Electric System Fault Calculations - MVA Method 3

[BigJake]

Circa 1975 there was an IEEE paper published outlining a novel way to perform short circuit calculations, called the MVA Method Of Fault Calculation(Exact title unknown).

Even though it was apparently not adopted by IEEE or Electrical Engineering Schools, it is an extremely simple system to use and to remember, unlike the p.u. system. It gives the same answers as the p.u. system. I still remember how to use most of its' features after all these years. (Now you can guess how old I am).

Anyone know where I can get a copy of this paper or a site that describes its' use?

Thanks!

 

[cuky2000]

The MVA method determine the equivalent SC load as follow:

a- Two load in parallel: MVA[sub]eq[/sub]= MVA[sub]1[/sub] + MVA[sub]2[/sub]
b- Two loads in series: MVA[sub]eq[/sub]= MVA[sub]1[/sub]xMVA[sub]2[/sub]/(MVA[sub]1[/sub] + MVA[sub]2[/sub])

An easy way to remember this method is using the same rule to determine the equivalent capacitance on serie or parallel circuits.

I believe that still I have and old paper that I got from the Ebasco Professional Development Program at the time I work in the NY office several years back.

Let me check in my baseman to see if I we are lucky to find a copy of this paper.

[sub]PS: I am not that old either even though Ebasco change names few times (Raytheon Eng, know Washington Group).[/sub]

 

[coingy]

Hi jake,

Years ago a Dutch colleague of mine when I was working in Holland gave a pamphlet for short circuit calculations which I refer to on and off as required. The address on the pamphlet where you may be able to obatin a copy is:-

Point-To-Point Method
For
Short Circuit Calculations

Graet Lakes Electrical Ltd.
14 Connel Court
Toronto
Ontario M8Z 1E7
Canada (416) 255-8161 Telex 06-967763

or

BUSSMANN MFG. Division
McGraw Edison Co, St.Louis, Mo 63107

Hope this helps.

Coingy

 

[DougMSOE]

I have the complete paper and it is very good!

 

[busbar]

I believe the author was Moon H Yuen {now deceased} of the Northern California bay area, but I cannot find any reference to the paper in IEEE's online author index.

 

[BigJake]

Cuky2000 you are correct on the calculation methods. Also I remember MVA for lines is kV^2/R or kV^2/Z. I have forgotten how the method was used for positive, negative, and zero sequence networks.

Coingy, thanks for your reply but I think the point-to-point method you refer to is not what I'm talking about. I may be wrong, though!

DougMSOE thanks for your reply. I have the greatest respect for your opinion of the article.

And Busbar thanks for your reply. I have no idea who the author was. I said the paper was published circa 1975, but it may have been a few years before that.

 

[james001]

Here is how I would calculate the MVA SC. Assume you are given 10000A of short current on the primary side of the 10MVA, 7.5%, delta-wye, 115kv-12.47kV transformer and you would like to calculate the short current (in terms of MVA) on the secondary side of the transformer

You need to convert everything to 100MVA base, so 10000A*1.732*115kV/1000 = 1992MVA, then take 10000/1992MVA to figure out the Z% = 5%, then you take the trans impednace of 7.5% * 100MVA/10MVA rating = 75%, then the total impedance is 5%+75% = 80% then take 10000/80% = 125MVA of fault on 12.47kV (assuming 3ph fault), it is aroung 5800A.

If you need additional info, let me know.

jt

 

[coingy]

Hi BigJake,

Further to my previous answer perhaps this may be of more use.

It is nescessary to convert all reactances to a common kVA or MVA which ever is your preference. The value of this base is quite unimportant and may be,
1. That of the largest plant
2. That of the total plant capacity
3. any arbitary value.

The conversion is as per the expression

% Reactance at base kVA
= Base kVA/Plant kVA x % Reactance at plant kVA

Thus a 5,000 kVA alternator with 8% reactance will have a 40% reactance at 25,000 kVA base.

Hope this helps, Coingy

 

[jetorange]

Hi,

Im new to this forum. I joined because of this thread.

I use the Point by Point Method as documented in the Bussmann SPD literature, but I am curious for academic reason to see this since unheard of method (I only been in the field for 7 years so im still pretty new )

Thanks

Tony

 

[Shortstub]

MVA technique was described by A.H. Knable, American Electric Corp, in his book:

"Electric Power Systms Engineering"

Published by McGraw-Hill, 1967.

