In this edition of FAST Facts, Kenny Green, technical support manager at Uninterruptible Power Supplies, explains why sizing a UPS correctly for a specific site takes more than just a knowledge of the site’s kVA
WHAT IS INVOLVED IN SIZING A UPS?
Superficially, sizing a UPS appears to be simple; add up the total kVA requirement of the critical load and allow for some spare capacity to arrive at the UPS’s kVA specification. Although this approach is certainly an essential starting point, there are other factors that must be understood and allowed for to establish the UPS’s true sizing requirement.
HOW DO I FIND OUT WHAT THE ACTUAL LOAD IS?
Most electrical equipment items have labels displaying their electrical characteristics. This information can be collated, and should include required supply voltage, number of phases, load current, power factor and power consumption. The number of phases is important, because if any part of the critical load is three-phase then a three-phase UPS becomes essential. A three-phase UPS can, however, supply both single – and three – phase equipment items.
kVA ratings can be noted and totalled using an online tool such as the UPS kVA Load Estimator.
For a more accurate view of the actual load and how it varies over time, a site survey using portable measuring and monitoring equipment becomes vital. Many UPS suppliers offer this service, including UPS.
WHY IS THE LOAD’S POWER FACTOR SO IMPORTANT?
Historically, loads have tended to present an inductive or lagging PF, typically of around 0.8. The more recent shift to blade servers and other equipment using switched mode power supplies (SMPSs) has resulted in load PFs approaching unity or even becoming capacitive or leading. Legacy transformer-based UPSs derate substantially under these conditions, while modern transformerless types are less affected. Fig. 1 below shows the impact of PF on both UPS topologies. Note that modern transformerless UPS, in this example, need no derating at unity PF; the 300 kVA system can deliver full 300 kW power while the transformer-based equivalent is limited to 240 kW.
|Power Factor (load)||kW Rating||kW Rating|
Fig.1: Impact of power factor on UPS derating
WHAT EQUIPMENT-RELATED PROBLEMS AM I LIKELY TO ENCOUNTER?
Increasing SMPS efficiency has created increased harmonic content in the load. All computer power supplies must have at least passive power factor correction to comply with EU standard EN61000-3-2. Equipment that imposes harmonic distortion can also create other problems such as high or pulsed currents during normal operation or start-up. Known ‘problem’ equipment in this area includes laser printers and some types of lighting.
WHY IS PHASE-BALANCING IMPORTANT – AND WHY ISN’T IT STRAIGHTFORWARD?
For three-phase UPSs, balancing the load across all phases is important to ensure that the UPS’s rating is fully utilised, as well as presenting a balanced load to the mains or generator if the UPS is bypassed. Most modern UPSs do not require careful phase balancing to operate correctly; even so, the load on any single phase must never exceed 33 per cent of the UPS’s total rating.
Computer network equipment can create very high currents in the neutral conductor of a three-phase Y power distribution system even if the loads are apparently balanced. This problem typically occurs when single-phase computer units with non-linear load characteristics are connected across line L1, L2 or L3 and neutral. High neutral currents can cause overloading of feeders and transformers as well as voltage distortion and common-mode distortion.