reduceing inrush current 5

[furm]

does someone know about "soft start" for 20MVA transformer 33kv.
we want to suppress the current when we connect it with "no load".
thank you

 

[waross]

Two ways at least, this has been discussed before so search this site.
1> The inrush varies and may be up to 25 times FLC depending on the strength and direction of the residual magnetism and the point on the wave that the transformer is energized. There is a circuit which will reduce the residual magnetism when the transformer is de-energized.
2> The transformer may be energized at a reduced voltage through series resistor which are then shorted out.
3> Point on wave switching may be another option.

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

 

[jraef]

I am not aware of anyone that is making a 33kV soft starter, at least not yet. 15kV is the practical limit right now. But for a price, there is someone out there who will build you a one-off if it's that critical.

Solcon is one of those that makes 15kV transformer inrush controllers, you might inquire on them about a 33kV if you are rich and desperate.

http://www.solcon.com/images/stories/Medium_Voltage_Soft_Starters/HRVS-TX_Transofter_Silver-v5.pdf

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[slavag]

What is a reason for reduce in-rush current?
Maloperetaion of protection? trip of generators?

 

[furm]

thank you all.
the reason is the duration of the sag during the inrush

 

[odlanor]

I guess power transformer standard, should support through fault 25In during 3sec.

 

[cranky108]

A few months ago I saw a method used on oil rigs to reduce inrush. They use a pony transformer to heat up the transformer from the low side prior to closing the transformer breaker.
The pony transformer is much smaller, so it has little inrush. And the heating up from the pony is small because of the impedance of the pony.
Because the transformer is magnitized from the pony, closing the transformer breaker also required little or no inrush.

So just remember little pony transformers.

 

[prc]

In a 20 MVA size transformer, the magnetiisng inrush current will be 6-10 times the rated full load current.

To reduce the inrush current, requirements are switch on when input voltage is at peak and reduce the residual flux in core from previous energisation.Residual flux will remain in core for several weeks or months.Reducing working flux density or using better core material grade etc will not change inrush current value.

Point of wave switching is quite common in EHV breakers( 400 Kv and above)-but in medium voltage it is first time that I see a device to limit inrush current.It looks like they are phase shifting to reduce input voltage. Another way is putting resitor in series in the first stage and then by pass contactor is closed to bring full voltage. It looks like that it must work for 33Kv also as vaccum switches are available at that voltage too.

I dont understand how heating the transformer will reduce inrush current.

 

[cranky108]

prc, are you talking about adding a fire to a transformer? Or energizing it? I was talking about energizing it.

 

[DTR2011]

Joslyn has a product for this application and voltage level.

http://www.joslynhivoltage.com/catPointonWave.htm

 

[furm]

craky 108 - thank for your answer but what is "pony transformer"?

 

[jraef]

Interesting on the Joslyn unit. I think its primary purpose is to survive the high duty cycle of an arc furnace, but I see no reason why it wouldn't work for this application. Interesting that they can do it with vacuum contactors, I would not have thought they could be fast enough.

Being that Joslyn was apparently bought by T&B (which I didn't know), who was themselves just bought by ABB, I guess this makes it an ABB product. But since ABB already makes vacuum contactors of their own, I ponder the fate of Joslyn now. Unless of course Joslyn has always been making ABB's vacuum contactors, as they do for many others.

"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
For the best use of Eng-Tips, please click here -> faq731-376

 

[DTR2011]

jraef-

I had recently read about ABB/TB (T&D World?). Add another reason be be an engineer / historian.

On a similar topic, imagine keeping track of the Alstom / Areva / Alstom product line.

 

[cranky108]

A pony transformer is a small transformer, maybe with a much higher impedance. It would be used only for energizing the larger transfofermer. It normally would not do any real work.

 

[electricpete]

I never heard of a pony transformer. Interesting

As a rotating equipment guy, I think of work as a torque acting through an angle. Since transformer does not move, it does no work.

