Inrush Current for Multiple Power Transformers in Series?

[doehl1]

I have an application where a generating station is providing power to a regional load via two power transformers.
Generation is at 400V, steps to 24kV at Trafo1, and then 600m later steps down to 400V at Trafo2. Both transformers are 2000kVA w/ impedance of 6.43%.

Assuming no cold load pickup, and only inrush/magnetizing current, is there a reliable way to estimate the inrush current out of the generating station to BOTH transformers in series on a single feeder line?

Or does the 10-15xIn rule hold for a single unit? Does Trafo1 act as a buffer against magnetizing currents seen at the generating station from Trafo2?

David

http://www.mavenpower.com

 

[prc]

This is an occasional dilemma faced by engineers.Can a 50 MVA 33/220 kV transformer be used for back charging a 250 MVA 220/11 kV GSU? Will the added up total inrush current blast away the 50 MVA unit?Though, I dont have fool proof evidence from site measurements or calculations,I believe the inrush current when the units are in series will increase only marginally. The inrush drawn by second unit will cause a voltage drop through first unit so as to bringdown the inrush current by second unit. This was the conclusion as per some tests on small, distribution transformers.If, some one, can do experiments and report,it will be interesting.

 

[Bronzeado]

Doehl1 and prc,

The attached doc may help you in this matter.

Best Regards,

Herivelto S. Bronzeado
Brasília, Brazil

http://www.linkedin.com/profile/view?id=46319837&trk=tab_pro

 

[prc]

Thank you Bronzeado for that part from CIGRE brochure.Let me put my doubts a bit more detail for better understanding.

Case.1 - 50 MVA energised fronm 33kv and the inrush current died out.This inrush current peak may be 10If on 50 MVA base. Then we are switching 250 MVA using 220 kV out put from 50 MVA. The inrush may be 5xIf (25xIf on 50 MVA base) 25If may be quite severe loading on 50MVA if it so happen. I dont think transformer will withstand this. But my point is 25 If may not happen as the voltage drop through 50 MVA may pull down the 220 KV voltage so much that inrush may be less.

Case 2- Both transformers are connected in series and energisation done at 33kV of 50 MVA. What will be the current intake? Will it be 35xIf ? or say 12 xIf ?

Which procedure will be safer from the overloading aspect of 50 MVA.

 

[Bronzeado]

prc,

First of all, remember that transformer inrush is a non-linear phenomenon so we can not think linearly on it!

Regarding your cases I would say:

Case 1) The transformer being energized in "series" is seen by the already energized transformer as a "load" or a "dying away short-circuit". So the 50 MVA transformer should be design to withstand it.

Case 2) It will be difficult to guess the magnitude of these currents as the phenomenon is non-linear. To calculate them is necessary to use good saturated transformer models in a EMTP like software.

The best procedure from the point of view of reducing the overloading of 50 MVA transformer should be the use of controlled switching strategy to energize the "series" transformer.

Best Regards,

Herivelto S. Bronzeado
Brasília, Brazil

http://www.linkedin.com/profile/view?id=46319837&trk=tab_pro

 

[jghrist]

The CIGRE cases are unlike the OP case in that the second transformer is energized after the first transformer is already energized. The switch is between the two transformers. In the OP case, both transformers are energized at once from the generator.

 

[prc]

Bronzeado,I also used to advise to go for controlled switching for such out of turn exercise.But the problem is controlled switching provision will not be there for such voltage ranges. These transformers are already there and one cannot design to suit such overloads. It is almost impossible to make 50 MVA transformer to stand 25 If. It has to be actually a 100 or 200 MVA active part.Some guys,even after my cautioning had gone ahead and came out unscathed.May be they are lucky as the mentioned maximum inrush will occur only when the contacts are made at the point when the voltage sine wave passes through zero.

It will be great help if some learned engineers like Bronzeado undertake a study and publish guidelines. At least which is more severe? Case 1 or 2? My view is when 50 MVA see 25 If, the voltage should dip instantly and the load will drop and 50 MVA may not experience such over currents.

 

[Bronzeado]

jghrist,

In the case both transformers being energized at once from the generator, I would recommend the ideal case of using a generator voltage ramp (if possible) to energize them. So, there will be no inrush currents!

prc,

Controlled switching is "only" a smart time relay that sends a order to a circuit-breaker in the right/calculated instant. So, I think a relay adaptation can be done to provision controlled switching for such voltage ranges.
One restriction is that the operating time of the circuit-breaker should be quite repetitive.

As I said, we cannot think linearly for transformer inrush phenomenon. It is complex non-linear phenomenon that depends on many variables.
Your point about the voltage drop through the 50 MVA is correct. It "will pull down the 220 KV voltage so much that inrush may be less".

I think it is a good idea to write something on this matter.

Best Regards,

Herivelto S. Bronzeado
Brasília, Brazil

http://www.linkedin.com/profile/view?id=46319837&trk=tab_pro