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Wednesday, October 31, 2007

How to recognise online UPS’ – Kleynhans

Regional director for Africa at APC-MGE, Mr. Carl Kleynhans, has explained the difference between the two kinds of online uninterruptible power supplies (UPSes) by APC, so as to afford the consumers the opportunity of having clear picture of what could solve their needs.

According to him, APC-MGE has two online UPS, namely the Double and Delta Conversion respectively.

For him, the Double Conversion On-Line UPS is the most common type of UPS above 10kVA and operates in much the same manner as the Standby UPS except that the primary power path is the inverter instead of the AC mains.

He added that in the design of Double Conversion On-Line operation, failure of the input AC does not cause activation of the transfer switch.

This, he said, is because the input AC is not the primary source, but is rather the backup source.

“Therefore, during an input AC power failure, on line operation results in no transfer time,” he said.

He stressed that the on line mode of operation exhibits a transfer time when the power from the primary battery charger, battery, inverter power path fails.

This, he explained could occur when any of the blocks in this power path fails, hence the inverter power could also drop out briefly, causing a transfer, if the inverter is subjected to sudden changes in the load, or if the inverter experiences an internal control glitch.

“Contrary to popular belief, Double Conversion On-Line UPS systems do exhibit a transfer time and in actual installations may transfer as frequently as standby type UPS systems; however on line UPS transfers are not related to AC input power failures as they are in a standby UPS,” he said.

Mr. Kleynhans emphasised that both the battery charger and the inverter convert the entire load power flow in this design, which causes undesirable heat and results in reduced efficiency.

Due to practical design constraints, he said, that UPS below 10kVA, which are represented as Double Conversion On-Line UPS are always working like the Standby On-Line Hybrid type.

On the Delta Conversion On-Line UPS, he said, was designed via a technology that was recently introduced to eliminate the drawbacks of the Double Conversion On-Line design and is available from 5kVA and greater.

“Like the Double Conversion On-Line design, the Delta Conversion On-Line UPS always has the inverter supplying the load voltage,” he said.

However, he warned that the additional Delta Converter also contributes power to the inverter output and under this condition of AC failure or disturbances, this design behaves identically to the Double Conversion On-Line.

While during steady state conditions the Delta Converter allows the UPS to deliver power to the load with much greater efficiency than the Double Conversion design.

“It obviously saves energy to carry the package only the difference (delta) between the starting and ending points,” he said.

Even as the Double Conversion On-Line UPS converts the power to the battery and back again whereas the Delta Converter moves much of the power from input to the output directly.

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