OnLine Power Power Wave 4 - Manual (Page 83)

6005-145 Rev. A 8-2
ONE TO ONE SERIAL REDUNDANCY
Serial (hot standby) redundancy consists of two ELI with one ELI’s (ELI1)
connected to the reserve/bypass input of the other ELI (ELI2). See the above figure.
Both ELIs are running in normal mode under normal conditions. When one of them
has a problem, the load will still have protection from the inverter and battery. If
ELI1 fails and ELI2 is running normally, the load is unaffected, and supplied from
ELI2. If ELI2 fails and ELI1 is running normally, ELI2 will transfer the load to
ELI1 and the load will continue to be protected by the inverter and battery of ELI1.
If both of them are running normally, ELI2 takes up all load and ELI1 bears no load.
Therefore, ELI1 has a longer MTBF than ELI2 (can be interchanged after a period
of time), and their MTBF multiplies to a very large MTBF.
This type of redundancy is employed most frequently. When mains fail, ELI2 is the
first to contribute its battery to back-up the load. When ELI2’s batteries are
exhausted, the load is transferred to ELI1. Therefore, this topology can make full
use of the batteries of both units. Users get a system that has redundant protection
and double the back-up time.
Another less expensive solution (see the figure below) is available if the load is
separated. A user may intend to install two ELIs, each with the same power, and
supplying half the load. A third ELI can be installed as backup to two main ELIs. It
is statistically improbable that both main ELIs concurrently break down. Therefore,
the third ELI can serve as a hot standby to two ELIs.
These topologies make full use of the resources to create the best protection and
longest MTBF.