Ground Fault protection for a wye-wye transformer 1

[nerb31]

I am working on a ground o/c coordination and i am having a bit of trouble on the wye-resistor grounded wye-solid grounded with delta tertiary transformer. Solidly grounded is connected at 230kV while the resistor grounded (50ohms) is connected at 34.5kV. Now, my questions is:
1. Will a ground fault of say 1000A at 230kV side appears as 1000A also at the 34.5kV side since both neutrals are grounded on the same grounding system(im a bit confused on the limit of the NGR at 400A)?

i have googled for answers and come up with a few topics here about wye-wye transformers but not exactly answers my question.
appreciate any inputs.

thanks

 

[waross]

The solid ground current of 1000 amps on the 230 kV side will divide between the ground at the transformer and the source ground. The division of current will depend on the location of the fault (line impedances on each side of the fault.
Also, a primary ground fault will cause a circulating current in the delta winding. This current will be fed from the healthy phases, will be driven by voltage errors and phase angle errors and limited by three times the impedance of the 230 Kv winding and the delta winding.
Determine the voltage magnitudes and phase angles of the primary phases at the transformer terminals and construct head to tail vectors. In a healthy system the delta vectors should form a closed triangle. Voltage and phase angle errors will result in an open triangle. The magnitude of the opening in the triangle is the voltage driving the circulating current in the delta winding.
A primary bolted fault at the transformer primary terminals may result in the voltage error in the delta winding being close to line to line voltage. The back feed current may be substantial, and if this has not been considered in the calculations to determine the magnitude of the primary fault, the primary fault current may be considerably higher than the calculated 1000 amps.
A sketch may make this easier to visualize. The math may be more difficult.
The source impedance, the transformer impedance to the delta winding, the line impedance from the source to the fault, and the line impedance from the fault to the transformer under consideration may interact to make the math somewhat challenging.
You may wish to trip off-line in the event of a primary fault to prevent damage to the tertiary winding and to stop back feeding into the transmission line fault.
On the practical side:
You can detect primary ground current contribution to the fault by monitoring the current from the primary X0 terminal or by the vector sum of the primary phase currents.
You can detect secondary ground current by monitoring the current from the secondary X0 terminal or by the vector sum of the secondary phase currents.
David Beach; Do have any similar transformers in your system?
Would you trip this transformer locally, or would you rely on the source protection to take the line out of service?
Thanks in advance David.
Yours
Bill

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

 

[davidbeach]

You're going to need to draw out the sequence diagrams and work out all of the currents. A a quick shortcut to get the basic picture (not the details) think of the transformer as two wye-delta transformers connected back-to-back delta-to-delta. The grounding characteristics on each side will be what it would be for that transformer alone. So, in the OP's case, there can't be more than 400A of ground fault on the 34.5kV side since that's what the grounding resistor does. What's on the high side doesn't much matter. Protection would be what every you'd use for protection of two transformers except that nothing needs to be done with the delta. My choice would be differential plus REF on both sides.

These are actually these are a quite useful beast. Makes it possible to derive a grounded station service source from an impedance grounded generator bus without introducing a phase shift.

 

[waross]

That helps to visualize the setup, David. Thanks.


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

 

[nerb31]

thanks for the replies guys.
however, there is another issue that I am in doubt. apparently, the other end of the 34.5kV is also resistor grounded. I have attached the single line diagram for reference. what would be the resulting ground fault current if there will be ground fault in the middle of the 34.5kV level as shown in the diagram? Am i correct to opined that the ground fault could be as high as 800A (double the NGR limit since the impedance seen by the fault is halve considering the NGR are in "parallel")?

thanks again.

 

[mgtrp]

The fault current will be close to 800A (a little less, depending on the impedance of the fault and of the line back to each transformer), but this will divide roughly evenly between the two transformers - 400A in each.

