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Operation of parallel UPS and power protection systems

Operation of parallel UPS

The operation of parallel uninterruptible power supplies (UPS), both parallel-redundant and parallel-fed, depends on criteria such as mode of operation (power present or absent), battery operation, operation with line regeneration, fault condition, and overload. It also depends on whether they are connect to a single or dual input power supply.

The rectifier and static bypass switch supply inputs are taken from a common AC supply point (building input), and each UPS module is equipped with disconnects or circuit breakers for individual disconnection. Each module also has its own set of batteries.

The output of each UPS module routed to a common AC bus, from where it is distribute to the critical loads. This UPS configuration also equipped with an emergency bypass switch (distribution panel). An additional connection point is integrate into the panel for future expansion.

Operating mode

In normal operating mode, with the mains supply or with an emergency power supply (e.g. diesel generator), each UPS module distributes the load evenly. Visit also: APC Easy UPS 3M  

In battery mode, when mains power and/or backup power is no longer available, each UPS module in the parallel reserve configuration will continue to share the load equally. Each has its own battery set with the same runtime and DC trip threshold. However, each battery set discharges at a slightly different rate. During extended power outages, the batteries discharge until they reach their DC trip threshold.

The uninterruptible power supply modules automatically restart when power is restore. To avoid tripping the breaker, the startup sequence includes a soft start. To reduce the effects of a high inrush current.

When a UPS module detects an internal fault, it automatically disconnects from the common output. In this case, the other modules in the configuration take over the load (without failure) and share it equally.

If the second module in the parallel configuration fails, it overloads the remaining modules. And the load is transfer to the bypass power supplies via a static switch. This method of operating the UPS in parallel ensures that sufficient power is automatically supplied to the load without interruption.

When an overload occurs, the entire UPS responds in one of two ways – depending on the size and duration of the overload. First, the UPS inverters have an overload capacity so that in the event of a small overload (between 100 and 150% of rated capacity), the UPS system continues to supply power to the load through its inverter.

Second, in the event of large overloads that exceed the UPS capacity, all modules automatically switch to bypass to clear the fault. They remain in this state until the fault is correct or, if it persists, a shutdown occurs.

Therefore, modern uninterruptible power supplies are design and programme to issue notifications in the event of an emergency that can monitored locally, over the network, or remotely.

Emergency bypasses are typically install in parallel UPS configurations. So that modules can electrically isolated and power can shut off as needed.

With twice the input power, the UPS system is power by a separate rectifier and static switching sources. Which contributes to fault tolerance and eliminates the possibility of a single point of failure. Associated with a single power supply.

The operation of an uninterruptible power supply with parallel capacity is similar to that of a system with parallel redundancy. Except for the way a failure of one UPS module is handle. In this case, if one module fails, all others, as well as the failed module. Enter an overload state and the load is switch to bypass. This ensures that the load is automatically supplied with sufficient power without interference.

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