Rating UPS Systems At Unity Power Factor

More and more UPS manufacturers are putting systems into the market with high Power Factors. What does this actually mean for the datacentre or comms room manager who simply wants an uninterruptible power supply to power their load and possibly integrate with a standby power generator?

When talking about uninterruptible power supplies, there are two Power Factor (pF) figures to consider: the Output Power Factor and the Input Power Factor. Firstly, we will consider output Power Factor. When looking at electrical systems, Power Factor is the ratio of Real Power (measured in Watts, kW or MW) that a circuit can deliver compared to the Apparent Power (measured in VA, kVA or MVA) which is itself a product of the voltage (V) and current (A) within the electrical circuit. Power Factor is measured on a range from +1 to -1 and most IT loads are from 0 to +1 in their specification with 1 referred to as Unity Power Factor.

So, let’s take a traditional 100kVA/80kW three phase UPS and consider that the Real Power that this type of UPS can deliver is actually 80kW. If the same system could be rated at Unity Power Factor and deliver 100kVA/100kW for a marginal increase in cost we are looking at a 25% increase in Real Output power from (20/80kW). The larger the UPS system, the more impressive the gain achieved by raising the Output Power Factor to near Unity.

Input Power Factor is another area to consider and here most UPS manufacturers claim high Input Power Factors as high as 0.99. This relates directly to the efficiency of the system in terms of power loses. The lower the Input Power Factor the higher the potential wasted energy drawn form the mains power supply. An additional point to consider here is the input Total Harmonic Distortion (THDi). A high Input Power Factor normally goes hand-in-hand with a low THDi and means that the UPS system is not only very efficient when drawing Real Power from the connected AC supply waveform but the distortion generated back into the AC supply waveform is negligible. This can be very important when coupling a UPS to a standby power generator and considering harmonic pollution from the UPS disturbing the operating of sensitive upstream hardware systems and electrical switchgear.

The Eaton 93PM modular UPS system is a Unity Power Factor rated uninterruptible power supply. The system adopts a modular approach from 30kW that can be scaled upwards from 40kW to 200kW or 200kVA if you prefer the traditional approach. With its small foot-print and in-row designed cabinet, the Eaton 93PM delivers one of the highest power density square-metre ratios in the UPS industry, with vertical scalability.

If we consider a comparable free-standing transformerless UPS system from another manufacturer rated at 100kVA/90kW or (0.9pF), a comparable Eaton 93PM 100kVA/100kW UPS comprising two 50kW UPS modules will support 10kW more load and within a foot-print which will be up to 50% smaller. It’s little surprise that modular and Unity Power Factor rated uninterruptible power supplies are becoming the system of choice within small-to-medium sized datacentres and comms rooms.