Upstream and Downstream Short Circuit

These withstand values are for the purpose of discrimination.

If you are referring to two 11kV CBs then you need to tell us whether there are any other loads on the downstream side ... i.e., are there more than one 11/0.4 kV transformers on the line? In that case, you would need to set them to trip earlier, maybe 1 sec, to ensure that supply to the healthy transformers remains unaffected. If there are no other transformers within your area, then both 11kV CBs can be identical.

This paper may be of help to you, please download and study:

MV-network.pdf

Thanks for your reply. i will down load it and read as suggested. I will come back to you then. It seems that we can discuss further and I can lear lot more.

With all respect to you...the document which you suggested is a formal description about Mv distribution network only, and nothing helpfull for the subject problem.

I will explain the situation . MV switch gear(11KV)shall be 20 KA for 3 seconds fault with stand rating as per project spaecification. switch gear designed based on this data. Power utility authority informed later, the 11KV (MV switch gear) side of the up stream substation from where our substation fed, is 25 KA for 3 seconds. The switch gear manufacture now says that, they have to redign the panels based on this 25KA for 3 seconds. what I want to know is ...Is this required ??...

• First, you need to register on CR4, and get yourself a user profile. That way, CR4 can send you emails whenever there is any comment on your post. Helps, believe me.
• Ok, so that paper was not helpful, but did it not have ANYTHING that you did not know already? If so, i am sorry, but then i did not know what you knew. It is nice to know more than what you need ... right?
• You are focussing on that 25kA for 3 sec rating....what does that mean, and why is it important ? i already asked you whether there are MORE THAN ONE transformers on the downstream side...you haven't answered it yet. Please do, and others will help also.
• If your original spec was for 25kA for 1sec or something like that, the panel manufacturer is justified in asking for more time and money, is he not?

Please take your time and answer, i will be glad to help....

1. I will register on CR4, as you said.

2. Yes, there is some thing helpfull about overall Mv distribution system.

3. We have one Mv switchgear panel which feed to 4 transformers.

4. Original spec was 20KA for 3 seconds.for our MV switchgear. 25KA for 3 seconds for the MV panel in the up stream substation. Not in our substation.

Please glad to help me...

Excellent! You are very prompt.

OK. The MV switchgear has a 25kA/3sec rating. No big problem for you. As i said before, in case a short circuit happens within your panel, you need the faulty circuit breaker to trip and isolate that circuit, so that the other three are unaffected. (This holds good for all 4 CBs, of course.) You can ask your vendor to tell you what he can do without any rework....20kA for 3 sec (1200A²S) or 25kA for 2 sec (1250A²S) ? If he is a technically good and honest supplier, he will not ask for more time or money, his existing design will be OK. Think about it.

Now, i have a feel for how good you are. So, it is VERY UNLIKELY that you will allow a short circuit to happen within your area. So, the SCs which happen will be on the LV side, and, IN CASE THOSE CBs FAIL, your 11kV CB will trip. VERY UNLIKELY!

So, you please familiarise yourself with the LV discrimination. Read ECT167. Nobody uses time-based discrimination anymore. Most common is ENERGY-BASED discrimination. After all, would you want 25kA to flow for 3 seconds for God's sake ???

Good. There you go. we do not need to think about LV side discrimination now. It is safe . well co-ordinated.

All what we say here the withstanding capacity. May be spec means that 25KA for 3 sec equal to much higher rating for 1 sec. (normally). Some clients/consultants thinking the peak value at the first cycles. to withstand that peak value at first cycles, they put requirement of 25 KA for 3 sec. Am I correct?...But still i do not understand what is the relation of the withstanding rating of MV panel at 3 Km away from our substation to rating (not short circuit value but the withstand rating)of our MV panel??

Good that you are ok on the LV side.

You are quite correct in worrying about the first-peak withstanding capacity of the busbars & their supports. Here is what a typical SC current wave will be like:

Being more familiar with LV, where the norm is 50 to 65 kA for 1 sec (with a first peak of 105 - 143 kA), i am not that much worried about the ability of your busbar+bracing. i do not have the MV standards with me, so i can't say what the peak will be, but it should not worry you much.

