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11 comments
Member

Join Date: Apr 2010
Posts: 9

Transformer Trip

04/18/2010 12:56 AM

Hello,

Our indoor air cooled transformer( 75 KVA) main MCCB 250 A got tripped suddenly, when it is in service.

On checking MEGGER are ok. but the winding resistance come lower as 0.2 ohm , befoe it is 0.3 ohm.

Also our load side MCCB is not tripped , can u send the reason

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Guru
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Join Date: Mar 2010
Location: In the pool because it is too hot.
Posts: 2732
Good Answers: 129
#1

Re: transformer trip

04/18/2010 1:25 AM

Has it tripped on temperature? If your ohms value is different, maybe some wires of the coil inside might be shortened? (this a Megger doesn't detect when you measure insulation

I usually measure impedances with 60Hz device or higher frequencies.

But that is me. Old habit when I made coils, but a good one. You will be able to see more difference.

If there is a shortage inside the primary, the voltage should be higher on the secondary. And also both currents, which explains why the primary tripped.

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Posts: 9
#2
In reply to #1

Re: transformer trip

04/18/2010 3:19 AM

there is no temperature measuring device to detect the temperature, temperature went upto 110 degree celcius. there is only MCCB. no other protection device both in load end and the the incomer.

resistance valuue is not different ,the values are same. before 6 months and now it is 0.1 ohm different

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Guru
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Join Date: Mar 2010
Location: In the pool because it is too hot.
Posts: 2732
Good Answers: 129
#3
In reply to #2

Re: transformer trip

04/18/2010 12:46 PM

0.1 ohms difference in value is not much for some and a lot for others.

Some multi -meters have even trouble detecting it. But, if your measurement is correct YOUR 0.1 ohms represents 33% of the original value and to make it back right 50% of the value to add.

That is why I like measuring impedances, because they show a lot more distinctions. I really hope for you that your measuring device gives you wrong information.

And prove my conclusion is wrong too. Replacement or re-calibration of the meter will be cheaper than getting a new transformer. Just get more reliable information - if possible from the manufacturer. Good luck.

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Member

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Posts: 5
#4

Re: Transformer Trip

04/18/2010 11:37 PM

You can check the transformer condition by following non distrutive method.

Check the Transformer Specifications or Name Plate.

Check % Impidance value

Apply same % of Primary voltage ( ie if pri is 400 v & % imp is 10 % ,then apply 40 v to primary ).Leave secondary without any load.

Measure the Primary No Load current.It should be in milliamps and fairly balanced.If the current value is measurable in amps,the possibility of turn to turn short is there.

If the current is negligible then leave the transformer in charged condition for an hour.

Check the transformer winding temp before & after 1 hour No load charging.

For healthy transformer there will be no noticeable change in Transformer Winding Temp.

If the test increases the temp by 5 Deg Cent or so, it indicates winding problem.

hemantphatak

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Active Contributor

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Posts: 14
#5

Re: Transformer Trip

04/19/2010 1:24 AM

Hi, I think you have checked only primary side winding insulation, and winding resistance, what about secondary winding resistance. What is the voltages of the transformer, if possible do the ratio test.And Conform the MCCB condition also.

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Participant

Join Date: Apr 2010
Posts: 3
#10
In reply to #5

Re: Transformer Trip

04/28/2010 3:31 AM

Please check the performance,conditions and parameters of MCCB. Try to test the MCCB and compare the result with the manufacturer's Confirm there is no problems with the MCCB and your Measuring Instrument (MEGGER) Then start the transformer again and try to load the same then if this transformer trips again , you should check the other relays if the relays are ok then you can go for other decisions.

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Associate

Join Date: Jan 2010
Location: India
Posts: 42
#11
In reply to #10

Re: Transformer Trip

05/03/2010 12:02 AM

All ardent engineers out there, visit www.maintenancecircle.com and register your email ID for subscription. There's a lot to read every fortnight !!!

Maintenance Circle Team(R)

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Guru

Join Date: May 2007
Location: Budapest, Hungary, HA5YAR
Posts: 611
Good Answers: 13
#6

Re: Transformer Trip

04/19/2010 3:55 AM

There might be a shortcut between the turns or the layers of winding. You should check the unloaded loss of the transformer or measure the primary impedance with a bridge what can measure both the Ohmic and reactant component.

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Power-User

Join Date: Nov 2009
Posts: 197
Good Answers: 18
#7

Re: Transformer Trip

04/19/2010 5:03 AM

Hi, You dont mention the load applied to this tx, or how much of the 75 kVa is online before the trip, or even for how long the transformer was online. However, this transformer may have picked up moisture in its oil, which will show as reduced resistance. There may also be a number of other nasties present due to the oxidation of the oil. If i am correct, all that will be necessary will be an oil change. But do have it serviced any way.

PS. I made the same mistake: Misreading. You dont have OIL. However the rest of the post still applies. You do need a service since you may have oxidation of your insulation material, which may be OK under no load conditions, but breaks through under some load.

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#8

Re: Transformer Trip

04/19/2010 8:41 AM

Dear,

First do the Meggar test and confirm the condition of the Transformer. ( Earth fault condition). And Measure the resistance values by Milli ohms meter. All are satisfactory then apply low voltage in the primary side of the Transfomer if the Transformer is a step down Transformer. Measure the voltage. If voltages are equal in all phases and measure the no load current of the Transformer. If current in all phases are OK, then apply actual voltage with out connecting load. If you get satisfactory result then connect the load. MCCB of Transformer Trips then problem may be in MCCB selection/faulty.

PRABAKARAN.N

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Participant

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Posts: 2
#9

Re: Transformer Trip

04/20/2010 8:32 AM

You have not indicated some important data about your transformer which you need to:

Is it a single phase or 3 phase transformer? What is the primary voltage? What is the secondary voltage? I understand that the 250 amps is the amperes trip rating of the MCCB. What is the ampere trip rating of your secondary? Your transformer being indoor, is it in a ventilated vault? temperature has effect on the tripping of the circuit breaker. Is the vault room hot inside? Is there a motor load connected on your transformer? These are relevant questions that you should have indicated so that a good evaluation of the problem can be analyzed.

harmonicfilter

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alagirisamyv (1); anandck (1); Anonymous Poster (1); dvmdsc (2); harmonicfilter (1); hemantphatak (1); jvrj (1); maintenanceanand@gmail.co (1); Qqberci (1); shamnas.aappa@gmail.com (1)

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