Sympathetic Trip at Grounded Y Delta Power Transformer 2

[TomScott]

I've got a bit of a head scratcher on my hands. Last night I had a ground fault trip on a substation caused by a fault on an adjacent feeder. See Pic below for diagram.

The station that tripped for the ground fault is a medium voltage 14.4kV to 2.1kV station (Grounded Wye Primary / Delta Secondary) that feeds a commercial customer. We are in the process of rebuilding this station and currently the transformers that tripped out were energized but not carrying load (The station it is replacing is still in service carrying the load and is not shown on the diagram). The feeder that experienced the actual fault (C to Ground) is a 14.4kV distribution line that passes by this station. Both are fed from a 115kV to 14.4 KV station a few hundred yards away up line. The fault on the distribution feeder occurred approximately a mile past the 2.1kV station. We did see substantial voltage sag on the bus however since the 2.1kV station isn't connected to load yet, I think I can rule out voltage drop induced sympathetic tripping.

Both ground relays at the 2.1kV station saw the fault current on the ground so I'm pretty sure it's not a fluke. I seem to recall reading about Wye/Delta transformers being sources of 0 and negative sequence currents but I think it only involved faults down line of the transformer in question.

Any help explaining why this possibly happened would be appreciated.

Tom

 

[davidbeach]

Those 14.4/2.1 transformers functioned as grounding banks and sourced enough zero sequence current to cause the trip.

The grounded wye side will source ground current even with nothing connected to the delta.

 

[dpc]

What David said. Been there.

 

[protoslash]

A ground fault at C will cause the red circled breakers to see zero sequence current in the reserve direction if your 14.4/2.1kV TX is Yg delta connected.(regardless of loading)

to avoid sympathetic tripping, you need to enable directional element on those relays. Voltage is not the issue here.

This effect can be explained with symmetrical component when you draw out the zero sequence network. Even better, get a power system modelling software and run the fault flow.

Intuitively, i don't have a good explanation. Think of the ground fault current at C needs to flow back to the source in multiple path and the Yg delta TX provides a path. When the fault happens, the fault current flow from source to breaker 4 to C to grounded then back up YgD tx to breaker 2 back to source. (someone correct me if im wrong here)

 

[waross]

That happened a lot when 2400 V delta distribution circuits were upgraded to 4160 V wye. Old texts mntion that a ground fault on a distribution circuit could result in blown fuses throughout the circuit.
That is one reason that grounded wye delta banks fell out of favour except as a grounding transformer.


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

 

[stevenal]

If the neutral is brought out and fully insulated, the simple solution would be to disconnect H0 from ground. Better 2.1 kV voltage balance, and no circulating delta current with its losses and capacity reduction.

 

[TomScott]

Thank you all, I've been reading all weekend and now see the issue. We are investigating a number of possibilities to compensate.

1. Supervise the ground trip with a directional element.
2. Float the high side neutral separate from ground.

I'm leaning towards floating the neutral for simplicity sake but I'm curious if there are any drawbacks to this? The main two I am aware of are ferroresonance if the transformer is run with an open phase and neutral shift on the ungrounded wye during certain fault conditions. Neither of these should be an issue, as we will not be single pole tripping for any faults detected and we have arrestor installed on the high side to prevent over voltage conditions. In addition, we could also enable over/under voltage tripping via digital relays as well. As always, I welcome any comments.

Tom

 

[DM61850]

Protolash,

A simpler solution would be to just remove the ground connection on the second set of wye-ground-delta transformers. I don't understand why they were ever grounded.

 

[TomScott]

Do you mean why did we ground the neutral?

We normally have the grounded wye on the load side of the transformer so the grounded neutral gives us a good source of zero sequence current for tripping. We mainly serve residential and light commercial so we really aren't that familiar with heavy industrial set ups like this. This is the only installation we've got with the grounded wye on the source side. I investigated other options for the feeding the load but their facility is quite old and I wanted to avoid increasing their available fault current so I went back with the same transformer type they currently have. The only thing that has changed is our protection scheme. The station we are replacing is protected with a 3 phase recloser with old form 1 controls and I think it has ground trip blocked, probably for this very reason. Our new equipment is all digital relays so we are now seeing an issue that has been around for a long time.

Tom

 

[DM61850]

Your generator transformer is acting like a ground source. Why do you need two other ground sources?

 

[waross]

The system is somewhat self rectifying when fuses are used for primary protection.
The fault current blows a fuse on one phase and the transformer bank happily continues as an open delta.
In a country where the grounded wye/delta is common, it is also common to see one fused cutout hanging down at each wye/delta transformer bank.

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

 

[RRaghunath]

Grounding of high side neutral in 14.4/2.1kV transformers is intended to improve the zero sequence impedance of 14.4kV system overall. This is required as there are overhead lines which pose a challenge in detecting single line to ground faults (due to high earth loop impedance and resultant low fault currents).
I have seen power systems without earth fault protection on Y-side of Yd transformers (with star point of Y side grounded). Think this is preferable if earth fault protection cannot be made directional (looking towards transformer).

 

[stevenal]

Lifting the wye ground on a load transformer will not alter zero sequence source impedance or line to ground fault current. The 115/14.4 transformer should remain wye grounded.

 

[waross]

The line to ground fault with the neutral grounded will be increased by the current supplied by the healthy phases and back-fed by the delta winding of the 14.4kV/1.2kV transformer.
It is this back-fed current which caused the ground fault trip.
In the absence of ground fault monitoring the back-fed current will often blow the a supply fuse on wye/delta banks remote from the fault location.
This will not appear as extra ground fault current at the 115kV/14.4kV supply transformer.
It will show up as added current on the healthy phases at the supply end.
The magnitude of the added fault current will be limited by the line impedance and the impedance characteristics of the wye/delta transformer.
The added current may or may not be significant.
It will often be greater than the holding ability of the primary fuses of the 14.4kV/2.1kV transformer.

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