Understanding UPS Transfer Time: Mains to Battery Support

This solution explains the nominal transfer time for a specific type of Uninterruptible Power Supply (UPS) system described in the question. We will analyze how the system operates during an AC mains failure to determine the time it takes for the battery to support the load.

UPS System Topology Explained

The question describes a UPS system with the following key components and operation:

• Inverter: Continuously powers the critical load by converting DC power to AC power.
• DC Bus: Serves as the central point supplying DC power to the inverter.
• Rectifier: Converts incoming AC mains power into DC power to feed the DC bus and charge the battery bank.
• Battery Bank: Connected in parallel to the DC bus, ready to supply power when needed.

In normal operation, both the rectifier and the battery bank supply power to the DC bus. The inverter then uses this DC power to supply the load. The rectifier also keeps the battery charged.

How AC Mains Failure Affects the System

When the AC mains power fails:

• The rectifier stops converting AC to DC.
• The battery bank, being connected in parallel to the DC bus, immediately starts supplying the required DC power to the bus.
• Crucially, the inverter is already running and connected to the load. It continues to draw DC power from the DC bus, which is now solely supplied by the battery.

This setup ensures that the inverter's output to the critical load is not interrupted. This specific configuration is characteristic of a Double-Conversion UPS (also known as an online UPS).

Defining Nominal Transfer Time

The nominal transfer time refers to the duration between the failure of the primary power source (AC mains) and the point at which the backup power source (battery) begins supplying power to the load without interruption. For sensitive electronic equipment, a zero or near-zero transfer time is essential to prevent shutdowns or data loss.

Analyzing Transfer Time in the Described UPS

In the described system (Double-Conversion UPS):

• The inverter is always online and powering the load.
• The DC bus is always supplied with DC power, either from the rectifier or the battery.
• When AC mains fail, the battery seamlessly takes over the role of supplying the DC bus. There is no need for the inverter to switch to a different power source because it is already connected to the continuously powered DC bus.

Therefore, the transition from mains power (via rectifier) to battery power on the DC bus happens almost instantaneously from the perspective of the load connected to the inverter. The nominal transfer time is effectively negligible.

Conclusion on Transfer Time

Based on the operation of the described uninterruptible power supply system, where the inverter is always powered via the DC bus and the battery is connected in parallel, the transfer from AC mains failure to battery support occurs seamlessly. The nominal transfer time for the load is essentially zero.

This corresponds to the option indicating approximately $0$ ms.