UNDERSTANDING CASCADE VS PARALLEL

The output of UPS A is fed into the bypass input of UPS B as shown in the drawing below. In the event that UPS B should malfunction, UPS A is always in a hot-standby position. In the extremely rare event that both units fail, the load will be transferred to the bypass source to maintain power to critical loads.​

​PROS: The transfer is natural to the design of the UPS. Provides very high reliability and maximum redundancy as four total power sources are available to the critical loads:

1) AC Main Input Source

2) UPS A Battery Bank

3) UPS B Battery Bank

4) Bypass Source

CONS: Each unit must be correctly sized to handle the entire load

UPS and Inverters in a Parallel redundant configuration require both units' outputs to be connected together. This is typically done by using a paralleling cabinet which consists of two inputs and a single output, as well as inductors that allow for load sharing during parallel operation.

PROS: Can be used to double the output of a UPS and can be a cost effective method when paralleling for capacity. Ex: Two 50kva UPS can be paralleled for a total output capacity of 100kva.

CONS: Not a very reliable means of redundancy. In an event that one unit unexpectedly transfers to a bypass source, the remaining unit can potentially be back-fed by the bypass source resulting in cleared fusing or internal damage. This is because the bypass source is typically a stiffer source of commercial A/C power. There are also additional losses due to the added inductors that are required to allow the units to share the load properly.

Modular Systems in Parallel: (Also known as an N+1) Modular systems are designed to provide a low cost solution with the reliability of a redundant system, however this design falls short. Their inherent design doesn't contain the same type of de-rated components as an independent or standalone UPS. The tight margin and low reliability of each individual module requires them to be designed in a parallel configuration. This in turn, reduces their overall MTBF, and while the initial cost of a modular system may be less, the cost of ownership and risk of premature failure is much higher.