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Active Contributor

Join Date: Jan 2010
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ONAN/ONAF/OFAF and short circuit currents

01/05/2010 5:10 PM

How do fans and pumps effect transformers short circuit current, meaning ONAN/ONAF/OFAF affect the transformer impedance? which will then effect the short circuit current. So I have a xfmr with a fan and the impedance is given based on the self cooling (ONAN) so does the impedance change linerally to maintain the same fault current?

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Anonymous Poster
#1

Re: ONAN/ONAF/OFAF and short circuit currents

01/05/2010 11:14 PM

Hi

I think the kind of cooling what we are using for transformer is not to maintain the short circuit current of it, cooling is for maintaing the contant and uniform temperature of the x'mer,

Impedance of x'mer normally given by considering its highest operating temperature, and cooling maintains the temperature of x'mer below its highest operating temperature, so impedance is sometimes higher than its given value, when temperature is higher,

regards

Divyesh patel

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Good Answers: 1
#2

Re: ONAN/ONAF/OFAF and short circuit currents

01/06/2010 12:47 AM

Hi!!,

[p] I hope this what you mean. Transformer impedance are expressed in terms of percent impedance that is referred to its ONAN rating. If the fans and pumps are activated, the MVA rating of transformer will increase. Since the MVA rating of the transformer has been changed, it will follow that the percent impedance of the transformer will change. But if you are to calculate its ACTUAL impedance, it is still the same. If you will solve the 2 second short circuit current rating of the transformer using the respective values of MVA rating, you find that the short circuit current is the same. I think you could say linearly, if you maintain the voltage level used and changing its MVA. But if you will make the voltage as well as the MVA rating the transformer variable, i think it is kinda quadratic.

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Member

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Posts: 7
#3

Re: ONAN/ONAF/OFAF and short circuit currents

01/06/2010 2:01 AM

i guess ONAN/ONAF/OFAF doesn't relate to short circuit current since ONAN/ONAF/OFAF is used as cooler in the transformer, so we can know the maximum temperature the transformer can endure.

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Anonymous Poster
#4
In reply to #3

Re: ONAN/ONAF/OFAF and short circuit currents

01/06/2010 3:28 AM

yes, it doesn't relate.. ONAN/ONAF/OFAF "names" only changes its per unit impedance, not the actual impedance...

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#5
In reply to #4

Re: ONAN/ONAF/OFAF and short circuit currents

01/06/2010 9:04 PM

To answer this question simply, let's first see the steps to measure the percentage impedance (%Z) (Also see http://cr4.globalspec.com/thread/41265):

(1) Short the secondary side of the transformer with a current measuring device (Ammeter)

(2) Apply low voltage in primary side and increase the voltage so that the secondary current is the rated secondary current of the transformer. Measure the primary voltage (V1).

(3) Divide the V1 by the rated primary voltage of the transformer and multiply by 100. This value is the percentage impedance of the transformer.

The transformer rating (MVA) is changed with the cooling systems: OFAF Rating > ONAF rating > ONAN rating.

As the rating changes, the rated secondary current in step 2 changes too. So, the V1 varies with cooling methods: V1 at OFAF Rating > V1 at ONAF rating > V1 at ONAN rating.

Hence, the percentage impedance (%Z) changes accordingly: %Z at OFAF Rating > %Z at ONAF rating > %Z at ONAN rating.

And so the short circuit current changes: ISC at OFAF Rating < ISC at ONAF rating < ISC at ONAN rating.

Hope it is clear now.

- MS

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Anonymous Poster
#6
In reply to #5

Re: ONAN/ONAF/OFAF and short circuit currents

01/06/2010 9:55 PM

Are you sure about this?.. can you site an actual calculation?... I know that the percent impedance changes. but it is in percentage.. but the actual does not change.. can you prove it to me?..

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Guru

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#10
In reply to #6

Re: ONAN/ONAF/OFAF and short circuit currents

01/07/2010 10:01 PM

This is what it is - if you are sure about your understanding of transformer percentage impedance (%Z) and then the rest is just pure math. There is nothing to be sure for 2 + 2 = 4.

- MS

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Anonymous Poster
#7
In reply to #5

Re: ONAN/ONAF/OFAF and short circuit currents

01/06/2010 9:59 PM

show me an example about this... including the short current changes..

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Commentator

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#8
In reply to #5

Re: ONAN/ONAF/OFAF and short circuit currents

01/06/2010 10:11 PM

Hi!!.

[p] For the sake of everybody tomorrow I will be attaching a sample of nameplate of a transformer. This way, maybe, we can clear this up. Till tomorrow then..

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#11
In reply to #8

Re: ONAN/ONAF/OFAF and short circuit currents

01/07/2010 10:29 PM

Here is a sample namepate from one of our substation, take a look a this %Z

25/33MVA = 7.8%/10.2% at 69kV

solving for Z at 25MVA

Z = 0.078((69kV)^2/25MVA) = 14.854ohms

solving for Z at 33MVA

Z = 0.102((69kV)^2/33MVA) = 14.715ohms

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Active Contributor

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#9
In reply to #5

Re: ONAN/ONAF/OFAF and short circuit currents

01/07/2010 3:18 PM

Thats what I thought and I am the only one in the office so far. I found some data somewhere that I can't find now that shows the impedance changing as fans/pumps are added. If you think about in terms of just heat the fans/pumps get rid of the heat that transformer is using to charge itself therefore it will take more heat which will increase V1. The example I found had the same Isc because the impedance increased linerally from like 7.8% @18MVA to 13.3% @ 30MVA. Thoughts?

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Participant

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#12

Re: ONAN/ONAF/OFAF and short circuit currents

01/08/2010 12:13 PM

hi, let's take it from this angel that, a transformer has certain operational issues

1) Temperature rise:

.Temperature rise is the difference between the transformer operating temperature and the ambient temperature.The transformer operating temperature is mostly related to these under listed areas:

- Top oil temperature

- Average winding temperature

- Winging hottest spot temperature

The continuous operation of the transformer with temperature rise may cause thermal degradation of the solid and oil insulation.

2) cooling classes

Transformers with triple rating (ONAN/ONAF/ONAF) have an increase of 33%MVA capacity for each cooling step.The 33% increased capacity is based on the first MVA base of the transformer,therefore transformer MVA rating is increased with additional cooling stages hence the purpose of the fan and pumps being employed to achieve this goal.

Now the impedance of a trafo is usually quoted as its %impedance which is the percentage of the normal terminal voltage required to circulate full-load current under short circuit conditions.That voltage drop on full load due to the winding resistance and leakage reactance expressed as a percentage of the rated voltage is affected by temperature as is established that increase in temperature causes a linear expansion in the conductors used to wound the trafo. This will then increase the trafos impedance( winding resistance plus leakage reactance) .

Short circuit current on the other hand, can only flow in the advent of short condition or fault condition hence there is no direct link with the effect of fans and pumps on trafo short circuit.

In conclusion, fans and pumps have same direct effecet on the impedance of the trafo but not with the short circuit current of the trafo which only flows when there is the accidental creation of a short circuit.

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#13
In reply to #12

Re: ONAN/ONAF/OFAF and short circuit currents

09/09/2015 4:39 PM

For an ONAN transformer that is single rated is it possible to make it ONAF and assume the power rating will increase by 33%? I tried checking it out i observed it is possible to increase the power rating by installing fans but other factors have to be taken into consideration. tried checking IEEE standard on this, found it, but no free copy to read.

Kindly help me out on this question and it's pitfalls.

Lastly, does transformer aging affect the power to be gotten after installing fans?

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