A Short Circuit Study is an important tool in determining the ratings of electrical equipment to be installed in a project. It is also used as a basis in setting protection devices. Computer software simplifies this process however, in cases where it is not available, alternative methods should be used. The per-unit and ohmic method are very tedious manual calculation. These hand calculations are very prone to errors due to so many conversion required. In per unit, base conversion is a normal part of the calculation method while in ohmic method, complex entities conversion.The easy way to do hand calculation is the MVA method.

In this example, we shall be presenting a short circuit study of a power system. Motors are already lumped with ratings 37kW and below assigned an impedance value of 25% while larger motors are 17%. A 4MVA generator is also included into the system to augment the utility.

Figure 1

Utility: 33KV, 250 MVA_{sc}

Transformer 1: 10 MVA, 33/11KV, 9% Z

**11KV Bus**

Generator: 3MVA, X"_{d} = 0.113

Transformer 2: 5 MVA, 11/6.6KV, 7% Z

Motor 1: 5MVA (Lumped), 17% Z

**6.6KV Bus**

Transformer 3: 2 MVA, 6.6KV/400V, 6% Z

Motor 3: 6.8 MVA (Lumped), 17% Z

**400V Bus**

Motor 4: 300 KVA (Lumped), 17% Z

Motor 5: 596 KVA (Lumped), 25% Z

## 173 Responses

Sir Ver,

I’ve noticed in the calculation for Single Phase to

Earth faults that you didn’t consider the contribution of the lumped

motors (@400V) but just the transformer 3 (Y-grounded side) for the Zero

sequence contribution. I’ve read Moon H. Yuen’s paper about MVA method

with the title Short Circuit ABC. What he did there was aside from the

positive and negative sequence he also considered particularly for the

zero sequence contribution the transformer and of the motor’s, please

advise Sir what are the consideration why you didn’t consider the

contribution for the lumped motors and just consider the transformer

only. Thank you and more power Sir Ver!

(Y)

The motors are considered to be connected “delta” thus it does not provide any fault contribution on single phase to earth faults.

Sir Ver,

I’ve noticed in the calculation for Single Phase to Earth faults that you didn’t consider the contribution of the lumped motors (@400V) but just the transformer 3 (Y-grounded side) for the Zero sequence contribution. I’ve read Moon H. Yuen’s paper about MVA method with the title Short Circuit ABC. What he did there was aside from the positive and negative sequence he also considered particularly for the zero sequence contribution the transformer and of the motor’s, please advise Sir what are the consideration why you didn’t consider the contribution for the lumped motors and just consider the transformer only. Thank you and more power Sir Ver!

Hi Sir Ver,

Good day, I’ve noticed. In the calculation for For single phase to earth faults, you considered the MVAsc1 and MVAsc2 which is the positive and the negative sequence components respectively plus the MVAsc0 which is the Zero sequence contribution, but in the example @ 400V you considered the 28.28 and the 33.33 (for the transformer since its Y grounded side is at the 400V) but why did you not considered the zero sequence contribution of the 2 lumped motors (300 & 596kVA respectively)? I’ve read MOON H. YUEN’s paper titled Short circuit ABC, what he did there was considered the total MVA contribution at a certain point but also considering the zero sequence contribution not only of the transformer’s Y grounded side but also the motor’s. Please advise Sir Ver, thank you and more power!

does this means for calculation of fault short circuit mva is If(p.u) * base mva

Please do not confuse Per Unit fault calculation with the MVA method fault calculation.

H sir thank you for this very informative article. May I ask, How do you assign a value for the impedance of lumped motor loads. Many thanks in advance.

Motors are lumped with ratings 37kW and below assigned an impedance value of 25% while larger motors are 17%. See https://filipinoengineer.com/blog/tutorials/mva-method/mva-method-short-circuit-calculation

Thanks Sir! here is additional information. distribution feeder is designed from AAAC with: positive impedace=0.598 + j0.39483ohm/km, negative impedance = 0.598+j0.39483 , zero sequence impedance 0.77218 + j2.03059. it is uniform for total of 110km long distribution feeder. here I am not considering for line down stream to each transformers or there is no line at low voltage side of transformer. I.e each transformer supply load directly. thanks!

With the positive & negative sequence impedance the same and with the zero sequence impedance much higher, I do not see any reason why LLG fault is much higher than 3L fault.

Thanks Ing.Ver. while I simulate short circuit analysis using ETAP for my power distribution system, I get result of fault current report of each fault type. In this result double line to ground (L-L-G) fault current is higher than three phase fault current. what is the reason for this? thanks!

It is due to the transmission line design. It may be that the negative sequence impedance of your transmission line is lower than the positive sequence impedance.

Sir! l have no word to say thank you! here is my last question! don’t hesitate me! does100000MVA simply inserted to MVASc given on ETAP? how to calculate MVASc value for single phase of this power grid point? l can’t get X/R from utility. so, what will be X/R for this power grid point! thanks!

