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effects of inrush current transformer

effects of inrush current transformer

effects of inrush current transformer

(OP)
I know that  very high inrush current during energization of an unloaded transformer can result in maloperation of protection.
What more? what kind of damage can result in transformer?
Do you had any field experience about this problem?
  

RE: effects of inrush current transformer

The core magnetic forces are limited by saturation. The windings and the leads from the bushings to the winding ends must be of a suitable size to withstand both the inrush current and the magnetic forces. I understand that transformer designers have rules of thumb to select the minimum conductor size on the high voltage side.  

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

RE: effects of inrush current transformer

(OP)
I guess this is a problem of  repetition of saturation of transformer magnetic core not overload in conductor.
 

RE: effects of inrush current transformer

If there is any concern at all for possible damage over time from repeated inrush over time, I would think the most vulnerable component would be conductors, which experience very high forces during inrush.  As mentioned above, since this is a routine evolution, the designer surely designs for this.  

=====================================
(2B)+(2B)'  ?

RE: effects of inrush current transformer

(OP)
I was worrying because of  thread238-302092: Change in Transformer Inrush by mgtrp:
     ....I presume that an increase in transformer inrush would be caused by the core/windings shifting, or alternately by the transformer becoming magnetised............
 

RE: effects of inrush current transformer

OK, so you're viewing change of inrush as a symptom of a problem rather than a cause (that's a different question).

fwiw, I would certainly be worried if I saw any detectable change in inrush behavior of transformer which was not attributable to external factors, system operation, or variability of closing angle. Unfortunately ruling out those external factors can be a challenge... many possible causes of "change" mentioned in that thread.

But if you are concerned about the transformer, certainly there are a number of tests available. I'd start with routine tests: dga and electrical TTR / power factor / excitation.  The one that would be most likely to show show movement would probably be excitation.  There are also more exotic tests geared toward detecting winding movement: frequency response analysis and leakage reactance.  I'm not very familiar with those, but I'm guessing trending is important.

=====================================
(2B)+(2B)'  ?

RE: effects of inrush current transformer

Apart from maloperation of protection, the main issue with large transformers is the mechanical forces on windings due to 4-10 times the full load current.Thermal effects are not an issue as the duration is only for few seconds or couple of minutes, still well below the time constant of winding.This over current creates dynamic oscillations in windings,cumulative effect can be detrimental if frequent switching is involved.Controlled switching is the solution followed with very large units.

Another pecular problem found recently is the marked reduction in content of second harmonics in inrush current of large transformers so that it will not actuate harmonic restraint feature in differential relays.It seems superior quality of CRGO with improved joints in core is contributing to it.

RE: effects of inrush current transformer

(OP)
eletricpete,
Very useful your opinion. Made me change my approach to the problem. Thank you.

prc,
You have hit the point!
I've been analysing an EMTP study  about 525/13.8 kV-350MVA transformer inrush  ready to be commissioned in the field.
Because of recently is marked reduction in the content of second harmonics in large inrush current of transformers leads to two situations:
1- 2nd Harmonic  showned in study is very high. It is likely that the model used to represent power in the study is outdated and consequently the values do not represent reality;
2 - Modern transformers use the step-lap type joint. Because of the high reluctance of the core joints, the remanence flux density levels of the transformer core is significantly lower than that of the core material itself.
Under the worst condition, the 15-20% 2nd harmonic restraint relays may not correctly restrain during the energization of the power transformer  that have rated higher flux density (1.5 - 1.75 T). For transformers which have the  higher flux density, it is recommended that the lower (5-12%) 2nd harmonic restraint relay setting be used.
See article: Power Transformer Characteristics and Their Effect on Protective Relays.
2.1 It is necessary to reduce the adjustment (15%) of the second harmonic.
2.2 here are several cases of inrush:
- Energization of the power transformer
_ Voltage recovery after the clearing of a heavy short circuit in the power system (recovery inrush)
_ Energization of another parallel connected power transformer (sympathetic inrush)
We are considering the possibility of recording the operation of  harmonics to collect subsidies for the new range of adjustments of the 2nd harmonic restraint relay setting be used.
Does anyone have suggestions for how to do this?


 

RE: effects of inrush current transformer

Hi.
For newer protective terminals low set of 2-nd harmonic is not problem ( Im put between 10-12%), becouse you have few unblock solutions.
In additional, not only 2-nd harmonic block is used, something like to waveform block is used too. Cross blocking, etc.

I would like recommend to you read documents of mr. Zoran Gajic from ABB Sweden on the issue.

RE: effects of inrush current transformer

(OP)
slavag,
I found it! thank you.

RE: effects of inrush current transformer

Regarding collection of harmonics - you might be able to do this using your protection relay's oscillography function upon initial energisation, and then adjust the relay setting to suit.

RE: effects of inrush current transformer

(OP)
mgtrp,
good idea! it is better then collection of harmonics!thank uou

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