Fault Current 2

[Danilo917]

Gents
can you help me figure out how the fault current at 11kv pole substation (first feeder out) say,2km out, returns to its source (Zone Substation)? Soil resistivity is less than 100 ohm m. Earthing at the pole sub is 2 x 2.4m electrode for HV & LV. Assuming a L-G fault happens at 11kV side.

regards,
Danilo

 

[dpc]

For a line-to-ground fault, the current has to return via the neutral conductor, through the earth, or some combination of the two.

 

[Danilo917]

Thanks DPC...thats correct but no LV neutral is installed
between first pole sub and the source(zone sub)

 

[davidbeach]

What is a pole sub? What is a zone sub? What equipment is in each, what voltages, what types of transformation, if any, etc.?

 

[collies99]

I think it is a pole mounted substation, 3 single phase transformers 25 or 12 kv primary and 3 phase 600v secondary ? That is about 2km from the main sub ?

need yo know the SCA and equivalent reactance at the main bus and the feeder reactance.

Primary fault current return via neutral and ground resistance to source former which may be 2 if it has a close breaker?

 

[davidbeach]

Not sure you can call anything with a 600V output a substation. But never the less, if it is configured delta on the high side, grounded wye on the low side, then the entire ground return goes to transformer, none would go further upstream.

 

[Danilo917]

Thanks Gents for your reply..the pole sub is 11kv delta-wye connected..can you explain please why if the high side is delta,the entire ground return goes back to pole sub? My understanding is, the ground fault should return to its source(zone sub)

 

[collies99]

Someone gave you bum information. The ground current goes back to the main transformer at the feeder sub. The xformer fuses at the pole sub is coordinated with the tap fuses (if they exist) and then coordinated with the feeder relays at the feeder sub.

The pole top sub fuses is to isolate the faulted transformer for internal ground faults near the terminals and for terminal faults if they are single phase xformers and offer limited overload protection.

Do you have series reactors at the feeder sub ahead of the feeder breaker ?

 

[davidbeach]

No, the ground (zero sequence) component of fault current does not go any further than the delta-wye transformer(s) on the pole. Between there and the next transformer upstream it will appear as a phase-phase fault but there will not be any zero sequence component on the delta side. Never is, doesn't matter at what level in the system you find the delta-wye transformer. Time to study up on transformer sequence connections.

 

[dpc]

Better post a one-line. Your terminology is non-standard for engineers in the US at least, and I think everyone who is responding is talking about something different. A big part of being a good engineer is being able to communicate with a minimum of jargon and three-letter acronyms. If in doubt, a one-line diagram is the best starting point.

One thing is universal however - there must exist a complete circuit path. For a line-to-ground fault, the current must return to the transformer neutral - otherwise there is no current. But it can find a variety of paths to get from the fault point back to the transformer - any grounded conductor will do.

 

[Danilo917]

Thanks again for your time..maybe DPC is right,anyway,just imagine an 11kv feeder out from 132kv/11kv station sub (zone sub)..the feeder is straight out from 11kv CB at station sub and feed the distribution system via 11kv U/G cable with metallic cable screeens (UGOH)onto first dist sub(pole sub) 2km away from station sub(zone sub)..my question is, if the L–G fault occurs between UGOH(underground/overhead)termination and dist sub(pole sub),how will the fault current goes back to the source (station sub or zone sub).?

 

[davidbeach]

OK, that just confused things more. Please post a diagram of the circuit, including all transformer connections, with labels, so that we can all understand what you're talking about. Fully define pole sub (many people here have never heard of such a beast), zone sub (also an unknown term), UGOH (underground overhead?), anything else. Remember this is an international forum in which everybody uses some version of English, but there are many versions of English, two wildly divergent sets of standard that at times seem to require two separate sets of laws of physics, and who knows how many sets of local jargons?

 

[PHovnanian]

Danilo917, we need to know how the zone and pole sub transformers are connected. The pole sub has a delta primary (I gather from a previous post) and there is no neutral returning to the zone sub. So, how is the zone sub connected and grounded? Specifically on the 11kV side. Is there a grounding transformer on the 11kV bus?

 

[Danilo917]

Thanks..just to clarify the terminologies...im using AS/NZS standards, so the terms I used is quite different from US standards. I understand that pole sub here is called distribution transformer in US and zone sub here is called Substation there while the UGOH here is called pothead there.

Anyway,i'll try to use terms where you are familiar so it wont confuse you. the question is , how is the L-G fault current occured at the HV side of 11kV/430v Delta Wye Distribution Transformer goes back to 132kv/11kv Substation (source)? The 11kV U/G first feeder out (with cable screens) from that substation terminated approx 1km to a pothead,then goes to OH Dist Transformer approx 1km from the pothead. So,the total distance from 132/11kv Substation to OH Dist Xformer is approx 2km. There is no grounding transformer on the 11kV bus but the cable screen of 11kV feeder at substation end is bonded to main sub earth grid and grounded on the pothead side.

Also,if the fault occurs at the pothead,how is the L-G fault current goes back to 132/11kV Substation?



Regards
Danilo

 

[davidbeach]

A ground fault on the 430V system returns, only, to the neutral of the 11kV/430V transformer. From there back to the 132/11kV transformer it is only a line-line fault with no ground component.

A ground fault at the pot head will return to the 132/11kV transformer through what ever path it finds. If it's a high impedance return path the fault current will be low.

 

[jghrist]

A ground fault on the high side of the 11kV-430V transformer will return through the earth, OH grounded neutral (if any), and the cable sheath (if grounded in more than one place). There does not have to be a separate grounding transformer on the 11 kV system if the Zone Substation is fed by a delta-wye transformer. The ground current returns to the neutral of the Zone Substation transformer.

 

[Danilo917]

Gents,
thanks for your replies..well appreciated.

regards,
Danilo

 

[7anoter4]

I think, on ungrounded systems, the fault current-phase-to-ground-returns to the source only through capacities of the other two healthy phases to ground.

 

[waross]

7anoter4; This is a special condition where a Petersen coil is installed. The Petersen coil must be tuned to the capacitive charging of the protected system line. Modern Petersen coil installations use a variable reactor and may include a control system to match the inductive reactance to the line capacitive reactance as the latter varies due to switching operations.

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

 

[7anoter4]

I agree with you, waross. Simply, I employed this sketch with the Petersen Coil-but, if it can see, I deleted the coil- only to demonstrate the fact that the fault current returns to source through capacities and not through Ground. Now I see was not so clear, I guess. I did not intend to use the Petersen Coil at all.