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IEE regulations recognise flaw with static UPS

A Power Systems International product story
Edited by the Engineeringtalk editorial team Oct 17, 2008

IEE Wiring Regulations require that static UPS must be capable of operating circuit protective devices when the inverter is operating in battery (emergency) mode during a mains power failure.

The latest (17th) edition is effective from 1 July 2008.

This means that it is no longer acceptable for distribution system designers to rely on fault clearing by the static bypass circuit of a UPS system when the inverter reaches its current limit handling a downstream fault condition.

The normal mode of UPS system operation is the mains power feeds the rectifier and bypass, the inverter is connected to the downstream distribution system critical loads.

The rectifier/battery charger keeps the battery maintained in the fully charged condition to provide stored energy to the inverter upon mains failure or during mains disturbance conditions.

If the inverter of the UPS system is, for example, 60 per cent loaded and a downstream fault should occur, the inverter has only 40 per cent of its rated capacity available, plus 25 per cent peak overload current to clear the fault.

Where the downstream fault condition imposes a sustained current level in excess of the available capacity of the inverter the 'current limit' set point in the inverter would be reached and the load is transferred without break in power to the bypass for fault clearing using the mains power source feeding the bypass.

But, what happens when there is a mains failure? There is no power source feeding the bypass, the battery is no longer being charged.

If the inverter is nearly fully loaded, the downstream fault clearing by the inverter then becomes a very serious consideration especially with transformerless inverter UPS systems.

Transformerless inverters in UPS systems do not have the benefit of inductance and stored energy from integral output transformers as used in the conventional and industrial UPS systems designs.

This change in the IEE regulations now poses interesting questions for distribution system designers to ask potential suppliers of UPS systems.

Simply, what ampere rating of fuse can be opened by the inverter, for example, when 90 per cent loaded and where a downstream fault occurs in one of the sub-circuits, and what will be the voltage reduction during the fault clearing process.

Engineers attending factory-acceptance tests should consider including this item of fault clearing by the inverter with the rectifier and bypass disabled as an important part of their test procedure protocol.

The RT and CEG series of industrial UPS systems from Power Systems International have fully rated inverters with integral output isolation transformers and have the abilty to clear downstream faults protected by F type fuses rated at 30 per cent of the 'In' output current of the inverter without the rectifier or bypass AC power source connected.

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