 

[nerb31]

so, say if I set the 51G or 51N of the 34.5kV system to 300A pickup, should I also set the pickup of the 51G at 230kV side at 300Amps? Or should I set it at the "equivalent amps at 230kV side which is around 47A?
I am a bit confused on how to plot that in TCC since they are of different voltage level.

appreciate any advise.

thanks

 

[waross]

34.5 kV sounds like distribution. Depending on the location the majority of the ground or L-N faults on the 34.5 kV lines may be tree branches or birds and small animals. The reclosers will generally allow these to clear in the first one or two reclose cycles. The second most common cause of L-N faults may be failed transformers. The reclosers will generally allow the transformer local protection (fused cut-outs) to clear in the third, fourth, or fifth cycle. You may wish to delay a circuit trip long enough for the reclosers to do their job.
In any event, you may not wish to trip the primary and subject your system to the energization surge of the transformer on account of a cooked crow out on a distribution line somewhere.
But systems differ. There may be local considerations which over-rule these comments.

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

 

[jghrist]

so, say if I set the 51G or 51N of the 34.5kV system to 300A pickup, should I also set the pickup of the 51G at 230kV side at 300Amps? Or should I set it at the "equivalent amps at 230kV side which is around 47A?

It's not that simple. Part of the zero-sequence current will come from the 230 kV system and part will come from the delta tertiary, so the current on the 230 kV side will not be at a 34.5/230 ratio.

You'll have to work out the sequence diagrams like davidbeach said.

 

[odlanor]

nerb31,
34.5kV and 230kV define two zones of faults protection independents except perhaps backup protection.
For Ground faults at 34.5kV system neutral current contribution will be :
34.5kV neutrals – from ground to neutral direction
230kV neutral – from neutral to ground direction
for Ground faults at 230kV system neutral current contribution will be :
34.5kV neutral at 13.8/34.5kV transformer–from ground to neutral direction
34.5kV neutral at 34.5k/230V transformer– from neutral to ground direction
230kV neutral – from ground to neutral direction
You should set it at the "equivalent amps at 230kV side which is around 47A
but with delaying(0.5s) as backup protection for faults at 34.5kV.
Perhaps , this could be maloperation for fault to ground at 230kV at time of
the 2nd zone and should be set for 3rd zone.
You have to verify how is the coordination of the 230kV protection system.

 

[davidbeach]

Look at the zero sequence diagram. That delta winding decouples the high side zero sequence from the low side zero sequence. For faults on the low side you have the entire high side zero sequence impedance in parallel with a short. The high side will see positive and negative sequence currents but not any meaningful zero sequence.

 

[odlanor]

reference:
symmetrical components for power systems engineering - J. Lewis Blackburn -1993
fig 9.4 pag 231 and fig 9.7 page 235.
I assume 230kV system is solidly grounded at neutral transformer and neutral source system.

 

[odlanor]

davidbeach
see attached

 

[odlanor]

davidbeach
attached is from protection book of Blackburn

 

[odlanor]

davidbeach
attached is from protection book of Blackburn

 

[odlanor]

davidbeach
attached is from protection book of Blackburn.
I dont know how transfer multiple files!

 

[davidbeach]

Ok, I was missing the Z_L impedance in my mental picture. So it isn't a complete decoupling, but certainly not a turns ratio relationship between low and high side currents.

 

[odlanor]

davidbeach,
Yes you correct.

nerby31
ERRATA:
.................
You should set it at the "amps at 230kV side seen by a fault to ground at 34.5kV".
.....................

I never saw setting 51N at 230kV for faults at 34.5kV.
Each protection engineer has its quirks!.

 

[nerb31]

hi odlanor,

while I am trying to digest all of the helpful tips by you and others, could you please clarify what you mean by:
You should set it at the "amps at 230kV side seen by a fault to ground at 34.5kV"?

thanks

 

[odlanor]

Its is the contribution of 230kV side for a fault to ground at 34.5kV side ,in 230kV Amps.