Transformer selection, Fault Current calculation

Firstly,

Not being horrible but how did you get that job if you can't do these simple formulas

Secondly

We need more information on what you are trying to achieve in order of you to get better and more relating anwsers to your question??????????

________________

Thank you and good night

Dear Sir,

I dont think this to be HORRIBLE to get this job. As a fresher I am very curious to know from the experienced people in this field. In current trend most Engineering graduates coming out from college are jumping into IT field. So atleast appreciate me for my curiousity rather than discouraging me by saying it as horrible.

Cheers

Hi guest ,

Don't worry .You have started only . Do not get panic. Everybody starts the job with lot of educational background and Will to learn. So start. First you must know what is your requirement. Find that out first. So start from the beginning. Do not jump to start designing. Your load assessment , regular requirement ,Type of load continuous process, intermittent load ,jerk load like metal, cutting, press, welding etc.

Next step availability of power , Distance from the source, From where the power to sourced ,either the supplier will provide power at site, what should be the incoming voltage , what is the load voltage.Above all what is the financial capacity of the company who give the order to you. Make an initial paper comprising all this and tell me then I shall tell you how to go ahead .Best of luck .Start your job

Hi,

Vi Abraham, I have read some of your postings and they look very knowledgeable. Would you mind sending me your e-mail add so that if i also have a question i will be sure to get in touch with you. I am in protection engineering...South Africa.

It's a little difficult to answer all of your questions but here are a couple of items:

1) There are tables in the NEC that provide data on how to estimate the demand of a facility. For example, the estimated demand of a school would be: Building square feet x 15 VA/square foot. (49,000 S.F bldg X 15 VA/S.F = 735 KVA)

2) The transformer to feed this load would be the next standard size of commercial transformer, assuming a 3 phase supply, this would be 750 KVA transformer.

3) The per phase current available from this transformer would be

2040 A for a line-to-line voltage of 208V, 902 A for a line-to-line voltage of 480V, etc.

4) The distributed generation would be sized similarly.

5) A simple short-cut for calculating short circuit, assume an infinite source bus, the maximum fault current available at the secondary terminals of a 150KVA 13.8KV-480V transformer with a 2% impedance can be calculated by:

Short Circuit Current = (150,000VA)/(sqrt(3)*480*(2/100))= 9,022 A

You should really get your hands on a copy of the IEEE Buff Book "The IEEE Recommended Practice for the Protection and Coordination of Industrial and Commercial Power Systems" to develop a solid understanding of fault current calculations.

Congrats!!

Please LIST THE CONNECTED LOADS(KW's+ load descriptions in time)

A description -what runs at same time,what loads are ON, all the time.

You are talking of Transformer & DG- you want to feed which loads from Diesel Genset?

Do you want to rate the Transformer for full utilization NOW or less- or keep Reserve for growth? How much reserve?

Fault level of Transformer/Line incoming/Of DG ?

There are various parameters for T/F selection

main are given below

1. Voltage level

2. Frequency

3. type of load

4. For genration / distribution

5. Indoor / out door

6. temperature

7. overload timings

8. Fault level

As far as DG size is concerned you can take help from this relation

KVA = E x I / 1000 ---------- Single phase

KVA = E x I x 1.73 / 1000 --------- 3 phase

You can calculate fault level KVA or MVA rating Divided by % impedance

Remember that Fault level is nothing but the amount of current that flows in the network during fault or short circuit

---Jazsy Dhariwal

Dear Sir

pls tell impendence selction on transformer 2500 kva ,11/0.440 kv. if we go on higher side impendence what is benfit or loss

Hi,.

The Transformer sizing will depend upon the nature of the load that is resistive ,inductive like motors then calculate the toatal current consumed by all the load in running condition and then use the formula P= sqrt3*.433* current calculated that will be the KVA of the Transformer ,also if the load are variable or intermittent than u have take consideration of the starting condition of the largest motor also.

Hi Every One,

I have following questions to this forum

What is the maximum & minimum fault lavel of a 400KV S/s? How it is calculated? If we know the Phase to phase & phase to ground fault MVA of a S/s how can we calculate maximum & minimum fault of the S/s?

It will be help full if any one reply.