Problem with transformers on cogeneration plant
Problem with transformers on cogeneration plant
(OP)
Hi,
I need help to understand a problem I have to solve in a cogeneration plant. The power transformer that do the conncetion between the generator and the network has problems which I can't understand. That transformer fails because it heat excessively.
Some help?
Thanks
Paulo
I need help to understand a problem I have to solve in a cogeneration plant. The power transformer that do the conncetion between the generator and the network has problems which I can't understand. That transformer fails because it heat excessively.
Some help?
Thanks
Paulo






RE: Problem with transformers on cogeneration plant
RE: Problem with transformers on cogeneration plant
Bill
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"Why not the best?"
Jimmy Carter
RE: Problem with transformers on cogeneration plant
Dear DanDel
In fact I'm starting collecting the information about the cogeneration plant. At this moment I'm trying to define an approach to study the problem.
Yes, you are right, I need to collect more information. However, I only will do it on the next week, because I'm going to visit the cogeneration plant.
At this moment I'm interested in tips that may help me defining an approach to solve the problem.
Bill, I don't hace certanty about the kind of connection of the transformer windings, but it must be a delta/delta or a wye/delta connection, without neutral grounded.
As I said, at this moment it is important to me to collect the possible tips in order to define an approach to solve the problem. Yours opinions and tips will be very important to me.
Thanks
Paulo
RE: Problem with transformers on cogeneration plant
Try to take a scope, spectrum analyzer or other instrument to check for harmonics.
Check voltage balance and check tap changers or fixed taps for equal settings.
If the transformer is properly matched to the generator and is heating disproportionately to the generator, suspect harmonics or unequal voltages or tap settings that are not alike.
Call from the job site when you have more info.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Problem with transformers on cogeneration plant
RE: Problem with transformers on cogeneration plant
rmw
RE: Problem with transformers on cogeneration plant
Measure the neutral current (if it is grounded). You can probably get line currents from the plant Historian which should be part of the control system. Look for long term trends of load or line currents. Are they greater than the transformer rating? Verify the CT ratios and check the correct scaling is applied in the control system such that one amp in the field equates to one amp on the control system. If you are lucky you may find that the oil and widing temperature indicators are connected back to the control system. It is certainly an option which can be retrofitted and it would make sense to make this addition and bring the data back to the control system both for the operator's information and for trending purposes.
Check the transformer spec against the local ambient conditions. Most transformers are rated up to 40C ambient per the IEC standard, but if this is somewhwere like the middle east then designing for a 40C ambient isn't adequate.
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If we learn from our mistakes I'm getting a great education!
RE: Problem with transformers on cogeneration plant
Thank you for yours good tips. I'm goingo to study the situation on-line and consulting the project of the cogeneration plant (and follow yours suggestions). Note that this plant is new, it was installed a few mounths ago.
Whem I have more information I'll post it at this site.
Thank you for your good tips.
Paulo
PS: I'm a new member of this community, but I became a fan.
RE: Problem with transformers on cogeneration plant
At thi moment I have some more information about the problem we are discussing. Unfortonately, I don't have, yet, the project of the plant. Despite this I did a visit to the plant, and I have some issues/questions that for which I need some support:
a) How can I get information about the effect produced by an increase in network voltage above the nominal value of the secondary of the transformer? This may contribute for heating the transformer?
b) How can I calculate the needed ventilation for a room where exist more than 1 transformer?
Thank you
PMAC
RE: Problem with transformers on cogeneration plant
The transformer primaries are normally provided with taps to account for increased input voltage.
How bad is the increase in your network voltage ?
Regarding the spacing between the trafos, it would depend on the size and the amount of ventilation, I guess.
RE: Problem with transformers on cogeneration plant
Thank you for your answer. Have you some expression that allows to calculate those effects?
I have no values at this moment, but my information is that the the voltage on network may be 5-10% above the nominal voltage of the secondary of the trasformer.
Thanks
PMAC
RE: Problem with transformers on cogeneration plant
RE: Problem with transformers on cogeneration plant
Well, I'm thinking on the secondary of the transformer. But, on the primary I have the value of 5%.
RE: Problem with transformers on cogeneration plant
RE: Problem with transformers on cogeneration plant
Thank you for yours good tips. At this moment I'm interested to undersantand how the network voltage may influence the losses of a transformer. Assume a transformer 0,4 kV/15 kV connected to a distribution network where the voltage may achive, for instance 16 kV. What I need at this moment is some expression that may help me to quantify the effect of that voltage increase on the losses of the transformer.
