Parallel operation describes a configuration whereby the outputs of two or more uninterruptible power supply modules are connected to supply the load via a common ac bus bar. For it to be successful each UPS module must be capable of parallel operation and not all are. To work in this configuration, modules need to be fitted with parallel cards, which are then interlinked via opto-isolator cables to form a closed loop data communication system. This allows data to be exchanged between the control sections of each uninterruptible power supply module. They can still be operated in isolation if necessary but the card will enable the whole group to be operated as one system. A group of parallel UPS modules like this is referred to as a UPS system and there are two basic configurations: parallel-redundancy and parallel-capacity.
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