He also presented a nomagraph that simplified some of the relevant calculations.

 

[Weimer42]

EC&M published a book explaining this method in detail entitled "Short Circuit Calculations - The Easy Way" by J.R. Seiver, P.E. and John Paschal, P.E.

 

[jbartos]

Suggestion: Reference:
1. Alvin H. Knable "Electrical Power System Engineering Problems and Solutions," McGraw-Hill Book Company, 1967.

Refers to the method as "short circuit kVA method", e.g. in Figure 8-5 on page 230

 

[jbartos]

Suggestion to cuky2000 (Electrical) Feb 12, 2003 marked ///\\The MVA method determine the equivalent SC load as follow:

a- Two load in parallel: MVAeq= MVA1 + MVA2
b- Two loads in series: MVAeq= MVA1xMVA2/(MVA1 + MVA2)
///Please, could you clarify the b- line\\
An easy way to remember this method is using the same rule to determine the equivalent capacitance on serie or parallel circuits.
///This reads and sounds impressive. What about the internal impedances of sources and their impact on MVAsc and MVAeq?\\I believe that still I have and old paper that I got from the Ebasco Professional Development Program at the time I work in the NY office several years back.

Let me check in my baseman to see if I we are lucky to find a copy of this paper.
///The constraints under which b- relationship holds seem to be important.\\\

 

[cuky2000]

Dear Jbartos

The MVA method is a modification of the ohmic method. This method calculates the SC MVA using each component power rating divided by the appropriate impedance. Ex Transf: MVAt = MVArated/Zpu, Gen: MVAg = MVArated/X”d pu, Cable: MVAc=kV^2/Z(Ohm). Notice that one of your concerns with the internal impedance is addressed during the calculated SC MVA.

To illustrate this consider determine the SC at the end of a feeder connected to a transformer with infinite SC power source. The SC at the transformer LV could be calculated as MVA1 = MVArated/Zpu and the cable: MVA2= kV[sup]2[/sup]/ZOhm).

MVA[sub]eq[/sub]= MVA1.MVA2/(MVA1+MVA2)
or 1/MVA[sub]eq[/sub]=1/MVA1 +MVA2
Z[sub]eq[/sub](Ohm)/kV[sup]2[/sup] = Z[sub]transf[/sub](Ohm)/kV[sup]2[/sup]+Z[sub]cable[/sub](Ohm)/kV[sup]2[/sup]
Z[sub]eq[/sub]= Z[sub]transf[/sub]+ Z[sub]cable[/sub]cable

The above relations presume that all MVA are in the same base otherwise the result could be less accurate. The MVA method is reciprocal of the impedance and proportional to the admittances. The end result should provide the same level of SC current as follow:

MVA Method: Isc=MVA[sub]eq[/sub]/1.73xkV (kA) Ohmic Method: Isc= kV[sup]2[/sup]/Z[sub]eq[/sub]. (kA)


Similarly to the PU method, the MVA method could also be used to determine phase-to-gnd fault or phase-to-phase fault calculating the sequences MVA based on sequence impedances. Depend of the level of precision, there are several simplifications that also could be implemented such as X=Z (R=0) for MV & HV, X/R large, motor less than 50 HP can be lumped together, etc.

Hope this could help.

 

[RJF]

As a follow up to Weimer42's comment "EC&M published a book explaining this method in detail entitled "Short Circuit Calculations - The Easy Way" by J.R. Seiver, P.E. and John Paschal, P.E. "

J.R. Seiver works (or worked) for Bechtel in Houston and I have a copy (somewhere) that I bought from him. I was unaware that EC&M was selling it.

 

[GrahamP]

You might also try Engineering Recommendation G74 -Proceedure to meet the requirements on IEC 909 for the calculation of short-circuit currents in three-phase ac power systems.

The document is available from the UK Electricity Association web site (pay for)

 

[jburn]

Hi All

Type in "short Circuit Calculations" into the Google search engine, and you will find a lot of information on this topic.

Below is information from one web site, I found, where you can purchase a book at a reasonable cost.

I am not selling or promoting the book, and I have no relationship with EC&M. I have not seen the book, so I can't even recommend it.

