2.3 Power Conversion System (PCS); how DC becomes AC and back again

The translator between the battery and the grid

Batteries store and release direct current. The grid runs on alternating current. The Power Conversion System is the bidirectional inverter that translates between the two. When the system is charging, the PCS converts AC from the grid into DC to charge the battery. When discharging, it converts DC from the battery into AC to feed the grid. It does both continuously and can switch between them in milliseconds in response to grid signals.

At utility scale, PCS is the correct term. You will sometimes hear inverter used on smaller systems or by people coming from the solar industry, where string inverters and central inverters are the standard vocabulary. On a utility-scale BESS, the PCS is a significantly more complex device — it manages bidirectional power flow, participates in grid frequency and voltage regulation, and interfaces with the EMS for dispatch control and with the BMS for safety coordination.

PCS units are typically skid-mounted. They arrive on site as a pre-assembled package that gets set on a concrete pad, connected to the battery containers via DC cable, connected to the medium-voltage transformer via AC cable, and integrated into the site controls system. The PCS skid module in this course covers the installation, inspection, and pre-commissioning requirements in detail.