UPS Applications in Data Centers ( Part 2)

Double conversion online UPS

This is the most commonly used UPS type for power supplies with a power range of 10kVA and above. In normal operation, mains power is converted to DC by a rectifier, and then to AC by an inverter (hence the term double conversion) to supply the load. Simultaneously, the rectifier charges the battery. In the event of a mains power failure, the battery/inverter combination provides uninterrupted power to the load. Power continues until mains power is restored (or the generator starts) and the system returns to normal operation, or until the battery discharges completely. When the rectifier/inverter fails or the inverter is overloaded, the load is temporarily powered via the UPS static bypass switch. To ensure uninterrupted switching, the inverter output must be synchronized with the static bypass power supply. This type of UPS provides excellent power output performance. However, the continuous operation of the power components reduces reliability compared to other designs. Furthermore, the input power from a large battery charger is typically non-linear, which may interfere with the building’s power supply system or cause the backup generator to fail.

Some double-conversion online UPS systems also have an Economy Mode (ESS) operating mode. This mode determines whether the UPS operates in Economy Mode based on whether the mains power quality exceeds the ITIC curve . The ITIC curve is a voltage tolerance curve and four related voltage disturbances (voltage drop, voltage rise, spike, and power outage) proposed by the Information Technology Industry Council (ITIC) to address the power quality requirements of computers and prevent equipment failure and damage. The ITIC curve has been adopted by IEEE as a US power quality standard and is also applicable to countries worldwide.

When the mains power quality is good, meaning the mains power specifications are within the range specified by the ITIC curve, the double-conversion UPS can operate in Economy Mode (ESS), where the UPS directly supplies power to the load via a static switch. When there is a mains power failure or quality degradation, and the mains power specifications exceed the limits specified by the ITIC curve, but are within the allowable fluctuation range of the UPS rectifier input voltage, it switches to double-conversion mode within 2ms. In ESS mode, the rectifier and inverter remain in a special “ready” state, meaning their control circuits are fully operational, such as continuing to synchronize the inverter with the mains power, but the power semiconductors of the rectifier and inverter are turned off, i.e., no drive signal is received. Therefore, no switching operation occurs. Because they do not handle power, no losses are generated. When mains interference occurs, it can switch back to double-conversion mode within 2ms without affecting the load. Bypass mode differs significantly from ESS. In bypass mode, the rectifier and inverter stop operating. Switching back to double-conversion mode from bypass requires the inverter to start and synchronize with the mains power, which takes a long time.

In data centers, parallel redundant UPS systems consisting of single double-conversion UPS units are generally used . When the data center load is powered by dual power sources, parallel redundant UPS units are used to form a dual-bus UPS power supply system .