Performance Evaluation

24.3 Performance Evaluation

There are four criteria for evaluating the performance of a UPS system: quality of output voltage, input PFC and current har- monic cancellation, transition time, and efficiency. The quality of output voltage is the most important factor. The output voltage of a UPS system should be sinusoidal with low THD in different loading conditions even with non-linear loads. The control system should have small transient responses to pro- vide appropriate line conditioning in different loading profiles. Typically, rotary UPS systems, which employ an AC generator at the load side, have better output voltage quality than static UPS systems. In these systems, there is no converter switch- ing frequency present at the output voltage. Among the static

TABLE 24.1 Performance comparison of different configurations of UPS systems Parameter

On-line

Line interactive

Hybrid Surge protection

Excellent Transition time

Excellent Line conditioning

Excellent Good Good Backup duration

Depends on battery

Depends on

battery

Depends on

battery

Depends on battery

Typically 0.1–0.5 s

Depends on battery Efficiency

Low around 80%

High up to 95%

High

High up to 95%

High typically

above 85%

High typically around 95% Input/Output isolation

Perfect Cost

Very high

Very high

UPS systems, on-line UPS configuration provides better output voltage quality. In this system, output voltage is provided by

a DC/AC inverter regardless of input voltage quality. Usually,

a pulse width modulation (PWM) method is used to regulate the output voltage. This kind of UPS should be designed to have minimum switching frequency at the output and provide pure sinusoidal voltage at different loading conditions. Fol- lowed by on-line UPS system are universal and line-interactive configurations. In universal topology, during normal mode of operation, the series converter provides voltage conditioning and regulates output voltage. In the backup mode, the par- allel converter provides the load with sinusoidal voltage. In the line-interactive topology, during normal operation mode, input voltage directly supplies the load and no voltage condi- tioning is provided. In the backup mode, the DC/AC inverter provides the load with sinusoidal voltage.

The second criterion is transition time from normal mode of operation to stored energy mode. On-line rotary and static UPS systems have superior performance in this regard. The output voltage is always provided by the output generator or output DC/AC inverter and there is no transition time between operation modes of the systems. However, some of the rotary and hybrid configurations shown in Figs. 24.10 and 24.11 can only provide power to the load for a limited time. This time is determined by the amount of kinetic energy stored in the mechanical system. The transfer time in universal and line-interactive topologies depends on the time necessary for converting the power flow from the battery bank through the inverter to the load. Improved performance is achieved by choosing the DC bus capacitor voltage at the battery side to be slightly higher than the floating voltage of the batter- ies. Therefore, when the AC line fails, it is not necessary to sense the failure because the DC bus voltage will immediately fall under the floating voltage of the batteries and the power flow will naturally turn to the load. For off-line UPS systems, the transfer time is the longest. It depends upon the speed of sensing the failure of the AC line and starting the inverter.

The next important factor is the input power factor and the ability of the system to provide conditioning for load power. Universal UPS system has better performance followed

24 Uninterruptible Power Supplies 635 by line-interactive and on-line UPS in this regard. During

normal mode of operation, the parallel converter acts as an active filter and compensates reactive current and current har- monics generated by the load. In the line-interactive system, the bi-directional AC/DC converter performs this task. In an on-line UPS system, an additional system must be added to improve PFC and mitigate current harmonics.

The last criterion for performance evaluation is efficiency. To emphasize this factor, it should be noted that losses in UPS systems represent about 5–12% of all the energy consumed in data centers. Efficiency in rotary and hybrid configurations depends on the topology of the system but typically for low power application due to mechanical loss in the motor and generator, the efficiency is not very high. Among the static UPS systems, on-line UPS system has the poorest efficiency due to double conversion. Line-interactive and universal topologies provide higher efficiencies since most of the power directly flows from the input AC to the load during normal operation.