UPS |
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Modularity offers many attractive features .
role , other components can be regarded as equally important . One of these is the static switch , which allows the UPS output to be transferred seamlessly between conditioned power and raw mains in response to different threats and failure modes . Below , we look at the static switch and how it contributes to UPS functionality .
Figure 1 shows how the static switch connects the critical load to either conditioned power from the UPS or raw mains from the bypass supply . In some installations , the bypass supply could be provided by an on site generator . The static switch has two operational states – ‘ On UPS ’ and ‘ On Bypass ’.
The static switch typically comprises a pair of inverse parallel connected SCRs linked in series with the bypass and inverter supplies as shown in Fig . 2 .
Fig . 1 : UPS components , showing static switch .
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UPS operational modes
The static switch ’ s role relates to the three operational modes for UPS systems ; ‘ online ’, ‘ offline ’ and ‘ lineinteractive ’. Products such as UPSL ’ s PowerWave 9000DPA can operate in any of these modes .
During normal online operation when the mains supply is present , mains power is used to feed the rectifier which generates a stable output DC voltage . This is used both to float charge the battery and supply the inverter . In the absence of the mains supply , the battery takes over and begins to discharge through the inverter which continues to support the critical load with well regulated AC power . This design is often referred to as ‘ double-conversion ’, due to the two conversion stages , AC-
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DC and DC-AC . It offers the highest possible level of critical supply integrity because the load is supplied with uninterrupted processed power from the inverter at all times . In addition to the battery ’ s support during blackouts , the rectifier and inverter protect the critical load from electrical disturbances originating from the mains supply .
While the UPS is online , using either mains or battery power , the static switch is in ‘ On UPS ’ state , connecting the inverter output to the load . However if a UPS fault should develop , causing either the voltage or frequency at the inverter output to exceed acceptable levels , or an overload beyond the UPS ’ s redundancy capacity appears , the static switch changes over without power interruption to ‘ load on bypass ’.
Once the overload is cleared or other problems are rectified , the static switch transfers the load back to the inverter output and normal operation continues . Overall , the online mode provides the highest possible level of power security , and is very strongly recommended for sensitive and critical IT loads .
However , offline operation is possible , especially if there is confidence in the mains supply ’ s quality and continuity . Offline mode means that during normal operation the static bypass switch is in ‘ On Bypass ’ mode with the load being fed directly from the bypass supply . The rectifier and charger remain powered to enable battery charging , while the inverter is also enabled and ready on standby .
If the bypass supply fails the static switch will transfer the load to the inverter output – within 3 – 5 ms for the PowerWave 9000DPA – and if all mains supplies are missing at the time of transfer , the inverter is supplied by the battery . When supplies return to normal , the static switch automatically transfers the load back to bypass while the battery recharges and the inverter reverts to standby mode . Offline / line-interactive systems are slightly more efficient than online configurations due to reduced rectifier and inverter losses , and ‘ offline ’ is sometimes referred to as ‘ eco-mode ’.
As static switch operation involves paralleling the inverter and bypass outputs for short periods , synchronisation between the two supplies is important to avoid subjecting the load to power disturbances . Usually , the mains voltage supply frequency in the UK is very stable . However this may not be the case for an on site back up generator , so checking a generator ’ s output frequency stability is important before use with a UPS system .
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Static switches and modular topology
Systems such as the PowerWave 9000DPA have a modular topology . Modularity offers many attractive features including easy scalability , efficiency in achieving redundancy , and high availability boosted by ‘ hotswap ’ online module replacement .
As such systems are implemented by up to five modules in a single rack , synchronisation between them is essential . This can be accomplished by connecting all modules to a control bus that carries signals for frequency synchronisation , load sharing and other functions . By using sensing signals passed over the control bus , each power module can electronically compare its own frequency and output current with its neighbour ’ s and carry out fine adjustments to achieve balanced conditions .
The module control logic always recognises one of the power modules as the ‘ master ’ and the others as ‘ slaves ’. However if the ‘ master ’ module goes faulty at any time the next module in the chain will immediately take over as master and the former master module will turn off . This multi-master capability eliminates any single point of failure , allowing very high levels of system availability .
In any case , the bus signal logic always ensures that all static switches are in the same mode , so that all modules feed the load either from the inverter or from the static bypass . Alternatively , all modules can be switched into maintenance bypass mode .
Conclusion
In this article we have seen how UPSs must offer a fast , flexible response to every situation that potentially threatens their critical load . These include power blackouts or unacceptable power quality , overload problems or even failures within the UPSs themselves . UPSs should also offer flexibility in their operational mode – online or offline – to best suit the requirements and priorities of their particular load .
Additionally , modern UPS systems are seldom single-channel architectures ; as we have seen , modular implementations offer many advantages . However , these installations must be internally synchronised to function successfully , with all modules either on inverter or on bypass . The use of one static switch per module is favoured , to avoid a single point of failure .
Today ’ s UPSs , especially those such as the PowerWave 9000DPA that use modular technology , are flexible and easily scalable while protecting their critical load from many different threat types . Static switches play a key role in assuring this resilient performance .
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Load on Bypass |
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LINE 2
LINE 1
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BY PASS
INVERTER
BATTERY
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LOAD |
Bypass Supply
Mains Supply
RECTIFIER
Figure 2.6 Load on bypass
MAINTENANCE BYPASS LINE
Charger
STATIC BYPASS LINE
INVERTER
BATTERY
STATIC SWITCH
IA1
IA2
PW9000DPACabinet
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Load Supply |
Fig . 2 : Static switch circuit .
Bypass supply
Inverter supply
SCR1
SCR2
SCR3
SCR4
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UPS output |
Alan Luscombe is sales & marketing director at Uninterruptible Power Supplies Ltd . A qualified electrical & electronic engineer , Alan has over 18 years ’ experience of sales and marketing leadership within several blue-chip B2B technical companies serving the global commercial , industrial , telecom , defence , infrastructure and medical market sectors . |
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