IEC short-Circuit Rating for Idc 2

[Schwatzernov]

Dear All,

Have anyone experienced the problem in comparison between the calculated Idc system short-circuit current and circuit breaker rating using IEC standard ?

I have one small power plant project which the owner had bought several small generators size from 8MVA to 17MVA and connected to the same system bus at 6.6kV voltage. The system is connected with utility grid through 69/6.9kV 25MVA transformer with high X/R at 45.49 as they want to have low loss. The result is that the DC component short-circuit current is very high.

I studied short-circuit current using ETAP6.0, the result shows that the Ib sym, Ib asym, Ip are within CB rating but very near its rating.
However Idc of short-circuit current exceed the Idc rating of CB approx. 4kA.
The CB used are the standard IEC CB having time constant 45ms, 50kA rated short-circuit breaking, 1 sec.Type ABB VD4.

Consultant said that the CB have not enough rating for Idc breaking even all other including Ib asym are within CB rating limit due to IEC requires all these four parameters be compared in considering the sufficiency of short-circuit breaking.

However there is the clause Annex-I section I.2.1 c)in IEC 62271-100 which seems to mean that " in case of Idc exceed breaker DC rating limit but the Ib asym is still lower than CB asymmetrical breaking rating, then this IEC deems as the CB have sufficient short-circuit rating for this application and Idc is not required to be considered ".

Is this understanding is correct ?

Could you please advise.

Thank you very much.

 

[ScottyUK]

You should consider the time to first current zero which is the first opportunity for the breaker to interrupt the current. A long time to first zero will place a large stress on the breaker contacts and may result in premature failure. I think one of the IEC publications gives some guidance on how to do this but maybe it was one of the ANSI docs. Discuss the result of the calculation with the breaker manufacturer.


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If we learn from our mistakes I'm getting a great education!

 

[odlanor]

I have IEC 62271-100 /2008-04: the clause Annex-I section I.2.1 c) estabilish: ... higher time constants than 45ms are expected but with a short ciruit lower then rated, such a case may be covered by assymetrical rated short-circuit current test using 45ms time constant.


For X/R=45.49 we have 120ms <= (1000*45.49/(2 & pi * 60) of system constant time.
A CB standard is 50kA-45ms, but is equivalente to 40kA-120ms. See Fig. 9 - percentage D.C. component.....

 

[Kiribanda]

Schwatzernov,

Per your post it is a generating station with step-up transformers having X/R= 45.49.Therefore, naturally, the fault currents at 6.6 kV bus should have very high peaks.Based on above and assuming your calculation is correct, it is little strange to hear that specially Ip value at the fault location (6.6 kV bus) is with in the limits of a "standard" VD4/ 50 kA CB having 45% DC component.

With my experience, if the I(Asym) value at the fault location is lower than the I(Asym) value of the CB, then you are ok.But at least you should have 10-15% margin due to growth and other factors.

So if the consultant still insists on the I(dc), then you should at least prove that when the CB is opened by a command of the fastest relay (may be 87G or 87B), the DC component that the CB will see at the moment of contact parting is less than 45% with a reasonable margin.

Have you checked about the possiblity of using these VD4 breakers as generator CBs against
1)Out of phase switching
2)TRV
3)RRRV parameters?

 

[Schwatzernov]

The calculated system Initial symmetrical short-circuit current is 48kA.

One hing I noted that the VD4 CB declared that the opening time is 45ms+arcing time 15ms =60ms interrupting time. However the 60ms is put in ETAP as minimum time delay.

However if 45ms is used as minimum time delay, the Idc is within CB DC capacity limit.

Does the 60ms is the mistake in the input data for minimu time delay or not ?

However I am noted from IEC62271-100 that the minimum time delay is the sum of minimum time of CB opening + half cycle delay time of protective relay(20ms/2) =55 ms. However with this 55ms, the Idc exceed the CB Idc limit again.

