Dynamic Analysis of a Transformer Bushing System

[MGutarra]

Hello there:

So basically, according to IEEE 693, I need to perform a dynamic analysis of a transformer-bushing system using FEA. I've done static analysis of these machines before but this is the first time I have to make a FEA of a transformer. The concept seems to be simple: model the ''structure'' and obtain the mass participating ratios and natural frecuencies, that is all I need. The problem I have is im not sure how to model this machine, I've seen some papers showing pictures like the one I attached, but it's not clear to me the nature of the modelling. Are those shells all the model? is it only the case being modelled with all the mass distributed in the exterior shells? Or are they modelling the interior parts of the machine too? How are the masses distribuited in these kind of models? What elements are considered in the stiffness of the model and what elements are considered only as ''loads'' for mass porpuses? If someone has experience with these kind of modelling, I would really appreciate their help.

Thanks in advance.

Link with model example:

https://www.youtube.com/watch?v=cRgSkXirBKM&fe...

Disclaimer: I posted this in the FEA section but no one answered and I really don't have much time left. Hope this doesn't upset any moderators. This section seems to be more active.

 

[r13]

I think this (equipment) modeling for dynamic effect is a very specialized field. My past experience was relying on the analysis provided by the vendor. As the minimum, the vendor needs to provide you with drawings and tables indicating the static and dynamic characteristics of each part, and operation modes/range of frequency, etc.

 

[WARose]

First off, if you are structural....what are you doing looking at designing something like this? Seems like this would be more in the realm of mechanical.

In any case.......

So basically, according to IEEE 693, I need to perform a dynamic analysis of a transformer-bushing system using FEA. I've done static analysis of these machines before but this is the first time I have to make a FEA of a transformer. The concept seems to be simple: model the ''structure'' and obtain the mass participating ratios and natural frecuencies, that is all I need.

If the code you are using allows that.....that's great. But as a structural engineer who has done a lot of time-history analysis.....that seems a little odd to me. Reason being you can have parts that don't have much mass (relative to the whole model) that are shaking at a unacceptable level.

The problem I have is im not sure how to model this machine, I've seen some papers showing pictures like the one I attached, but it's not clear to me the nature of the modelling. Are those shells all the model? is it only the case being modelled with all the mass distributed in the exterior shells? Or are they modelling the interior parts of the machine too? How are the masses distribuited in these kind of models? What elements are considered in the stiffness of the model and what elements are considered only as ''loads'' for mass porpuses? If someone has experience with these kind of modelling, I would really appreciate their help.

No offense but: you may want to get someone else involved.....these are fundamental type questions.....but I'll take a shot at them anyway. If you can represent something that has no impact on the dynamic behavior of the machine as a mass/load.....it makes sense to do so. You don't want to make the model too cumbersome. A lot of this is based on (experienced) judgement calls. You may want to try the mechanical board too.

 

[SlideRuleEra]

By all means, get someone else involved... who is familiar with this type equipment, and knows what they are doing.

A transformer is not a "machine" (with moving parts), it is basically a static laminated iron core with copper wire windings, probably with (relatively small) cooling fans and maybe small circulating pumps for an oil cooling system. There are appurtenances, like the fragile bushings (the three sets of brown cylinders on your fist image), that could be damaged by seismic events... therefore the reason for the analysis.

http://www.SlideRuleEra.net

 

[JoshPlumSE]

Huh? This is a weird one. My thoughts:

1) I have seen structural engineering do a dynamic analysis on the structure (including some mass and stiffness consideration of the equipment) in order to determine natural periods / frequencies, mass participation and mode shapes. Or, to determine the dynamic response of the structure to the equipment forces.
2) I have not seen a structural asked to perform dynamic analysis of mechanical equipment that has moving parts. I've done dynamic analysis on mechanical equipment like vertical vessels because our group was more comfortable with doing that type of analysis than my company's vessel guys. But, those didn't really have any moving parts.
3) If this equipment doesn't have any moving parts then it's not outside the realm of possibility. Provided that you know the following:
a) detailed information about the equipment parts.... material, thickness, et cetera.
b) There is likely a lot of oil inside. I would suggest that any "sloshing effects" of the oil be considered out of scope. I would just lump the mass of any liquids into the structural items that contain them.
c) Those images look to me like it's using a lot of plate elements. It's possible that there are solids in there. But, at first glance, I'd say mostly plates and the occasion frame element.
d) This looks pretty tricky to model. So, I wouldn't take this project lightly. If you're not getting much clear direction and don't have an example to fall back on, then (if it were me) I would probably pass on the project.

