Load Flow Cases: Overload % 3

[IndianTechEngg1001]

I am involved into load flow studies for a system with Generator, Transformers and Transmission lines. I am referring a book "overhead power lines, Planning Design , Construction" by F. Kiessling, P. Nefzger, J F Nalasco and U Kaintzyk)

I am taking following cases. In book following cases are mentioned.

1. No Overload
2. Loss of Transmission Line and Generator: 20% overload.
3. Loss of Transformer:40% overload.

It will be great if someone can give me the idea about 20% and 40% overload mentioned in point 2 and 3 above. What it is actually.

 

[lume7006]

Hello,

What I think the cases 2 and 3 refer are the following:

1) When you are planning a transmission network, you should verify many things and doing several evaluations like:
a) Determine no overload on the networks
b) Determine voltage on busbars
c) and so on.

2) Starting from the above mentioned, you should evaluate some possible conditions (contingencies) in which several elements are out of service , like some "key transmission lines" and see the effect into the remaining elements.
In your second point, you may take out of service all of the transmission lines (once each time), perform load flow and determine if there are some elements overloaded more than 20%.

In your third point, all the xfmrs are out of service (once each) and see the effect on the rest of the network, in order to determine possible overloads above or up to 40% in the remaining elements.

As you may see, you would need to do many simulations, save many results and compare them, in order to reach a result.

In case of lines it would be useful you to know the thermal limit of each one,and in that way determine if the overload is tolerable or not having a reference.
For the xfmrs is the same, they do typically have some overload tolerances and also depending on the cooling system (ONAN, ONAF, etc) they may tolerate or not the overloads.
I hope this may have helped you!

 

[bacon4life]

Looks to me like the emergency ratings of the transformer. Under normal condition you do not want to exceed nameplate rating, but under N-1 conditions you may accept some loss of life in the transformer by overloading it. Our criteria for transformer overloading allows higher overloads for shorter duration events.

The loss of a transformer is much less likely than a line or generator, so the author is allowing for a more significant loss of life in the short event.

Do be aware that some transformers cannot tolerate that much over load and that some components of a transformer may not allow for a 40% overload such as bushings, leads, LTCs, heating by leakage flux, or the bus work feeding the transformer.

Here is a link to the referenced passage:

http://books.google.com/books?id=Zb... Kaintzyk&pg=PA5#v=onepage&q=overload&f=false

 

[tlrols]

Presumably the percent overload quantities are intend to relate to some type of short term overload capability.

In reality the use of overload capability is evaluated on a case-by-case basis and should NEVER be blindly applied to any line, transformer, or other piece of equipment prior to evaluating its capability.

What you have described is an academic example and should be viewed as such. Do NOT apply such overload constants in the field. You would need to evaluate each piece of equipment in detail to determine its overload capability (if any). Be aware that if you use that overload capability in the Planning realm then the Operations folks have NO capacity left for their needs. This is BAD Planning and most certainly is not a good way to make friends in Operations!

As a practical matter any Planning department worth its pay will screen all of its simulation results using loading percentages LESS THAN 100%...typically a 90% figure may be used. The point is to have a plan of service ready to go BEFORE something overloads. If Operations has to develop procedures to manage a problem then one can assume that Planning has not done its job very well (or that the utlity in question is unwilling to spend money at that time).