LV Bare Conductors

[Danilo917]

Gents,
if an underground 11kv feeder cable screen from substation is earthed and bonded to existing bare LV neutral on pole approx 120m from substation and then a fault occurred along overhead LV conductors,could the fault be seen by feeder's protection relay ?

Many thanks,

 

[davidbeach]

Where are you? In my experience, there isn't any low voltage leaving a substation.

 

[Danilo917]

the underground feeder is coming from a zone substation and its first connection out is terminated to existing pole with HV and LV overhead conductors..the 11kv cable screen will be bonded to overhead LV neutral.

 

[waross]

It depends.
Probably not, Probably the safest. You don't want a service may to encounter a serious voltage difference between the local ground/neutral and the screen and drain conductors.
BUT. In a neglected system where grounds are missing and are in poor condition, part of a fault current may find its way back through the cable screen via the station end ground. Not likely but possible.

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

 

[Danilo917]

Yes I agree..but, between a 7/4.5 AAC conductor and 11kv cable screen,which has the least resistance for ground fault return path?

 

[7anoter4]

On my opinion, if the neutral would be disconnected from pole grounding electrode and interrupted from LV supply transformer neutral point the Grounding Fault Current could flow through the 11 KV cable screen towards Substation Grounding.
But the current induced from the screen current in the main 11 KV circuit will be negligible so no protection will "see" the fault.
The L.V. current flowing through the 11 KV cable screen could heat the cable indeed ,so a good grounding electrode has to be provided at the screen connection pole and to connect the cable screen directly to this ground instead of LV neutral directly.

 

[Danilo917]

I agree ,that's my opinion as well..I believe it's not the safest approach to do because it opens up the HV feeder to potential multiple faults at LV circuits .Even if the induced current is relatively small, a zero current condition is still better than this.

 

[jghrist]

Is the LV transformer primary connected to the 11 kV feeder? If so, the feeder will see the LV fault reduced by the turns ratio of the transformer. The ground fault current will flow through the LV ground/neutral to the LV transformer, not through the 11 kV cable shield to the substation.

It is normal to ground the cable shield at every transformer, where it naturally gets connected to the LV ground.

 

[Danilo917]

Yes , a pole sub 200 m away is connected via HV bare conductors to pole with 11kv u/g cable and this feeder is approx 150 m from zone sub..aerial services are there also on that said pole..you reckon a bonded L-N fault current at LV mains or aerial services on that pole would have a return path thru pole sub ? I think it's a clear short circuit between 11kv cable screen and LV circuit, isn't it?

 

[7anoter4]

It could be ?

 

[Danilo917]

thanks for that illustration..i think even if neutral is connected (although very likely it can be otherwise) and pole is grounded,a substantial fault current may pass still pass thru less resistive cable screen onto zone sub protection system..the distribution of fault current among several return paths can be an issue here I guess. I assume that more or less 70% of fault current at HV will go thru cable screens but im not sure for the LV system.

 

[jghrist]

this feeder is approx 150 m from zone sub..aerial services are there also on that said pole..you reckon a bonded L-N fault current at LV mains or aerial services on that pole would have a return path thru pole sub ?

I'm not familiar with the terminology, but a LV ground fault would return to the LV transformer through the LV ground wire and would not flow to the 11 kV source.