Transformerless Inverters and BESS

[tcleg]

I discussed with a DC-block BESS company about pairing their product with a 480VAC-1500VDC Power Conversion System (inverter).

They have a requirement for an AC-side, ungrounded isolation transformer. The DC-side also operates ungrounded which is expected. They said it was because of common mode voltage that they require the isolation transformer.

I would like to take the PCS and directly interconnect at 480V to facility grounded distribution similar to how it is commonly done with PV, without a dedicated 480V to 480V ungrounded secondary isolation transformer.

As I see it, the ungrounding of the secondary changes the distribution served by the inverter to capacitively-grounded. It would cut down on common-mode current a bit but I don’t see how it significantly affects anything when the DC-side is ungrounded to start with very short conductors connecting to the inverter (very low capacitance +\- to ground on the DC-side).

Am I missing something with what the isolation transformer is doing? And, how is this application different from VFDs, PV inverters, and AC-block BESS which do not require it? Is the DC-block BESS manufacturer being overly cautious?

 

[electricuwe]

Basically the decission to operate a system ground or ungrounded has three implications:

- voltage stress across isolation
- capacitive currents driven by common mode voltages
- fault currents

If you compare PV and batteries, PV is not able to drive large fault currents from the DC-side, hence protection is more simple

 

[waross]

Your inverter output is a separately derived system which by code may have to be grounded.
If both the neutral and an output of a transformerless inverter are grounded, the bridge rectifier will be shorted.
example: If the negative side of the DC line is grounded, then when the negative diodes conduct, there will be a line to ground short, and probably blue smoke.