Transformer In rush current 1

[Pgee630]

Hi all,

May I get some advices that, normally under routine power transformer (e.g. 2MW) factory test, is the in-rush current testing essentially to be involved?

Thanks
Pgee

 

[argotier]

Hi Pgee630,

In-rush current testing is not required by the main transformer standards: IEC 60076 and ANSI/IEEE C57.12.00, not even as a design or special test.

Hope it helps.

 

[jraef]

But is is nice to know what it is, especially for the protection circuits involved.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[RRaghunath]

I have seen vendors furnishing the inrush current magnitude and waveform spectrum, if the same is requested by client.
For relay setting purpose, inrush current is considered as 12 times the rated current.

 

[che12345]

Dear Mr. jraef

Q. "But is is nice to know what it is, especially for the protection circuits involved."
A. Valid, it is "nice" to know, but (unfortunately) it is {unpredictable? with precision]. It depends on too many factors that are not within the transformer designers' control. Even any recordings provided by the manufacturer/ or lab during testing does NOT ensure that further subsequent tests will repeat the same result.
For these reasons, as learned Mr. argotier stated " In-rush current testing is not required by the main transformer standards: IEC 60076 and ANSI/IEEE C57.12.00, not even as a design or special test".
Che Kuan Yau (Singapore)

 

[Shanaya]

When a transformer is energised, it may draw a large transient current from the grid supply, resulting in a temporary voltage dip at the point of connection (POC) where customers are connected. The voltage dip is dependent upon the magnitude of the transformer inrush current which in turn depends on:
[ul]
[li]Transformer design (i.e. its construction and materials).[/li]
[li]Residual flux in the transformer, which may be as much as 50% to 90% of the maximum operating flux. This is the amount of flux remaining in the core due to the properties of the magnetic core material.[/li]
[/ul]

 

[bacon4life]

In addition to the value of 12x at 0.1 seconds, some references also list 25x at 0.01 seconds.

 

[waross]

The point-on-wave when the transformer is energized is an important factor.

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

 

[emoltzan]

The reference I have states the following:

Xfmrs <= 2500kVA have an inrush of 8xFLA @ 0.1s
Xfmrs > 2500kVA have an inrush of 12xFLA @ 0.1s

 

[Alms88]

No , not required,
inrush current test is done only in factory or type test

 

[itsmoked]

The point-on-wave when the transformer is energized is an important factor.

Because of the point-on-wave that the transformer was disconnected at.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[crshears]

Although there are some exceptions, for the most part we take our two and three significant digit MVA transformers off potential with motorized air-break disconnects, meaning that as the switch blades open the arc gets longer and longer until it finally breaks completely. Would this type of removal from potential have the same sort of point-on-wave residual imprinting effect on the transformer?

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[waross]

Correct Keith. Thanks for pointing that out.
The rest of the story:
The point on wave of the disconnect will determine the amount and direction of the residual magnetism left in the core.
The point on wave of the reconnect will determine the effect of the residual magnetism on the inrush.

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

 

[crshears]

The point on wave of the disconnect will determine the amount and direction of the residual magnetism left in the core.

The implication then being that the seconds-long process of removing a transformer from potential with a motorized disconnect will lead to an arc of steadily increasing resistance and therefore minimal to moderate current flow and thus minimal to moderate residual core magnetism.

Correct?

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[waross]

The motor operated disconnects that I am familiar with are of two types:
1. The motor charges a spring and the spring provides a "snap" action or quick disconnect.
2. The motor operated disconnect is for isolation only and is never used for interrupting current.
I have never seen a disconnect designed to interrupt a current with a slow opening action.
(But I haven't seen everything yet.)

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

 

[crshears]

Come to Ontario sometime, Bill, that is, if/once interprovincial travel becomes safe and unrestricted [ sigh ] and you'll see all manner of them. We do indeed have some snap-action ones, TransRupters [tm] and other such devices, but those tend to be exceptions rather than the rule. As to whether these slow-opening disconnects were "designed" for this purpose, that is unknown; I remember seeing past documentation stating that the sole reason this was done was for cost, even though the technical sub-optimality [ is that even a word? ] of this was acknowledged; I'll see if I still have a hard copy of that somewhere in my files.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[waross]

Thanks cr.
As I understand it, the residual will depend on the current at the point in time when the arc extinguishes.
Do you find that these disconnects are effective in reducing the residual magnetism?
Has anyone ever checked the inrush under operating conditions?

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

 

[crshears]

It being the weekend, there's nobody around to ask; I'll see who I might be able to e-mail about that and get back to you.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[waross]

Thanks.
I suspect that there may be a fair amount of residual magnetism left after an arc fails to re-strike after a zero crossing.
I may be wrong.
Waiting to see your findings.
Bill

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

 

[electricpete]

> The point-on-wave when the transformer is energized is an important factor.
> Because of the point-on-wave that the transformer was disconnected at.

I just wanted to mention that it is not soley because of that.

You can model a transformer with no residual magnetism and still get a high-magnitude non-sinusoidal inrush waveform whose magnitude is highly dependent upon the point of energization.

If you model a single phase transformer suddenly energized at no-load, you are applying a sinusoidal voltage to the inductive magnetizing impedance:
I_magnetizing = (1/Lmagnetizing)*int{v(t)} dt where v(t) is a sinusoid.
If you close at v(t) = Vmax, the current only increases for a quarter of a cycle before turning around when v(t) crosses zero, so you get no dc offset in magnetizing current. If you close at v(t)=0, the current increases for a half cycle before turning around when v(t) crosses zero and you get max dc offset and more importantly that max dc offset pushes the transformer far into saturation when the ac component peaks in the same polarity. When that happens there is not just addition of the dc and sinusoidal component but the sinusoidal component screams higher by a factor of 10 or more since saturation limits the rate of change of flux to oppose it. If you want to simulate/visualize it by integrating i=(1/L)*int{v(t)dt then you have to consider L not as a constant but instead as a function of i... and that L(i) decreases dramatically when |i(t)| goes above the normal peak magnetizing current. What you end up with is huge unidirectional current spikes once per cycle... that's what comes out of the model and that's also what I have seen on real world traces. The pattern is more noticeable and lasts longer on larger transformers energized with no load on the opposite winding because the circuit is overwhelmingly inductive with little resistance to help the dc decay.

So there is definitely a sensitivity of inrush to angle of closing that is unrelated to the residual magnetism. But I'm sure residual magnetism can make the worst-case worse as you guys know, probably more than me.

I don't think I'm telling anybody anything they didn't know, just felt like chiming in.



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