 

[oldrunner]

IEEE 693-18 is the latest edition of the IEEE Recommended Practice for Seismic Design of Substations. There are quite a few structural engineers that model transformers. Whether they model them correctly is always an issue. One of the problems is that the transformer has a definite life and the life is determined by the insulation which is oil, solid and paper and in some cases probably the wood supports of various items inside the tank. One needs the shop drawings to accurately model the tank. One new issue are the bushings. According to the new bushing criteria, many will have to be re-certified. Another criteria of the IEEE 693 code is that it is to be certified that the transformer will survive and continue to operate after a seismic event. From a structural standpoint this only focuses on the no structural failure and the purchasers may assume that the transformer is earthquake proof (or resistant) - basically the whole transformer is to sustain shocks and violent shaking.

But if the transformer is 15 to 20 years old, then the insulation failure can be the culprit. Oil insulation can be monitored and treated or changed out for new. Paper and solid insulation degrades (ages) over time and becomes brittle and weak and susceptible to shocks. The solid insulation is difficult to physically inspect and the damage cannot be repaired. The solid insulation is used in the construction of the windings. Initially the windings are compressed by heavy long bolts. As time passes, the insulation degrades, shrinks and looses the compression allowing the assembly to be somewhat flexible and vulnerable to shocks - so much so that required clearances between some elements can promote pyrolysis - possibly starting an internal fire and resulting in an explosion. From this view, it appears that transformers are very fragile. The possibly remedy for this is complete isolation to avoid seismic impacts.

Porcelain bushings are a big issue also. I have never seen a properly illustrated free body diagram of a bushing. One of the issues is that the metal flange is bolted to the tank cover - basically making it part of the tank. Then there are only three main elements to the bushing: the upper and lower ceramic tubes with a gasket between the tubes and the flange and the internal metal tube - either copper or aluminum that is unrestrained to vibrate independently of the transformer inside the ceramic tubes. This tube is tensioned, resulting in compression in the ceramic tubes - which have reasonable compression properties but poor shear characteristics resulting in unexpected rupture. (does anyone have any shear values for porcelain?) Therefore studying the free body diagram one may reason the possibility of self-damage.

Another problem is that the internal assembly shall have continuous load paths. The problem is that with the heat and other considerations, there may be some gaps in the load path resulting in impacts. ASCE 7-16 recommends seismic restraint mechanisms. What one is attempting to do is to minimize internal pounding.

 

[MGutarra]

First off, if you are structural....what are you doing looking at designing something like this? Seems like this would be more in the realm of mechanical.

My company designs lots of Electrical Substations, as a Structural we design the foundations for the machines, the steel structures and the main RC structures. Also, sometimes, not always, we are required to do verifications on the bushings, I've done several of that, but, as stated in the picture attached, the type of Transformer we are using requires a dynamic analysis, which will have to be using FEA according to the code. Usually we use static analysis, but the current supervision has been very clear about the type of analysis we have to make. So I, as the structural engineer of the project, have the responsability to make this analysis. I know is not ideal, and I am aware is a tricky analysis, but we don't have much time left. It is obvious to me that the guidance of someone with experience in this type of analysis is necesary, that's why I am asking for your expertise.

 

[oldrunner]

MGutarra:

IEEE 693 recommends fixed anchorage of a transformer such as bolts or embedded fixtures welded to the base plate. Do you consider the thermal stresses of the transformer base when it is energized? Do you consider the variation in the coefficient of expansion of the concrete, which depends upon the coefficient of expansion of the aggregate which depends upon the particular quarry that the aggregate is obtained? Review ASCE 7-16, Section 1.3.4 - Self-Straining Forces and Effects.

 

[WARose]

My company designs lots of Electrical Substations, as a Structural we design the foundations for the machines, the steel structures and the main RC structures. Also, sometimes, not always, we are required to do verifications on the bushings,.....

Ah, I got you now. (Didn't realize you were doing this for seismic purposes. Was unfamiliar with the code you cited.)