As per ANSI Std C37.010, typical X/R ratio is the range of 5-12.

Thank you very much! your great Engineer! Sir don’t hesitate me! l have my last questions! what does 100000MVA mean? can l simply insert 100000 value to MVASc place on ETAP for three phase(my feeder is 3phase)? If so, how I calculate for single phase? I can’t get X/R data for power grid. from I can get this data also? Thanks!

Thank you sir for soon response!

Have 15KV radial power distribution feeder for certain rural area with 50 distribution transformers(15KV/400V) of different rating. most of the loads are small rating and residential loads. I assume there is no any fault currents from generator or motors.so, the only fault source will be feeder supply point or grid point according to ETAP circuit model. From our utility I can’t get MVASc value that is grid MVASc is unknown. but, to run this short circuit analysis ETAP require MVASc value. so, can you help me sir! Thank you in advance!

If you don’t know the MVAsc, do the followling

Sir do you have any version of etap software that I could purchase or pirate that I can use?

Bakit po 100000MVA di ba dapat 100MVA base

MVA Method does not use base values

how to guess MVASc for feeder line analysis. if there is no MVASc data at utility! how to guess MVASc?

If you do not know the MVAsc of the utility, then assume that it is an infinite bus.

Hi Sir! l want to short ckt analysis15kv feeder ETAP

Can you add more details of the 15kV feeder?

Hi sir,

This is my scenario, I use MVA method to calculate the 3-phase short circuit of my electrical system medium voltage 13.2KV to low voltage 400/230V.

Utility : MVAsc= 500MVA

Transformer : 225KVA , %z = 1.12

Fire Pump Motor : 50KVA , %z = 25%

Genset : 70KVA, %xd = 9%

Unitized Panel ‘MDB’ , 220KVA %z = ?

the unitized panel composed of 6 branches,

panel 1 : Combination of loads (motor and lighting)

panel 2 : ACU1

panel 3 : ACU2

panel 4 : ACU3

panel 5 : L.O and C.O

panel 6 : spare

My problem is i don’t know how to calculate or identify the percent impedance of every panel, in this scenario it is possible to use the mva method?

How can i identify the impedance of every panel?

Regards,

Ian

For the unitized panel, for purposes of short circuit calculation, do not consider the L.O.s and C.O.s. For the ACUs and motors at Panel 1, just lump the total KW of the motors then use the most suitable %Z value. I believe it will be 25%. From there, you will be able to calculate the fault current at the unitized panel mdb.

For load flow, if you wish to do it, then lighting and c.o.s need to be considered. I hope this helps.

hi mr. ver,

how can i identify the impedance of the lumped load?

Caution should be exercised when dealing with lumped loads. Lumped motor loads provide upstream fault contribution, static lump loads such as lighting or resistive loads do have this characteristics. Mixing these two type of loads will make analysis very difficult. The impedance of lump motors loads has been provided in the article.

Hi Sir Ver,

Thank you Sir for the verification, i see now that in a case where we should find the fault at the 33kV bus obviously the MVAsc coming from the utility itself will be towards the bus and also the downstream contribution will also be towards the 33kV bus hence treating it in parallel. Thank you so much Sir

Hi Sir Ver,

Thank you Sir Ver for the prompt response, and pardon me for my late reply. Sorry if i may sound like a newbie but i’ve noticed some calculations where if the MVAsc is coming from the utility (generator) the KVA is treated as series but if the MVAsc is coming from a bus (just like here in our example) the treatment is in parallel. I’m very confuse with this. Also for the direction of fault is there a method on determining its direction. What i’ve experienced is that such fault (directional) is associated with PTs since the reference angle will be coming from the PT hence the fault will have a direction. I’m sorry i have to many questions since i am very interested in this kind of study. Thank you so much

Hi Rudolf,

I may have confused you a little bit with my previous explanation. MVAs are in parallel if they come from opposite direction toward the fault (see Point A). MVAs are in series if they are in the same direction towards the fault (see Point B).

Hi Sir Ver,

I would like to verify the computation at the 33kV bus the 250MVAsc for the utility is in series with the 50.3 MVAsc downstream contribution, Since as per the summing of KVAs in series it should be (250 x 50.3)/(250 +50.3) right? please advise , thank you so much

Hi Rudolf,

At the 33kV bus, despite the connection looking like in series, the 250MVA (upstream) and 50.3MVA (downstream) are actually connected in parallel. The reason for this is the flow of fault current are both toward the 33kV bus. For MVAs to be in series, the flow of current should be in the same direction. Hope this helps.

Ver

Sir,

we have 3 rectifier transformers of 2 x 9.80 and 1 x 12.20 MVA and these are used for electrolysis process. The impedance of theses transformers are 8.20% and 7.9 % respectively. Which actually injecting huge fault current into system as it is directly connected to 11 kV bus.

As there is no motor load on these transformers, shall i consider there fault MVA as it is???

