Grounding design for energy storage systems with high fault currents 2

[mb3928]

For a typical large-scale lithium ion battery energy storage system, a battery container may have a rating of 5 MWh usable, 2 MW DC continuous discharge power, with a nominal 1000V DC bus voltage ranging from 700V-1100V DC in service. DC fault current can be as high as 200 kA. The DC system is ungrounded and incorporates fusing on both poles as well as GFD / insulation monitoring.

I am wondering if anyone has any insight into design criteria for the premises grounding system & equipment grounding connections - neither the manufacturer nor IEEE literature seems to have much guidance on grounding design for high DC fault current sources. Should I be designing for a multiple ground fault scenario? Do I need to worry about touch voltages in the event of multiple ground faults? Is that overkill given it's an ungrounded system? Any input is appreciated.

 

[FPelec]

hello mb3928,
we installed several large energy storage systems, facing the very same issues.
In the end, the grounding system design is not that important: you are not talking about an interconnected dc system (as in dc railways or HVDC), but instead you are investigating single energy sources.
Therefore, fault current will not flow into the ground (the reclosing path will only involve the battery container chassis) and there will be no potential rise: the grounding system design will thus be mostly dictated by the ac fault constraints.
By contrast, bonding to the metallic masses of the storage systems should take into account the massive currents arising from a double fault.
Consider also the need for maintenance and repair after a first fault to ground: how to keep the personnel safe during such operations, involving voltages up to 1 kV (the risk is direct contact, not earth potential rise) and large arc flash energy?
One solution we adopted is to include disconnectors in the battery racks, in order to sectionalize the battery strings into 120 V blocks, which should mitigate arc flash and electrocution risks.


Si duri puer ingeni videtur,
preconem facias vel architectum.

 

[mb3928]

Thanks FPelec!

The DC architecture of the product we are working with does sectionalize each rack with a DC contactor, so that helps with some of the maintenance issues.

Perhaps I'm overthinking things since a double ground fault scenario is extremely unlikely. But wouldn't grounding/bonding the ESS and PCS enclosures through a resistance help eliminate the arc flash risk from such extremely high fault currents? By my calculations arc energy, even at a working distance of a few feet, could be as high as 400 cal/cm2 for a solid (second) fault. Obviously this creates a shock hazard if touching the enclosures but perhaps that's a risk worth taking when arc energies are so high?