Delta system short circuit

[james001]

I have a delta wye transformer with generators connected on the wye side. I'm wondering if there is a single line to ground fault on the delta side (assuming there is no other source on the delta side), how do I calculate the fault contribution from the wye side. I know there is no current flow on the delta side except charging current. Also instead of installing the 59G relay on the delta side, can I monitor the current on the wye side and trip off the breaker for SLG fault on the delta side

thanks a bunch

jt

 

[dpc]

If the delta side is not grounded anywhere else, the ground fault current will be vanishingly small - it must flow through the capacitive coupling that naturally exists. There would be no appreciable contribution from the delta winding.

There would be no ground current flowing on the wye side of the transformer, so nothing to detect.

Current on each side of the transformer must have its own complete path to flow. And due to the transformer action, for any significant current to flow in one winding, it must have a path on the other side of the transformer.

 

[busbar]

I agree with dpc — there will be very little change in zero-sequence current on the delta side, so no appreciable change on the wye side, giving no reliable way to detect it. {Normal load characteristics will mask the ground fault.}

Within limitations, your best bet may be to install IEEE C57.13 “Group 2 — Figure 6d” VTs on the delta winding, with a 59G (and loading resistor) on a broken-delta secondary configration.

 

[james001]

Thanks for the answer.

The reason I ask that is because everything is set and done and there is an ungrounded section (13.8kV switchgear) between the GSU transformer (delta connected) and the generator breaker, which does not have SLG detection. The switchgear also has couple aux transformer to feed aux (BOP)loads. The aux trans are also delta-wye.

As you can imagine, there are no ground detection on this. Also the main bus PT are connected open delta, phase to phase. I don't think I can derive the 3V0 out of this. I need to install the additional (3) PT and connect them in the broken delta and put the resistor across to monitor the voltage.

My other questions are
1.) how critical to have the 59G relay? Can I live without it. Would it cause damage to the GSU transformer etc..

2.) Also if I plan to operate my combustion turbine as the synchronous condenser mode, which means keeping the generator breaker close majority of the time, I can eliminate the the 59G installation. The gen is grounded on the low side.

Thanks again for your comments.

jt

 

[peebee]

1) Not critical, but should be considered a really good idea. One possible but less attractive alternate: periodic testing of your system. Without constant monitoring or periodic testing, you'll never know if you develop a ground fault on one phase. But you'll definitely know it when you develop the second fault, even without relaying.

2) Your second question is a little unclear. It appears to me that your generator is on the wye side of the transformer. If so, this has no impact on the grounding of the separately derived delta-side system.

 

[busbar]

Assuming the bushing/fusing/busing arrangement on the existing VTs would allow it—there is a very long shot you could use the existing VTs reconnected ø-g—adding a third VT to the pair to change the open-delta primary and secondary to grounded-wye. The line-to-line rating of the existing VTs is what is needed for the described case. The existing VT loads would be reconnected to the secondary “phase” terminals, but then add a non-mainstream auxiliary VT like at page 22-23 in

http://www.geindustrial.com/products/buyersguide/GEP-9186-2.pdf

for ground detection.

The problem with having a bus without ground detection is that for a fault, there is no appartent problem with the initial fault, but the second fault has potential for significant damage. Also, closing a breaker or switch into another bus—all hell may break loose.

As brilliantly observed in another recent post here, "...free advice from anonymous sources is a dangerous way of engineering..."

 

[peebee]

"...free advice from anonymous sources is a dangerous way of engineering..."

They should add a frame to this site that displays that quote on every single page in 20-point bold-face capital letters.

 

[busbar]

10-4 and Boy Howdy, peebee!

 

[bjo]

What if there line is also feed from the Delta side??