The consultant also argued that the clause I.2.1 c) does not mean that " in case of system short-circuit current having high time constant but have lower than rated, the 45ms CB may be used ".
This clause can not be read alone but instead it must be read from the second paragraph of section I.2.1 which said that
" When specifying higher time constant(CB)...."
then clause c) shall be read following. So clasue c) actaully mean that
" When specifying higher time constant CB but with the system short-circuit current less than rated, the 45ms CB may be selected ".
which later in clause g) also give the expanded explanation on the meaning of the above clause c)
So totally it means that " in system with high short-circuit time constant, instead of sepcifying high time constant CB(e.g.120ms) but system have short-circuit current less than rated, the 45ms CB may be applicable ".

What is your opinion about this matter ?

Thank you all gentlemen's opinions.

 

[odlanor]

Schwatzernov
You are right!
All this parameters refers to T100a type test for circuit breakers; - You should require from manufactory the symmetrical short-circuit current for 120ms of time constant, equivalent to CB standard 50ka-45ms.
So, You optimized your system studies for getting that value.Certify that bus short circuit is different from through fault seen by CB.

 

[Schwatzernov]

Odlanor
Can you give more details ?
What part you mean right, the consultant's opinion or not ?

How about your opinion on the keyed input for minimum time delay 60ms, should it be 45ms, 55ms or 60ms ?

Do you think that strange result(only Idc exceed, all other are within rating imit) came from the wrong input of minimum time delay 60ms ?

Thanks for your opinion.

 

[Kiribanda]

Schwatzernov,

If 45 ms is used the breaker DC capacity is within limits. But if 55 ms is used, the breaker DC capacity is exceeded!

I cannot explain how the above happens.Because, the more the time passes, the DC component is reduced.

Anyhow you have to use 55 ms as the minimum time delay.At 55 ms if the DC coponent at the fault location is exceeding the CB dc capability,then

1)you may intentionally delay the relay operating time so that,the breaker will not exceed its DC capability when the contacts are parted.For this you should have an idea of stability and other related issues.
2)there is no choice other than specifying a CB with a higher time constant.(As far as I know the manufacturer used to change the type of the vacuum bottle when higher time costants are requested)

Could you please post a copy of the complete SLD with all network parameters required for a complete SC study?

 

[ScottyUK]

Hi Kiribanda,

Option 1 is an awful solution and the only reason for doing this is to save money by using a cheaper circuit breaker. To me the idea of allowing a major fault to persist on the system by deliberately introducing a delay into the clearance time when a solution exists which does not require this delay is indefensible from a technical perspective. Siemens and ABB both have circuit breakers specifcally designed for generator duty with very high X/R ratios; not using them simply to save money is bad engineering but is frequently proposed by EPC contractors wanting to minimise the build cost.

Somewhere I think I have a MathCAD file which calculates the time to first current zero - I will see if I can find it.

Schwatzernov,

Can you disclose who the consultant is? Until recently I worked for a well-known consulting engineering company and am aware that this subject was frequently discussed by the switchgear group. I'm just curious if you are dealing with my former employer but I understand if you need to keep this anonymous.


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If we learn from our mistakes I'm getting a great education!

 

[odlanor]

Schwatzernov,
1- type test normally are made only one time by Institutes of Eletricity that create a document official for that model of Circuit Breaker.
2-Institutes only obbey Standards. Minimum time delay is that they decided to make the tests, not what you are thinking it could be.
3- The critical measuring for T100a test is de % Idc component.For your application I figured out:
tau %Idc
45ms 26%
120ms 60%
4-To reduce Idc=60% to Idc=26% you should reduce your Isym. This is only an aproximation.
The actual tau value is function of system constant time (120ms) and the Arc Resistance that appears during CB opening.
This arc resistance forced an imediatelly attenuation of current wave in order to cross zero current and to possibily extinguish the arc.
This arc resistance is inerent to design of each model CB. So, only manufacturer can give you the final decison.

 

[Schwatzernov]

Gentlemen,

I enclosed the detailed calculation made by ETAP softwaare for your reference. The problem is at SUB#5 having 6.6kV switchgear SGM-5-01, SGM-5-02 and SGM-5-03 which only Idc exceed the rated while all other short-circuit current are within limit. To be noted that all 6.6kV VCB are ABB VD4 type having 45ms opening time+15ms arcing time =60ms interrupting time, however the minimum time delay input to the software was 60ms.

ScottyUK,
For the consultant you are asking for, may I tell you this way, this is the European consultant main expertise in power plant business, No.4 in the world of Top 200 design firm(ranged by ENR) in power plant business.