Thanks
RE: Problem with transformers on cogeneration plant
May be trafo experts like prc, Dandel, Kiribinda, ScottyUK et. al. will chip in later to answer those questions.
RE: Problem with transformers on cogeneration plant
RE: Problem with transformers on cogeneration plant
You are right. The problem should be the room temperature. The transformers are wet and the protection systems gives temperature alarms.
I don´t know if someone knows where I can get information in order to calculate the factor to derate a cast coil transform in function of the room temperature.
Thanks
RE: Problem with transformers on cogeneration plant
Also a 5% voltage increase would probably not cause the xfmr to go into saturation, I assume?
"Throughout space there is energy. Is this energy static or kinetic! If static our hopes are in vain; if kinetic — and this we know it is, for certain — then it is a mere question of time when men will succeed in attaching their machinery to the very wheelwork of nature". – Nikola Tesla
RE: Problem with transformers on cogeneration plant
RE: Problem with transformers on cogeneration plant
The temperature on the room, at this moment, is about 30ºC, but when the transformers did fail it was greater than this (someone told me that it achieved 40-45ºC). At this moment I don't have the temperature in the winding (its a wet transformer).
PMAC
RE: Problem with transformers on cogeneration plant
RE: Problem with transformers on cogeneration plant
Yes, I agree that the temperature may contribute for the problem. However I have some difficulties to accept that it is the only cause of the problem. In fact, two wet transformers have been detroyed, the first six months after being in operation and the second one month after the failure of the first one. Is that possible?
Not that, at the time there was three transformers each one supplied by a syncronous generator and with the secondarys in paralell.
Thanks
PMAC
RE: Problem with transformers on cogeneration plant
In the previous post, wher we read "In fact, two wet transformers have been" should be "In fact, two DRY transformers have been"
Thanks
Paulo
RE: Problem with transformers on cogeneration plant
Are the vent louvres clear? Is there adequate air circulation? Have you considered forced air cooling?
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If we learn from our mistakes I'm getting a great education!
RE: Problem with transformers on cogeneration plant
You need to sort out whether these are oil-filled or dry type trafos. There are many contradictory posts.
RE: Problem with transformers on cogeneration plant
Summarizing the situation:
a)The cogeneration plant had 3 DRY transformers of 0,4/15 kV
b) Each LV side of each transformer was connected to a synchronous generator
c)The MV sides of the transformers are connected to the main grid
d)Each set generator/transformer is connect and disconnect some time a day
e)The first DRY transformer fails after six months
f) the second transformer one month later
g) the room temperature at the time teh transformers fail was 40 to 50ºC
Thanks
PMAC
RE: Problem with transformers on cogeneration plant
Also, how did the earlier trafos fail ? earth fault, turn-to-turn short, cooked, fire ?
RE: Problem with transformers on cogeneration plant
RE: Problem with transformers on cogeneration plant
I now have new data about the problem.
To facilitate the analysis, I summarized the matte:
a) I have a cogeneration power plant
b) There is three generator-transformer sets (In attach I present two of the systems)
c) The transformers are of dry type and Class F insulation
d) The primary of each transformer is connected in a star connection and linked to a generator. Note that the connection between the primary of the transformer and the generator is made including the neutral conductor.
e) The secondary is connected on a delta connection and linked to a commum bus that allows the connection to the main grid.
Facts:
A few months after putting into operation the power plant, one of the transformers burned and a month latter another had heat problems. The thermal protection did not work.
The temperature in the room at the time would be 45ºC (maybe 50ºC).
One of the transformers has evidences that a electric arc happened between one of the LV poles and the neutral bus on the LV side of the transformer.
Questions:
a) What may have led to the electric arc between a phase and the neutral bus in the LV side of the transformer? Overvoltages?
b) DanDel said before that the room temperature may have led to the failure of the transformers ... with fire? And just the temperature? The issue is that the third transformer had no problems.
c) Any idea about the reason why the thermal protection of the transformers did not work? Well, it may be due to bad connections, but assuming that is not the case, there are other reasons?
d) In the secondary of the transformers (grid side), the voltage has periods of over voltage in the order of 10%. This contributes for the problem?
Thanks
RE: Problem with transformers on cogeneration plant
The high ambient temperatures are not helping either.
Float the wye neutral. The delta secondary will keep the voltages balanced. Bring the transformer up with the generator to avoid switching transients if you must energise the wye side first.
Try more ventilation to cool the transformer room, a lot!!
