LeapZipBlog: Servostabilizer's blog: Why isolation transformer is used at the output of online UPS?

Why isolation transformer is used at the output of online UPS?

May 21, 2019 by Servostabilizer  

Isolation transformers as per their name isolate the electrical device from their power source to provide complete safety to them from the harmonics present in the bus which is feeding the device. These transformers rather also eliminate harmful harmonics generated from the device to pass back onto the source or the distribution bus. Thus isolation transformer acts likes an interface between the source and the device and saves both of them independently from each other’s harmful harmonics, noise, poor signal impact or power quality. In this manner, it acts as a safeguard for the equipment and bus both.

Online UPS is the uninterrupted power supply which works online. These UPS keep a tab on the incoming power supplies quality and rectify them to provide clean power at the output thus saving the devices connected. The online UPS is generally connected with an isolation transformer at its output to safeguard the connected load from the UPS as well as to save the UPS from the faults arising downstream.

There are many reasons for connecting an isolation transformer at UPS output, but let’s chalk out a few worth discussing from them -

1)    Different system voltages – The UPS may be designed for different outputs as per the requirement. Therefore, it is highly probable to connect a UPS with different output to the load than required by the load and put it in the risk zone. Thus it is advisable to provide with an isolation transformer at the output of the online UPS as an extra safeguard of the equipment connected.

2)    High resistance grounding – The UPS supplies are generally on high resistance grounding systems while the load requires a solid grounding system. Therefore it becomes vital to isolate them and thus isolation transformer is used at the output of online UPS.

3)   Eliminating fault current – The isolation transformer connected at the output of the online UPS protects the UPS from fault currents generated at the downstream equipment. Otherwise, any fault current generated downstream will result in damage to the UPS also along with the equipment failure.

4)    Easy and safe maintenance – The isolated transformer connected at the output of online UPS makes it easier and safer for the maintenance of the load connected as well as of the UPS itself.

Most of the UPS are ‘buck and boost’ type with the battery connected to the rectifier in parallel rather than connected in series between the rectifier and the inverter. Thus the UPS earthling makes the unit useless for isolation of harmonics. This leads to violation of critical infrastructure norms to have continuity of ground conductors. Therefore in such a case, a separate grounding is provided for the equipment connected.

These isolation transformers though provide safety to the connected load in more than one way or the other, but at the same time they also, make the entire system voluminous and bulky making them acquire more space in addition to making them more expensive. Therefore, it is advisable to put a UPS with an isolation transformer exclusively for sensitive devices where noise on the grid is harmful to the correct functioning of the instrument in addition to making it prone to damage. This generally happens in the case of costly and sensitive medical equipment which may include various monitoring devices such as for heart and life monitoring.