Recently the market has seen requests for UPSs with a unity power factor (PF 1). At the same time, input and output PF, inductive or capacitive are also mentioned.
Why are all these values important? What is the best output PF rating? 0.8? 0.9? 1?
INPUT PF
The UPS is an electrical load, and its input PF represents how it draws power from the main power line. It depends on the rectifier technology used and can vary from a low 0.75 for 6-pulse rectifiers with batteries discharged to 0.99 for IGBT technologies, regardless of energy stored in the accumulators. To complete the picture, low PF’s cause unnecessary oversizings of the power distribution.
OUTPUT PF
It is common to refer to the output PF of a UPS. What it actually refers to is the range of power factors of the loads connected to the UPS determining the nominal active power available for work.
There are exact limits which determine the operating conditions of the UPS based on the active or reactive power of the load. These limits depend on:
- Current limits of inverter components.
- Current limits of rectifier components.
- UPS output filter which is also a reactive load.
Figure 1 helps to understand these limits. Based on the current limits of inverter components, the XZY arc circumference would be admissible if the following didn’t also occur:
- The output filter adds a reactive component to the active current of a resistive load and the current resulting from the vectorial sum is external to the XZY circumference.
- There is a reduction in the performance of the rectifier explained in detail in the following paragraphs.
Why are there UPS units on the market referred to as having a capacitive output PF of 0.9 or PF 1?
The full title should actually state that they have a capacitive to inductive value to reflect points H and K in Figure 1.
The selection of these values depends on technical and cost considerations which in turn are related to the load served by the UPS. In fact, you will see a change over time or in the size of the UPS together with the load. The capacitive PF has only become relevant in recent years, the value more commonly used was the 0.8 inductive one. This value has remained in many industrial applications although it is gradually verging towards 0.9 capacitive in ICT applications[1].
There are UPS devices with an output power factor 1, but the ratio of cost to performance limits their popularity. In fact, loads with a unity power factor are actually very scarce. Server PSUs (Power Supply Units), the devices that are most likely to reach this value, draw current at the same phase as the voltage only under full load, a condition which does not often happen in practice.
On the other hand, power factors, tolerances, battery charge and cost are four interdependent considerations in the selection of a UPS: changing one will automatically alter the others. Whilst this is evident below, let’s not forget that all UPS’s currently distributed can supply resistive loads by accepting compromises. In this regards, the following equivalence may be useful.
Where:
- Prect is the rated real power of the rectifier.
- Vmin and Imax are the minimum voltage and maximum current tolerable by the rectifier.
- PFinput-UPS is the input PF of the UPS.
- Pbatt is the power needed to recharge the batteries.
- Pload is the active power of the load.
- SUPS and PFload are the apparent power and output PF of the UPS.
It’s worthwhile to remember that the cost of a UPS and the space it occupies are in essence linked to the apparent and not active power. Which is why two UPS units of the same cost and size with output power factors of 0.9 and 1 will perform the same; this also applies to the rectifiers.
Reduction of input voltage tolerances
One of the limits of the rectifier is the rated current of its components. This equals the maximum current (Imax) that the rectifier can tolerate. Whilst overlooking battery charge for the purposes of simplicity and remembering that given the same cost and size UPS units have the same apparent power, the following relations apply:
- (UPS with PF 0.9);
- (UPS with PF 1 ).
As a result
or given the same cost and size of the UPS, the minimum voltages of PF1 UPS’s are 11% higher than UPS with a power factor of 0.9. The most common values found for three-phase systems are:
- Vmin =320 V (UPS with PF 0.9);
- V’min = 355 V (UPS with PF 1 ).
Every time the input voltage is outside the tolerated limit, the load will be supplied by the batteries, shortening battery life and available reserves that are genuinely necessary.
Increased battery recharge times
For similar reasons to those outlined above, with the same rectifier the sum Pbatt +Pload must be constant. An increased demand for active power from the load equals a reduction in recharging power for the batteries and subsequent increase in the time needed to restore the charge in the accumulators after they have been used.
Cost and size
The various problems mentioned above can be avoided by increasing the capacity of the rectifier in the UPS with associated increase in the cost and size of the device.
CONCLUSIONS
Whilst a UPS with unity input power factor is the preferred solution to lower installation costs, the output factor must reflect the actual requirements of the load. Energy Star itself, in its consumption guidelines (ENERGY STAR® Program Requirements for Computers – Version 5.0) requires a minimum PF of 0.9 at full load for ICT applications. This would mean at loads lower than 100 % Pn, the PF could be lower.
Furthermore, the fact that UPSs with an output PF of 1 are marketed at a higher price or at a lower capacity than those with a PF of 0.9 should make us reflect on the real utility of PF1.
Do we really need to compromise on UPS capacity for such a limited if not inexistent number of cases? Is an output PF of 1 really relevant for all applications?
The answer is no given that the majority of loads are nowhere near this value, and a 30% margin is usually left with respect to actual load requirements when selecting an uninterrupted power supply.
Tech Info – August 2010
CHOOSING THE POWER FACTOR IN UPS
by
MATTEO GRANZIERO
Technical Communication Specialist
SOCOMEC UPS