Is the transformer that DIDN'T burn out closer to a door or ventilation inlet?
Bill
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"Why not the best?"
Jimmy Carter
RE: Problem with transformers on cogeneration plant
Thank you for your tip. Please, help me to clarify some matters:
a) when you said that "the transformer with the neutral connected on the wye primary and a delta secondary is prone to circulating currents", are you referring to the primary or to the secondary? i.e. the circulating currents happens on the primary (LV) or on the secondary (MV)? Have you some book or paper that allows me to better understand that issue?
b) What may be the consequences of floating the wye neutral? Well, in fact the MV side is connected on dealta wich should ensure the balance between voltages and so, the primary voltages will be also balanced. As well, the primary currents will remain balanced and so the neutral wire is not necessary. Is that correct?
c) Do you believe that the high voltages that happen on the secondary has no influence? Why the evidences that a electric arc happened between one of the LV poles and the neutral bus on the LV side of the transformer?
Thanks
PMACP
RE: Problem with transformers on cogeneration plant
Please post a few photos of the transformers here.
RE: Problem with transformers on cogeneration plant
RE: Problem with transformers on cogeneration plant
If the wye neutral is connected, a small primary voltage unbalance will result in a heavy circulating current in the delta.
If the wye neutral is left floating, there may be 173% transient voltages when the bank is energized. These transients will be avoided if the transformer is connected to the generator before the generator is started.
this may not be an issue, but it is well to be aware of the possibility.
I would look first at ambient temperatures.
I would look second at the possibility of saturation. At 110% Voltage you don't have much safety factor left. If high voltage is saturating the transformer it will burn out very quickly. Do you have taps on the transformer that may be used to better match the voltages?
Another possibility is poor workmanship. I have been aware of several transformer failures that were attributed to lightning strikes and poor grounding. I found that hard to believe as the transformers were fed by long underground feeders and the grounding was up to north American standards.
I later encountered an installation done by the same crew. The main cables were so loose that had the installation been energized, it would have soon failed. Of course the failure would have been blamed on lightning. (The customer pays for lightning damage.)
To my mind it is unlikely that over-voltages on the 15 kV side would cause a flash-over on the low voltage side without damaging any generators. More likely that a loose connection was arcing internally and the arc spread out or tracked to engulf the neutral bus. (But I have found over the years that almost anything is possible.)
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Problem with transformers on cogeneration plant
Thank you waross, edison123 and Dandel for yours good tips.
DanDel, I'm not sure what thermal protection is installed (I'll check). However, a protection, no matter its type, should act, right?
Waross, I think that the circulating currents may be the problem. Well, I'm not sure but It is a strong issue. I'll made some measures namely concerning currents, voltages, harmonics and neutral currents.
Waross, hen you said "a small primary voltage unbalance" you are thinking on a unbalance of what magnitude? 5%, 10%?
Since the primary is supplied by a synchronous generator, is that likely that a unbalance with that origin may occur?
Waross, concerning saturation you said "I would look second at the possibility of saturation. At 110% Voltage you don't have much safety factor left. If high voltage is saturating the transformer it will burn out very quickly. Do you have taps on the transformer that may be used to better match the voltages?" Have you some idea that allow mw to understand if the saturation is ocurring? Perhaps measuring the harmonics and, if the 3º harmonic is present?
Thank you
PMAC
RE: Problem with transformers on cogeneration plant
Circulating currents are limited by the transformer impedance so if the percentage voltage unbalance approaches the transformer percent impedance you are in trouble.
Saturation.
At the saturation point the current rises disproportionately to the voltage. A small voltage increase results in a large current increase. You need to know the saturation curve of the transformer and the history of overvoltage surges on the 15 kV side.
If the voltage on the 15 kV side is normal then a 5% voltage surge for a few minutes is no problem.
If the voltage on the 15 kV side is 10% high already, then a 5% voltage surge for a few minutes may destroy a transformer by over heating.
When a transformer is fully saturated, a 1% increase in voltage may result in a current increase of over 5000%.
Bill
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"Why not the best?"
Jimmy Carter
RE: Problem with transformers on cogeneration plant
Intresting thread.
All possible, but from my point of view, PMACP you MUST strat from trafo mnf.
Not once, I see troubles with dry trafos and usually it's what Edison saied:
"I think the transformers are poorly manufactured (premature failures, flash-overs, non-working RTD's point to that) with insuffcient clearances."
I suggest, are some no name company and w/o any type-test.
of course ambient temp., overvoltages increase reasons of faults, but isn't primary reason.
