Most of the transformer failures that I see are grounded wye primary/delta secondary connected. Primary phase loss may cause primary fuse blowing, transformer burnout, refrigeration compressor burnout in other premises served by the "lost" primary phase or some combination of the above.
respectfully

 

[stevenal]

Primary voltage phase unbalance does (or should) fall into the PQ category. In my experience, portable PQ recorders do not report this figure directly. The raw data must be dumped in a spreadsheet or to find the percentage. Might be worth asking the utility about this. The heating waross spoke of is one good argument against grounding the neutral of these transformers.

 

[Zogzog]

The transformer is a silicone oil filled 2500 kVA, 24.94 kV/14.4 kV primary 480/277V secondary (Y-Y). It is running at about 70-80% loading and they have had to replace this transformer 3 times over a 4 year span. We are seeing high oil temps (80C) and excessive gassing (H2, CH4, C2H6).

I have not seen the power monitoring data from the utility, they just reported that there were "No problems found", I do not know what equipment they used. We use RMS line monitors and software and we would see the imbalance graphically, but the customer wont let us install them, they are happy with the utilities results.

I plan on re-requesting to see the data gathered by the utility today, I seems that is the missing piece of the puzzle.

P.S. Waross gets a star

 

[CarlPugh]

4 TRANSFORMERS IN 4 YEARS. ?or is it 3 transformers in 4 years?

Transformers are 2500 KVA, silicone oil filled.
You have a problem.

Why do you believe that it's a harmonic problem?

What was failure analysis of failed transformers?

What does the transformer manufacture say about the failures?

Transformers replaced for gas analysis? Is the gas analysis for a silicon oil filled transformer different than for a mineral oil filled transformer? Was gas analysis for mineral oil filled transformer used instead of that for a silicon oil filled transformer?

80 deg C rise in a silicon oil filled transformer using noxex, isn't that high. What is rated temperature rise of transformer?

When transformer was filled, could a small amount of mineral oil have contaminated the silicon oil?

The user won't let you make measurements! The user doesn't give you the utility reports! Is someone trying to hide something?

From the information given, I would be suspicious about the information given you by the user and very suspicious about the transformer design.

Don't believe anything that you are told. Double check any measurements that you have made, cross check any assumptions you make.

 

[waross]

Hello Zogzog;
Does the transformer have a delta tertiary winding? A delta tertiary winding will make the transformer overheat with unbalanced primary voltages and in the event of phase loss on the primary, the delta winding will try to support the load on the disconnected phase. The transformer will be supporting the plant load on two phases and the tertiary winding will be supplying the third phase. The transformer will also be backfeeding to any load on the primary back to the point of phase loss. The phase loss current will depend on the load on the transformer and on the load on the primary back to the point of phase loss, but on a transformer with 300% primary fusing the current may be as much as 300% of rated current until the lost phase is restored. If the tertiary winding KVA rating is less than the KVA rating of the main windings, the heating will be even more.
respectfully

 

[Zogzog]

Carl,

4 TRANSFORMERS IN 4 YEARS. ?or is it 3 transformers in 4 years?

4 in 4 years=3 replacements

Transformers are 2500 KVA, silicone oil filled.
You have a problem.

Well, technically I dont, the customer does.

Why do you believe that it's a harmonic problem?

I suspect it is a PQ problem, I dont know whatr else makes sense.

What was failure analysis of failed transformers?

They did not fail, they had excessive gassing (H2, CH4, and C2H6), the test lab suspects due to overheating followed by insulation breakdown and PD.

What does the transformer manufacture say about the failures?

"Replace it again" I guess, we recently just entered the picture. The first 3 were remanufactured units, the last one was a new unit from a different manufacturer.

Transformers replaced for gas analysis? Is the gas analysis for a silicon oil filled transformer different than for a mineral oil filled transformer? Was gas analysis for mineral oil filled transformer used instead of that for a silicon oil filled transformer?

Silicone, the right tests were performed per 10 different ASTM standards and a DGA per D-3612.

80 deg C rise in a silicon oil filled transformer using noxex, isn't that high. What is rated temperature rise of transformer?

55/65C rise, ambient was 18C.

When transformer was filled, could a small amount of mineral oil have contaminated the silicon oil?

I dont know for the first 3, but we filled the last one and use seperate equipment for silicone and mineral oils. So I doubt it for the last one.

The user won't let you make measurements! The user doesn't give you the utility reports! Is someone trying to hide something?

Thats what I suspect.

 

[tommom]

What is the primary load of the customer - do they have harmonic generating loads? Is their phase loading severerly unbalanced?