Moreover, the impedance of 3km of cabling...will it not attenuate the fault at your panel ? In any case, you will set your CBs to trip in 1 sec or so, so that there is no nuisance tripping of the far-away MV CB won't you?

i do not get the feeling that i am answering your main worry still...maybe someone else will be able to. Good luck.

Thanks for your inputs Mr.KVS. I am continouing with this problem .I will try reading and in some other forums too. Once I get satisfied with good answer, i will post it here.(May be this is silly question.Thats why so many viewed but only you tried to contribute here).......tnx

If it is the understanding of what happens to the busbars, their supports, the plug-contacts to which withdrawable breakers connect etc, a very good paper on this is here: ect162.pdf

This refers to LV switchgear, but then these forces are current-dependant, so should be the same.

You can also google for other Cahiers Technique publications, some of then MV, which are superb and free. Cooper-Bussman also has many good downloads available. Good luck.

Dear Guest,

You have to run calculations to check the max. short circuit current on your 11 kV busbar (downstream) based on the upstream short circuit power (MVAsc). Thus, you can determine the withstand short circuit level of your switchgear. During running of these calculations, you have to consider the case of parallel operation of feeders (if any).

Good luck...

I got 19.89KA fault current based on the max.short circuit level at 11 Kv side 381051 KVA. please tell me how can I determine the withstand short circuit level of my MV switchgear?...ignore the parallel operation as we consider only one feeder.

It is the duty of the MV switchgear manufacturer to certify this withstand capability. You are not the designer of that i assume. If you still want to calculate, this is not the forum.

Ok. agreed . I am not the designer. According to you it is the duty of MV switchgear manufacturer. Is there any data/input for them to say time of one second is enough and 3 seconds not required?...

PS. I didnt knew that this forum is your family property. I will be verymuch thankful to you, if you could tell me who granted you the voice to say this is not the forum.

You are a matured man, and so it is not good to discourage others, who may be willing to give more details. Is it because of you couldnt give correct answer me?.

My friend, you misunderstood me.

i meant you should read the ECT 162 that i gave the link to, and you will see how many parameters you need to put in to calculate. If you had done all that, you would have probably found your answer, or at least asked more specific questions like how close the busbar supports should be, is a M10x50 bolt of grade 8.8 that i am using adequate etc.

i look forward to someone answering your question, which as you say, i could not answer. Good luck and goodbye.

Dear Kvsridhar,

I respect you and your postings. I am verymuch thankfull for your contribution to this thread and overall forum. No problem with you.

I know what is fault level, and how to calculate it.All those things were doing years ago. Some body answered like they are big shots knows everything and this question is nothing for them.( not you). My question was very simple. If one substation rating is given for 3 seconds is it required to count 3 seconds for the down stream also?...my answer is no. I was trying to find some back up clarification for my decision to go for NO.

This is rare situation as most people will not worry about such things. They simply forward the variation cost added by manufacturer towards client. I can also do that.

I just tried to get something from this forum. Thats all...

Anyhow,..dont be angry with me.

Dear Guest,

If you are confident of your calculations so, you can select switchgear with withstand short circuit current of 25 kA. Please note that during fault, the protection scheme is supposed to work properly to clear the fault in m-seconds. Therefore, having switchgear with 3 seconds duration is a cost effective, since the protection will never allow the fault to be sustained for more than m-seconds. Thus, I would suggest the switchgear in your substation is to be 1 s, unless otherwise instructed by your authority.

Good luck...

Well, thank you, i feel somewhat better. i was not angry, just hurt

Anyway, reading my own post, i realise you had a right to react, since it was rather abrupt. My apologies.

i have designed some schemes with time-based discrimination in the old days, in LV. My trip times were generally: Main incomer ACB- 0.5s, next ACB downstream - 0.2s and thereafter all MCCBs which were current limiters, so no time setting possible. The MV side was often 1s for sake of economy in such cases.

Mind you, these times were for the relay to operate and signal the CB. The actual tripping time of the CB (usually < 30ms) needed to be added.

In any case, other CR4 members like TITO are getting involved, so you may get some good answers.

Good luck.