Sorry for being a stickler. I think I know what was meant (pony transformer does not transform any significant amount of power for any significant time?).

My wife would probably say that I don't do any real work.







=====================================
(2B)+(2B)' ?

 

[ScottyUK]

I've also never come across the pony transformer, although I am very familiar with the pony motor. I guess the idea is that the impedance of the pony transformer is sufficient that it limits the magnetising inrush of the large transformer to an acceptable value. Depending on the ratio of sizes I expect that the pony transformer would see something approaching a through-fault and would need to be braced to withstand routine operation in such conditions.


----------------------------------

If we learn from our mistakes I'm getting a great education!

 

[prc]

1) In US with large interconnecting ( 765 or 500 kV) auto-transformers, they sometime use a small transformer, connected to tertiary terminals, to cater to the station lighting.This transformer is normally mounted on the main transformer near to tertiary bushings to avoid any faults on the line connection between the tertiary terminals and small transformer terminals which can result in heavy fault currents in auto transformer.The small transformer will be of high impedance to limit fault currents in case of a fault in secondary lines. They call these as pony ride transformers, probably because these are mounted on main transformer.

2) Point of wave switching is known for many years and popular with EHV systems.

http://www05.abb.com/global/scot/sc...1/$File/ABB B.G. Controlled Switching Ed3.pdf

Now it is being developed for MV systems too. Normally in MV system, transformer inrush current is not taken seriously,probably becuse the system impedance will cause sufficient voltage drop so as to limit the magnitude and duration of inrush current flow.In fact feeding system fault level is also a factor deciding the inrush current value.

3) Dear cranky , no offence was meant.We are from different parts of the world and when you write "pony transformer to heat up the transformer from the low side prior to closing the transformer breaker." I take it literally.We sometime heat the transformer by applying low voltgae to a transformer with shorted secondary.

I am still not clear how pony transformer will reduce inrush.If you energise transformer from HV or LV the inrush value as % will be same, may be slightly more for LV as air core inductance become less then.If you energise through another transformer, the only effect is inrush may come down little due to voltage drop through transformer impedance.It is not going to reduce the residual flux as it depends upon the point on voltgae wave at which the supply is switched off. Even if residual flux is zero at the time of Hv switching, still the inrush current will be there if switched when voltage is zero.Residual flux is adding up to the twice normal flux density that will happen when switched at voltage zero.

 

[GEORGE2311]

My personal opinion is that switching resistors are the best choice as you don't have to worry on oscillations or sympathetic inrushes, etc. What HV side breaker you are using? The most cost effective way is to use the controller (IED) to open and close the breaker in the appropriate time (controlled switching).

George Fotiou

http://www.Engineering-services.gr

 

[rcw retired EE]

A challenge in using a Point-On-Wave controller on Medium Voltage equipment is single phase operation of the contactor or circuit breaker poles is needed to obtain proper timing of the close. A standard breaker with three poles closing simultaneously will always have one pole making contact at an inappropriate point on the voltage wave.

Maybe the poles can be sequenced mechanically.

The ABB POW controllers I have seen provide individual closing signals for each phase. They can also stagger the trip signals to open each phase at the optimum point of the current waveform to minimize contact duty.

The optimum closing phase angle changed based on system conditions- whether the breaker was energizing the transmission line from the station bus or was energizing a station transformer from the line.

The extra complexity of monitoring the voltages, currents and closing times of the breaker poles makes the cost benefit ratio high for MV systems. Resistor closing is a lot simpler.

 

[cranky108]

Why would a resistor reduce inrush any more than a reactor? If that reactor were energizing the transformer from the secondary side, would that make any difference?

Let me make it clear I have never used a pony transformer, or this application. I had heard about this at an IEEE PES meeting just a few months ago.

Working on power systems, I don't haven't had the need to try this. So if you use this, let me know how it works.