fluctuation in secondary voltage of transformer

[tareksabry]

we have a problem during testing the transformer the
data is 1600kva,delta/star 11kv/380v while we are testing the secondary side we found the voltage is fluctuated from 400V to 500V or 600V or 700V in a (ms)seconds then return back to 400V.

 

[Skogsgurra]

Is that transformer connected? Is there a load?

Regarding the millisecond variation. As you know, the voltage of a 380 V grid changes between -537 and +537 V fifty times every second. Or once every 20 ms.

On top of that there are usually transients caused mostly by load changes. Those transients are normally in the 700 V range (peak) and not to worry about.

What measurement equipment do you use? And why are you testing this? Did you have a problem?

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[slavag]

If this trafo connect to load?
If the star earthed now?
If the RMS or peak meas?
Regards.
Slava

 

[tareksabry]

btw it`s no load

 

[tareksabry]

1-it`s no load.
2-the star is earthed.
3-the measurement is rms.

 

[Skogsgurra]

Do you really see RMS over milliseconds?

Please tell us what instrument you are using.

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[tareksabry]

we use avo meter

 

[tareksabry]

not milliseconds but it appear for while then return back to 400Volt

 

[slavag]

Hi Tarek.
Few steps for to be sure.
1. Please provide fixing contact of AVO meter, maybe is contact.
2. Please for second phase use another AVO meter.
Are you have this on all phases.
For first, it's seems as bad meas or AVO meter problem.
Slava

 

[tareksabry]

only in one phase.

 

[slavag]

Stop, stop.
380V it's phase to phase voltages.
Please check again all meas:
R-N
S-N
T-N
R-S
S-T
T-R
and back to us with results.

 

[tareksabry]

R-S= GIVE VERY HIGH VALUE.
S-T=GIVE VERY HIGH VALUE.
T-R=GIVE VERY HIGH VALUE
IT GIVE A VERY HIGH VALUE IN THE SECONDARY SIDE THEN RETURN BACK TO 400V,SOMETIMES IT REACHED 1000 VOLT IN THE SECONDARY SIDE.

 

[slavag]

Something crazy.
I strongly recommend stop test and check this trafo again.
Megger, transmission ratio, etc.
Phase to phase voltages are very high and phase to earth/ground are OK????
I haven't any idea on this moment, 3 times voltages and back to the normal??? Earthed star w/o any load, maybe some capacitors are connected on the bus???

 

[tareksabry]

we removed all the capacitors.

 

[Skogsgurra]

Is this a test you do routinely? Or is it the first time you do it?

Why do you do it? Did you have some kind of problem with this transformer?

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[waross]

Your transformer could be failing, turn to turn in a primary winding. It may well be an arcing or intermittent fault.
Is this a dry type or oil filled transformer?
Is this a new transformer or one that has been in service and is developing a problem?
Are there any current transformers in the primary circuit that you may use to compare primary phase currents?
In another thread, you asked about a short circuit test.
A short circuit test on a failing transformer may lead to a catastrophic failure with property damage, fire, injury and death. At least one of our regulars here has lost friends who died when a transformer exploded.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[tareksabry]

this is a new one,an oil filled transformer.

 

[waross]

Does this transformer have automatic tap changers?

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[slavag]

Hi.
It seems as turn to turn problem.
But what about some Buhholtz relay?
Best Regards.
Slava