Dear Kvsridhar,

Thank you for your comment, which shows that you have a respected personality.

We all in this forum are trying to lend a helping hand to the people who are looking for answers to their questions, even if some of their questions were intuitive, simple or even repeated. But, these questions may serve as a stumbling block on their way to understand things. Therefore, should not we disregard these questions; on the contrary, should anyone who has the experience to try to answer the question because it is obligatory for him, and I believe it is a part of his life's mission to exchange his experiences and knowledge with others. I know the easy come easy go; but at least you have done your part by putting him on the right track. However, the rest shall be on the shoulders of the second party to start digging to get more and more.

Thank you again for your decent and civilized attitude, and I am very proud being a member of this esteemed forum, which includes respected people.

Good luck...

I think the question is slightly different to the answers that have been given..

If the switchgear rating is given as 25kA for 3 seconds this is just a rating. The fault level, ie the current that will flow during a fault may be anything up to 25kA. The switchgear is therfore rated to allow that sort of current flow for up to 3 seconds if it flows for more than three seconds copper bars are likely to melt and buckle.

BUT the actual fault current will nearly always be much less than this. Maybe as low as 10kA.

Standard ratings for 11kV switchgear are 250 and 350 MVA.

Your downstream substation MUSt have a fault level withstand of more than the actual fault level at that part of the network. So you must find what the FL is at the source. The supplier should be able to tell you this and then you must calculate the loss due tot he cables between the substations. If only a short length of cable then you will not see much difference. If you are asking what the FL should be for the Low voltage side fo the transformer then this will have dropped by a very large amount but any circuit breaker and the transformer itself must be able to withstand the actual FL on the HV side.

1. Ask your supplier for the actual FL

2. Calculate the FL downstream

3. Select appropriate FL for the switchgear. You wont have much choice it will be 13kA or 25kA

4. Then do a discrimination survey.

One post on here said that you dont need switchgear to be rated at 3 seconds as the protection will operate in less time than that. This is such a dangerous thing to say, 3 seconds is a very short time and any slow opening or poor pickup in protection could result in DEATH and DESTRUCTION.

Dear powersystem,

First of all .. nice post.

However, i am a little perplexed and i hope you will explain and clear my doubts here.

In the last paragraph you say that 3 sec is a very short time and in case the trip pickup and/or circuit breaker is slow, this much time is needed... (this is news to me. As i have mentioned in this post, my entire experience is in LV, in which 1 sec Icw ratings is the norm. This is a rated thing, and in real life, the fault is cleared by current-limiting devices in LV in millisecinds)

However, i note that the ABB vcb trips in maximum 50 ms, after receiving the command. And the microprocessor-based relays of today are very accurate and reliable. Other makes of cbs are comparable i am sure.

Also, i understood from my colleagues at that time that energy-based discrimination is usually used nowadays, and time-based discrimination is not so common. i wouldn't know..

Please note that i am not questioning what you say, just trying to understand a little more of the MV world. Thanks in advance.

Hi, nice to hear from you.

the current limiting is mainly a feature of LV systems and of course fuses. We do use fuses at high voltages even up to 33kV and they are good at what they do but for circuit breakers fitted with relays they still do a time based discrimination.

Tripping is as you say extremely fast and any really high curents would be tripped using a High set or instantaneous relay.

All switchgear at 11kV is rated at 3 seconds. this is due to the need to allow many stages of downstream protection. IE if you have 3 circuit breakers in series downstream you may get around .5 of a second between each one tripping. If there is afault on the source busbars then this may last for 1.5 seconds before it trips which is getting close to the 3 second limit. In practice its not a problem unless somthing goes wrong and the protection does not operate correctly.

I am not sure about the energy-based discrimination. Its really a function of current and time which is proportional to the energy anyway. In all recent installations i have done they are IDMT ( time based relays) BUT we do tend to use a lot of unit protection that copmpares what goes in with what goes iut and this really is very fast. And it gets to the 50mSec you mention.

Hope that helps if not try me again.

Kev

Thank you. i worked for a while for the Indian arm of English Electric (later GEC, then GEC Alsthom, the Alstom, then GE+Areva etc) and i had the privilege of knowing intimately a master of HV fuses in Liverpool. What technology! And what a range of superb products!

