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samsam_sam45
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Join Date: May 2011
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Transformer Size Selection

05/16/2011 3:31 PM

I make my calculation for STP plant . During my load calculation stage , I calculated total connected load of depart was 1750KW, 380V (estimated 2150KVA @ 0.8pf). the transformer calculated according to connected load.

Now when all the motors are in operation, I checked that the running load never exceed 525KW (estimated 650KVA @ 0.8pf). Now i have added service factor of 25% on it and i selected the size of transformer as 650x1.25 = 800kVA say 1000 kVA .It means our transformer is 3 times > max connected load demand . If we talk about starting current of big motors then all the big motors of 45kW are connected with DOL. So in this case can any one help me that the size selected as 1000kVA is correct or not . Please give me technical answers for the mention subject.

My question is that what should be the best practice/rule of thumb during selection of transformer in such cases?
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Tornado
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#1

Re: Transformer Size Selection

05/16/2011 4:53 PM

Déjà vu?

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lyn
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#2

Re: Transformer Size Selection

05/16/2011 7:12 PM

Next time, do it the same way.
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hien.nguyenquoc
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#3

Re: Transformer Size Selection

05/16/2011 9:40 PM

samsam_sam45:

You forgot one most important thing and I will tell you right now. After consideration of various things you described in the OP. Checking of motor starting against the voltage drop is neccesary. Requirement of voltage drop says it shall less than 10 percent during motor starting and less than 5 percent during normal operation.

If the requirement is met. Thing would be Ok.

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soebfatehi
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#4

Re: Transformer Size Selection

05/16/2011 10:48 PM

if your largest motor is 45kW . . .

do you have only one such motor? and if you have more than one then will all 45kW motors (or all large motors) ever start simultaneously? even if they do will they have loads with a high starting time? chances are that they wont. your calculation method is correct. keep doing what you are doing and sleep safe.

next time you can select a smaller transformer IF you have enough data to correctly estimate your ACTUAL TOTAL RUNNING LOAD (MD) and add to it the DOL starting current of your largest motor (or motors if they are expected to start simultaneously).
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V.I.Abraham
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#5

Re: Transformer Size Selection

05/16/2011 11:49 PM

The basic loading factor for design purpose is continuous loading at rated KVA.

The transformer loading depends on design,temperature rise at rated load , cooling medium , duration of over loading, load factor, ambient temperature and so many other factors. The transformers are designed to withstand over load for limited period however it should not any time cross 200 percent rated KVA. In case of motor load the starting current of the motor and shot circuit rating of the transformer to be used have a specific relation to calculate the actual kVA rating of the transformer to be designed. You can get a practical knowledge about the actual calculation from the link below.

http://www.powerqualityanddrives.com/short_circuit_transformer/
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electricalexpert65
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#6

Re: Transformer Size Selection

05/17/2011 10:04 AM

Good thing that you posted it now! Very recently, I had reviewed the transformer sizing of one of my clients (a water treatment EPC Organisation) - similar to your query. The calculation is too large to post here. If you can send me your mail ID thro this forum's mailbox, I will send you the same.
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samsam_sam45
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#7
In reply to #6

Re: Transformer Size Selection

05/17/2011 10:22 AM

Dear Friend ,can u send me the information which u have ask to send me in your comment ,my email id is amalik.electrical@gmail.com

Thanks

Sam
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hien.nguyenquoc
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#8
In reply to #6

Re: Transformer Size Selection

05/17/2011 10:30 AM

electricalexpert65: Could you drop it to my id? I also want to have a look at it.

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electricalexpert65
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#9
In reply to #8

Re: Transformer Size Selection

05/17/2011 10:38 AM

What is your mail ID?
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hien.nguyenquoc
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#10
In reply to #9

Re: Transformer Size Selection

05/17/2011 10:42 AM

Hey, I have sent you for the training package. Could you please check your inbox?

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samsam_sam45
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#11
In reply to #10

Re: Transformer Size Selection

05/17/2011 11:01 AM

dear friend ,

I didnt receive your training package could you please send me through your personal email please find my personal email : amalik.electrical@gmail.com
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hien.nguyenquoc
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#13
In reply to #11

Re: Transformer Size Selection

05/17/2011 11:09 AM

samsam_sam45: Sorry making you confused. I did not send you any training package. I said to EE65 only.

Please read the CR4 Code of conduct at here: http://cr4.globalspec.com/faq/cr4rules

Don't publicly put personal email address otherwise your post will be removed.

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samsam_sam45
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#12
In reply to #10

Re: Transformer Size Selection

05/17/2011 11:07 AM

can you send me the training package Mr.Hein my email id :

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electricalexpert65
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#16
In reply to #10

Re: Transformer Size Selection

05/17/2011 12:27 PM

Pl. wait till tommorrow evening.
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rathakrishnan05
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#18
In reply to #16

Re: Transformer Size Selection

10/11/2011 4:37 AM

dear electrical expertise,

as i m new to this forum, i need transformer sizing calculation model as u mentioned in the previous post. Please post it in my id. I wil be very thankful to you.
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samsam_sam45
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#20
In reply to #9

Re: Transformer Size Selection

10/12/2011 2:27 PM

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samsam_sam45
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#14
In reply to #6

Re: Transformer Size Selection

05/17/2011 11:52 AM

electricalexpert65:PLEASE SEND ME THE INFORMATION WHICH YOU ASK ME TO SEND MY ID :

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electricalexpert65
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#15
In reply to #14

Re: Transformer Size Selection

05/17/2011 12:26 PM

Pl. wait till tommorrow evening.
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rathakrishnan05
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#21
In reply to #15

Re: Transformer Size Selection

10/19/2011 9:20 AM

Dear Electrical expertise,

I need the following clarifications,

1. I have done 3 times transfomer sizing for various projects for various clients. During that sizing i have done bus bar sizing calculation also. Now my new client is asking to submit voltage regulation calculation along with Transformer sizing calculation. As per the bid stage we had selected 5MVA,34.5/4.16kV, ONAN,Dyn5 with 7% of voltage impedance. My major loads are as follows,

1. One LV transformer (3MVA, 4.16/0.480kV, ONAN, Dyn5 with 6.5of %Z)

2. Two motors of rating 1050kw and 910kw, respectively.

I have done transformer sizing and busbar sizng of switchgear which is connected to this transformer. Now i want to calculate Voltage regulation for this Transformer.

Can you explain in detail to find regulation. I will be very thankful.
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34point5
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#17
In reply to #14

Re: Transformer Size Selection

05/17/2011 10:52 PM

samsam_sam45

If you click on your forum ID (as above, or anywhere in CR4 it appears) it will take you to your 'profile page'.

At the head you will find;

"Check Your Mailbox"

There is also a link on the right and column of every thread page under "Logged In"

To send a message to a member: Click on their ID, go to their profile, at the top right is "send a message" which links through your CR4 mail box, to theirs.

If you have received mail, it will show up here

as a box above "updated discussions" - which when clicked - takes you to your box.

Use this system - rather than posing you email address - (which no member will reply to)

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samsam_sam45
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#19
In reply to #6

Re: Transformer Size Selection

10/12/2011 2:23 PM

dear electrical expert kindly please find my email id :

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21 comments

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Users who posted comments:

34point5 (1); electricalexpert65 (4); hien.nguyenquoc (4); lyn (1); rathakrishnan05 (2); samsam_sam45 (6); soebfatehi (1); Tornado (1); V.I.Abraham (1)

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