TRANSMISSION lines

[mbrooke]

The only reason I see for having a g-wye/g-wye 115/13.8kV distribution bank is phasing or perhaps a super high ground impedance from the soil (ie rock or pure sand).

How would ground impedance effect the primary?

 

[mbrooke]

Not sure what you mean. I've seen large ground current on the neutral of a distribution bank for unbalanced loads on the 13.8kV side. I wouldn't expect to see any ground current on the 115kV side. The xfer to phase imbalance on the highside would be in the form of negative sequence current.....yes?

In wye wye neutral imbalance goes right through, in delta wye any neutral/ground current remains on the 13.8kv system?

 

[Ingenieur]

Not sure what you mean. I've seen large ground current on the neutral of a distribution bank for unbalanced loads on the 13.8kV side. I wouldn't expect to see any ground current on the 115kV side. The xfer to phase imbalance on the highside would be in the form of negative sequence current.....yes?

for example if you have 115k wye-g to 13.8 wye-g (although I did not think this was common, we do not allow it for mine distribution) if the 13.8 is unbalanced would that not be reflected to the primary?
so you would have neutral/ground current.

or as you say the imbalance would occur as negative sequence currents on the lines (not neutral) on the primary?

 

[Ingenieur]

In wye wye neutral imbalance goes right through, in delta wye any neutral/ground current remains on the 13.8kv system?

this is my understanding also
wye-g 115kv to wye-g 13.8kv
any imbalance will be reflected to the primary

so the static/shield wires are not solidly grounded?
only thru LA's
are they tied to the X0 at each end?

 

[mbrooke]

this is my understanding also
wye-g 115kv to wye-g 13.8kv
any imbalance will be reflected to the primary

so the static/shield wires are not solidly grounded?
only thru LA's
are they tied to the X0 at each end?

The static shields connect to the metal towers which are basically grounded through the foundation. The static wires connect to the station structures (bays) which are bonded to the equal potential ground mesh.


As for wye-wye you are correct. If I have a 10kv system with 50-50-100amps on phases A,B,C respectively and 50 on the neutral then I will have 5, 5, 10 amps on primary A,B,C and 5 amps on the primary neutral on a 100kv primary system.


It is for this reason (among others) why wye-wye is avoided on the transmission to distribution levels. A bolted fault on the secondary of a 115kv-13.8kv 60MVA transformer will reflect on the primary, and in some cases can cause mis-operation of ground relays on the transmission lines especially where high speed high sensitivity protection schemes are employed. Where a zero degree phase shift is required POCOs will go through the trouble of ordering a special delta-zig zag or wye ungrounded-delta-wye gorunded transfomer which has a zero degree shift without passing zero sequence currents.

 

[Phil Corso]

MBrooke...

Unbalanced phase-currents in any 3-phase system, including transmission-lines, only occur if the Delta-Load it supplies is unbalanced. Since there is no neutral connection to the load, no current will flow in any of the grounding "circuits", nether the static line or the ground-plane!

However, I do agree that a ground-fault on any phase-line will cause current to flow in the in the static line, the tower grounding conductors and the ground-plane!

Regards, Phil Corso

 

[mbrooke]

MBrooke...

Unbalanced phase-currents in any 3-phase system, including transmission-lines, only occur if the Delta-Load it supplies is unbalanced.

Correct, as that does not equate neutral current which is what I was trying to say.

Since there is no neutral connection to the load, no current will flow in any of the grounding "circuits", nether the static line or the ground-plane!

Correct.

However, I do agree that a ground-fault on any phase-line will cause current to flow in the in the static line, the tower grounding conductors and the ground-plane!

Regards, Phil Corso

Correct, but in a wye-wye a single L-G fault on the secondary reflects on the primary.

 

[Bugman1400]

this is my understanding also
wye-g 115kv to wye-g 13.8kv
any imbalance will be reflected to the primary

so the static/shield wires are not solidly grounded?
only thru LA's
are they tied to the X0 at each end?

I believe static wires are tied to ground at each tower.