Energizing Power Transformer 1

[enggines]

Hi All!

What is the correct or better way of energizing a 15/20 Mva, ONAN/ONAF, 69-13.8 KV, DYn1 Power Transformer serving at least feeders? Without Load or at least one (1) feeder is initially loaded? My concern is the inrush current. One guy advice me to energize with load to damp the inrush while another is saying, should be without load to minimize. Which is which.

Any comments or alternative method would be highly appreciated.

Thanks.

 

[DanDel]

I've heard several people in the field propose that one small circuit left connected to the secondary will decrease primary inrush current upon energization, but I haven't been able to find the reasoning to support the proposal. Circuit analysis of a transformer would indicate the opposite, as any secondary load as seen from the primary would serve to decrease the primary impedance.
Anyone else have any comments?

 

[SidiropoulosM]

An impedance Z connected to the transformer secondary will reflect as nnZ on the primary, where n is the turns ratio. The magnetizing inrush current is therefore reduced with the load connected. Michael Sidiropoulos

 

[DanDel]

SidiropoulosM, then the larger the impedance on the secondary, the larger the reflected primary impedance, and the lower the inrush current. The largest secondary impedance would be no connected load at all.

 

[GordS]

From the primary side, the electrical circuit representing a transformer has the magnetizing branch in parrallel with the secodary load reflected to the primary by the ideal turns ratio. Energizing an umloaded transformer means you just supply current only to the magnetizing branch. Energizing a loaded transformer means you supply current to both branches. Therefore, energizing a transformer with a load will result in greater inrush.

What could change, I think, is the duration of the inrush current. The magnetizing branch is highly inductive so its time constant (L/R) will be large. Adding some resistance to the ciruit via secondary load should decrease the time constant.

 

[SidiropoulosM]

DanDel, thanks for the observation, I believe you're right. On second thought, here is something to consider: When the transformer is energized under no load, only magnetizing current flows on the primary circuit and the resulting voltage drop across the transformer leakage reactance is negligible. The magnetizing branch therefore sees the entire source voltage. When the load is connected during energization, the total load current flows on the primary circuit and the voltage drop across the leakage reactance is significant. This reduces the voltage seen by the magnetizing branch and the resulting inrush current is smaller. Any other ideas would be appreciated.
Michael Sidiropoulos

 

[1x1x1x]

I agree with GordS, my experiance is that the duration of the inrush current will be effected by the ressitive components of the load.

 

[jbartos]

Suggestion: It is better to analyze the circuit from the transient standpoint, e.g.
v(t)=L x di(t)/dt
Now, if a resistor is in parallel with the inductance L then the voltage v(t) cannot increase as much since the current will be flowing in the resistor in parallel thus reducing the inrush. Check the intent of MOV, back to back Zener diodes, snubber circuits, etc.

 

[DanDel]

jbartos, it seems to me for the purposes of this question that v(t) and L are the constants and di(t)/dt is the quantity in question.
And I don't believe you can add a resistor in parallel with the inductor(L, in Henrys) in the formula as stated, since there are no values of impedance present.

 

[enggines]

My experience is that when a Power Transformer is being energize without load, humming or rattling or whatever sound (i mean, the usual or normal one) is more louder as compared to a lightly loaded unit. But like DanDel said, I haven't had the technical justification.

Perhaps, somebody out there could just share their method (which may not be perfect, but at least, its what they practice).

Thanks for all the post.

 

[Andiri]

While on the topic of inrush currents on power transformers. We have (2X) 40MVA, 66kV/30kV transformers in tandem. When re-energising one of the trf's from the primary side while the other is still in circuit we experience a heavy "dip" (probably inrush related volt drop). If the same trf is energised from the secondary first, no "dip" is experienced. The same does not apply for the other trf though.
Any comments?

 

[DanDel]

Adar, what do you mean by 'in tandem'? Are they connected in parallel on the primary and secondary also?
When you say you experience a 'heavy dip', does the voltage actually drop(primary and/or secondary?), or is this mostly associated with a change in the operating noise of the running transformer?
You say that if this transformer is energized from the secondary, no dip is experienced, but the same does not apply for the other transformer. Does this mean that there is no dip from the other transformer, or that there is a dip if it is energized from either the prinary or secondary?
What is connected(loads and sources) on the primary and secondary?

 

[Andiri]

DanDel
-They are paralleled primary and secondary (buscouplers on pri and sec).
-Secondary voltage drops enough for motor drive contactors to "drop out" at plants 7km and 32km away.
-My mistake, further enquiry reveals that the other trf does the same when re-energised.
-Source: 66kV via 47km of o/head delta line.
-Load: 25MVA worth of ore treatment plants (mostly DOL drives) and 5MVA of residential township.

