Short circuit current calculation at MV bus (34.5kV), Tesla Megapack 2

[adi 3291]

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?

 

[JJ_Roy]

Tesla has a document that describes the short circuit contribution and how to model it in ASPEN. This document is only available from Tesla under an NDA.

The short circuit contribution from inverter-based resources is manufacturer specific and is based in their inverter hardware and controls design.

 

[adi 3291]

@JJ Roy Thanks for the info. We are working with Tesla to get this info, any chance you have the approximate values on the Tesla Megapack short circuit contribution?

 

[7anoter4]

I never worked with lithium battery for power supply. I used only lead acid open vessel of 220/360 Ah for control in power stations. The lithium battery was for iPhone only.
However, to start a discussion, I drew a possible charging and distribution of 300 units of 1 MW 480 V lithium batteries schematic diagram.
At first, it seems to me, the MPT is a three-windings transformer and 34.5 is the lower voltage winding.
Let’s take the base voltage then 34.5 kV.
The Utility System Scc=3) *230*40=15935 MVA
Then Zcc=34.5^2/15935=0.07469 ohms.
The transformer impedance ZAB=34.5^2*10.5%/210=0.595 ohms.
The System supplied short circuit current on 34.5 kV busbar it is then:
Isac=34.5/sqrt (3)/ (0.07469+0.595) =28.74 kA.
On the Battery side the limiting position will be the inverter for 1 MVA -each-maximum. In parallel will be 300 MVA short-circuit power. Zinv=34.5^2/300=3.97 ohm
In my opinion, the 0.480/34.5 kV transformer power will be maximum 3 MVA and connected with 3 inverters in parallel.
The minimum transformer impedance as per IEC 60076-5 Table 1 is 7%
Then one transformer impedance will be Ztrf=34.5^2/3*7%=27.77 ohms
ZtotX=Ztrf/100=0.2777 ohm.
So, the short circuit current from the battery will be: Iacdc=34.5/sqrt (3)/ (0.2777+3.87) =4.8 kA

 

[Parchie]

At the battery terminals, the short circuit current is only limited by the internal resistance of the battery. At 480VDC and an internal resistance of 20 milli-ohms, you'll have 24 kA. Available short circuit currents from inverters (at the AC side) are in the range of 1.2 to 1.5 times the rated current of the inverter. Therefore, a short-circuit at the 480VAC output of the inverter rated 1.1 MVA (1,323A) will be 1,720A. Since there are 3 megapacks pooled into one collector transformer, expect a short-circuit of not less than 5.16 kA. That is because the collector transformer contributes to the fault. Assuming the system behind the collector transformers as infinitely large, a fault on the 480VAC bus could reach more than 55 kA.

 

[7anoter4]

In my opinion, the OP required: What is the short circuit current contribution by all these Inverters at the MV bus level?
I agree with Parchie: 1.5 times the rating may be the maximum.
Then, 300 inverters in short-circuit case supply 1.5*1.1*300=495 MVA
It will be the System power from the Battery source.
Then the System impedance at 34.5 bus bar will be 34.5^2/495=2.4 ohm.
100 parallel transformers Zxfr=34.5^2/3*7%/100=0.2778 ohm
Then the short circuit current from Battery on the 34.5 kV bus it will be 34.5/sqrt (3)/(2.4+0.2778) =7.4 kA

 

[JJ_Roy]

@adi3291. From what I "remember" , the lower number in the range that Parchie gave is a good place to start until you get the info directly from the OEM.

 

[adi 3291]

So 4 tesla megapack inverters connects to a one MVT rated for 4400KVA, 34.5kV/480v Z=7%.Total 78 MVTs

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:

 

[Parchie]

@adi 3291,
IDK if the data you've provided changed. Having 4 megapacks connected to a 4.4 MVA inverter transformer requires you to have 300/4.4 = 75 inverter transformers, not 78. 78 inverter transformers with 4 megapacks per transformer will require 4 X 78 = 312 megapacks, not 310!
Still, any fault occurring at the 34.5 kV POCC will be sum of the fault contribution of the 75 inverter transformers plus the contribution of the system your BESS is connected to thru the 210 MVA 230/ 34.5 kV main transformer.
Simply, with an available short-circuit fault from the system of 40 kA (230 kV) plus the 210MVA / 10.5%Z, total contribution coming down from the system for a fault at the 34.5 kV POCC is 1,777 MVA! That would be 1,777MVA/(1.732 x 34.5kV)= 29.7 kA. Add the fault contribution of the 75 inverter transformers backed by 300 megapacks, you' get about 36 kA @ 34,5 kV. Please check the numbers and see if they are correct.

 

[adi 3291]

@parchie

That is correct. For reference here, I only added short circuit contribution from Inverters only. If we add both MPT and inverter contribution, it will come around 36kA. The inverter count has been changed, we are going with 310 Inverters, and going with 78 Transformers, as the last transformer only connects to 2 megapacks only.

Can you share any reference document highlighting about short circuit contribution of the inverters are in the range of 1.2 to 1.5 times the rated current of the inverter?

 

[Parchie]

@adi 3291,
Sorry for the late reply.
IEEE has a document on that but IIRC, grid-forming IBRs run around 2 to 3 pu Isc contribution. For grid-following IBRs, that is what I mentioned to be 1.2 to 1.5 pu. LINK