To be honest, i haven't understood fully the energy-based discrimination either. There is a superb paper from Schneider which i have referred the OP here to study, but i haven't studied it myself Must do it today...

Thanks again for the response.

Dear Powersystem,

Please note that the case of our guest is different; as his substation is considered the end substation with respect to the authority one. Therefore, the time grading in the protection devices of the interconnection feeder(s) is simple (i.e. there are no manny time stages). Accordingly, it would not be feasible to specify 3s switchgear.

It is not a normal practice to specify the MV switchgear with 3s withstand duration; as this is subjected to network and protection coordination studies. But it might be applicable in cases similar to the one you have mentioned in your post. In any case, if the protection scheme is not a reliable one, neither 1s nor 3s switchgear would help.

...and there is a 3km (1.8 mile) cable from the upstream CB....

HI TITO

Thanks for your comments. Made me think what i had said and I no way want to argue. I found that after 34 years in an english utility that there are more than one ways of doing stuff and the English way is more onerous and not always best but..

Can I respond...

first is that "3 secs is not usually specified:" we usually do specify 3 secs and to be honest its hard to find switchgear that is not rated for less. can you recomend any plant that is advertised at less than 3 secs? I recently closed an 11kV switch to earth and managed to close to earth a live supply. The switch was rated at 13.1kA for 3 seconds and it never made me think I had done a mistake.

The gear was 30 years old and rated at 13.1kA for 3 secs, current switchgear will be 250/350MVA at 13.1kA or 25kA for 3 seconds

The issue here is the 3 second rule.. 3 seconds relates to the energy let through. The equation is.......... energy is proportional to the current squared times time. The 25KVA at 3 seconds is what the switchgear is rated at after tests.

I am really worried that anyone would consider less values than this. BUT somnone said the cable is 2kM in length so that will deplete the Fault Level massivly. So I would say specifiy that the fault level is 250MVA less what ever the loss is in the cable. This may take the fault level down to... I guess 8MVA

But the gear is designed for 250MVA for 3 secs so the Isq t is much less so the gear can withstand the fault for much longer.

We can get into a cuircular arguament.

The last post on this said ........ 3 sec or 1 sec does not make difference as the board would be ruined....I have to disagree.

After 3 sec the upstream protection should have operated and cleared the fault. Thats why we specify 3 seconds as a reasonable time for the upstream protection to operate.

Ok consider this.. you operate the gear and close onto a 3phase earth fault. i.e. Its got a downstream fault. The fault will last for a finite time. .1 to 1 or even 2 seconds before the upstream protection operates. What rating DO YOU want? Do you want your switchgear to last for 1 second or 3 seconds? The fault level is 200MVA the rating of the gear is 250MVA for 3 seconds

The switch is rated for 250MVA or 13.1kA for 3 secs

If the protection takes 4 seconds to operate then it will have melted and will explode.

If the fault level is 125MVA then it will melt in 6 seconds as the energy let through is proportional to time,

I am really struggling to explain the impact of fault level and perhaps you guys can help me here.

1. DO A FAULT LEVEL CALCUL:ATION

2. Check 1 above!!!

Dont rely on your protection to operate to clear the fault. I am working in England and thats a route to a term in prison for negligence. Assume the protection will not work and YOU ALWAYS assume that,,,,,, you then assume that the upstream protection will operate and this may be much more that 1 second..

OK a summary

Interesting questions and comments but I cant agree with, but it made me rethink my answers

I dont think you can buy switchgear rated at less than 250MVA for 3 seconds.. Let me know any models as Id like to offer to clients as it would be cheaper.

Ignore any issues of discrimination. You need to consider the ratings of the gear and not assume that it is protectoed by protecition. (Thats an English perspective and am open to comment)

Loved the comments so please argue.

Kevin , Power System

Hi Powersystem

I'm glad that you responded to my comment and thanks GOD that nothing happened to you or to one of your colleagues as a result of the terrible mistake that you have done; however, it seems that we have different points of view.

