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28 Jul Technical Q&A: Unbalanced loads and input currents

 Q: Can Riello UPS’ accept 100% unbalanced load? If so what effect does this have on balance of input current?  

Our technical services manager, Jason Yates, has the answer: 


 A: Yes, all Riello UPS can accept a 100% load imbalance across the output phases, for example 100% load on phase 1 and 0% load on phases 2 and 3. In this condition the output performance is still maintained in terms of output voltage and angular displacement between phases. 

 During normal operation (when the load is being supplied by the inverter), the loads applied to each of the three output phases are indirectly passed onto the incoming three-phase supply via the inverter and rectifier. This effectively means that the total load applied to the output is evenly spread across the three incoming phases, even if the output load is unbalanced. For example, if a 30kVA UPS was supplying a total load of 10kVA (10kVA on phase 1 and 0kVA on phases 2 and 3), then a minimum load of 3.33kVA (10÷3) would be applied to each of the three incoming phases (obviously ignoring any losses). 

 This operational characteristic is due to the online double conversion mode, where the UPS first converts the incoming AC (Alternating Current) to DC (Direct Current) and then re-inverts the DC back to AC in order to supply the load.  

 However, if the load was transferred to bypass, the 10kVA load would be directly connected to the incoming phases, resulting in there being 10kVA on phase 1 and 0kVA on phases 2 and 3.  

 It should be noted that a three-phase output UPS will allow a maximum of one third of the total power available to be applied to each outgoing phase i.e. a 30kVA UPS can only have up to 10kVA load applied per phase.