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Mains Supply
Mains Supply
Mains Supply
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Bypass – common bypass configuration
Charger
Static Switch
Inverter
Battery
Bypass
Charger
Static Switch
Inverter
Battery
Bypass
Charger
Static Switch
Inverter
Battery
Fig . 1 : Off-line UPS topology
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Output to Critical Load
Normal operation
Output to Critical Load
Mains failure
Output to Critical Load
UPS failure
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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 ’ s important to inform the generator supplier that their equipment is intended for use with a UPS and ensure that it ’ s designed and tested accordingly .
UPS topologies While the UPS plays its essential part in securing blackout backup 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 the 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 off-line UPS design . If the mains supply transgresses preset limits or fails , the load is transferred to the inverter output .
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See Figure1 |
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Mains Supply
Mains Supply
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Bypass supply
Rectifier
Rectifier
Bypass – split bypass configuration
DC Bus
Battery
DC Bus
Battery
Bypass
Inverter
Inverter
Static Switch
Static Switch
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Output to Critical Load
Normal operation
Output to Critical Load
Mains failure
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Offline designs are attractive in some circumstances because of their high efficiency , as the UPS components are bypassed during normal operation . Their capital cost may also be reduced due to use of lower-rated components . However , they ' re not viable in applications where sensitive ICT equipment can ' t 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 off-line design – any transfer between mains supply and UPS battery is entirely invisible to the load , with no break in power .
See Figure 2
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Mains Supply |
Rectifier
DC Bus
Battery
Bypass
Inverter
Static Switch
Fig . 2 : On-line UPS topology
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Output to Critical Load
UPS failure ( on bypass )
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Best possible protection We ’ ve 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 backup system comprising an on-line 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 backup specifications . n |