Operation of parallel uninterruptible power supplies (UPS), whether parallel-redundant or parallel-capacity, centres on such criteria as mode of operation (supply present or supply failure), battery operation, handling of supply restoration, fault condition and overload. It also depends upon whether they are connected to a single or dual input power supply.
The rectifier and static switch bypass supply inputs are taken from a common ac supply point (building incomer) and each uninterruptible power supply module is fitting with isolators or circuit breakers to allow individual disconnection. Each module will also have its own battery set.
The output of each UPS module is taken to a common ac busbar where it is then distributed to critical loads. This configuration of UPS is also fitted with an emergency bypass switch (switchgear panel). For future expansion, a spare connection point is built into the switchgear panel.
Operational Mode
In normal operation, where mains power supply or back-up power (diesel generator, for example) is present, each of the UPS modules shares the load equally.
During battery operation, when mains power and/or back-up are no longer available, each of the uninterruptible power supply modules in the parallel-redundant configuration will continue equally to share the load. Each one has its own battery set with the same runtime duration and dc voltage cut off threshold. However, each battery set will discharge at slightly different rates. During extended supply failures, the battery sets will discharge until they reach their dc disconnect threshold.
Uninterruptible power supply modules will automatically restart when mains power is restored. To avoid upstream breaker tripping, start up sequence includes a soft-start action to reduce the effects of high in-rush current.
Should any of the UPS modules detect an internal fault, they will automatically disconnect from the common output, in which case the remaining modules in the configuration pick up the load (without disruption) and share it equally.
If a second module in the parallel configuration fails, it will force the remaining modules into an overload condition and the load will be transferred to bypass supplies via a static switch. This method of parallel UPS operation ensures the load receives an adequate source of power automatically and without disruption.
When faced with an overload, the entire uninterruptible power supply will react in one of two ways – depending upon the magnitude and duration of the overload. Firstly, UPS inverters have overload capabilities, so in the case of a small overload (within 100 – 150% of nominal rating), the UPS system will continue to power the load from its inverter.
Secondly, for larger overloads that exceed the capabilities of the uninterruptible power supply, all the modules will automatically switch to bypass in an attempt to clear the fault. They will remain like this until the fault is cleared or, if it continues, shutdown.
It is important to point out here that when in bypass, system resilience is compromised, which is why modern uninterruptible power supplies have been designed and programmed to generate alarm notifications, which can be monitored locally, via a network, or remotely.
Emergency bypasses are commonly installed in parallel UPS configurations to allow modules to be electrically isolated and powered down when necessary.
With a dual input supply, the UPS system is supplied from separate rectifier and static switch sources, which helps to enhance resilience and negates the single-point-of-failure associated with single supply.
Operation of a parallel-capacity uninterruptible power supply is the same as a parallel-redundant system – except for how it handles UPS module failure. In this case, if one module fails, all of the remainder, plus the faulty one, will be forced into overload condition and the load transferred to bypass. This ensured that the load continues to receive adequate power automatically and without disruption.
There are a couple of useful accessories that can be incorporated into parallel UPS operation: a UPS Group Synchroniser (UGS) and a Parallel Systems Joiner. A UPS Group Synchroniser synchronises the outputs from two separate groups of parallel UPS, which may be supplied from separate ac sources. It enables the outputs from the groups to be established into a dual bus format. A Parallel Systems Joiner operates in a similar vein but enables the connection of the two separate groups into a dynamic dual bus format, which facilitates system expansion, fault tolerance and ease of maintenance.
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