That is correct. You need to consider the cable impedances as well.

Thank you Sir, Godbless…

Sir, How can i get the MVAsc of Induction motor if the given are 2000hp, 3600rpm, X”d=16.7% and X/R = 30…? what are the formulas? Thanks

For motors, it is assumed that the HP rating = KVA rating. Thus for a 2000HP motor, it will be rated 2000KVA or 2MVA. The MVAsc therefore will be MVA/Z = (2 / 0.167) = 11.98 MVA.

Thank you very much sir ver for the information.

God Bless you.

Sir Ver,

Where the 17% Z and 5.9 came from? is there specific formula that i need to derived?

Thanks again sir.

These are rule of thumb estimation for equivalent motor impedance. If you need to find out the actual values, refer to manufacturer’s data sheet. You could also refer to IEC 60909 for such data.

Sir ver,

what is the exact value multiplier for motor contribution to fault? is it 4 or 4.8 times the rated current (for induction motors).

Thanks

Glenn Antonio

Aspirant to be an PEE

Hi Glenn,

For small induction (37kW & below) motors the multiplier is 4x or 25% Z. For larger sizes is about 5.9x or 17% Z.

All the best to your PEE.

Hi Engr Ver,

I have local 3x1000kVA Gensets on synchro pannel, The manufacturer has given X”d, X0, X2,

Does X0 contributes to Zero sequence at any fault points or levels? what about X2 or negative sequence? since X”d is already considered in the calculated MVAsc equivelent which is the same as equiv MVAsc=MVA1=MVA2 value?

Additional, what about the genset SC contribution on ATS panel? do they (pos,neg,zero) really contribute pass through the ATS?

Thanks

Engr. Ver if the transformer zero sequence is equal to positive and negative sequence, how about the Cable zero sequence? I have a lot of research but i cannot find the exact formula.

Cable zero sequence impedance is usually provided by the cable manufacturer.

OHHH Engr. Ver thanks… Didn’t see it that further.

Hi ver,

I think with the new diagram, there is much clarity to it that i guess all will appreciate. Thanks a lot.

I have another question… would you mind share your expertise on this. Suppose the same diagram and values for the same ELEMENT as above.can you provide the SINGLE LINE-TO-GROUND FAULT using MVA method. Thanks again… MABUHAY KA.

Gil, Page 5 presents phase to earth faults.

Hi ver,

How did you arrive at these values 133.39, 139.38 and 31.17 MVA? THANKS

76.87+27.11+29.41=133.39MVA

76.87+35.4+27.11=139.38MVA

Hi Sir Ver,

I would like to know if where did you get the X/R ratio of transformer in your tutorial with 5.662 and how did you solve it?

regards,

jo

Thanks ver.

Hi Sir Ver,

How can i get the X/R ratio (MVA base-given) of utility fault duty? Should i base it on the IEC-60076-5 table 2…. we’ve been asked to show the X/R ratio during our technical interview and we are using here is 500MVA..

Jo, the utility fault duty should be provided by the utility.

Thanks for quick reply. So far my load consists of 3kW pump and 108kW motor and few single phase fcu and exhaust fan. Is my calculation limited to motor and chiller only since fcu and exhaust fan have no significant contributions? Thanks

If you have a single FCU, then the contribution is negligible. But if you have lots of them, then you need to consider them as lump motor loads. You can combine all the ratings of all motors rated 37kw (50HP) or less.

Hi Ver,

Is there short circuit contribution from chiller with 108kW 415V 3 phase?

Thank you and more power.

@exzvi, yes, a 108kW motor for a chiller will contribute to the SC. However, new chillers now are controlled by variable speed drives (VSD), depending on the VSD make, motor contribution may not be considered.

Hi Ver,

Regarding short circuit calculation. Is the FCU or fan coil unit single phase, 240V 200 Watts connected at DB panel can be also be calculated as contributor to short circuit? I have 6 nos. of DB with 6 nos. of FCU in each DB. How I can calculate the contribution if any? or is it negligible? please need your help. Thnaks.

Hi Victor,

The FCUs are too small to contribute fault current into the DB.

Regards,

Ver

Sir,

are the motors driven by frequency drive will include in the calculation since there will be no current that will pass through frequency drive if fault in the system occur before the VFD.

Erwin, it all depends on the VFD. For non-regenerative types, motor contribution is not included but for regenerative types, motor contribution needs to be included into the calculation.

There is a typo for the formula for the reactor and cable KVASC under you Short Circuit Tutorial.

It reads: KVASC = (1000 x KVA2) / Z (ohms)

It should read: KVASC = (1000 x KV2) / Z (ohms)

where the number 2 of course is a superscript (exponent).

There is a typo for the formula for the KVASC for Reactor and Cable under your Short Circuit Tutorial.

the Formula reads: KVASC = (1000 x KVA2) / Z (ohms)

It should read: KVASC = (1000 x KV2) / Z (ohms)

The number 2 of course is a superscript (exponent)

Dear Mr. Ver Pangonilo,

Thanks a lot to your article.

