Earthing Design - 66/11kV Substation

[Yogima]

Dear experts,

I have been reading the IEEE Std80:2013 (free version) and have compared it to one of our company's recent projects design files and have noted some issues I hope the experts can shed some light on. For example, the consultant had used the fault levels from the 11kV side to size the earthing tails(risers) as well as the main earth grid, which in the IEEE Std80 states need not be the case if the MV side is Y-connected with neutral solidly bonded to earth grid and HV side Delta-connected. In which case the earth grid conductor should be sized based on fault level of the 66kV side (system) and in this project's case, much less than the 11kV side.
My question may be too obvious but just wanted to know from the experts, did the consultant overdesign the main earth grid?

Background: Substation is a single-bus configuration with 2 transformer bays, the transformers are of Dyn11 type with the neutrals solidly bonded to the earth grid. 66kV bus fault level was 5kA and 11kV bus fault level was 26kA. There is no plan for expansion of the substation within the next 5-10 years.

Thank you in advance.

 

[bacon4life]

Sizing a 66/11 kV sub for only a 5 kA fault current sounds really low for sizing of the conductors. In some cases the worst case fault location for sizing from a thermal/mechanical perspective can be different that the worst case fault for calculation of acceptable ground potential rise (GPR). As an example of a thermal/mechanical worst case, consider a line-to-ground fault from the 11 kV to the bus support structure. The riser to bus structure will carry 26 kA so sizing the riser to the bus structure for only 5 kA would be way too small.

Upgrading the conductor sizing of existing ground grid later is extremely expensive since every conductor has to be dug up and replaced. Typically I see substations ground grids designed with thermal/mechanical ratings for a 40+ year design lifetime. Dealing with future increases in GPR is somewhat easier because it may be possible to increase the overall footprint or add incremental conductors/grounding rods to the existing grid.

Also keep in mind that buried conductors corrode over time. Smaller conductors have a larger surface area to volume ratio, so corrode relatively faster than larger conductors. Detecting degradation from corrosion is challenging, and replacing failed ground grids is very expensive.

 

[Kiribanda]

In fact, the main ground grid should be designed using 5kA earth fault current because it is the return current going back to the 66kV remote substation through the soil during an earth fault at the 66 kV side at the substation. Now, the 26kA earth fault current is going back to the wye side of the transformer during an earth fault at the 11kV low side. This 11kV low wye side is normally connected to a 11kV switchgear/ MCC using a 11kV cable system/ cable bus/ busway etc. Therefore, during an earth fault at the 11kV side, since the neutral is solidly earthed, this 26kA earth fault current is taken back to the wye side of the transformer by the PE of the busway/ cable shields/ cable armour etc etc and not by the soil. Hence no touch, step potential issues. That means,
1)If you have a 11kV cable/ bus way system + swgr/ MCC, and if the ground grid is sized for 26kA, in my opinion it is an “oversized” grid.
2)If you have a 11kV overhead line system for the low side of the transformer then, the grid has to be sized for 26kA which is the highest earth fault current.

 

[Yogima]

Thank you Bacon4life and Kiribanda,

I note both your comments.

After reading both replies I now believe the consultant designed for 26kA accounting for probable phase ground fault along the short span of overhead 11kV conductor that runs from the transformer's 11kV bushing to a pothead where it transitions to underground conductor and runs into the 11kV panels.

Thank you for your expert opinions, much appreciated.

 

[jghrist]

the consultant had used the fault levels from the 11kV side to size the earthing tails(risers) as well as the main earth grid

The consultant was correct in sizing the conductors for the higher fault current. An 11 kV line-to-ground fault will flow through the conductors to the transformer neutral. The spacing of the grid to reduce GPR, step-voltage, and touch-voltage does not need to consider the 11 kV fault level because the return current does not flow through the earth and does not cause a rise in voltage with respect to remote earth.

 

[Yogima]

Dear jghrist,

Thank you for your comment. My replies to bacon4life and Kiribanda noted that there is a pothead located between the 11kV bushing of the transformer and the 11kV panels in the control building. If there was an accidental phase-ground connection/fault (highly unlikely) of the outdoor section of this circuit, would 26kA be directly injected into the ground (causing GPR) and into the grid (that determines Main Grid conductor size) and travel back to the neutral. In this case would 26kA be considered for calculating the grid current and hence GPR?

Your expert opinion would be appreciated.

Regards,
Yogima

 

[Yogima]

Dear jghrist,

Please note in my above reply that the phase-ground fault I was referring to here is not via the pothead or any structure on the 11kV side for that matter as they are all bonded to the earth grid but via a direct phase-ground fault (fallen 11kV line, accidental short circuit via external metallic object, etc, all highly improbable events but thrown here for the sake of discussion).

Regards,
Yogima

 

[bacon4life]

For an overhead line with a multi-grounded neutral, the fault current will split between the neutral conductor and the actual earth(dirt/rocks). The exact split is dependent on many factors. Here is an example for a specific situation is

https://www.google.com/imgres?imgur...hUKEwjX9Y7ExIjxAhWuiJ4KHbCgBt0QMyheegUIARCcAQ

 

[jghrist]

If the accidental short circuit was via a metallic object that was grounded to the grid, the current would return through the grid and not through the earth. For a fault on the 11 kV line outside the substation, some of the current would flow through the earth as noted by bacon4life and would cause some GPR and touch-voltage. It also would cause touch-voltage at the point where the short circuit occurred.