Inrush current of transformer

[Kanu_01]

Should I consider the impact of inrush current while doing settings for the secondary side of the transformer breaker?

 

[waross]

No.

 

[crshears]

Not unless you're energizing from the low tension side, then loading on the high tension side . . . which I've seen done as a routine practice often enough to not flat-out dismiss the possibility.

 

[Kanu_01]

No.

Thank you!

I am sorry to ask again for clarity. Is it because the primary side of t/f will draw the inrush current to develop the magnetic flux? That’s why impact will be seen majorly on the primary side, not on secondary.

 

[waross]

Using the convention that the primary side of the transformer is the supply side, the secondary will not see the transformer inrush. The secondary will see any inrush caused by loads that are connected when the transformer is energized.

 

[crshears]

waross: The secondary will see any inrush caused by loads that are connected when the transformer is energized.

Um . . . what am I missing? Typo?

I was referring to the "conventional" configuration in my response, and specifically intended to say "placing on potential with the secondary [ LT ] breaker, and loading on the HT side by some appropriate means."

I have also seen GSU Txs direct connected to the generator's "isolated phase bus" without LT breaker, where the trafo is slowly brought to operating potential along with its associated generator . . . but I do not understand that scenario to be in play in the situation being described.

 

[waross]

If a motor or another transformer is connected to the transformer secondary, the inrush of that equipment will be seen by the transformer secondary.
Consider a distribution circuit being re-energized after an outage. All of those refrigerator and furnace fan motors trying to start at once.

 

[crshears]

Ah, that's what you meant; difference of terminology, I guess.

I picked up many an interrupted feeder over the course of my career, but it was never part of our jargon, ever, to refer to feeder cold load pick-up current as supplied by a trafo's secondary windings as any kind of "inrush;" that was reserved specifically for transformer energization and the harmonics etc. that are generated when a trafo is placed on potential.

 

[waross]

Terminology;
For over 60 years I knew the primarily energized side of a transformer as the primary.
It is only in the last 5 years or so that I have heard the hi-voltage side referred to as the primary, regardless of which side is energized.
Yes, there may be some confusion when a transformer is back fed but I would expect to see the SLD show the energized side as the primary if it was specified.
I want to know which side of a transformer is energized. Calling the high voltage side adds absolutely no information.

CR, Yes, I would tent to think of that load as a surge or temporary overload rather than a secondary inrush.
However the OP questioned breaker settings on a secondary breaker.
I hoped that my suggestions would be helpful when deciding the settings.

 

[davidbeach]

I picked up many an interrupted feeder over the course of my career, but it was never part of our jargon, ever, to refer to feeder cold load pick-up current as supplied by a trafo's secondary windings as any kind of "inrush;" that was reserved specifically for transformer energization and the harmonics etc. that are generated when a trafo is placed on potential.

Did you ever look at a raw event capture of that? You don't see the load initially, all you see is all the transformers being energized (aka inrush). Yeah, there's a bunch more load to carry for the first bit than there was before the circuit tripped, but most of what a cold load setting really does is keep you from tripping on the transformer inrush. I see that getting above the instantaneous setting, but I rarely see true load getting above the instantaneous setting.

 

[crshears]

davidbeach: Did you ever look at a raw event capture of that? You don't see the load initially, all you see is all the transformers being energized (aka inrush).

Yeah, saw traces several times over the course of my career.

My utility's practice predominantly favoured placing transformers on potential using high side disconnect switches designed for the purpose; the charging currents tended to be mitigated by the resistance in air of the arcs when they first struck.

davidbeach: Yeah, there's a bunch more load to carry for the first bit than there was before the circuit tripped, but most of what a cold load setting really does is keep you from tripping on the transformer inrush. I see that getting above the instantaneous setting, but I rarely see true load getting above the instantaneous setting.

Often enough, when an LT bus lost its supply due to HV circuit lightning strikes, it was routine practice to attempt to re-load a bus using a transformer secondary breaker without opening all of the downstream feeder breakers [ often as many as six of them ]. It was not uncommon to successfully restore ~100 MVA of interrupted load that way.

Loss of load diversity due to outages of extended length, on the other hand, could cause feeder pickup currents of as much as eight times the normal load, necessitating cold load pick-up in stages by breaking the feeder into sections so as to preclude tripping the feeder breaker on high set instantaneous.