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Muthukumar
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Join Date: Sep 2008
Posts: 9

Voltage Drop Problem Occured in LV Side of Transformer on 50% Load

12/01/2013 12:37 AM

Dear friends.

we are facing one problem on transformer. Transformer specification is 0.750MVA, 11/0.433 KV. Vector Group: Dyn 11. During load, the transformer's LV side voltage is dropped on one of the phase. Voltages are approximately RY-432V, YB-433V, BR-350V. Breaker also tripped on unbalance voltage. Load on transformer is 50% only. But on no load, voltage on all three phase is 431 V balance.

Then we conduct transformer test, voltage ratio, magnetic balance and excitation current test results are satisfactory results. But in winding resistance test on HV side , when injecting 1A DC, RY-4.5ohm, YB-4.5ohm and BR-9.3ohm on tap no.1. In other tap positions also same pattern of winding resistance are obtained. But injecting 5A, RY-4.5ohm, YB-4.5ohm and BR-5.8ohm. in some times measuring once again by injecting 5A, RY-4.5ohm, YB-4.5ohm and But BR-0.1 ohm are obtained, Same pattern is obtained for all taps.what will be reason for reduced winding resistance of BR phase when injecting 5A.
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Winfield Hill
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#1

Re: Voltage Drop problem occured in LVside of transformer on 50% load

12/01/2013 5:39 AM

Whoa, you are indeed facing a problem!

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Muthukumar
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Join Date: Sep 2008
Posts: 9
#2
In reply to #1

Re: Voltage Drop problem occured in LVside of transformer on 50% load

12/01/2013 6:02 AM

Yes..
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RAMConsult
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#3

Re: Voltage Drop problem occured in LVside of transformer on 50% load

12/01/2013 8:22 AM

Have you inspected the tapchanger yet? I suspect you will find a mechanical linkage problem inside the compartment.

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Fredski
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#4
In reply to #3

Re: Voltage Drop problem occured in LVside of transformer on 50% load

12/01/2013 11:31 AM

or bugs gumming up the works
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Muthukumar
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Join Date: Sep 2008
Posts: 9
#5
In reply to #3

Re: Voltage Drop problem occured in LVside of transformer on 50% load

12/01/2013 11:36 AM

yes. we lift the top cover with bushing slightly. Only the off load tap changer is visible which is located on the top. But we didn't get any problem on the tap changer. we didn't lift the whole winding with core till now for inspection.
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avra.setcl
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Join Date: Nov 2012
Posts: 21
#10
In reply to #5

Re: Voltage Drop problem occured in LVside of transformer on 50% load

12/05/2013 3:15 AM

There may be core saturation during injection of 5A for RB phase of delta
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Johny451
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#6
In reply to #3

Re: Voltage Drop problem occured in LVside of transformer on 50% load

12/01/2013 10:20 PM

GA from me. I suspect the tap-changer as well.

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PWSlack
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#7

Re: Voltage Drop Problem Occured in LV Side of Transformer on 50% Load

12/02/2013 4:47 AM

Somewhere in the installation is either a loose connection or a damaged winding or perhaps both. The trips are doing their job in protecting the installation.

On a transformer that size, do not energise it without having removed the fault beforehand.

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V.I.Abraham
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#8

Re: Voltage Drop Problem Occured in LV Side of Transformer on 50% Load

12/02/2013 3:40 PM

Since the voltage is dropping in one phase on load means the defect has to be with tap changing switch. Loose contact on the connection or switch contacts of tap changing switch is expected as the HT side winding resistance shows unbalance and no load voltage shows balance and voltage drop happens when load is switched on. There can be loose contact on the main HT connection leads to the winding which can also cause similar fault.
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horace40
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#9

Re: Voltage Drop Problem Occured in LV Side of Transformer on 50% Load

12/02/2013 3:58 PM

I guess it could be a loss of insulation between coils in the windings especially when under load and running hot causing some windings to short out.

But when tested 'cold' with 5 volts there are no 'shorts' to speak.

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Users who posted comments:

avra.setcl (1); Fredski (1); horace40 (1); Johny451 (1); Muthukumar (2); PWSlack (1); RAMConsult (1); V.I.Abraham (1); Winfield Hill (1)

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