 

[DanDel]

Adar, let me understand you: If one transformer is energized and supplying load, and the other is de-energized and has both primary and secondary connections open, and you close the primary device with the secondary device still open, you get a severe voltage drop on the secondary, but if you close the secondary device with the primary device open, you don't get the voltage drop. Is this correct?

 

[Andiri]

DanDel, yes that's the way it happens.

 

[electricpete]

The question of why a loaded transformer has lower inrush:

I think Gord and Sirido had the right answer. The inrush is entirely dependent upon the dc component of flux. If we include in our model a primary reactance in series with a parallel magnetizing reactance and load resistance, it can be shown the transient dc current in the magnetizing branch will decay faster as load resistance decreases.

It is a little bit of a cheat because the whole idea of the inrush depends upon the non-linearity of the magnetizing branch (saturation in presence of dc), but it's the basic idea I think.

Now on to Adar's question.
The magnetizing impedance is primarily inductive. It goes with N^2 (turns ratio squared). Primary impedance is factor of N^2 higher than the primary. So in steady state the magnetizing inductance of primary winding (with secondary open circuited) would be "equivalent" to magnetizing inductance of the secondary winding with primary open circuited.... so far no answers.

Now... look at the resistance. Not important in steady state but very important to inrush since it is what allows the dc to decay.

The resistance goes with N.
Remembering inductance goes with N^2....
we conclude the L/R ratio is factor of N higher on the primary... dc decays N times slower. Inrush lasts much longer. You probably had the same magnitude of inrush when energizing from the secondary only it didn't last as long.

Hey I found this fantastic link on the subject of transformer inrush. This guy must really know what he's talking about. (you'll get the joke if you look at it).

http://geocities.com/pschimpf/temporary_stuff/transformer_inrush_simulation.htm

 

[electricpete]

I'm not 100% sure of the explanation provided in my last post but it sounds reasonable to me. To add some credibility, we have large generator stepup transformers that have inrush on the order of 30 seconds. I'm not sure but I think the reason is tremendously high L/R due to transformer construction.

 

[neps3]

Hi all

GordS's explanation is correct. The irush is not dependent on the secondary impedance. Dandel is correct. The inrush is just magnetising current 12 - 15 times the FLC sustaining for about 300ms. Transformers are always switched on the high voltage side. Particularly interconnecting transformers in switchyards are switched on the higher voltage side becaiuse of the inrush.

 

[electricpete]

Hi kantor - Hard to follow exactly what you're trying to say.

"The inrush is not dependent on secondary impedance."

"The inrush is just magnetising current 12 - 15 times the FLC sustaining for about 300ms."

Don't you think the duration can vary based on the following?:
transformer L/R
power system L/R
connected load (effect on decay of dc component)

Another question:
"Transformers are always switched on the high voltage side....becaiuse of the inrush."

My discussion above would suggest that the magnitude of the inrush current (referenced to hi-side) would be the same when energized from either hi or lo side.

In general the impact of that similar-magnitude in-rush current on low-voltage system would be less on when energized from the high side (because current doesn't have to flow from hi-side through impedance of a parallel transformer to get to low-side of the switched transformer), which seems like a good reason to energize from the hi side.

But I think (may be wrong.... looking for comments) that the duration of the inrush is longer when energized from the hi side based on L/R considerations described above. This would be a possible explanation for Adar's scenario, particularly if there is high supply system impedance (low fault levels) on his high voltage system which help to create the voltage drop. (I realize the total impedance from supply to low side is always more, but the duration would be the key). What do you think?

 

[electricpete]

One more comment on Adar's scenario along a completely different line:
It is possible that the per unit voltage at transformer terminals is less when energized from low side than from hi side. One reason would be a pre-existing steady state reactive power flow through the parallel transformers, which depresses the per unit voltage of low side compared to hi side. Another reason might be different tap setting of the parallel transformers compared to the switched transformers, although we rule this out if it happens to both transformers.

Lower per unit voltage at transformer terminals when switched from low-side means lower dc component, less saturation, less inrush.

We might also consider the voltage drop associated with the inrush current itself as it travels through parallel transformer in establishing lower voltage at transformer terminals, but I'm not sure if that would lead me in a circle.

I think the dramatic L/R difference between hi side and low side and it's effect on duration of the inrush would likely be more significant than slight difference in per-unit voltage at terminals when comparing switching from hi and lo sides.