Does grounding a wye increase inrush? 1

[BigJohn1]

I had to ground the low-side wye on a 5MVA 13,200Δ:480Y transformer, afterwards we began tripping out on inrush when trying to energize with no secondary load.

Nothing else about the installation changed except bonding the 480V wye to ground.

I don't know what inrush was when the wye was floating, but once grounded we recorded slightly over 300A on the high side, which was tripping the 300A instantaneous protection.

We enabled a cold-load pickup feature that temporarily increases instantaneous to 2X for 30 cycles after closing, and this allowed us to stay online.

Why would a ground reference on a transformer cause this?

 

[beyond86]

No, grounding neutral point on LV side doesn't affect to inrush current.
You can put in service 2 harmonic blocks for your protection.

 

[RRaghunath]

I don't think the tripping has anything to do with 480V winding star point grounding.
What is the basis for Instantaneous Overcurrent protection pickup of 300A?
I see 5MVA transformer full load current at 13.2kV is ~219A. Considering that inrush can be as high as 12 times the full load current (depending on the point on wave at the moment of transformer switching), any pick-up setting lower than 12x could cause mal-tripping.
Check the protection setting calculations.

 

[BigJohn1]

What is the basis for Instantaneous Overcurrent protection pickup of 300A?

I don't know, those are old legacy settings and I'd have to do some digging to discover the origins.

While I agree the protection seems set very tight, the trick is that this transformer repeatedly switched and loaded just fine for years as a floating wye.

Once grounded it would not close in, despite repeated attempts, until the protection was changed.

The protection was undoubtedly seeing something different. So if unrelated to the grounding, then what?

 

[Kiribanda]

5MVA, 13.2-480V when energized will generate an in-rush at least 2000A for 10cycles. So it is obvious that it will trip during the inrush.
The only difference is generating zero seq currents on the high side when the low side star point is grounded.
Was it not tripped on ground fault?
Could you please upload a copy of protection schematic drawing indicating the AS LEFT settings?

 

[RRaghunath]

"...if unrelated to the grounding, then what?"
Some of the points that come to my mind are as below:
Could it be the bus voltage is being maintained > rated in recent days.
Higher terminal voltage at the time of switching on causes higher inrush current.
Is the transformer with On Load Tap Changer? If yes, you could look at the tap position at the time of switching on. A wrong tap can apply over voltage to the transformer though the bus voltage is normal.
Was the transformer switched with load connected at its LV terminals earlier? Load at the time of switching on has damping effect on inrush.

 

[beyond86]

When you energize transformers you have to:
1. Close HV CB.
2. Close LV CB.
So, when you close HV CB, a circuit for flowing zero-sequence current doesn't exist.

 

[Bronzeado]

beyond86!

You are absolutely RIGHT!
Please, forget my answer. I have deleted it.

Cheers,

Herivelto S. Bronzeado
Brasília, Brazil

https://www.linkedin.com/in/heriveltobronzeado/

https://www.researchgate.net/profile/Herivelto_Bronzeado

 

[BigJohn1]

So when you close HV CB, a circuit for flowing zero-sequence current doesn't exist.

Can you expand on this? What condition would allow a zero-sequence circuit to exist? Because that sounds like it might be a plausible answer:

I know from experience that inrush on these substations will trip ground-fault protection. I don't know why, but it's a proven problem across multiple substations with this ungrounded wye setup. Is it in any way possible that same mechanism is at work here, only exacerbated by the bonding?

 

[jghrist]

Zero-sequence current in the relay could occur from mismatched CT ratios or saturation of one phase CT during inrush.

 

[Kiribanda]

Yes, I agree with jghrist. But those zero seq. current components on the high side can exist,
1) only if the low side bkr is already being closed while the transformer is energized by the high side bkr. But per the post it has not happened.
2) only if the high side is WYE grounded.
It is my understanding that in both cases the high side residual GF protection should operate creating "nuisance tripping". But per the post only high side 50 has operated.

 

[davidbeach]

But in this case the high-side is delta.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[Bronzeado]

As generally accepted, when a transformer is switched onto the system, the magnitude and duration of the inrush current depend basically on:

• The point on the voltage wave at which the transformer is energised;
• The amount of residual flux in the transformer core and its polarity with respect to the first half-cycle of the steady state alternating flux;
• The saturation condition or maximum flux density of the ferromagnetic material of the transformer core;
• The total impedance of the circuit through which the inrush current flows.

However, in case of having one or more transformers already connected to the system, the duration and magnitude of the inrush current may change significantly due to "sympathetic interaction" between transformers.

BigJohn1, do you have any voltage and current waveform from those energizing?
Are there others transformers connected on the same busbar?

Cheers,

Herivelto S. Bronzeado
Brasília, Brazil

https://www.linkedin.com/in/heriveltobronzeado/

https://www.researchgate.net/profile/Herivelto_Bronzeado

 

[beyond86]

Can you expand on this? What condition would allow a zero-sequence circuit to exist?

When you close LV CB before closing HV CB. In this case in LV side can flow zero-sequence current when you energize transformers, but even this condition doesn't increase inrush current.

 

[davidbeach]

The OP says nothing else changed, but I have to wonder how accurate that statement is.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[BigJohn1]

Do you have any voltage and current waveform from those energizing?
Are there others transformers connected on the same busbar?

No, I don't have any oscillography.

There are also voltage regulators between the switch and the transformer. These were in manual mode at 4L which is also the normal condition they would be in during past switching operations.

The OP says nothing else changed, but I have to wonder how accurate that statement is.

I'll investigate any suggestions. I was the guy who shut down the sub and installed the bond, so I know no other manual changes were made to the system.

After the first failure to energize, I even lifted the bond and did insulation-resistance testing to be sure we didn't have an unknown fault.