UPS
If the facility is ‘ shut-down tolerant ’ then it can survive with a UPS alone . During short term blackouts , the UPS simply switches its inverter to battery power on detection of utility supply failure . If power is restored well within the battery autonomy , the inverter is switched back to the incoming mains supply . The load has enjoyed uninterrupted operation , with complete protection from the temporary problem .
However , if the blackout period starts to threaten the battery autonomy , the UPS has no choice but to signal the load , allowing it to shut down gracefully in the remaining battery time . System hardware and data has been protected from damage , although the organisation suffers from lack of ICT resource .
If loss of this resource is unacceptable , the only solution is a UPS in combination with a back up generator , which , with sufficient fuel , can outlast any blackout .
Such generator-UPS combinations need reliable communications between the generator , UPS and critical load , as well as a suitable control system on the generator . During short duration blackouts , the UPS uses its battery power to avoid unnecessary generator start-ups . If a blackout becomes extended , the UPS signals the generator through an Automatic Mains Failure ( AMF ) panel to start up . The UPS battery autonomy allows the generator to come up to speed and synchronises with the UPS ’ s voltage output , while the load suffers no power interruption . Once the generator is online , the UPS takes its power to feed the load and recharge its batteries . When the UPS detects restoration of mains power , it typically waits for a few minutes before switching back to the mains . This is to ensure that the power is sustained rather than the temporary result of , a utility company fault location procedure for example .
The generator itself must be adequately prepared in several aspects to perform successfully as a power back up component . It must have an electronic governor to regulate the engine speed and therefore the alternator output frequency , to ensure sufficient speed of response and accuracy . Mechanical governors are lower cost , but lack the responsiveness , accuracy and stability needed by the UPS and critical load .
If the generator ’ s output frequency is outside the UPS ’ s critical limits , or is changing too fast ( excessive slew rate ), the UPS will not be able to synchronise . It will also generate a warning that in the event of a fault the load will not be transferred from the UPS to the raw generator supply .
To avoid such problems , it is important to inform the generator supplier that their equipment is intended for use with a UPS , and ensure that it is designed and tested accordingly .
UPS topologies
While the UPS plays its essential part in securing blackout back up appropriate to an application , its role in assuring power quality during normal utility mains supply is equally as important . Different topologies , with varying impacts on power quality , are available . Similar to the ‘ generator or not ’ discussion , the ‘ best ’ solution is the one that makes most sense for the target application .
In some applications , for example , power quality is not seen as a critical issue , either because the equipment is resilient to transients , noise and other problems , or the operators have complete faith in the quality of the incoming power supply . In such cases , an ‘ offline ’ UPS configuration , as shown in Fig . 1 , may be considered as most appropriate . The key point is that during normal operation , the critical load is directly exposed to raw power , flowing from the mains through the bypass line and static switch . While there may be some transient suppression and radio-frequency filtering in the bypass circuit , protection is not inherent in an offline UPS design . If the mains supply transgresses preset limits , or fails , the load is transferred to the inverter output .
Offline designs are attractive in some circumstances because of their high efficiency , as the UPS components are bypassed during normal operation . Their capital
Mains Supply
Fig . 1 : Offline UPS topology
‘ Mechanical governors are lower cost , but lack the responsiveness , accuracy and stability needed by the UPS and critical load .’
Fig . 2 : Online UPS topology
Mains Supply
Bypass supply
Bypass – common bypass configuration
Charger
Rectifier
Battery
Battery
Inverter
cost may also be reduced due to use of lower rated components . However they are not viable in most applications , where sensitive ICT equipment simply cannot be exposed to raw mains supplies . Instead , these sensitive applications need an online UPS as shown in Fig . 2 . During normal mains operation , the incoming supply is fed through the UPS rectifier and inverter components , which means that the load receives processed power at all times . These components act as a barrier to mains borne noise and voltage transients , while also providing a well regulated output voltage . Additionally – and unlike the offline design – any transfer between mains supply and UPS battery is entirely invisible to the load , with no break in power .
Conclusion
Static Switch
Output to Critical Load
Normal operation
Above , we have seen how consideration must be given to both extended power blackouts and power aberrations during normal mains operation . The best possible protection , which is essential to most mission critical applications , comes from a power back up system comprising an online UPS and on site generator . Cost saving compromises as described can be made , but only if the site operators are entirely confident that their load will not be exposed to unacceptable risk as a result . The possible consequences of inadequate protection could far outweigh any savings made through reduced power back up specifications .
Bypass – split bypass configuration
DC Bus
Inverter
Static Switch
Output to Critical Load
Normal operation
April 2017 | 33