Switching or turning on a 3000 kVA transformer with a soft starter (SS) - to reduce inrush current 2

[Spgema]

Hi Participants out there,
I have an interesting problem to solve , as you all know that switching on an unloaded txf can see 5 to 10 time its rated current, depending on any remnant magnetism , and point of voltage at instant of turning on.
I intend to use soft starter, and ramp the voltage up within say 10 to 20 second, starting with 0 volt to 100 %, essentially means every second, voltage is increse by 10% or 5% rated voltage depending on my ramp periods of 10 or 20 seconds.
In this way,I believe I will not have trip the main supply upstream, which is the utilities company who can fine me.
Any help appreciated.

John

 

[itsmoked]

Do a search in this forum for transformer inrush. Lots of good discussions.

You may find unloaded inrush to be considerably more than 10X. 20X is a possibility. It's fast so reducing it may not need you to take 20 seconds but only a couple will do it.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[che12345]

Dear Mr. Spgema

Q. " I intend to use soft starter, and ramp the voltage ....."
A. Have you considered the ( [cost] of the soft starter unit) with suitable voltage and current ratings for a 3 MVA transformer? Consider that you would need [one soft starter per transformer] and use it only during switching on, and then [by-pass] it by the soft starter internally or external, during loading.

It would be impossible to use [only one soft starter unit] and move it around for numerous transformers. A soft starter suitable for switching 3 MVA transformer would [not be portable]. It would not be practical to move around [a ] single soft starter unit to be connected to the switching on transformer, disconnecting it and move on to the next transformer. The soft starter unit and the labour cost would prove the intention to be impractical.

Che Kuan Yau (Singapore)

 

[davidbeach]

How does the soft starter do its soft starting? Probably by part wave conduction of the full voltage. Could that mean you actually get the inrush over and over again?

 

[cuky2000]

Not sure if a soft starter is a practical and economical application to mitigate the inrush current on power transformers. If you should provide better details of the operation mode and equipment, perhaps someone could provide a bit of technical advice.
Below is an example comparing different techniques for inrush mitigation of MV Distribution transformer.
>>>>>>

 

[jraef]

I have used MV Soft Starters for transformer inrush mitigation. It's an expensive option, but in the times I have been involved, it was the only viable solution. "Ramp" time for us was only 2 seconds typically, so it's basically almost a stepped response; an initial voltage setting, ramping to full in 2 seconds, then closing the bypass contactor. 10-20 seconds is unnecessary, there is no "load" that needs accelerating, all you are doing is magnetizing the windings.

That said, when I did this it was now almost 20 years ago. Looking at cuky2000's post and seeing "CSD", I looked it up (Controlled Switching Device). That looks a LOT simpler to me. As far as I know that technology didn't exist yet or if it did, nobody in my circles had heard of it. That to me looks MUCH more viable than a Soft Starter, especially if you already have vacuum circuit breakers involved. In fact even if you don't, it may be less expensive to add a vacuum contactor and a CSD controller than a soft starter.


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

 

[cuky2000]

Jraef,

We had been used electronic switching synchronous control for more than a decade for HV application with IPO circuit breaker.
This technology is migrating to MV application including 3 pole gang operated breakers.

Breakers manufacturers including GE, ABB, Siemens and others already have the electronic Synchronous control device and some of them have MV breaker suitable for inrush mitigation.
One interesting device for switching control is available to be implemented with circuit breakers of the owner preference and even could fit in an existing installation. Detail could be explored in the following link:

https://www.vizimax.com/products-services/controlled-switching

 

[Spgema]

Thank you all for replies, and insight provided.
I do not have the budget for CSD, and also this is LV switching unlike HV and MV with more budget. I will adopt an economical yet difficult method as I am switching on LV Breakers which do not have CSD functionality found in HV and MV.
What I propose to do is
1) to demagnetise the transformer,
2) use a SS, and ramp the voltage up in say 2 seconds, as indeed it's true that the inrush will be some 6 to 10 times;
3) my concern is still the initial voltage setting ( as also mentioned by Jraef ) %, any comment if I use 30% of rated voltage, so at the instance of switching ( time 0+), the inrush would be 1.8 X to 3 X of transformer rated current if on 100% voltage setting,
4) as the inrush last from 6 to 20 cycles ( 120 to 400 ms) depending on transformer X/R ratio, I would set the switching to full voltage at 500 ms,
5) the other problem I had to contend with is what davidbeach said below :-
* How does the soft starter do its soft starting? Probably by part wave conduction of the full voltage. Could that mean you actually get the inrush over and over again?* Anyone can help on this ??.

