FEATURE UPS & STANDBY POWER
SIZING A UPS TO MEET specific site conditions
Correct UPS sizing depends heavily on knowing the total kVA demand of the critical load. However, many other factors related to the load profile, UPS topology and expectations of the application must be allowed for as well. Here Kenny Green of Uninterruptible Power Supplies (UPSL), discusses how to correctly size a UPS for its specific application
S
uperficially, 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.
ESTIMATING AND DETERMINING THE ACTUAL LOAD Most electrical equipment items have labels displaying their electrical characteristics, though a review can be performed to collate this information. The review 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 3-phase then a 3-phase UPS becomes essential. Such a UPS can, however, supply both single- and 3- 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.
LOAD FACTORS In the past, 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 will derate substantially under these conditions, while modern transformerless types will be less affected. As SMPS efficiency has grown, the penalty has been increased harmonic content in the load. All computer power supplies must have at least passive
14 JUNE 2014 | ELECTRICAL ENGINEERING
Left and below: this article sets out to review the factors involved in correctly sizing a UPS for its target critical load. It reveals that although a knowledge of the load’s size in kVA is certainly necessary, it is not sufficient
common mode distortion. Some equipment such as motors can cause problems through inrush current as well as the harmonic distortion mentioned above. If such devices lack soft-start circuits, they can draw up to 15-20 times their steady state current. Significant over sizing of the UPS is required to support these high inrush loads.
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 categories include laser printers and certain types of lighting. For 3-phase UPSs, balancing the load across all phases is important because it ensures that the UPS’s rating is fully utilised, as well as presenting a balanced load to the mains or generator when installed if the UPS is bypassed. Most modern UPSs do not require careful balancing across their three phases to operate correctly - even so, the load on any single-phase must never exceed 33% of the UPS’s total rating. Computer network equipment can create very high currents in the neutral conductor of a 3-phase Y power distribution system even if the loads appear to be balanced. This problem is typically created 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 and
TOPOLOGY AND EXPECTATIONS As we have seen, UPS topology is important because transformer-based types are so much more adversely affected by poor power factor loads than their modern transformerless equivalents. However, modular transformerless installations offer future-proofing benefits too, because additional modules can so easily be plugged into the UPS frame as the need arises. This means UPSs do not have to be excessively oversized during initial installation but are adaptable to cope with future power demands. In some applications the UPS may simply
be required to support a safe system shutdown during a power failure. Other users will expect the UPS to maintain power throughout an extended blackout, allowing its load to remain in service. It is important to understand these diverse demands on autonomy as they will affect the size of the battery needed. This article set out to review the factors
involved in correctly sizing a UPS for its target critical load. It revealed that although a knowledge of the load’s size in kVA is certainly necessary, it is not sufficient. The topology of the UPS itself can affect the sizing requirement, as can the electrical environment in and around the target site. Current and future expectations of the UPS’s role must also be taken into account to ensure that the UPS is correctly sized and future-proofed.
Uninterruptible Power Supplies (UPSL)
www.upspower.co.uk T: 0800 731 3269
Enter 200 / ELECTRICALENGINEERING
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