Comparison of Reliability of Power Distribution System with UPS & STS

In an conventional UPS configuration with N+1 configuration, where N=2, we put in place a parallel configuration with 3 UPSs in parallel and an common output from the parallel bus is taken and provided as a input to the critical loads.

Although this configuration provides the redundancy at UPS level, there is still a possibility of a single point of failure, which is the common parallel bus (a rarest scenario).

Disadvantages of the configuration includes

o Concurrent maintenance is not possible
o Critical parallel bus is not a fault tolerant design, should there be fault generated down stream, the same can be propagated to the critical bus impacting the availability for other critical loads.
o Single point of failure in parallel bus

These above mentioned disadvantages can be overcome by optimising the system design using an STS and the common parallel bus is replaced with a three independent STS with a criss-cross connection as shown in the below table.

This configuration helps us to have

• a concurrent maintenance of the distribution system as the UPS can be removed without disturbing the load.

• compartmentalisation of fault in the downstream distribution of the load is possible as the STS doesn’t allow the fault to transfer(example a fault in load connected to static switch C, will impact only the loads connected to that specific static switch).

• no single point of failure as we migrate to multiple N configurations instead of conventional N+1 configuration.