 

[odlanor]

Schwatzernov,
I never saw a system like that in my life! I print a copy of main unifilar to apreciate!
All my life I worked with hydro generation, HV or EHV system. I am at opposite side:- Just now , I am envolved with a 500kV system with 2 hydroelectric power plant, each one with 44*80 and 46*80MVA-13.8kV - bubble turbines. Those generation(at Amazon) are sent by 2xGIS 500kV that is converted to 600kV DC and sent by 2 or 3 HVDC bipole transmission line of 3500km at center(Sao Paulo-Brasil).
I am sorry. I have no idea about your system!

 

[Kiribanda]

Schwatzernov,

What I can see from your SC report is that you are comparing the CB device capability with the "TOTAL BUS FAULT CURRENT" which I feel is not correct.As an example, SGM 5-01 BUA A/ CB#S5-SM1-SUB#2,the DC component of the CB is 18.639 kA after 60 ms time delay.But the DC component of the fault current is 23.99 kA for a fault current of 44.651 kA. This 44.651 kA is the TOTAL BUS FAULT CURRENT with all contributions at that bus and not the MAXIMUM THROUGH FAULT CURRENT via the CB#S5-SM1-SUB#2.So you should compare the device capabilty (18.639 kA)with the maximum through fault current (5.68 kA)of the device.Similarly, for SGM 5-02 and 03 buses too.But it is not happening for the other buses because the difference between the Total Bus Fault Current and the Maximum Device Through fault Current at those buses is very small depending on their X/R ratios.If you check the X/R ratios of these remaining buses they must be lower comparing to SGM 5-01, 02 & 03 buses. Inother words the fault currents of thoses buses decay faster.(You have not included X/R ratios of all buses)

Therefore, when are you doing the SC calculation you should select the option "PROTECTIVE DEVICE DUTY-BASED ON MAXIMUM THROUGH FAULT CURRENT"

 

[Schwatzernov]

Kiribanda,

Sorry, those file were not correct as they reported by comparing with total bus fault.

However with the updated files as herewith enclosed with through fault current report, the Idc still exceed the CB Idc capacity. Please see the attached files.

 

[Schwatzernov]

Kiribanda,

I attached two files, one is SLD and another is the complete report. However I have seen my post has only one file which is SLD, so this is another file of the complete report.

 

[Schwatzernov]

Kiribanda,

The herewith enclosed file was simulated to have three-phase fault only at bus SGM-5-02B and see the current flowing through CB S5-SM1-B2, the result show as in the attached file which Idc exceed the CB Idc capacity(21.441>18.639kA)while Ib asym is within CB capacity(42.464<53.361kA)

 

[Kiribanda]

Schwatzernov,

It is a very strange situation.

Bus#SGM-5-02B:

X/R=9.8.Therefore, k=1.74.
With Ik3"=43.067 kA, Ip=107.97 kA is accepted.

But "due to some unknown reason"(which I am unable to find),after 60 ms,the Idc=21.441 kA whereas it should be 8.899 kA so that it is within the DC capability of the CB.That means for calculating the DC component after 60 ms, the program is taking a diffrent X/R ratio (X/R=19?) at the faulted bus.

May I suggest,
1)Insert a new 6.6 kV bus between CB#S5-SM2-B2 and cable131.
2)Select protective device duty based on maximum through fault current.
2)Make a 3-phase fault ONLY on this new bus.DONOT fault any of the other buses.
3)Check for any change
4)Also run transient sc program (IEC61363)and check the decay after 60 ms.
5)If possible upload both OTI and MDB files to check.

Hope this helps.

 

[Schwatzernov]

Kiribanda,

Thank you very much for your kind help.
I re-studied according to your advice and enclosed the report here below.

Yes, it is somehow strange that X/R=9.8 but it give the very high Idc. Is this because of using IEC 60909 Method C or not ?

 

[Schwatzernov]

Kiribanda,

I enclosed the OTI and MDB as you requested.

Thank you for your kind help.

 

[Kiribanda]

Schwatzernov,

I have found that since lot of cable impedance data is missing, the program assumes some default values. Therefore,either you have to delete all cables or input realistic impedance data for them.

Please refer the uploaded zip file.