Problem with transformers on cogeneration plant 1

[PMACP]

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

 

[DanDel]

An overload condition? Any more info for us?

 

[waross]

Why? Delta? Wye/Delta? Delta/Wye? Wye neutral grounded?

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[PMACP]

Dear Bill (waross)
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

 

[waross]

I hope you can go online at the job site. We will be happy to help and the response time is quite good here.
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

 

[alehman]

And check the load while you are at it. Are cooling systems (fans, pumps, if applicable) working properly?

 

[rmw]

Check the operating conditions against the name plate data. Someone may have supplied the wrong transformer. Was this plant relocated from another location?

rmw

 

[ScottyUK]

Look for silly mistakes, like closed isolating valves to the radiator banks, tripped auxiliary power to the cooling system, etc.

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!

 

[PMACP]

Hi,

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.

 

[PMACP]

Hello:

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

 

[edison123]

If the primary voltage exceeds the nominal rated voltage, you get into saturation leading to high core losses and excessive no-load primary currents. Not much increase on the secondary voltage due to saturation.

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.

 

[PMACP]

Edison123:

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

 

[edison123]

you mean 5-10% above the primary of the transformer ?

 

[PMACP]

Edison:

Well, I'm thinking on the secondary of the transformer. But, on the primary I have the value of 5%.

 

[edison123]

OK. Do you know what was the tap position on the primary side ? Normally, you see primary taps for +5%, +2.5%, Nominal, -2.5% and -5%. If the tap was at nominal and the input was consistently high at +5%, then the tap should be changed to +5%.

 

[PMACP]

Edison:

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

 

[edison123]

Once the trafo goes into saturation, it is difficult to estimate to estimate the losses and the no-load current. It would depend on the properties of iron, frequency and other factors.

May be trafo experts like prc, Dandel, Kiribinda, ScottyUK et. al. will chip in later to answer those questions.

 

[DanDel]

You will not get severe overheating issues from a 5-10% primary overvoltage. You have to look at the loading, winding/oil temperature, and room temperature, and compare those values to the nameplate rating and temperature rise first before you do anything else.

 

[PMACP]

Hello DanDel:

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

 

[VTer]

Ok, please correct me if I am wrong, but I’m just wondering? 5%-10% higher transformer secondary voltage under normal conditions means that your current would be lower at particular load, therefore the heat created would be less because of I^2R. But still this small increase in voltage cannot possibly create that much heat to cause any problems. There must be something else involved, and it probably is not the 5% voltage increase.
Also a 5% voltage increase would probably not cause the xfmr to go into saturation, I assume?


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