Short circuit current calculation at MV bus (34.5kV), Tesla Megapack

[adityalagudu3291]

In the process of calculating SCC at MV bus for a BESS plant. Utility POI fault current is at 40kA. Our only MPT size is 230/34.5, 210/280/350MVA, %Z = 10.5 @210MVA. Based on this the SCC at MV bus is coming around 30kA.

From the BESS Inverter side planning to use TESLA MP2XL megapack rated for 1100KVA, 480V, 979kW and MP2XL qty is 300. What is the Short circuit current contribution by all these Inverters at the MV bus level?

 

[Elect117]

I found this:

https://www.nrel.gov/docs/fy10osti/46698.pdf

I would ask Tesla to provide some kind of documentation. I don't know of any IEEE papers that cover above IEEE 1547. You will most likely have to create your own modeling.

 

[adityalagudu3291]

I found this:

https://www.nrel.gov/docs/fy10osti/46698.pdf

I would ask Tesla to provide some kind of documentation. I don't know of any IEEE papers that cover above IEEE 1547. You will most likely have to create your own modeling.

Thanks, checking with them too.

 

[kingpb]

In the process of calculating SCC at MV bus for a BESS plant. Utility POI fault current is at 40kA. Our only MPT size is 230/34.5, 210/280/350MVA, %Z = 10.5 @210MVA. Based on this the SCC at MV bus is coming around 30kA.

From the BESS Inverter side planning to use TESLA MP2XL megapack rated for 1100KVA, 480V, 979kW and MP2XL qty is 300. What is the Short circuit current contribution by all these Inverters at the MV bus level

Inverters cannot supply more than around 1.4 x rated current during a SCC situation, so that would be around 555kA, but that is at 480V. But for calculating in amps you are missing a critical piece of information, the step-up transformer(s) for 480V-34.5KV. I would use MVA method and you thus don't need it, per se. 1.1MVA x 1.4 = 1.54MVAsc, 300 x 1.54MVA = 462MVAsc, combined with the utility xfmr of 210MVA/0.105 = 2000MVAsc so ttl is 2462MVAsc. @34.5KV; then it is 41.2KA. You will likely need 50kA gear to be safe.

However, the current will be less because you have not taken into consideration the step-up transformer impedance of all the units on the BESS, nor the impedance of the cable which will all lower the amount.

 

[adityalagudu3291]

So 4 tesla megapack inverters connects to a one MVT rated for 4400KVA, 34.5kV/480v Z=7%.

Base current of Inverter is 1324A, multiply with 1.4 times SCC, then Isc-LV side for each inverter is 1854A.

Reflect this short-circuit current to the MV side (34.5 kV) using the voltage ratio, then Short-circuit Current at MV (I_sc_MV)=I_sc_LV×Current Reflection Ratio=1854.8×0.01391=25.79Amps
For all 310 inverters, the total short-circuit contribution at the 34.5 kV MV bus is:

Total I_sc_MV=25.79×310=7,995 Amps, So getting around 8kA contribution from all the 310 Inverters.

Following is what referenced in SMA inverter Short circuit contribution document:

​

 

[adityalagudu3291]

@kingpb It is hard to get the MV breakers rated for more than 40kA.

Do you have any reference document showing about the Inverter’s SCC contribution wont be more than 1.4 times of nominal rating?

 

[wwhitney]

This google hit says "A 1.2 PU fault current is used for short circuit analysis, from the Tesla Application Note supplied by the IC."

https://www.nspower.ca/docs/default-source/pdf-to-upload/ir578-feas.pdf?Status=Master&sfvrsn=800ab826_4

Sounds like you need to get the relevant Tesla Application Note from Tesla.

Cheers, Wayne

 

[kingpb]

4.4MVA/0.07 = 62.86MVAsc
(1.1MVA x 1.4) x 4 = 6.16MVAsc
Using MVA these combine by the inverse of the sum of the inverses; so
1/(1/62.86MVA + 1/6.16MVA) = 5.61MVA x 75 = 387MVAsc

ttl = 2000MVAsc + 387MVAsc = 2387MVAsc; or 39.95kA

@ 1.2 SCC multiplier instead of 1.4 it would reduce to 39.6kA, not much difference.

Based on those numbers, it would not be recommended to try and use 40kA gear, but as pointed out @ 34.5KV that is difficult.

Workaround; The most contribution comes from the utility, thus it would be very prudent to increase the transformer impedance to 11.5%, gets you to 37kA ttl, then with cable you'll be a little lower, and thus 40kA would probably work.

 

[adityalagudu3291]

@kingpb Using this method the SCC from the Main Power transformer is coming around 1762MVA, rather than what was calculated as 2000MVA before. The utility side POI Short circuit current taken was 40kA for this calculation. This is giving some bandwidth on the overall fault current at the MV bus.

 

[kingpb]

Now take into consideration that the utility voltage can very well be 103% and the transformer impedance unless that is the nameplate can be -7.5% lower, you would be looking at 1900MVAsc. The 2000MVAsc is infinite bus.

If you want to get into detailed design there are a lot of things to consider, like cable. But from a back of the envelope approach it's close enough to say further analysis is probably required before committing $$$ to a expensive swgr order. I personally would model it in ETAP and dial it in before specifying equipment.