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Transformer Inrush
6

Transformer Inrush

Transformer Inrush

(OP)
Has anyone ever used any transformer inrush elimination techniques?

I think in the past I have read about preflux circuits and also maybe zero current crossing circuit breakers (not sure if I dreamt the last one though!).

If anyone has any experience or weblinks I'd appreciate it.

Application = 31.5MVA 15kV (primary)

RE: Transformer Inrush

I used to work for Joslyn Hi-Voltage as a field serviceman and have worked on many of their POW (Point on Wave) controls before.  They were used for several electric arc furnaces I worked on.  They are nothing more than a modification of their ZVC (zero voltage close) controls made for cap bank switching.  They were accurate to 1 ms with their VBM/VBT switch and fairly easy to calibrate.

The website is here:  http://www.joslynhivoltage.com/

I tried to find the Point on Wave product, but I got tired of navigating the site.  Perhaps you could call them?   

------------------------------------------------------------------------
If it is broken, fix it.  If it isn't broken, I'll soon fix that.

RE: Transformer Inrush

davva,

Forgive me if I am wrong, but I am assuming you want to know about any techniques to block protections from tripping during transformer energization (due to in rush currents)?

the method I am familiar with is to detect a 2nd harmonic and block the protections when it is present.

RE: Transformer Inrush

marks1080,

Nope, I think he is talking about eliminating the actual inrush.  If you can close each breaker pole at precisely the right phase angle (near peak voltage), there is no inrush.

But at 15 kV, I've never heard of it being done.    

RE: Transformer Inrush

Hi.

Is it really possible to fully eliminate inrush for a 3-phase transformer (i.e. one three limbed core) by point-on-wave switching? The Alstom Network Protection & Automation guide claims that it isn't. I haven't thought about it too carefully, but it doesn't look simple.

Note also that due to core remanence, your next switch-on points-on-wave depend on your last switch-off points on wave.

Hmm!

RE: Transformer Inrush

Wow this is some interesting stuff.  I am fairly green at this so you'll get more questions from me than answers...  How would it be possible to eliminate the inrush?  Is the inrush current there initially to magnetize the core?  Seems to me you almost need a seperate source to energize the core before connecting the transformer to the grid.  

On a slightly different topic, isn't it desired for other reasons to switch elements on during the zero crossing point?

RE: Transformer Inrush

I am right with you marks (green), but I think the inrush current is not eliminated and it has to be still there to create the magnetic field. Only thing is that the magnitude is not as high, and probably takes longer to energize the transformer. Did not look at the exact definition of "inrush" and don't know if in this cause you would even refer to it as such, but I do know that you still need Vars to energize the xfmrs, and for that reason you will still have "inrush/current". It's like limiting inrush on motors with a soft starter but probably using different principles to achieve same outcome. Please correct me if I am wrong, but that's the way I see it.

Anyway why do you need to limit the inrush on transformers? If u have diff protection, than marks has to answer in his 22 jan 09 13:50 post.
 

"Throughout space there is energy. Is this energy static or kinetic! If static our hopes are in vain; if kinetic — and this we know it is, for certain — then it is a mere question of time when men will succeed in attaching their machinery to the very wheelwork of nature". – Nikola Tesla
 

RE: Transformer Inrush

Hi marks1080,

Consider the easiest case: A single phase transformer.

The flux in the core is the integral of the applied voltage.
It is helpful to consider two cases:

Case A:
If you begin integrating sin(wt) from a zero crossing, you end up with another sinusoid which is fully offset (i.e. oscillates from 0 to 2).

Case B:
If you begin integrating sin(wt) from a peak, you end up with another sinusoid with no offset (i.e. oscillates from -1 to +1)

The steel in a transformer core can be magnetised "forwards" to +Bmax, and "backwards" to -Bmax. Total allowable change in flux before saturation is 2*Bmax.