John

 

[itsmoked]

Why not just use some resistors and a contactor? Figure out your maximum allowable starting hit for your network and size up three resistors to to limit current to that if there was no transformer. Then close on them into the transformer and a couple of seconds later bypass them.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[LionelHutz]

What is the voltage? Trying to figure out what LV is to you.

A soft-starter could work. You could possibly save some money by using one rated for less than the TX rated current as well.

You simply start at as low a voltage as possible and ramp to full-voltage.

The answer to the question davidbeach posted is no, the inrush won't happen every cycle.

 

[MacCB]

Hi Spgema,
Yes, the SS works with anti parallel thyristors in each phase, and by that control the URMS value. Usually the SS applications are for drives, where you want to limit the torque (T=f(Usquared) at starting to protect the working machine. The disadvantage is the longer starting time Energy = f(Isquared x R) which will heat cable a.o. in the circuit. You also create harmonics in your network!!
If possible, leave the load on secondary side of the TRF connected as the Isec will provide a reaction in the core and damp the inrush current considerably.

I would recommend you to use the a.m. proposed solution with the inrush impedance or resistans(the better) and shorten this resistor by a contactor or breaker after T=time(simple), or I<Imax(more technical details.
Good luck, have fun
BR MacCB

 

[jraef]

Similar to what Lionel said, you have a 3000kVA transformer with a LOW voltage primary, as in 690V or less? That seems extreme...

The only cautionary issue if it is indeed a LV primary is to avoid using cheap soft starters that only put thyristors on 2 of the 3 phases. That sort of works for motors but will not give you the full benefits you need for this application. Also, get one with a bypass contactor.

Initial voltage should be under 10%, ramp time 2 seconds or less.


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

 

[Spgema]

Thank you all, and before I close this matter, like to share a different case of switching on a much larger transformer rated 20 MVA, 22 kV primary voltage with secondary open circuited, as follows:-

Afrer demagnetize the 20 MVA transformer,the inrush current was in the range less than 350%, instead of suspected 600% to 1000 %, thus no tripping and no adverse volatge dips experienced by the grid. We just turn on the MV circuit breaker !!

Now I will use demagnetization for this 3000 kVA 400 V transformer, and may not use SS, in order to cut down my budget.
With demagnetization carried out, and if SS is still used, I would prefer to set my voltage still at 30% of rated voltage, so at the instance of switching ( time 0+), the inrush would be around 105% of transformer rated current if the demag done is as effective as that done on 20MVA txf ( which give 350% Inrush on 100% voltage switching)
Like to know from LionelHutz how he conclude that a lower rated/smaller SS could do the job ?- I fear that if I use a half rated SS ( ie use a rating of 1500 kVA), then if no demag is carried out, and yet set at 30% voltage starting, the Inrush would be some 360% to 600%, and that would trip the internal protection feature built in SS, won't it ?? ; if so, is it possible to set the trip level much higher ??. Can Lionel provide/point to sources that indicate no inrush on every cycle with the pulse switching of SS
5) To MAcCB & Itsmoked: I could not use series resistor and bypass them as budget is limited. SS I hope can work as it's a luxury too, and may be a waste if it cannot do its work without demag. On the other hand, if demag work as good as my 20 MVA txf, I may decide to do same on this 3000 kVA txf without the use of SS.

 

[cuky2000]

Caution is recommended to determine the inrush current since is a random event that is not predictable and or repeatable.
The main causes of inrush current depend on the following variables:
a) Instant in went the voltage is switched on.
b) The magnitude of the voltage.
c) Level of residual flux in the transformer.
For reference, check-in the following Link

 

[itsmoked]

Spgema; How are you carrying out the demag process? Isn't that cumbersome and... tricky?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[LionelHutz]

I didn't say to undersize and start at 30% voltage. That 30% is something you made up that frankly makes no sense. Since a heavy duty rated soft-starter can handle 600% current for 30 seconds, it's quite easy to make it work with an undersized starter.

I have no idea where you get a soft-starter that will trip instantly if the current exceeds that value range you listed values. Must be some poor rated low end thing that can't handle decent motor starting currents.

A motor has a similar magnetizing current inrush as a transformer. The soft-starter works great to greatly eliminate it in a motor. It'll work just as good on a transformer. I have no sources, and I'm not going to try and find any.

3MVA @ 400V is 4300A give or take. Not sure where you expect to source a soft-starter that size without under sizing it or how a soft-starter that size would be within budget but using simple resistors is not.

 

[waross]

Hi Lionel I wondered that. Possibly he is adding the cost of an additional contactor to the price of the resistors.
But then, resistors plus a bypass contactor or a soft start with a bypass contactor.
Back to your qualms.

how a soft-starter that size would be within budget but using simple resistors is not.

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