how to calculate system fault MVA ?

Hi,

System Fault MVA = Sqrt(3)*Isc(three-phase)*line-to-line voltage

They have calculated dis fault mva for 66/11 kv s/s.....so we can guess line to line voltage is 66kv. wat is d value of short circuit current? how dey calculated 31500ampere sys. fault current?

Hi,

The electric utility in your area knows the short circuit level on each busbar (substation) on the system. So you can get these values form the electric utility supplying the 66/11kV substation.

Ok...thanks but electric utility have also calculated it by some mean. like dey hav consider d impedence of generator,transformer etc. plz let me know if u know further.

Happy new year,

To make the short circuit calculation you need the system data (the sequence impedances) and a software tool to help in calculating the fault currents for a large system. The system data are available with the electric utility. You may read any Power System Analysis book for more details.

The other simple way is by assuming the fault current in your substation equal to the maximum short circuit current of the circuit breaker. The duration of this fault current is selected as 3 seconds as recommended by IEEE std. for small substations.

Happy new year. thanks..ok our sf6 rating is 31.5k amp..dats y dey r using dis fault current i think.

we have 2x30 MW GT having 33kv voltage level how we calculate faultof the system the generation is 11kv and direct going to generating transformer having 45MVA and voltage level is 11/33kv

You require reactance values of the Generator to find out the short circuit current at the Generator terminal.Also required complete SLD with Device particulars to calculate the Short circuit current ( Short circuit MVA) at various BUses of the system.

Hi

I have 3x1.65MVA @440V 50HZ + 1x 2.2MVA @440 V 50HZ generators operating in parallel. Each generator output is connected to the switchboard using a 4000A rated Bus Riser of length 20M each.

What will be the fault current by each generator for a fault on the switchboard

Krishnan S

Sydney

Hi Sandeep,

You may pl. try with the following formula :

Fault MVA = Base MVA / Per unit impedance

For single source, Base MVA = Transformer / Generator rating

For multi souce feeding the fault, pl. calculate individual Fault MVA & sum up.

Fault Current = Fault MVA / Sqrt(3) x VL

My question is from where dis evaluation process starts. how dey calculate system fault current 31500ampere for 3 sec. y not 25 ka ?

Kindly gothrough Power Engineers Hand Book,different types of calculations,fundamentals of Short circuit current,Fault MVA ets are given in detail.

I dont need d whole details.so i think i dont need d book..i wana only know d single formula to calculate system fault current. .i wana came to know y dey havt use 25amp instead of 31.5amp as a system fault? if u think i need d book plz tell me writer name .

Hi Sir,

Can you please tell me any good power engineering handbook???

my email id sadhana25aps@yahoo.co.in

If you are from India,please try Power Engineers Handbook published by Tamilnadu Electricity Board Engineers Association .It is costing Rs 1500/- only.

You may get it from Engineers's association office in TNEB complex,Annasalai,chennai 2.

ramesh

The fault level calculation is required for proper selection of circuit breakers and interrupting devices. The short circuit current consists of two components

AC component and DC component. To calculate the fault current the transformer KVA rating and percentage impedance must be known.A simple formula to calculate i is

I =

1.1 x V_L

_{-------------------} Amps

√3 x Z

I Fault current

v_L Line Voltage

Z percentage Impedance

1.1 factor of multiplication to take care of possible 10 % rise inn voltage

you can get a fairly good idea if you read the following link.

http://wiki.myelectrical.com/index.php?title=LV_Fault_Calculation_Overview#Maximum_Fault_Current

hi friend...thx

I= 1.1*66/√3*0.846

= 49.6 KAmpere

where VL=66kv , Z= 8.46%

Is It right??? its fault level??? or we have done something wrong

sandeep,

The formula stated 1.1*VL where 1.1 is c factor value according to IEC909 for the greatest value of initial symmetrical short circuit current three phase to phase fault clear(I"K) from ground. Other method is VL/√3*Z, where VL here is no load volatage which is commonly 5% to 10% over than load voltage.

peak value of SC : ip= κ*√2*I"K, where κ is limited to 1.8 in low voltage network, and 2.0 in high voltage network.

The impedance is total impedance of the network, not transformer impedance only. One by one impedance of the line must be calculated to get the total impedance.

The term of fault level is subjected to MVA fault i.e √3*VL*I"K for symmetrical and √3*VL*ip for peak MVA (all in MVA unit).

c factor above is substitution value of uncalculated factor.

The problem is focused not on the formula, but on the accuracy of analizing the fault-network (impedance method, per unit method, equivalent circuit), thus to properly calculate the impedance value.

All above is for fault far from generator.

I don't know if these might help?

Thx friend...it is very helpful.....plz tell me where vl means no load voltage is of generating station no load voltage?

Say secondary side of transformer voltage 380V(system voltage), so take 400V for short circuit calculation only.

where is 3~5% from? Assumption.

You have wrongly calculated the value of Z . The calculation of of impedance is based on the % Z of transformer ,KVA and transformer secondary voltage.The final calculation.

Z =

%Z x 10 x kv ²

^{---------------------------}

kVA

As an example for a 1000 kVA , 22/0.45 KV transformer with % Z of 5.25

the calculated value of Z will be 0.009 Ω and I = 29.3 kA

Hope this will be clear now.

To arrive at fault current u need to calculate taking into account the Impedance of the line up to your location.You will need the line parameters.Normally this is obtained from the Electricity boards who have the fault level at their bus bars.Then either u can ask for line parameters or take the fault level at EB substation itself if u are not far away.

short circuit MVA = common base / total reactances

but if you don't have these values then you can transpose the following equation to calculate short circuit MVA...

fault current level = short circuit MVA / (squr root x voltage)