Best Regards.
Slava.
Sorry, I hope, you don't work for this company
RE: Problem with transformers on cogeneration plant
Thank you for your opinion. Well, the company that manufactured the transformers is recognized on the market. I'll not say the company, at least until understand the problem. Note, I'm not saying that the transformers has no problems. Is that possible to test the transformers? How?
Thanks
PMACP
RE: Problem with transformers on cogeneration plant
This is a GSU trafo. The high voltage on the secondary side (which is what the OP says) will not affect the saturation. May be the tap on the HV side is set to produce high voltage.
@OP
This is a trafo which is already overated given that the generator KVA is only 1750 KVA. Why would such a trafo with only 80% load should overheat ?
For such a long thread, I am sorry to tell you that you have not provided any hard numbers so far (voltages, currents, temperatures, frequencies, thermal protection set limits) which would go long way in finding the root cause and solution. Otherwise all this theorizing is just looking for hidden ghosts.
My two cents.
RE: Problem with transformers on cogeneration plant
At this moment I have some new data (measured). I still treating the information collected.
Note that the collection was made on the LV side of the trasnformer.
The first major revelation of the data is that the currents in the three phases of the LV winding are unbalanced. The unbalance is of 14%. The unbalance on LV voltages is small.
The currents unbalance may result (as much I think) from the existance of some ancillary services that are supplied by the generator before injecting power on transformer (as we can see in the attach figure).
What may be the consequence of the unbalanced currents for the transformer? It may contribute for the heating problem? In fact, the unbalance will produce a homopolar current that tend to circulate on the delta secondary wedding. is that correct? Is there some method to unrate the transformer?
RE: Problem with transformers on cogeneration plant
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If we learn from our mistakes I'm getting a great education!
RE: Problem with transformers on cogeneration plant
Thanks for your advise. Until the moment there's no problems relating to generators. How much should be the unbalance in order to achieve a dangerous situation? And concerning the transformers is that some problem resulting from the unbalances?
Thanks
PMCAP
RE: Problem with transformers on cogeneration plant
In my previous post I should say that the generator has no current unbalances. In fact, the unbalances are to transformer. As you can see in the scheme I sent in the previous post, the unbalances result because before the transformer there are some ancillary services that are supplied by the generator. The remaining current on each phase is injected on the transformer.
Sorry for the late explanation
PMAC
RE: Problem with transformers on cogeneration plant
The reason for the small voltage difference is that the circulating currents are acting to correct the voltage unbalance. The circulating current is driven by the voltage unbalance and limited by the transformer impedance, and tends to correct the voltage unbalance. Can you measure the current in the neutral?
Bill
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"Why not the best?"
Jimmy Carter
RE: Problem with transformers on cogeneration plant
Yes, I may measure the neutral current. I have that value in my office. Tomorrow I'll put the exact value here. However, as I can remember, it is in the order of 50A.
Thanks
PMACP
RE: Problem with transformers on cogeneration plant
Without knowing much about the generator, 10% NPS current would be enough to be of concern.
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If we learn from our mistakes I'm getting a great education!
RE: Problem with transformers on cogeneration plant
I have now some data which is in attach to this post. There are some interesting results. For instance, the power factor is different on each phase. The currentes are unbalanced.
Any new suggestion?
Thanks
PMAC
RE: Problem with transformers on cogeneration plant
A couple of unknowns remain:
Actual primary voltage. Can you take the generator and the ancillary loads offline long enough to infer the actual primary voltage balance?
Are the ancillary loads heavier on this transformer than on the others?
What is the nature of the ancillary loads, KVA and PF?
Can you get a meter inside the transformer and measure the actual current in the windings rather than in the leads?
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Problem with transformers on cogeneration plant
Waross, You think that the phases are switched because of the power behaviour? On yours opinion, which phases may be switched? I think I didn´t make any mistake...
Switch off the loads of ancillary services may not be possible because I think some of those loads corresponds to refrigeration services of the generator. But, i'll check it.
But, you think that those results do not allow to conclude nothing? For instance, the total power os the transformer is 2000 kVA and the actual power is near 1800 kVA. If we derate de transformer due to ambient temperature (40-50ºC), the transformer is overloaded...
Thanks
PMAC
RE: Problem with transformers on cogeneration plant
R is the lowest current,
R is the lowest kW,
but
S is the lowest KVAR,
T is the lowest voltage.
It doesn't take much load to drop the voltage on a generator due to the relatively high impedance. Normally the AVR corrects, but it can't correct for unbalanced voltages.
