UPS Applications in Data Centers ( Part 3 )

Parallel redundant UPS system

A parallel redundant UPS system consists of two or more individual UPS systems, with the outputs of each UPS connected in parallel to a common bus. The system is typically configured as N+1, where N individual UPS units supply the entire load, with one additional unit added as a backup; this is called an N+1 parallel redundant system.

Even if only one unit fails, the N+1 system can still function normally. An N+1 parallel redundant system can expand output capacity and add redundancy. Multiple units connected in parallel can also be expanded without increasing redundancy; this is called an N+0 parallel system or an “N parallel capacity system”.

In a parallel redundant UPS system, all individual UPS units operate in parallel, sharing the load equally. If one individual UPS unit fails or needs to be disconnected from the system for maintenance, the remaining individual UPS units have sufficient capacity to supply the load, ensuring uninterrupted power supply. Therefore, it is not necessary to switch the load to a bypass power source.

Because the output of each individual UPS unit is synchronized with its bypass power supply, if the bypass power supply for all individual UPS units is the same AC power source, the individual UPS units will naturally operate synchronously. However, considering that during a AC power outage, each individual UPS unit will synchronize with its internal crystal oscillator, in which case they will not synchronize naturally. To ensure that all individual UPS units can operate synchronously and the load is evenly distributed under any circumstances, necessary synchronization methods are required, such as master-slave synchronization or masterless synchronization.

Based on the bypass type of parallel redundant UPS systems, parallel redundant UPS systems can be divided into centralized bypass parallel redundant UPS systems and distributed bypass parallel redundant UPS systems .

Centralized bypass parallel redundant UPS

In a parallel redundant UPS system with centralized bypass, each individual UPS unit does not have a static bypass. The entire parallel redundant UPS system is configured with a centralized static bypass and maintenance bypass, installed in a separate parallel cabinet. The capacity of the static bypass switch and maintenance bypass switch should be configured to meet the system output capacity; that is, the capacity of the bypass switch in an N+1 parallel redundant UPS system should be greater than or equal to the capacity of N individual UPS units. The diagram below shows a parallel redundant UPS system with centralized bypass, consisting of three individual UPS units and a parallel cabinet equipped with a static bypass switch and a maintenance bypass switch.

Distributed bypass parallel redundant UPS system

In a distributed bypass parallel redundant UPS system, each individual UPS unit is equipped with a static bypass switch. No parallel cabinet is needed; the individual UPS units can be directly connected in parallel. The diagram below shows the structure of a distributed bypass parallel redundant UPS system.

Dual bus UPS power supply system

Dual-bus UPS systems are best suited for powering dual-power loads, as power can be drawn from each of the two buses to the dual-power load. For single-power loads, a static transfer switch (STS) is required. Powering a single-power load via the STS improves power supply reliability.

As shown in the diagram, UPS1, UPS2, UPS3, and UPS4 are two independent UPS systems, with their outputs forming two independent buses, each capable of handling the entire load. The load is unaffected by the failure of one bus. The UPS outputs supply power to the server via a Power Distribution Unit (PDU). Loads with dual power inputs can draw power directly from both buses via the PDU. Loads with single power inputs are powered via a Static Transfer Switch (STS). When the bus currently supplying power fails, the STS automatically switches to the other bus. This power supply system achieves an availability of 99.9999%.