Could you give me title of the books that explain short circuit calculation like your article above.

Thank You

and

Best Regards,

Imam

Hi Ver, How can I design the rating of 33 kV Autorecloser used in 33 kV line?

Gostei muito do seu trabalho, parabéns e um grande abraço.

Enjoyed your work, congratulations and a big hug.

Hi Ver,

If you want to consider the effect of the distribution line, for example 11kV between the 10MVA transformer and 11kV bus, like 20 miles, Z= 0.35ohms/1000ft, how do you insert this into the equation?

Thanks.

Jojie

Hi Jojie,

First calculate the total impedance for the distribution line. Then using the formula for a reactor, calculate the MVA for the line. The result will be connected in series with the MVA of the 10MVA transformer.

Hope this helps

Regards,

Ver

Dear Engr. Ver,

How can i get the MVA value for a cable if the z% is not given from the formula: MVAcable = V²/z

Allan Camello

Saudi Arabia

Hi Allan,

The impedance (Z) of a cable are provided in cable catalogues or by manufacturers. The resistance (R) & reactance (X) are values are usually in ohms/km values. If you know the cable length (L), you will be able to calculate the impedance (Z).

Z = (R + jX) * LHope this helps.

Thanks for the Info Ver. More power.

Hi Mr Ver,

I’m happy to see great kabayan engineers like you here in Australia, anyway,

I just want to ask you about the downstream MVAsc calculation base in the above power system single line (fig. 1). Was this down stream MVAsc calculation was only to identify the bus short circuit current in 11kV bus and 6.6kV bus and for checking purposes only? What are the other reasons why we need to do the down stream SC calculation?

Regarding the Upstream MVAsc calculation, like for example, we have computed the available MVAsc @11kV = 1/(1/250+1/111.11) = 76.87 MVA or we have a 4.03 kA.as our Breaking capacity in kA rms, and as per the IEC practice @ 50Hz, Closing capacity Isc x 2.5(in kA peak) which will give us 10.08kA peak at the 11kV bus of 10 MVA transformer.

So it means that our Breaker, fuse etc located in the secondary side (11kV) of 10MVA transformer will not be less than 10.08 kaic AF? Am I right or?

Or we will used the equipment MVAsc 10MVA/.09=111.11MVA or 5.83kA? in where we base our KAIC?

Sorry for so many silly questions Ver. Hope that you have a spare time to read my query.

Thanks,

Frances Tesorero, REE, MIEAust.

Hi Frances,

I hope I understood your query right.

The fault on a bus will always be the total of downsteam (from supply) plus upstream (from motor contribution). This fault will be used from selecting equipment ratings.

Refer to the recommendations of IEC for initial selection and manufacturers information for the final selection.

Ver hi

One thing about the MVA method looks intuitively wrong.

It looks like The higher the bus voltage, the lower the short circuit ampacity. How is that possible.

from my engineering experience, it was always the opposite.

The higher the bus voltage – the higher the SC ampacity.

Kind regards,

Eric Stark

RNItechnology

Sr. Eng. Trainer & Consultant

Protection & Control Engineering

1-416-546-546-1

[email protected]

http://www.RNItechnology.com

Hi Eric,

The calculation assumes that upstream contribution remains the same, thus at higher voltages, the fault current is smaller compared to the lower voltages fault currents.

Regards,

Ver

Dear Sir,

How we calculate voltage dip/sag during shortcircuit using MVA Method?

Any advise please?

Thanks,

That may be a good topic to discuss. Watch this space.

ABOUT YOUR COMMENTS ON VER LEDESMA LAST 26-NOV.-2012 REGARDING IF UPS AND P.F.CORRECTION CAPACITORS CONTRIBUTE TO THE FAULT AT THE BUS, IS I THINK I DISAGREE WITH YOUR REPLY THAT IT DO NOT. HOWEVER, THE STORED ENERGY IN THE CAPACITOR BANK IS NOT DIMINISHED AND ACTUALLY IS CONSERVED AND SO WHEN A BOLTED FAULT OCCURS ON THE LV BUS WHERE IT IS CONNECTED, WHERE WILL THE ELECTRICAL ENERGY OF THE CAPACITOR BANK GOES BUT TO DISCHARGE ON THE SHORTED BUS? FURTHER, ABOUT THE AC-UPS, IT MAY OR MAY NOT CONTRIBUTE TO THE FAULT ON THE BUS PROVIDED THE INTERNAL SWITCHING CIRCUIT IS NOT REGENERATIVE AND THAT DEPENDING ON WHERE THE FAULT IS LOCATED FOR EXAMPLE IF IT IS AT THE INPUT SIDE IF THERE’S A REVERSE BLOCKING DIODE THEN THE BATTERY WILL NOT BE ABLE TO CONTRIBUTE AS THE DIODE BLOCKS THE FLOW OR IF THE INTERNAL CIRCUIT OF THE RECTIFIER CHARGER IS DESIGNED NOT TO BE REGENERATIVE THEN IT WILL NOT CONTRIBUTE BUT IF OTHERWISE THEN IT MAY CONTRIBUTE TO THE FAULT ON THE INPUT SIDE AS THERE IS CAPACITOR AS WELL INSIDE THE RECTIFIER CHARGER ASSEMBLY.

