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Help identifying a strange transient upon closing of a breaker
4

Help identifying a strange transient upon closing of a breaker

Help identifying a strange transient upon closing of a breaker

(OP)
Hi guys!

I've looked around a bit to find a good forum to post this question, and after reading through a few other threads here I think I may have found the right one!

I was analyzing a disturbance we had in our distribution grid (20 kV) and while examining the TFR:s I found a strange transient in one of our feeders when reenergizing it after having removed the faulty cable. We have a couple of customer owned stations on this specific feeder and I can't really say what kind of setup they are running, except that they are customers on the medium voltage level and may have medium voltage loads other than transformers.



In the image one can see that the current transient on phase 3 seems to be half wave rectified, inductive and of fundamental frequency. It is, in fact, the inverse of the sum of the currents in phase 1 and 2, which is quite easy to see as the transient is dampened. The transient eventually dies out, but the load current on this specific feeder is quite dirty with harmonics and I will have to investigate it in more detail eventually. For now, though, I'm just really curious on what kind of load that may be causing this kind of transient. Any ideas?

Best regards,
Henrik

RE: Help identifying a strange transient upon closing of a breaker

Hi Henik,

This is a typical wave signature of magnetizing inrush current with small harmonic superimposed. This shown primarily during stating of the 3phase power transformer and decaying rapidly in a few seconds.
It looks good to me.

RE: Help identifying a strange transient upon closing of a breaker

As cuky suggested, the inrush of a customer's transformer.

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

RE: Help identifying a strange transient upon closing of a breaker

3
As said, it's transformer inrush, but it can be any number of transformers that were all deenergized simultaneously.

Transformer inrush is typically characterized as being random, and if it's truly random one might expect that the simultaneous energization of multiple transformers to have a muddled wave shape without the sharp characteristic seen in single transformer inrush captures. Where the randomness actually comes in is the point on wave where the transformer was last deenergized.

A feeder full of transformers that were simultaneously deenergized by the tripping of a feeder breaker will all have the same inrush characteristic if energized simultaneously by the closing of the breaker. It will look like the inrush of a single transformer. Next trip and close will produce a different point on wave and a different inrush waveform, but it will still look like a single transformer.

RE: Help identifying a strange transient upon closing of a breaker

A good point and a good explanation David. Thanks.

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

RE: Help identifying a strange transient upon closing of a breaker

How long does residual magnetism last in a transformer? How long does it take to decay?

RE: Help identifying a strange transient upon closing of a breaker

Quote:

How long does it take to decay?

Weeks at a minimum. It only leaves thru thermal activity.

Keith Cress
kcress - http://www.flaminsystems.com

RE: Help identifying a strange transient upon closing of a breaker

(OP)
Thanks for your answers, everyone!

I guess we can consider this one solved. It proves that I still have a lot to learn about AC! (I have a background in HVDC)

RE: Help identifying a strange transient upon closing of a breaker

Hello Henrik_S:

Be aware that the HVDC converter station is also subjected to inrush current and overshoot of the output voltage which could exceed far larger than the rated values. The good news is there is available technology to mitigate that.

Quote (ABB: Due to capacitors used in the HVDC converter, its energization produces a higher inrush current causing a severe voltage dip in both the ac and dc systems potentially exceeding the allowable limits. Similarly, dc cable and HVDC transformer energization also causes a higher inrush current that can damage the semiconductor devices used in the converter system.)

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