How to choose transformer based and Transformer less UPS

UPS system has become vital component for managing critical downtime of all industrial applications and guarantee the power continuity. The question is what topology UPS has to use –

·         Transformer based UPS

·         Transformer less UPS

Transformerbased UPS are basically a technology where transformer is inbuilt at the output of UPS and provides galvanic isolation between the rectifier and output of the UPS, that helps in connecting two sources to the rectifier and bypass without any disturbances to straight to the earthing system.

The rectifier and the bypass inputs are the two AC inputs UPS has which can be connected as a form of common input inside the UPS. Normally, when main are available, the normal power flows via rectifier, inverter and transformer to the output which supplier critical load which is termed as “Double conversion mode”.

In this mode, the battery is constantly is kept fully charged and at the time of power outrage, this battery feeds the inverter which supplies power to critical load without any interruption via transformer.

·         Flexibility to connect two independent sources for increased availability to have redundancy of power at the UPS to the loads and to have the possibility of concurrent maintenance of the sources.

·         Galvanic isolation of DC (Battery) from the AC input

·         Short circuit characteristics of UPS

·         Safety margin in design

·         Local grounding of neutral

·         High efficiency

 Transformer less UPS is a technology is without galvanic isolation between rectifier input and output. An external isolation is required to have flexibility to connect two different sources to the rectifier and bypass unit that supports power to flow to the existing earthing system without any interruption due to which the neutrals of both sources gets connected together and leads to  -

-         In-active of ground fault detectors.

-          EMCs issue due to circulating current on the neutral of both sources.

-          Nuisance tripping of ELCS’s upstream of the UPS.

In the event of short circuit, Transformer less UPS is based on the external current limiting circuits of UPS. To have the similar characteristics like Transformer based UPS. The higher rating IGBT technology is selected wherein Transformer based UPS is based on the devices and impedance of transformer that have better phase to neutral short circuit and it support the fault via two primary windings.

	Transformer based UPS		Transformer less UPS (with external transformer)
		UPS efficiency	UPS efficiency	Transformer efficiency	UPS global efficiency
Full load	No isolation	92-94%	93-96%	Not required	93-96%
	Two different sources	92-94%	93-96%	97%	92-93%
	Full isolation	92-94%	93-96%	97%	92-93%
50% load	No isolation	91-93%	93-95%	Not required	93-95%
	Two different sources	91-93%	93-95%	97%	90-92%
	Full isolation	91-93%	93-95%	97%	90-92%
25% load	No isolation	89 - 91%	90-92%	Not required	90-92%
	Two different sources	89 - 91%	90-92%	97%	88-90%
	Full isolation	89 - 91%	90-92%	97%	88-90%