With the neutral connected the delta voltages are proportional to the wye voltages and may not sum to zero, hence circulating currents. But the currents tend to correct the voltage unbalances causing them.
I suspect the prime cause of the failures may be high ambient temperatures, but that may be made worse if there are circulating currents. I would ground the generator neutral either directly or through an impedance and float the transformer neutral. I had a unique industrial application where I was able to interrupt the circulating currents by using a "Broken delta" connection. The drop in transformer temperature was dramatic.
But we don't have quite enough data to determine if you do have a circulating current problem.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Problem with transformers on cogeneration plant
RE: Problem with transformers on cogeneration plant
Waross, you said: "R is the lowest current,R is the lowest kW,but S is the lowest KVAR,T is the lowest voltage.".
Well, the lowest current and active power on phase R is consistent. Also the lowest voltage on phase R is consistent because that phase has the higher value of current.
alehman: Yes, the tasnsformers are switched on and off once a day. They are switched on at 6 a.m. and switched off at 23 a.m..
Thanks
RE: Problem with transformers on cogeneration plant
How long is the cable between? Any surge arresters or capacitors?
RE: Problem with transformers on cogeneration plant
RE: Problem with transformers on cogeneration plant
"Did the faults happen when they were switched?"
I think no, it happen when the trasnformer is operating. But I'll check it.
"How long is the cable between?"
Between the generators and the transformers it should be 10-12 meters.
"Any surge arresters or capacitors?"
NO. The synchronous generators work with a leakage power factor.
Thanks.
RE: Problem with transformers on cogeneration plant
RE: Problem with transformers on cogeneration plant
In attach I present some photos of one transformer.
Thanks
RE: Problem with transformers on cogeneration plant
Some more photos on attach.
Regards
PMAC
RE: Problem with transformers on cogeneration plant
Now, the photos. Sorry.
Thanks
RE: Problem with transformers on cogeneration plant
I don't think flashover at that location would have been the result of overheating. The most likely cause would seem like a voltage transient from switching operation. Based on the burn marks along the vertical bars, it probably started near the 90 degree bend just above the coils. The space between phase and neutral looks rather small. In the U.S. 1 inch (25 mm) would be required.
RE: Problem with transformers on cogeneration plant
Hi. The manufacturer inspect the transformers, and point out some possible causes, namely: harmonics; transformers working in a permanent overvoltage, possible saturation of the nucleos, and the temperature of the room.
In my measures I don't see any problems of harmonics (THD-I < 2% e THD-U < 2%). The voltages in the primary are in the range of 403 V to 435 V for a rated voltage (on primary of the transformer) of 400 V. I think that overvoltages are not sufficient to justify saturation. What's your opinion?
The room temperature, at the time the transformers were destroyed were 40-50ºC, wich is significant. Is that possible that the room temperature has weakened the insulation? Moreover, the overvoltages may also contribute for that.
The defect you refer could ignite the transformer? As far as I know, one of the transformers was in fire and the other only was cooked.
The voltage transientes are a good possibility, but i need more details to understand that. One of the transformers had the problem two hours after the generator was switched on (when the transient on voltage could have occurred). It is possible that the transient has had this behavior?
Thanks
RE: Problem with transformers on cogeneration plant
RE: Problem with transformers on cogeneration plant
Thanks for your opinion. Please, can you give me some more details about your tip? Note that the transformer datasheet refers a maximum ambient temperature of 40ºC, imposing also a daily average temperature of 30ºC and 20ºC for annual average temperature. In fact, the transformers were operated above those values (in fact, above the maximum permitted). Also, in the datasheet we may found that for an ambient temperature of 30ºC (anual average), the transformer should be derated in 10%. If we assume a linear derating process, this means that we have a 15-20% derating factor for the present situation. You think that this kind of issues were not sufficient to damage the transformer?
Thanks
PMAC
RE: Problem with transformers on cogeneration plant
Please read the transformer standards about the temperature limits and what they mean.
RE: Problem with transformers on cogeneration plant
I agree that 50C ambient with 80% load should not cause immediate failure.
RE: Problem with transformers on cogeneration plant
Has there been any estimate of the magnitude (Size) of the switching transients?
Is there any chance of a transient reflection at the insterface points of the cable and could this have contributed to a higher transient than the transformers surge rating.
Are there any surge protectors installed on the Transformer or Sub?
