TS_622_02 PowerWAVE 9500DPA User Manual 26/2/19
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System expansion
Some UPS applications present a low initial power requirement which increases over time as the application grows; so it is
essential that the installed UPS system can be expanded to meet the growing demand without compromising the existing
load. This requirement is well met with the ‘hot swappable’ feature of the PowerWAVE 9500DPA UPS, whereby an
additional module can be inserted into a vacant slot in an existing cabinet without disturbing the load.
Note: If the expansion requires an additional cabinet the system will have to be shut down while the cabinet is installed.
‘Capacity’ versus ‘redundant’ module system
A parallel UPS system can be operated as either a ‘capacity’ or ‘redundant’ module system.
A ‘capacity’ system is rated such that ALL the UPS modules are required to furnish the specified full load power and the
loss of one module will automatically transfer the load to the bypass supply.
In a ‘redundant-module’ system, the system contains at least one UPS module over and above that required to supply the
full load and it is therefore possible to lose one module without needing to transfer the load to the bypass supply or in any
way disrupt the UPS output. A system operating with a redundant module is inherently the most reliable.
A parallel UPS system operating with one redundant module is known as an ‘N+1’ system.
Parallel control bus
All the UPS modules within a cabinet, and between cabinets, are connected to a parallel control bus which carries several
control signals used for frequency synchronisation, load sharing etc, as shown in Figure 2.9. This allows each UPS
module to electronically compare its own frequency and output current with that of its neighbouring module and make any
necessary fine adjustments to its control system to achieve balanced conditions across the system.
The parallel control system observes one UPS module as being the ‘master’ and the others as ‘slaves’. However if the
‘master’ module goes faulty the next module in the chain (a former ‘slave’) will immediately take over the role of ‘master’
and the former ‘master’ module will turn off. The ‘master/slave’ configuration is set during commissioning.
2.5
Xtra VFI mode
2.5.1 Introduction
The parallel control mechanism described above ensures that the operational UPS modules in a PowerWAVE 9500DPA
system equally share the load current. However, when the connected load is small in comparison with the system
capacity, the current required from each module can be sufficiently low that it adversely affects the module’s efficiency.
The graph below shows that the efficiency of a standard PowerWAVE 9500DPA UPS module remains at around 96% for
loads of 90% down to approximately 35%, but thereafter the efficiency begins to fall off to approximately 88% @7.5%
load. This reduced efficiency at low load represents wasted energy and expense, and is alleviated by the Xtra VFI feature
which dynamically controls the number of on-line UPS modules in line with changes in load demand.
Xtra VFI calculates the number of UPS modules necessary to supply the prevailing load such that current drawn from
each one places it in the higher range of its efficiency curve and only turns on the required number of modules. Surplus
modules are placed in a ‘standby’ mode, with their inverters turned off, and can be brought back on-line within 40~50ms
when the system load increases. The overall effect of Xtra VFI can improve the system efficiency by up to 5% at very light
loads, as shown in Figure 2.10.
Xtra VFI is an in-built feature, available in every PowerWAVE 9500DPA system, and can be enabled or disabled by a
qualified service engineer with access to the password-protected Service menu on the system control panel.
Key Point:
When planning a multi-cabinet system, it is not necessary to fully populate one cabinet with UPS
modules before installing the next cabinet. For example, if it is known at the outset that a 400kVA initial load
requirement is likely to increase to a maximum of 900kVA, it makes sense to install and cable-up two cabinets
and distribute the initial requirement of four UPS modules between them then add further modules as required.