Applications
Transformers for battery energy storage
A BESS transformer is specified for a duty cycle, not a load. Charge, discharge, reactive support and standby each load it differently, and the datasheet rarely describes any of them.
BESS step-up transformer sizing for the actual duty cycle
The common practice is to size the transformer at 110–120 % of the PCS AC rating. That covers the continuous case but not the thermal history. Repeated charge–discharge cycles accumulate hot-spot temperature in a way a single continuous rating does not capture. We size from the dispatch profile.
Where the transformer sits
- Battery racks (DC)
- PCS (0.4 / 0.69 kV AC)
- BESS step-up transformer
- collection at 11 / 22 / 33 / 34.5 kV
- main step-up transformer
- grid
BESS transformer specification: the design points that matter
| Vector group | Dyn11 is standard for BESS step-up duty. |
|---|---|
| Impedance | typically 4–6.5 %, selected against fault level and voltage regulation. |
| Harmonic duty | PCS switching produces harmonics that raise eddy losses; derating follows IEEE C57.110. |
| Ambient | containerised BESS often sits in high ambient, which compounds the derating. |
What we supply for storage
- BESS step-up transformers: 0.4–35 kV, 30 kVA to 31.5 MVA, dry-type or oil-immersed, Dyn11.
- Main step-up transformers: 110–500 kV, up to 600 MVA.
- Station service and auxiliary transformers.
Dry-type or oil-immersed for a BESS?
Both are supplied. Dry-type is common where the transformer sits inside or next to a container and fire load matters. Oil-immersed is common for larger blocks and for the main step-up. The decision usually follows the fire strategy and the ambient profile.