A transformer is designed for minimum core steel, so 2*Bmax corresponds to integrating one half-cycle of applied voltage. The core flux moves from -Bmax (at a voltage zero crossing) to +Bmax (at the next zero crossing).

Suppose you switch your transformer on at a voltage zero-crossing, and the core flux with the transformer de-energised was 0. Case A occurs and you attempt to magnetise the transformer core "forwards" to 2*Bmax. The core will saturate at roughly Bmax, and Bmax worth of flux is forced through the air. The result is a high inrush.

Suppose you switch your transformer on at a positive voltage peak, and the core flux with the transformer de-energised was 0. Case B occurs. The first 1/4 cycle magnetises the core forwards to +Bmax, the second (negative) 1/4 cycle brings the core back to 0, the third magnetises the core to -Bmax and so on. The result is no inrush.

The above explanation does not include effect of remanence.
Real transformer cores have memory, and do not always contain zero flux with the transformer de-energised. Hopefully you can see how remanence would affect the two examples above.

 

RE: Transformer Inrush

Inrush to induction motors is limited by soft starters. An induction motor is just a transformer with one winding on the rotor.

At higher voltages, perhaps pre-insertion resistors or reactors could be used.

RE: Transformer Inrush

Good explaination submonkey... I didnt actually follow along with pencil and paper, but I think I understand now.  It's not necessarily that there is no in rush, its just that if you take the average current over 1 cycle, if switching at the +ve V peak, than it (close) to zero?

 

RE: Transformer Inrush

Inrush is not the same as magnetizing current.  If the transformer windings are energized at precisely the right voltage, then normal excitation current begins to flow with no current transient.  


 

RE: Transformer Inrush

(OP)
Sorry been busy since original post, thanks for all the replies.  I will have a look at the links when I get time.  
DPC is correct I want to eliminate (ideally!) but otherwise reduce the transformer inrush current on a island power system.  There may be a case where we need to energise a 30MVA transformer from a 35MVA gas turbine alternator.  I was worried about transient voltage drop out of tolerance on the system.

RE: Transformer Inrush

2
We just implemented Point-On-Wave controls for four GSU transformers using a sophisticated ABB relay that monitors voltage, current, control voltage, ambient temperature of breaker mechanism, time since last operation and other variables that might affect the breaker closing time.  The device monitored A phase voltage, calculated the anticipated closing time and sent the A phase close signal so contact flashover occurred within 1 mSec of voltage peak. B&C phases were closed together a precise time later.

After several operations and adjustments we were able to get inrush currents reduced from 3600+ amps to about 200 amps on 240 MVA, 230 kv transformers.  The transformers were unloaded which made things simpler. The breakers were 350 kV class GIS breakers with single pole operating capability.

We're writing a paper about the results and the problems.

Biggest problem is a small change in timing makes a large change in inrush.  After the breaker is operated 2-3 times, it closes a little faster.  Supposedly, the adaptive relay accounts for this. The relay was making setting changes the same time as the test engineer so it took a while to get the optimum setting installed.

The relay can also be set to open the breaker poles at current zero's.  We did not implement that feature.

Settings are more complicated if there is a load or the transformer bank is three single phase units.


If inrush is a problem - use closing resistors.

RE: Transformer Inrush

rcwilson,

As soon your paper is ready, I would appreciate if you send a copy to me. Thank you!

Regards,

Herivelto  

RE: Transformer Inrush

(OP)
Is there anything suitable out there for MV applications, i.e 15kV?  This ABB POW device seems like it is used with HV transformers where single phase circuit breakers are used.

RE: Transformer Inrush

Well, first of all, your switching device would have to capable of independent single pole closing.  That is going to limit your options drastically.  

I've never heard of this being done at 15 kV.  

I think the pre-insertion resistors may be your best option.   

RE: Transformer Inrush

We looked into the ABB system for a cap bank. They offered  a three phase breaker with the poles staggered (not independent). 121kV was the minimum.

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