Transformerless UPS technology, once established, rapidly became dominant because it enabled much smaller, lighter power solutions. These allowed uninterruptible power supplies to be built from several relatively small UPS modules rather than a single monolithic unit. The two key advantages of ‘True Modularity’ are scalability and rightsizing, and the ease of building in N+1 redundancy. Both are facilitated because true modularity allows total capacity to be adjusted in small increments.
However, not all modular UPS systems architectures are the same. In some, the UPS power supply modules are not truly independent; they share some common components, notably the module control circuitry and static switch. This approach is known as Centralised Parallel Architecture or CPA. Originally parallel UPS power systems with this architecture were the only type available due to the complexity of data and signal processing required to effectively control a decentralised parallel architecture UPS system.
Its attraction, even now, is that it offers a cost-saving advantage since duplication of uninterruptible power supply components and subsystems is reduced. Its major drawback, however, is that the centralised nature of the control and power switching components introduces a greater number of “single points of failure” into the power systems, which adversely affects its availability.
To overcome this, decentralised parallel architecture (DPA) has been developed to yield inherently greater availability; true DPA systems have few single points of power loss. All essential UPS power supply functions, including system logic, control panel and display, static bypass power units and UPS battery connections are provided within every module, allowing each module to work entirely independently. In most data centre solutions or other applications where the data processing load’s uninterrupted power supply uptime is absolutely essential, the extra component cost is considered as insignificant compared with the improved power protection it provides. The consequences of power loss could seriously damage a business enterprise, or even bring it down.
KOHLER PW 9500DPA 100 kW module
One argument sometimes advanced in support of the centralised parallel system is the fault clearing capacity of the Central Static Switch. However, modern decentralised parallel architecture UPS systems incorporate internal static bypass switches capable of matching or bettering the fault clearing capacity of the Central Static Switch.
Decentralised Parallel Architecture UPS Systems Configuration
UPS modules in a decentralised parallel architecture system are practically identical to stand-alone uninterruptible power supplies. Some uninterruptible power supply manufacturers design their uninterruptible power supplies to be used in either configuration without modification.
Decentralised parallel architecture UPS systems always have one module performing the master role with the other modules in the power system being Slaves. If at any time the Master becomes faulty or is isolated for maintenance or other reasons, the next uninterruptible power supply in the system (former Slave) will immediately take over the Master function and the former Master will switch off.
In DPA parallel UPS systems, all the modules feed the critical load directly. The capacity of uninterruptible power supplies depends upon the UPS module rating and the number of modules used. The uninterruptible power supply modules are usually mounted within a frame; by providing the frame with spare capacity, more modules can quickly be added for future load expansion or electrical redundancy. This expansion is easy because it can be done without having to modify or shut down the system.
By contrast, it may be necessary to upgrade the static switch and circuit breakers when adding UPS modules to a CPA system. Additionally, the centralised parallel system’s typical need for a separate central static switch (CSS) cabinet makes it physically larger, more complicated to install and more expensive than its decentralised parallel architecture UPS system counterpart.
When these considerations are added to the DPA UPS systems’ inherently superior availability, it’s not surprising that decentralised parallel architecture UPS systems have become the most popular choice.