Cheers,
Drew
RE: Problem with transformers on cogeneration plant
edison123: I read the standards about the temperature limits, but its meaning is not clear. They set 3 limits: anual average ambiente temperature (20ºC); daily average temperature (30ºC) and maximum temperature (40ºC). All that values refer to the cooling fluid, that in this case is the air. There's no information about the meaning of those values. Obviously, I think the meaning and the second temperatures are clear, but not the last one. What means the maximum temperature? How long a transformer may work at 40ºC ambient temperature? That maximum must not be violated? or if it is violated only a reduction on life expectacy od the transformer occurs?
Is there someone that has the standard IEC 76-1 to IEC 76-5?
Thanks
PMAC
RE: Problem with transformers on cogeneration plant
Since the ambient temperature variations are not within our control, the standard specifies multiple limits from a maximum of 50 deg C to maximum daily average of 40 deg C to maximum yearly average of 30 deg C to even a minimum of -5 deg C.
Per standard, the transformer is supposed work continuously at rated load with a normal design life expectancy (which is around 20-25 years)
In your case, 50 deg C ambient should not destroy the windings prematurely especially it is only loaded up to 80%. The temperature is the major killer of any winding (motors, generators and trafos) and hence it is imperative you have a working temperature sensor/protection (even a redundant one) to monitor and save the trafo.
Assuming yours is a class F, dry-type trafo, the maximum winding temperature allowed is 130 deg C with a normal design life. Have your trafos ever recorded temperatures beyond 130 deg C ? Did you witness the factory acceptance test of these transformers where they are supposed to do temperature rise test ? If yes, what was the maximum temperature in that test ?
RE: Problem with transformers on cogeneration plant
Thank you again for your help. First of all, I would say that the transformers had a Z protection against excessive temperature, but them didn' work.
"...the standard specifies multiple limits from a maximum of 50 deg C to maximum daily average of 40 deg C to maximum yearly average of 30 deg C to even a minimum of -5 deg C."
OK. I don't have the standard (IEC 76), so, I don't know how should I derate the transformer in order to avoid excessive heating. Can you help me on that? The maximum value os maximum temperature defined on the standard is 50ºC for a average daily temperature of 40ºC and average annual temperature of 30ºC?
"Per standard, the transformer is supposed work continuously at rated load with a normal design life expectancy (which is around 20-25 years). In your case, 50 deg C ambient should not destroy the windings prematurely especially it is only loaded up to 80%."
Well, if the transformer works some months with maximum temperature of 50ºC, the life expectacy is reduced, right? How can I quantify that?
"The temperature is the major killer of any winding (motors, generators and trafos) and hence it is imperative you have a working temperature sensor/protection (even a redundant one) to monitor and save the trafo."
That's an interesting issue. The Trafo was protected using a Z protection with 6 PTC, one for winding. Those protection, as long I know, doesn't work...
Moreover, there are the arcing effect that I presented before (you can see the photos on a preview post in a pdf document - 14 Feb 09 4:21 ). On yours opinion, what may be the reason to that arcing effect?
"Assuming yours is a class F, dry-type trafo, the maximum winding temperature allowed is 130 deg C with a normal design life. Have your trafos ever recorded temperatures beyond 130 deg C ?"
In fact it is a class F transformer, but I don't have any information about those temperatures.
"Did you witness the factory acceptance test of these transformers where they are supposed to do temperature rise test ? If yes, what was the maximum temperature in that test ?"
I don't have that information. I have information for another transformer of the same manufacturer, but this kind of information doesn't appear. However, I think it should be in accordande with the standard, right?
Thanks
RE: Problem with transformers on cogeneration plant
You have already derated the transformer (either by design or by accident) to take care of 50 deg C ambient. So again, this is not a cause of your trafo burn-out.
Arcing is probably due to insufficient spacing, which leads us back to poor design/manufacturing.
As I said previously, you need to post some hard numbers here to go any further. Otherwise, both of us are just wasting time.
RE: Problem with transformers on cogeneration plant
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Problem with transformers on cogeneration plant
I'm finishing the collection of data on two transformers. Today I have some data that may help understanding what could happened. However, I have some questions for which your help is welcome:
a) When a transformer saturates, there are harmonics in the currents of magnetization. The fifth harmonic in particular, is often used to detect situations of saturation. What amplitude of those harmonics should indicate saturation?
b) What level of overvoltage could lead to saturation of the core? 5%? 10%? 20%? ... For instance, a transformer with a secondary voltage adjusted to 15400 V could be inm saturation when it is subjected to a voltage of 16500 - 16750 V?
Thanks
PS: I'll put some data here en the next days.
PMAC