Emerson 7400 One plus One Parallel Control, Synchronisation, Current sharing, Reverse current

Series 7400 Single Phase 1+N UPS

1.2.6One plus One Parallel Control

When two of the standard 7400 modules just described are connected together to form a one-plus-onesystem, each module is fitted with an additional circuit board which allows the two modules to communicate with each other. Communication takes place via a single ribbon cable connected between the modules as illustrated in figure 4.

Note: - Modifying a standard module for use with a one-plus-one system also involves replacing certain other circuit boards with modified versions, and relocating certain minor assemblies. This means that although it is not impossible to modify an existing 7400 module to form part of a one-plus-one system it is not a straightforward proposition.

The inter-module parallel control responsibilities are complex but can be summarised as follows:

Synchronisation:

As the outputs from both UPS modules are connected together to provide a single load supply, it is imperative that the inverters are fully synchronised both in frequency and phase. This is achieved by digitally locking the two inverter control oscillators. Similarly, as has already been mentioned, it is necessary for the inverters to be synchronised to the bypass supply to enable a “no-break” transfer to be achieved when the static switch transfers the load to the bypass supply. The inverter control oscillators are therefore not only locked together but are also made to track the bypass frequency.

Current sharing:

The parallel control circuit compares the module’s output current with that of its partner and is thereby able to effect current sharing between the modules by making fine adjustments of an individual module’s output voltage.

Redundancy configuration:

A link in the parallel control logic determines whether the one-plus- one system operates in a “redundant” or “non-redundant” configuration. If a non-redundant mode is selected the two static switch sections are effectively locked together in that both the static switches are turned off or on by a single control signal. Thus if one module develops a fault, when running, its static switch control logic will transfer its output from the inverter to the static bypass line and simultaneously send a signal to the static switch control logic in the second module to do likewise. This does not happen if the system is configured as a redundant system, in which case the second module is allowed to continue supplying the load from its inverter when the first module trips its inverter off line.

Reverse current:

A reverse current monitor circuit detects current flowing into, rather than out of, the module’s output terminals. Such a condition can arise if a module develops an internal power fault or if for some reason the two modules become unbalanced, and is liable to further damage the module and also degrade the load supply. If a reverse current is detected the inverter on the affected module is immediately shut down and load transferred to the bypass supply depending on the system redundancy configuration.

7