Sizing a Static UPS System In Data Center

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 and which cannot be made before, or independently from, the choice of distribution scheme.

The main elements to consider can be summarised as follows:

1. Two from the following parameters of the loads to be supplied: active power, apparent power or power factor
2. Type of load power supply (voltage, frequency, number of phases)
3. Load coincidence factor
4. Required back-up time
5. 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 with switching power supplies, is the current waveform and phase.

Since these power supplies only absorb current close to maximum voltage, the typical waveform, far from being sinusoidal (see Figure 1), has a rather reduced base line and a vertex in correspondence with the voltage peak.

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

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

With regard to the current phase it should be noted thatthe power factor of the loads under consideration is leading, and therefore specific precautions must be taken when sizing the UPS. Current computer loads have input power factors up to 0.9 leading. Note however that there are UPS systems on the market 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 fixtures with fluorescent bulbs, this must duly be taken into account.

The parameters are:

• I_UPS – Maximum current value from the UPS
• t_UPS – Time for which I_UPS is sustainable
• I_load – Overload current required by the load
• t_load – Theoretical overload time required by the load

The apparent power required for sizing the UPS will be:

Typical values of I_UPS and t_UPS can range from typical current values of 150% of nominal current for one minute up to 200% per 100 ms without mains power or for UPS systems not equipped with bypass.

Autonomy

Autonomy is essentially linked to the time that the data centre is able to guarantee continued operation and 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, a rotary generator should be provided to supply power to the static system, implementing procedures to ensure fuel replenishment if necessary.

APC UPS Symmetra PX 160 – Overview

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