Our latest Tech Talk sees Riello UPS Technical Services Manager Jason Yates explain how an online UPS deals with unbalanced loads.
Jason outlines the concept of a dual conversion online Uninterruptible Power Supply (UPS) system. In its most basic form, this sees Alternating Current (AC) taken from the mains supply, converted to Direct Current (DC) through the rectifier, before the inverter reconverts back to AC.
In an online UPS configuration, this dual conversion (AC to DC, DC to AC) means the input and output are always independent of each other. That’s why online UPS is often referred to as VFI – voltage and frequency independent.
The video sees Jason talk viewers through two examples of a three-phase UPS system, the first with a balanced load and the second with an imbalanced load. He spells out how dual conversion effectively balances the input regardless of any unbalanced loads.
Hi and welcome to this week’s Tech Talk.
Today we’re going to be talking about how the online three-phase UPS system distributes the input power being taken from the mains supply equally across the three incoming phases, regardless of how the load connected to the output from the UPS system is balanced across the three outgoing phases.
Now on the board here we have a very basic diagram of an online dual conversion UPS system. The online dual conversion UPS system operates by taking the AC voltage from the main supply, converting the AC voltage to DC voltage via the rectifier (which is referred to as the first conversion) we then reconvert that DC voltage back to AC voltage again by the inverter (which is referred to as the second conversion) and hence the term dual conversion.
The term online simply means that the load is being constantly supplied via the inverter and therefore should there be a break or disturbance occurring on the incoming side of the UPS system, this is never seen by the connected load because during these conditions, the inverter simply transfers from taking power from the rectifier to taking power from the connected batteries.
Now this will occur seamlessly and there is never any break or disturbance to the connected load, hence the term online.
One of the main benefits of the online dual conversion UPS system is the AC to DC, DC to AC conversion. Because we make this conversion anything that is passing into the UPS system is never replicated on the output side of the UPS system and therefore both the input and the output remain completely independent on one another. This is referred to as VFI – voltage and frequency independent.
If we apply loadings on to this UPS system and we consider that we’re not providing any power to charge our batteries and we have no losses through our UPS system, if we consider this UPS system to be say a 30 kilowatt UPS system and we applied 10 kilowatts to each of the three outgoing phases, the 30 kilowatts in total on the output side of our UPS will be applied across to our DC bus, which in turn will be applied on to our three incoming phases via the rectifier.
Under those conditions, we would get 30 kilowatts on the output and we would apply 30 kilowatts to the input again, assuming no losses. So therefore we would have 10 kilowatts on phases one, two, and three on the output, and ten kilowatts on phases one, two, and three on the input.
However, if we imbalance the loading on the outgoing side of our UPS system, say we apply 10 kilowatts on to phase one, five kilowatts on to phase two and zero kilowatts on to phase three, then effectively we’ve only now got a maximum load of 15 kilowatts.
Now the 15 kilowatts again will be passed through onto our DC bus within our UPS system. But this time the rectifier again will convert these across the three incoming phases but this time we’ll only apply five kilowatts to phase one, five kilowatts across phase two, and five kilowatts on to phase three, effective providing that 15 kilowatts are required by the load and hence our unbalanced load has now appeared balance on our incoming supply to our UPS system.
That is how the online dual conversion effectively balances the input regardless of how the outgoing side of the UPS system is imbalanced.
We also must consider most online dual conversion UPS systems do have internal static bypass switches and therefore if the UPS system is in bypass, then any imbalance on the outgoing side of the UPS system will be passed through directly on to our input supply, and therefore the infrastructure must be designed in order to support this.
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