In my example, only the utility is the source of fault current. There are no fault contribution from downstream (i.e. motors). So a fault on the 22kV will only affect the 66kV if there is a fault contribution from 22kV motors.

Hi Mr Ver,

Thank you very much for your quick response. I have download read your article about MVA Method for three winding transformer.

I have question regarding to page 7/9 of your article, on the paper mention that three winding capacity 150 MVA (Z=9%) on Primary side, 100 MVA (Z=8%) on Secondary side and 50 (Z=7%) MVA on tertiary side. When you calculate the MVA equivalent for each winding, you always use 150 MVA / Z of each winding. Is that mean you try to calculate the equivalent from upstream side to down stream?

If there is fault in 22 kV side, as I understand that the other two side (132 kV and 66 kV) will take contribution, am I correct? The same case if fault in 66 kV side, the other two side (132 kV and 22 kV) will take the contribution. How you calculate for this condition? Please give me advice.

Thank you so much sir.

Hi Sir,

I have a system using three winding transformer, how to determine the MVAsc for all side of the transformer? Is that same as delta/way impedance circuit?

(i.e primary side 11 kV, Secondary side 150 kV and tertiary side 6.6 kV. Transformer capacity 40/60, 6.5 MVA, ONAN/ONAF, there is generator in 11 kV side with X”=13.5 % and 6.6 kV side connect to switchgear)

Thank you so much sir.

@hurricane193

Try this article (https://filipinoengineer.com/blog/index.php/tutorials/mva-method/mva-method-for-3-winding-transformer).

@LUC

I am not familiar with railway. Probably some other readers will be able to help you.

Dear Ver,

i am trying to calc. the short circuit current protection (SCCP) for railway transmission line. getting feed from 132kV from municipal, and feeding signaling line of 6.6kV. my problem is, i have 2 old and 2 new substations, i want to calculate the SCCP for the 2 new subs, how do i go about using the distance between them

thanks

sir Ver,

from your example and formula for motors, is it an option not to multiply the subtransient reactance of motors with the ansi multiplier for first cycle? say, 1.2 X”d or 1.67X”d. and if motor code letters are available, would I use the value for that given code to get my motor kva? or should I only use them during load flow calculation?

thanks in advance.

@jules picarzo

NEMA Code letter should be used for load flow calculation.

thanks

Dear Engineer Ver,

In 400 volts bus bar, if i have a 3 phase UPS and Power Factor Corrector will they contribute to the fault? if so what is the formula? like in cable the formula is kv2/Z.

UPS and Power Factor Corrector do not contribute to system fault.

Hi Ver,

Thank you very much for your reply…

Based on your calculation, I calculate the fault current at 11kV & 33kV side for single phase…

I got 7.545kA at 11kV side and 7.88kA at 33kV Side… (by eliminating the zero sequence impedance at 33kV side)….

Advice me, if i am wrong…

Thank you in advance…

You need to consider the following in addition to positive & negative sequence faults:

1. Utility zero sequence fault level.

This is not provided in my example.2. There is no upstream contribution for the zero sequence fault as the 33 kV transformer is delta / wye.

3. Calculate the fault similar to the examples above.

How will i calculate the single phase fault at 33 kV Level?

DATE: 27-SEPT.-12:

YOUR ARTICLE ABOUT MVA METHOD IS REALLY GOOD AND INTERESTING APRTICULARLY FOR THE 3-WINDING POWER TRANSFORMER. SO, KUDOS FOR A FILIPINO ENGINEER AND WITH SALUTATION, CUM PEE, SUCH AS YOU AND KEEP UP THE GOOD WORKS, TO YOUR COMPANY, AS A PROFESSIONAL ELECTRICAL ENGINEER OF PHILIPPINES AND AUSTRALIA AND MOST IMPORTANTLY YOUR SERVICE TO THE WORLD. NOW THEY CAN LOOK UP TO A FILIPINO ENGINEER OF PHILIPPINE ORIGIN IN MATTERS OF ELECTRICAL ENGINEERING THEORY AND IT’S APPLICATIONS TO THE POWER INDUSTRY, RESEARCH AND STUDIES AND MANUFACTURING. HOWEVER, KEEP YOUR FEET ON THE GROUND AND LET GRAVITY MOVE OR ASSIST YOU.

Sir,

How is an autotransformer modelled in ETAP?

Thanks.