So many folks wanted information on this topic, I just wanted to show everyone a source. Please do the Google search, yourself. You will find a lot of other books, articles, and software etc, to help you with shirt circuit calculations.

EC&M's Short Circuit Calculations - The Easy Way
Covers: Short Circuit Calculations
Item ID: 7456
ISBN: 0-87288-745-6

Price: $26.95

Additional Details: This book is written specifically to simplify short circuit calculations. It contains the most streamlined, simplified method of short circuit calculations ever made available. Although in the past the subject of short circuit calculations has been a difficult one, this book shows just how straightforward it can actually be, and how amazingly little time it can take to make highly-accurate short circuit calculations for an entire electrical power system. Forget what you thought you knew before about abstract short circuit calculations, and instead adopt this new intuitive and understandable “Easy Way” method. It saves time, provides a “feel” for what is happening in the circuit, and is so simple and timesaving that “what if” scenarios can be easily and swiftly done. This book also contains a disk with sample short circuit calculations.
© 1999, 124 pgs, Paperback, By J.R. Seiver and John Paschal P.E.

 

[jbartos]

Suggestion to cuky2000 (Electrical) Mar 9, 2003 marked ///\\The MVA method is a modification of the ohmic method. This method calculates the SC MVA using each component power rating divided by the appropriate impedance. Ex Transf: MVAt = MVArated/Zpu, Gen: MVAg = MVArated/X”d pu, Cable: MVAc=kV^2/Z(Ohm). Notice that one of your concerns with the internal impedance is addressed during the calculated SC MVA.
///I meant MVAeq of two sources in series, not one and power distribution downstream.\\\

To illustrate this consider determine the SC at the end of a feeder connected to a transformer with infinite SC power source. The SC at the transformer LV could be calculated as MVA1 = MVArated/Zpu and the cable: MVA2= kV2/ZOhm).

MVAeq= MVA1.MVA2/(MVA1+MVA2)
or 1/MVAeq=1/MVA1 +MVA2
///Please, clarify 1/MVAeq=?\\\
Zeq(Ohm)/kV2 = Ztransf(Ohm)/kV2+Zcable(Ohm)/kV2
Zeq= Ztransf+ Zcablecable
///Yes, this is impressive mathematics; however, the short circuit current through the Ztransf in series with Zcable has to be taken the smaller one, since the larger value of the current would not materialize in the series circuit. Therefore, the meaning of MVAeq is questionable. One must select the smaller of MVA1 corresponding to transformer or MVA2 corresponding to cable. Then what is the meaning of MVAeq????\\

 

[jbartos]

Cont'd:
Considering:
Battery B1:
V1=8V, R1=4Ohm (internal), Isc1=2A
VAsc1=(V1**2)/R1=8**2/4=16VA
Battery B2:
V2=4V, R2=2Ohm (internal), Isc2=2A
VAsc2=(V2**2)/R2=4**2/2=8VA
Now:
VAtotal=VAsc1+VAsc2=16+8=24VA
VAsc12=[(V1+V2)**2]/(R1+R2)= =[(8+4)**2]/(4+2)=144/6=24VA
However, the discussed MVAeq or VAeq for small batteries will be:
VAeq=(VAsc1 x Vasc2)/(VAsc1 + VAsc2)=16 x 8 /(16+8)=5.3333VA
It can be seen that VAeq is considerably smaller than VAsc12.
Why to use MVAeq so small in the short circuit MVA method?

 

[cuky2000]

Hi Jabarto:

Appear that we need to work harder to clarify the concept of equivalent short circuit MVA (kVA or VA).

The impedance at the point where the fault is postulated could be calculated. If this is possible, the available SC current could be determine as
Isc= V/Zeq or MVAeq=Iscx V = V**2/Zeq (V= constant)

As you could see, there is no mystery on this approach.

Considering your example of two batteries, the equivalent SC available is as follow:

- Batteries in parallel: VA.eq1=VA1+VA2= 24W (You call this VA.total)
(Most practical applications, the parallel batteries are identical)

-Batteries in series: VA.eq2=VA1xVA2/(VA1+VA2)=5.33W

Please notice that your equation, VA.total = VAsc12 only for the particular cases where V1/R1 =V2/R2. Otherwise, VA.total (Different) VA.sc12.