What are the load limits of reverse feeding/loading power transformers 1

[gentech1975]

I read an article in a Ferranti-Packard publication years ago, of which I seem to have lost.
Can someone remind me of the limits of reverse feeding/loading power transformers? e.g.
600 VAC to 208 VAC stepdown; change to reversing it.
I read there are limits to what can be loaded in reverse on the xmfer such as you cannot load it to 100% in reverse due the original design. Thanks.

 

[jghrist]

The loading limits are the same regardless of the direction of power flow.

 

[tulum]

Inrush current tends to be higher when a power transformer is reverse fed.

Regards,
TULUM

 

[ScottyUK]

Hi Tulum,

Hadn't heard that before - any idea why? I can imagine that the numerical value of inrush current will be higher on the LV side than on the HV side but I would expect the inrush to have a similar P.U. value regardless of which side was energised. I suppose there could be a significantly different leakage reactance on one winding, due perhaps to a discrete tapping winding? Now I'm curious!


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Sometimes I only open my mouth to swap feet...

 

[stevenal]

I don't think the pu current is higher, but the pu voltage dip can be higher due to the higher source impedance on most lower voltage systems. Back fed a 14 MVA for test and dipped the lights all over town. No complaints when we energize it normally.

 

[JBinCA]

I've never heard anything about a difference in pu magnitudes of inrush current associated with whether the HV or LV winding is energized.

The magnitude of inrush upon energization varies widely each time the XFMR is energized, however. It is dependent upon the point in the cycle during energization, residual magnetism, point in cycle when XFMR was de-energized, etc.

Based upon the above-mentioned factors, there is no basis for comparing subsequent energizations unless these and other fators are controlled.

Regards,

JB

 

[RalphChristie]

Unfortunately I can not answer the original poster's question.

Regarding inrush, it seems to have an effect if been energized from the HV-side or from the LV-side. (like tulum said)

From an old ASEA manual: (Transformer differential relay, type RADSE)

Trsf connection, Trsf size, Instantaneous setting if energized from HV side, Instantaneous setting if energized from LV side.

- , <10MVA, x20, x20
Yy, 10-100MVA, x13, x13
Yy, >100MVA, x8, x8
Yd, - , x13, x13
Dy, <100MVA, x13, x20
Dy, >100MVA, x8, x13

From the table quoted above it seems as if a Delta-star connected transformer has higher p.u. inrush-currents if energized from the LV-side.

If I remembered correctly it was also discussed on eng-tips, but a few years ago. I think (someone might just correct me if I am wrong) it has something to do with the positioning of the windings, eg. HV-winding inside and LV-winding outside. I'll check if I can find the thread again.

Regards
Ralph

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[RalphChristie]

Ok, found it:
thread238-101496

Check cuky2000's response: his explanation seems to be reasonable

Regards
Ralph

[red]Failure seldom stops us, it is the fear for failure that stops us - Jack Lemmon[/red]

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Read the Site Policies at FAQ731-376

 

[ScottyUK]

Hi Ralph,

That sounds like an interesting thread.

The winding position would have some influence on leakage reactance, and the LV winding is usually nearest the core so it would have lower leakage. It would make sense that the inrush would be higher if there was less leakage reactance to limit the current.


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Sometimes I only open my mouth to swap feet...

 

[prc]

To the first question of any load limits for reverse power flow- Normally no restriction.But there are cases where on-load tap-changer may limit the loading limit.OLTC based on assymetrical pennant cycle operation are not suitable for bidirectional power flow and in such cases reverse power flow has to be severely limited.Such tap changers are rarely used nowadays.

To second question of excitation inrush current -this will change depending on the winding excited.If inner winding( winding near to core ) is excited inrush current will be high.This is due to lower air core inductance of inner winding.Air core inductance is directly proportional to winding diameter.Inrush current will also change with connection of windings, whether three phase or single phase,and whether auto-connected or two winding.Eg Y/y connected 3 phase unit will have only 60 % inrush current compared to a D/d connected transformer,both excited from HV side.

 

[dcdtn]

The low side winding may not be rated to handle full load PLUS transformer losses.