PB

Sir

For Incoming 11KV distridution system to a plant from a DGPower Plant if the system design is for40KA 1sec at the distribution station Bus, can the 11KV downsteam units such as switch gears, breakers etc be cosidered for a 31.5 KA 1sec fault level for cost saving.

BK

@BK,

the answer to your question is not simple. it is dependent on so many parameters. You need to verify your system fault levels. Also you can use this article as a reference.

@Hill Portez

If I understand right, your question may be similar to a previous comment by

Pio A. Lozano.These are derived from industry standard values. Using these values in your MVA SC calculations wil provide a comparable result using computer based softwares.

Hi, Sir:

May I ask how did you get the upstream values for Motors? and why is it that the given MVAsc of the Motors are placed at the downstream side? Just like to know if my understanding is right.

Thanks

Hill

@Ian, Yes you are correct. Thanks for pointing the error.

Hi Ver,

Correct me if I’m wrong, but should the 33kV 3ph bus fault be 5.25kA?

i.e. fault = (250MVA + 50.3MVA) / (?3 x 33kV) = 5.25kA?

Thanks!

Ian

Hi, where can I find a book/standard where the MVA method is referenced? Thanks you.

Try using this search: http://www.google.com/search?q=MVA+Method&btnG=Search+Books&tbm=bks

@shozy

That is the sum of all currents towards the 11 kV bus of the generator (76.87+27.11+29.41)

How did u calculate Downstream for generator. I can’t find out any supportive calculation for 133.32MVA as u mentioned in generator downstream calculated value.

thank you Mr Ver..

Hello mr Ver

i want to asked about MVAsc Utility, can you sharing how to calculated ?

if i had different voltage little bit higher in 20kV, 66 kV or 115 kV, how much i must put MVAsc ?

@riza,

please refer to this article.

Thanks

Hi, Mr Ver,

I have a doubt, how did you get MVAsc1 and MVAsc2 at 400V and 6.6kV?

Thanks for excellent work.

@Larroya, the MVAsc of each branch is inversely proportional to the MVAsc of each branch in the opposite direction.

paano po i compute ang MVAsc ng GEnerator at Motor 1

@norato:

Check these equations.

Treatment of Resistor will be like a transmission line?

R will be dominant in the total Z.

So %/ MVA will be Z(R).100%/ Un^2.

In case of a single phase fault i shall add Z1+Z2 +Zo +3 * %/MVA of the NGR.

Is this treatment right?

can this method account for NGR grounded system say at 6.6kv level?

@rrphatak

Yes you can. The NGR is a resistor so it’s MVA will be .

But remember that it does not contribute to the fault current but instead limits it.

Can u pls put some details documents for eath and unearth cable.

Sir,

If my system is 4.16kv, is there any formula for BIL so I can specify the right cable BIL rating?

BIL is normally 3-4 times the KV rating of an equipment. Refer to standard to ensure you are using the right rating.

how do you know that MVAsc utility is 250 MVA ?

@dian,

the 250MVA in this example is just assumed. Ask your utility for the actual fault level at your point of connection

thanks a lot and more power & blessings

Hi! Eng.r Ver, just a follow up question regarding this example in (fig 1) . suppose I have one standby Diesel generator connected to 400 volts bus that will only run and supply power to 300 kva, 400 volt motor (M5) only ,in case of breakdown in utility company supply. Is this generator will contribute to the SC?

Thanks & regards,

Dalton

Hi Dalton,

If you use the diesel generator as a standby, it means that it has a transfer switch. This means that SC will only be coming from the diesel generator. So the answer is yes but fault current is limited to the 300kva load.

Thank you very much Mr.ver…

hello sir if you can please guide about some softwares that can help in finding the short circuit faults or any type of faults that occur in power system. please help me, or if anyone knows about it please contact me, my id is [email protected]

Thanks

ETAP (etap.com), Power Analytics (poweranalytics.com) or SKM Power Tools (skm.com) could do all the analysis your talking about but they are not cheap.

An open source version is InterPSS (interpss.org).

Mr. Ver, what do think , could we use a estimation to define the utility’s MVAsc that the values 1.5 up to 3 time of the bigger-transformer’s voltage

What you think, we can use a estimate to determine the current value of this short circuit (utility) by setting 1.5 to 3 times the size of a short circuit in the transformer closed to the utility (with the largest voltage transformer)

In this case: Transformer 1: 168.79 MVAsc and then 168.79 x 1.5 = 253.185 MVAsc .

whether it is safe?

Thank you Mr.Ver

Rully, there is no relationship between the transformer MVAsc and the Utility-SC so we could not really make an assumption of either 1.5 or 3.

If you can’t get an actual utility-SC, assuming a 2000 MVA is very conservative for voltages 33 KV and above. For lower voltages, it will be less than that.

OK. Thank you Mr.Ver…

Hello, Sir Ver,

how to calculate MVAsc for utility in this example you stated 250 MVA, from which this value Mr.Ver?

Thank you for helping

The MVAsc for utility are provided by the utility themselves. You can asked them if you want the exact value. It varies depending on the location and voltage of the incoming supply.

