When a train or moving vehicle begins braking, the traction motors become generators and produce an amount of energy that can be fed back into an Overhead Cantenary System (OCS) or dissipated as heat through a set of braking resistors. Instead, this energy can be recaptured using AMT's Energy Storage System (ESS) that will intercept the remaining regenerated energy in onboard capacitors to be used during the next acceleration event.
The ESS uses Double-Layer Supercapacitors to capture, store and discharge 0.7kWh of energy for use in commercial transit applications. The ESS consists of one box with three components inside:
- A Double Layer Capacitor "DLC" bank grouped in a specific array to provide the appropriate source of energy, including cooling fans, capacitors, high voltage protection fuses, test ports and communication circuitry.
- The Power Control Unit that monitors the input voltage and power to allow energy to be stored and used by the propulsion motors. The PCU includes cooling fans, IGBT choppers, protection fuses, control relays and a control unit.
- Isolation contactor that protects the unit in the event of a system fault and will isolate the ESS from the train. The isolation contactor is very similar to those used inside existing propulsion containers.
The ESS has two modes of operation, Energy Saver Mode (ESM) and Voltage Stabilization Mode (VSM).
- ESM Mode: allows the system to capture excess braking energy. This mode relies on the capacitor bank being kept discharged, enabling it to be receptive of the regenerative braking energy. Any energy the ESS captures during a braking event will be fed back to the overhead line during the next acceleration cycle.
- VSM Mode: the capacitors are kept at a full charge to provide the vehicle an on-board energy supply during low line voltage levels or in the absence of overhead power.
The ESS is an embedded system on the vehicle and does not require any direct effort from operations and maintenance personnel for successful use.