Ground fault on a Delta System 3

[CuriousElectron]

Hi Guys,
I just wanted to discuss theoretics of what happens to a medium voltage system that has one phase inedvertently grounded. Say the phase A is grounded. LL 'A-B' and LL 'A-C' voltages would elevate to 173% of nominal.The insulation level of the cable should have been rated for this scenario already. The equipment downstream of and upstream of the faulted node is now subjected to high overvoltage condition and needs protection to "kick in" and trip and isolate the ground fault. Am I correct in my assestment?
Now, if the MV feeder had a sigzag transformer to function as a ground return path of the system, the system would not be subjected to the overvoltage, which would be preffered over the ungrounded delta system. Lower level insulated cable can be used, smaller conduit, etc.It seems to me that the zigzag transformer is equivalent in theory to the HRG system in low voltage system.
Please poke holes in my write-up.
Regards,
EE

 

[7anoter4]

CuriousElectron said:
“LL 'A-B' and LL 'A-C' voltages would elevate to 173% of nominal”
Do you mean B-N [or G ] and C-N [or G] would elevate to 173% of nominal.
The equipment-supply cable for instance withstand- usually the overvoltage according to insulation thickness for zero time and then the neutral has to be solidly grounded, for up to one hour for 133% or for indefinite duration if insulation is 173% rated.

 

[dpc]

The use of grounding transformer to establish a ground reference on existing ungrounded delta systems is pretty common. Grounding issues at medium-voltage are somewhat different than at 480 or 600 V, but principles are the same. Grounding is generally low-resistance (50A to 1000A) or high-resistance (<5A) at medium-voltage.

Also, just as an aside, a wye-delta transformer works just as well as a zig-zag autotransformer for this purpose and are much easier to source. In theory, the zig-zag transformer is less expensive, but in practice, this is not generally the case these days.

Cheers,

Dave

 

[CuriousElectron]

What would be some of the advantages of establishing a ground reference for a MV ungrounded delta system if it were possible to design an effective ground fault detection scheme? If the objective is to isolate a feeder with imbalance, it appears all you need is a sensitive/effective ground fault detection/protection scheme. Why the trouble with the grounding banks..

Regards,
EE

 

[dpc]

To reduce voltage transients. Also, with low resistance grounding it's possible to selectively coordinate ground overcurrent relays. If you are relying on ground detection on ungrounded delta do you trip the entire system on a ground fault or do you keep operating? If you keep the system energized with a ground fault, there are overvoltage issues and when a second phase goes to ground you have a particularly nasty line-to-line fault.

 

[cuky2000]

At difference than the delta source, a zig-zag power transformer will provide significant fault current to trip the protective device in the event of SLG fault.

As an example predicating a SLG fault in ΦC, the SC currents are as follows: I[sub]A[/sub]=I[sub]B[/sub]=I[sub]f[/sub]/3 and I[sub]C[/sub]=2/3*I[sub]f[/sub].

For an illustration of zig-zag connection, vector diagram, and representation of SLG fault see the figures below.

>>>>

 

[cuky2000]

For an existing or new "ungrounded" delta system, there are options to derive a neutral with a grounding transformer to create a high resistance system. The appropriate selection should be based on economics or other site specifics such as space, weight, size.

The sketch below shows a couple of examples to illustrate the concept to derive a neutral point through grounding transformers.

>>>>>

 

[CuriousElectron]

Can the zero-sequence CT be installed on the neutral point of the zig-zag?

Thanks,
EE

 

[GroovyGuy]

You betcha Curious;
The zero-sequence CT is always installed on the 'neutral point' of the zig-zag transformer. (ie in series with the ground resistor).
GG

"I have not failed. I've just found 10,000 ways that won't work." Thomas Alva Edison (1847-1931)

 

[7anoter4]

 

[CuriousElectron]

7qnother4,
May I ask what the reference is for the excerpted diagram?
Thanks,
EE

 

[7anoter4]

UNDERSTANDING ZIG-ZAG GROUNDING BANKS by JOHN R BOYLE, PSA

https://www.scribd.com/document/268936634/Georgia-Tech-Zig-Zag-Grounding-Transformers