Dear Sir,,

Thanks for providing the “MVA method short circuit calculation”, My concepts become much more clear.

But i couldn’t understand the positive, negative & zero sequence impedance.The calculation u provided @400V & @6.6kv bus, it hardly to be understand.

Can you please elaborate more in depth, so that calculation become more clear likewise MVA method……

Help me out

hi sir,

i’m coby, also form philippines. I just want to ask how do we compute for short circuit MVA of lumped loads? i mean, for example in a manufacturing plant where the are lots of manufacturing equipment installed, how do get the short ckt MVA given only the load KVA?

salamat sa inyong website!

@coby,

Use the principles stated in this article to deal with lump load.

Maraming salamat sa pagtangkilik!

thanks for the detailed reply. I already calculate the 3ph fault in 11kv & 400v bus. Now i got struck with SLG fault.

The delta primary of the TR1 / 2 will block the upstream contribution of zero seq.

But the delta primary of the 11KV trafo will also block the zero seq contribution of the the trafo & motor while calculating fault at 11kv bus. Is that true.

Also when calculating SLG fault at TR3 400v bus, only TR3 & the connected motor will contribute. Others trafo TR5,TR7,TR9,TR11,TR13 & their respective loads will not because of delta primary. Is that true.

@Raman, your analysis is correct.

Hello Sir,

Thanks for the earlier reply.

I started calculating the fault currents. I had a doubt. Considering the above SLD, the transformers TR3 ~ TR14 are of 11KV / 400V. While calculating the fault at the 400V bus (keeping Bus coupler open) of say TR3, will the loads connected to the other transformers TR5,TR7,TR9,TR11,TR13 & the transformers itself will contribute to the fault or not?.

Hi Raman,

The motor loads on TR5,TR7,TR9,TR11,TR13 will contribute to the fault current on TR3 @ the 400V bus in addition to the fault contribution of the motors connected to that bus.

The same is true if you calculate the fault current on the other transformers.

BTW, i have notice that your generator bus is not split. On generator power, even if the bus coupler is open, it is still connected at the generator bus.

@raman:

Please find attached mark up on your SLD. The heater group does not contribute any

upstreamfault current.thank you for this valuable information.

Sir,

Thanks for the response & kindness.

Please guide how to post the attachment as i find no option.

Single Line Diagram

Thanks for the reply. I still want to know that the case i describe above consists of a DYn11 2500Kva transformer which will feed the 35 sets heater systems.

Each system consists of Power Thyristor feeding to a 150KVA Yd1 transformer which in turn feed to 120Kwheater. The total connected heater load in similar fashion is 4000kw.

The impedance of the DYn11 & Yd1 transformer also can be neglected as the load is resistive. Is my understanding correct?

If we have some motor loads along with the heaters then the motors alone will contribute to the SC & not the heaters.

Also please refer me some more books.

Hi Raman,

Send me a single line of your system for me to be able to understand better. Then I could provide you the right answer to your query.

Thanks for the Great Job. Can you suggest some books which deal with MVA method in detail. I already refer to

1. Short circuit ABC-Learn it in one Hour Use it Anywhere, Memorize no Formulaâ€ from Moon H.Yuen Senior Member IEEE.

2. Electrical Calculation Handbook by John M. Paschal, Jr

But still I need some more worked examples based on practical industrial networks.

Also you said heater loads don’t contribute to SC. In our case we have 120Kw of heaters connected through Yd1 Transformer of 150KVA. Total Installed capacity of the heater load is 4000Kw. I want to know whether this contribute to SC or not.

Hi Raman,

Heaters do not contribute to short circuit as these devices are resistive.

Thanks for the Great Job. Can you suggest some books which deal with MVA method in detail. I already refer to

1. Short circuit ABC-Learn it in one Hour Use it Anywhere, Memorize no Formulaâ€ from Moon H.Yuen Senior Member IEEE.

2. Electrical Calculation Handbook by John M. Paschal, Jr

But still I need some more worked examples based on practical industrial networks.

sir! can you post short circuit calculation for low voltage system for commercial building design, how to rate circuit breakers using ansi standards

Hi Edison,

I am a little bit tied up with work. I will post your request as time permits.

Thanks

how can you apply the mva method to a three winding transformer

Hi Thomas,

I have not used MVA method on 3-winding transformers yet. I will get back to you soon as I can.

Thanks

Update:Check this new tutorial for 3-winding transformer MVA Method fault calculation – https://filipinoengineer.com/blog/index.php/tutorials/mva-method/mva-method-for-3-winding-transformer.

Sir, excellent example for MVA calculation. Can you suggest some books for High voltage switch gear and also model question papers for the same subject

High voltage switchgear: analysis and design, J&P Switchgear or ABB Switchgear Manual will be a good start.

I am also a practicing electrical engineer based here in southern part of Mindanao, Philippines.

