TS_622_02 PowerWAVE 9500DPA User Manual 26/2/19
9
:
2.3.3 UPS system operating modes summary
Section 2.3.2 described the operating modes for the individual UPS modules within the PW9500DPA cabinet: but entire
UPS systems are also categorised according to the way in which they operate at a ‘system’ level, and are typically
described as being either an ‘on-line’, or ‘off-line’ (‘line interactive’) system.
The PowerWAVE 9500DPA can be operated in either of these categories.
On-line UPS system
An ‘on-line’ UPS system provides the highest degree of load protection, especially if the utility mains supply suffers a
disturbance or complete failure, and we always recommended this mode of operation if the critical load will not tolerate
even a very brief supply interruption – e.g. in the case of a computer system.
When the PowerWAVE 9500DPA is used as an ‘on-line’ system, the UPS modules normally operate in their ‘on inverter’
mode (Figure 2.4), and switch to the ‘on battery’ mode if the input mains supply fails (Figure 2.6). The changeover to
battery operation is totally transparent at the UPS output and an audible and visual alarm warns the operator that the
battery is discharging to enable any intervention to be taken to protect the load integrity.
The UPS then continues to provide its rated output until the battery discharges to a low cut-off point at which time the UPS
attempts to switch to its ‘on bypass’ mode (Figure 2.7). If the bypass is unavailable when the UPS attempts to transfer to
it, the UPS shuts down in a controlled manner.
It is usual, especially in larger installations, to provide an alternative UPS input supply from a standby generator which
starts automatically following a utility mains failure; and where this secondary power supply is implemented the batteries
only discharge for a short period, until the generator comes on-line. This not only avoids the UPS shutting down due to a
fully discharged battery but also helps maximise the battery life cycle.
If the UPS experiences an internal fault during ‘on-line’ operation, the inverter turns off and the static switch transfers the
load to bypass mains automatically and without interruption – provided the inverter is synchronised to the bypass. If the
problem is due to an output overload the inverter can supply the overload for a limited time, depending on its severity, and
if the rated time is exceeded the UPS transfer the load to bypass. The additional power available from the bypass supply
will attempt to clear the overload but if it persists it will ultimately rupture the bypass mains supply fuses. However, if the
overload condition clears while operating on bypass the UPS re-transfers the load to the inverter and the UPS returns to
its normal ‘on-line’ mode of operation.
Off-line (On stand-by) UPS system operation
When the PowerWAVE 9500DPA is used as an ‘off-line’ system, the UPS modules are normally operated in their ‘on
bypass’ mode (Figure 2.7) with the load supplied via the static bypass line. However the rectifier and battery charger are
still powered up and maintain battery charging, and the inverter section is turned on and operating on standby.
Operating in this mode is slightly more energy efficient than when operating in the ‘on-line’ mode due to the reduced
rectifier and inverter losses during normal system operation; and it is sometimes referred to as the “ECO” (economy)
mode. However, this mode is recommended only if the connected load equipment can tolerate power interruptions of up to
3~5 ms during the load transfer period.
If the bypass supply fails, the inverter is immediately brought on line and the load is transferred from the static bypass line
to the inverter within 3~5 milliseconds. If the UPS bypass mains and input mains are connected to separate sources (dual
feed) and the input mains is still live when the load is transferred, the UPS modules will operate in their ‘on inverter’ mode
(Figure 2.4). However, if the input/bypass mains supplies are connected to a common feed, or the input mains is
unavailable in a dual feed system, the modules immediately revert to the ‘on battery’ mode (Figure 2.6).
When the bypass supply returns to normal, the load re-transfers to the static bypass line (without a break) and the inverter
returns to its standby operation.
Note: if the bypass is unavailable it is unable to take over the load supply if the inverter fails, or assist the inverter handle
an output overload. It is therefore important that the cause of the load transfer from bypass to inverter is quickly rectified.
WARNING:
The ON-LINE mode should always be used for critical load protection.