Firstly, I'm wondering that you do not considering what you have done is a mistake; this makes me curious to ask what about the interlocking scheme adopted in your system, which should prevent such human error; or such act is a normal practice in England?

Secondly, the current MV switchgear has many different levels of short circuit withstand current and not limited only to 13.1 & 25 kA. You can find 16, 20, 25, 31.5, 40 & 50 kA and in some special applications you may find 63 kA.

Thirdly, I have mentioned that sizing of switchgear is subject to network researches and protection coordination study. I understand the impact of the short circuit current on the switchgear; however, we cannot ignore at any case the protection system; as we pay much amount to keep our people and system safe.

In my opinion, the English advice to ignore any discrimination and protection, is unfair and exaggerated advice. Nobody would be happy, when a fault at downstream causes the upstream to trip. This means that there is a problem, either in the protection scheme or the switchgear itself. It is noteworthy that most of nowadays protection relays have many functions, features and faster; so, you can rely on their action in clearing the faults based on the applied coordination and settings.

What I'm trying to say, it is an integrated system; so, we cannot ignore any part of it. The system design should be optimized in light of your studies. Of course you know that the initial cost to have 3s switchgear, reliable protection system and of course the cables in this case shall be rated for 3s as well, will be higher compared to 1s switchgear, reliable protection scheme and 1s cables; which shall carry out the same assignment.

By the way, are you specifying the short time duration of the cables as three seconds as well?

My conclusion is, 3s switchgear has advantages than 1s switchgear as you mentioned; however, it is not feasible to recommend it unless your network really need it as it means unjustified cost.

Please find here below some links for 1s (Primary & Secondary) switchgear from different reputable manufacturers for reference.

http://www05.abb.com/global/scot/scot235.nsf/veritydisplay/905d9b3b1384a3ef85256df10066153c/$File/Advance%20-%20Safegear%20Tech%20Guide.pdf

http://www.areva-td.com/solutions/liblocal/docs/Products/SDS/AREVA_T&D_SDS_Secondary-Distribution-Switchgear_Offer-EN.pdf

http://www.energy.siemens.com/hq/pool/hq/power-distribution/medium-voltage-switchgear/ais-primary/air_insulated_switchgear_E_33.pdf

http://www.energy.siemens.com/hq/pool/hq/power-distribution/medium-voltage-switchgear/gis-primary/GIS-Brochure_Dach_en.pdf

Tito

I don't think I disagree with anything you said I think we have strayed a bit off topic... All I really mean is that you must make sure that all switchgear is suitably rated for the fault current that will pass through it. Understanding the fault level will therefore be essential in selection of switchgear. If 13.1kA switchgear is chosen and the FL at that point is say half of this then the switchgear will be able to withstand that fault for much much longer.

in my earlier posts I did not mention cables as I think it is odd that WE specify 3 sec switchgear and 1 sec cables. However we don't stand in front of cables and apply earths to live cables (!) like we do with switchgear.

As for my incident.. the switch was a stand alone switch and it was not clear which way the earth was applied. I ASSUMED WRONGLY and potentially lethally that the earth was away from the source. My mistake totally, if I wasn't certain then I should not have operated it.

There should have been an interlock but in general the supply companies here would not allow any interlock between a customers and their gear. And anyway I think the switch was installed incorrectly many many years ago.

The list of 1sec switches is interesting. The fault currents are much higher than I would have expected but of course that is a function of the current and time. So a 40kA 1 Second switch will also have a comparable but much lower rating at 3secs. Siemens switchgear brochure you refer to says Up to 17.5 kV, 25 kA, 1 s Up to 24 kV, 20 kA, 3 s

So I think we do actually totally agree. I will look at the standards again!

Thanks for comments I look forward to the next

Kevin

Dear All,

There a e few topics in electrical engineering such as Short circuit rating, earthing protection rating etc., the design of which have to be done in mutual consideration of all such equipment.

I have a simple question, may be a stupid one, but still an intriguing question. On what basis do we decide on the time factor SC rating i.e., 1 sec of 3 sec? Most of us know that the effect of SC of even 10 kA for 1 sec can be a disaster. Then why do 3 sec are specified?