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Sizing a Static UPS System In Data Center

Sizing a Static UPS System In Data Center (on photo APC UPS device; by Matthew Landry via Flickr)

UPS system serving a data center

Choosing the power of a static UPS system serving a data center is a decision that involves various elements andwhich cannot be made before, or independently from, the choice of distribution scheme.

The main elements to consider can be summarised as follows:

Two from the following parameters of the loads to be supplied: active power, apparent power or powerfactor

Type of load power supply (voltage, frequency, number of phases)

Load coincidence factor

Required back-up time

Type of mains power supply (voltage, frequency, number of phases)

Power

The fundamental characteristic of IT equipment loads, and more generally of all loads equipped withswitching power supplies, is the current waveform and phase. Since these power supplies only absorb currentclose to maximum voltage, the typical waveform, far from being sinusoidal (see Figure 1), has a rather reduced

Figure 1 – Typical current waveforms of computerloads compared with a sinusoidal wave (dotted

line)

base line and a vertex in correspondence with the voltage peak.

Given the same effective value, this waveform has the peculiarity of having a much higher crest factor than thatof a sinusoidal wave.

The static UPS must be able to supply this peak current value, which is normally indicated in the product’stechnical specifications as the ‘crest factor‘.

In accordance with standard EN 62040-3, the system must not bederated for standardised non-linear loads with a crest factor lower thanthree (3:1)

Sometimes the nominal power of a static UPS system is confused withterms such as ‘switching power’, ‘computer power’ and ‘actualpower’. These expressions were probably coined in an attempt todefine a parameter capable of modelling system power even inconditions where the current and voltage waveforms are distorted,nevertheless it should be borne in mind that these parameters do nothave an official regulatory definition and therefore cannot have anycorrelation with the apparent power and nominal active power of thestatic system.

Consequently, they cannot be used to size a static UPS system.

With regard to the current phase it should be noted thatthe powerfactor of the loads under consideration is leading, and thereforespecific precautions must be taken when sizing the UPS. Currentcomputer loads have input power factors up to 0.9 leading. Note however that there are UPS systems on themarket that are able to supply power without derating even this type of load.

If the static system is also required todeliver a large inrush current, as inthe case of lighting fixtureswith fluorescent bulbs, this must duly be taken into account.

The parameters are:

IUPS – Maximum current value from the UPS

tUPS – Time for which IUPS is sustainable

Iload – Overload current required by the load

tload – Theoretical overload time required by the load

The apparent power required for sizing the UPS will be:

Typical values of IUPS and tUPS can range from typicalcurrent values of 150% of nominal curren t for oneminute up to 200% per 100 ms without mains poweror for UPS systems not equipped with bypass.

The typical cold inrush current of an appliance of thetype under consideration is equal to 6 – 8 times the nominal current , and in practice is only limited by theimpedance of the UPS and the conductors comprising the part of the distribution network affected by the event.

Autonomy

Autonomy is essentially linked to the time that the data centre is able to guarantee continued operation and

Central Operations Centre – 10 x Apc in row cooling units DX were installed. 3 x500kVA APC UPS devices (photo credit: dieselelectricservices.co.za)

if necessary perform a controlled shutdown (typically 30-60 min) .

If service must be guaranteed for a long time(roughly 60-90 min), as is often the case, arotary generator should be provided to supplypower to the static system, implementingprocedures to ensure fuel replenishmentif necessary.

APC UPS Symmetra PX 160 – Overview

Cant see this video? Click here to watch it onYoutube.

Reference: Planning and design for a datacenter // Electrical Power Infrastructure – EATON

About Author //

Edvard Csanyi

Edvard - Electrical engineer, programmer and founder of EEP. Highly specialized fordesign of LV high power busbar trunking (<6300A) in power substations, buildings andindustry fascilities. Designing of LV/MV switchgears. Professional in AutoCADprogramming and web-design. Present on

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