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The AC inputs of all UPSs in the parallel system are connected to the same mains, and all the AC outputs are
connected to each other. Each UPS has its own battery group. The critical load is connected to the common
output of the parallel system. There are also some signal cable connections between the UPS units necessary
for parallel operation, and will be described later.
4.2 Parallel Operation Modes of PDSP Series UPSs
NOTE : Parallel operation hardware (parallel controller boards and some other accessories) are not
installed in a standard PDSP series UPS, but parallel operation option is available and can
easily be added to all UPSs.
Parallel operation is possible in one of the three modes:
1) PARALLEL operation (power increase)
2) REDUNDANT parallel operation
3) N+1 REDUNDANT parallel operation
NOTES:
1- The preferred mode for parallel operation can be selected using OPTIONS MENU.
2- For parallel operation, each UPS must be assigned a unique UPS No again using OPTIONS MENU.
4.2.1. PARALLEL mode
This mode should be selected for increasing the available UPS power capacity. If this mode is selected for
parallel UPS operation, the total VA rating of the parallel system is the sum of the VA ratings of all the
UPSs connected in parallel. The output load is equally shared by each UPS in the parallel system. Mains
power is used as the spare energy supply (BYPASS).
In case of a failure in one of the UPSs, the Static Bypass Switches of all UPSs are turned on
and the load is transferred totally to the spare energy supply (usually mains voltage) without any interruption. If
the fault is cleared and if everything returns to normal operating conditions, the Inverters of all UPSs in the
parallel system resume the load together at the same time.
The output load of the parallel system is shared equally by each UPS, both during Inverter or Bypass
operation.
If this mode is active, you will see ‘’PARALLEL MODE/MS’’ on the LCD panel of one of the UPSs, and you will
see ‘’PARALLEL MODE/SL’’ on the LCD panels of all the other UPSs connected in the parallel system (MS
meaning Master and SL meaning Slave).
In PARALLEL mode, the Static Bypass Switches of all the UPSs in the parallel system act (turns on or off)
simultaneously. During normal PARALLEL operation, the load is shared equally by the inverter outputs of each
UPS provided that the output switches of all UPS units are turned on. In case of a failure in one of the UPS
units or if the output switch of one of the units is turned off, the static bypass switches of all the parallel units
are turned on simultaneously and the critical load is transferred to the bypass supply without any interruption.
In other words, during operation in PARALLEL mode, the parallel UPS system behaves like a single
(stand-alone) UPS with a higher power rating.
4.2.2. REDUNDANT mode
This mode is used for operation of two (or more) UPS units connected in parallel, for increased reliability. If
this mode is selected, the load is shared equally by each UPS available in the parallel system, during normal
operation.
In case of failure of one (or more) of the UPSs, the faulty UPS is isolated from the parallel system and the
critical load is still supplied by the remaining UPSs. If the faulty UPS returns to normal operation , it joins the
parallel system again and starts sharing the load.
In REDUNDANT parallel mode, the total VA rating of the parallel system is equal to the VA rating of a
single UPS used in the system. Only after all the UPS units in the system fail and are isolated from the
system, the critical load is transferred to Bypass supply without interruption.
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