I have question regarding the lumped motor x” and generator Z% , where did you get this figure (=17% and 25% ) and for the generator which is X” = 0.113. Can we find it from the book? Thanks.

These are derived from industry standard values. Using these values in your MVA SC calculations wil provide a comparable result using computer based softwares.

Thanks a lot. This will be a big help to me. Maligayang Pasko sa yo @ sa yung pamilya.

Dear Kabayan,

I am a practicing electrical engr here in KSA but not as good as you. I would like to ask you regarding this MVA Method. Suppose you dont know the Fault MVA of Utilty Company. And you want to design a new switchgear say; 1600KVA, 13.8/0.4 KV, 3 phase and the total connected load let say 1200 KW the bigest of which is 400 KW. How to determine the busbars SHORT Circuit Current in Medium & Low Voltage. All motors are low voltage.

Hello Dalto,

Maraming salamat sa pagtangkilik mo sa aking website.

If you don’t know the fault MVA of the utility company, the rule of thumb I have been using for different voltages are:

However, if you are using a transformer like your example, 13.8/0.4KV – and you don’t know the utility fault current, you can assume an infinite bus (pu=1) on the transformer primary then use the transformer impedance to calculate the fault current on the secondary. You will be getting a little bit higher fault current but if you will be using this to select your equipment rating, then you will be fine.

To determine the motor contribution to short circuit, I have given examples in my article, just follow the steps and you will get the fault current at any voltage in your power system.

I hope this helps.

this is an good example

thanks a lot for your information

The switchboard (600VAC) is powered by diesel generator and supplied power to 1000HP, 750VDC motor through 3 phase rectifier (SCR). How to compute DC motor’s contribution if fault occurs in switchboard side.

Theoretically, there will be no motor contribution if fault occurs at the switchboard. The SCRs will block any back flow of current.

Dear Sir

This is a good example. Can you suggest me some text books which can cover entire system fault calculation, relay co-ordination part?

Thanks

Pramod Chavan

Here are some books I could recommend:

1. Protective Relaying Theory and Application – Walter Elmore

2. Practical Power Systems Protection – Les Hewitson

3. Protection of Electrical Networks – Christophe PrÃƒÂ©vÃƒÂ©

The total fault MVA (168MVA) in the 11kv branch are conbtributed by all sources, the normal power, the generator and the motor.

To determine the Faults Current at any bus on the power system, add the MVA values above and below the arrows. The sum should be the same on any branch.

Example:

11 KV Bus:

From Transformer 1: MVAsc = 76.87 + 91.92 = 168.79 MVA

From Generator : MVAsc = 35.4 + 133.39 = 168.79 MVA

From Transformer 2: MVAsc = 141.68 + 27.11 = 168.79MVA

From Motor 1: MVAsc = 139.38 + 29.41 = 168.79 MVA

Hi, I just need a clarification. From you answer, are you telling that Generator and Motor contributed up to 168MVA short circuit? Let’s say we us your example, and I wanted to know how much Motor 1 contribute to the fault on the 11kV bus. You’re saying it’s 168MVA? I know how you got the answer, but it’s hard to believe that a small motor can contribute that much fault current. Help me understand here. Thanks!

Your example is an interessant one for the beginning of the power system analysis.

The two books where i find are:

Electric power system Protection and coordination from Michael A.Anthony 1995 Mac Graw Hill

The second one is a german Book

Elektrische Krafwerke und Netze from Happoldt Oeding 1978 or Oeding Oswald 2004 Springer Verlag

I search also in the Brown Book from IEEE, but don’t find it.

How can we bridge the MVA Method with the %or per unit method?

See on a research motor the document “Short circuit ABC-Learn it in one Hour Use it Anywhere, Memorize no Formula” from Moon H.Yuen Senior Member IEEE.

You will find there the comparaison between MVA method and ohmic and/or per unit method

I don’t know if any softwares use it.

Anyway it is a good tool during commissioning.

How can we connect the transient to the MVA method is another question.

Joseph

The relationship between % or per unit method is just OHM’s Law as I have explained in this topic.

What will be the Single Phase Fault Calculation at the 11 KV Bus ? Can you please explain.

In the MVA method of short circuit calculation

The generator MVAsc = 35.4 + 133.39 = 168.79 MVA

From the calculation it is not clear that where from the value 133.39 is derived. Is it possible to explain that where from the value derived.

The motor 1 MVAsc = 139.38 + 29.41 = 168.79 MVA

From the calculation it is not clear that where from the value 139.38 is derived. Is it possible to explain that where from the value derived.

Both values are calculated:

133.39 = 76.87 + 27.11 + 29.41

139.38 = 76.87 + 27.11 + 35.4

Very good article!……I presume that the single phase fault mentioned in page 5 is a line to ground fault. How about a line to line fault? a line to line to ground fault ? Mabuhay ka kabayan!…

this is very good example of short circuit calc. by MVA method.i like this.thanks a lot!