It is widely believed that there are only two types of UPS systems, namely standby type UPS and on-line type UPS. These two terms are often incorrectly applied to many UPS systems on today’s market. Many misunderstandings about UPS systems are cleared up when the different types of UPS topologies are properly identified. The major ones are defined below, and these will be added to our Word for the Day archives for your future reference.
Standby UPS – The Standby UPS is the most common UPS type used for Personal Computers. The transfer switch is set to choose the filtered AC input as the primary power source and switches to the battery / inverter as the backup source in case of a failure of the primary source (AC). In the case of power failure, the transfer switch must operate to switch over to the battery / inverter backup power source. The inverter only starts when the power fails, hence the name “Standby.” Benefits of this model include high efficiency, small size, and low cost.
Line Interactive UPS – The Line Interactive UPS is the most common UPS used for small business, Web, and departmental servers. In this design, the battery-to-ac power converter (inverter) is always connected to the output of the UPS. Battery charging is provided by operating the inverter in reverse during times when the input AC power is normal. When the input power fails the transfer switch opens and the power flow is from battery to the UPS output. The fact that the inverter is always connected to the output provides additional filtering and yields reduced switching transients when compared with the standby type UPS. The inverter also provides regulation, operating to correct brownout conditions, which would otherwise force the UPS to switch to battery operation. This allows the UPS to operate at sites with very poor power. The inverter can be designed such that its failure will still permit power flow from the AC input to the output, which eliminates the potential of single point failure and effectively provides for two independent power paths. This topology is inherently very efficient, which leads to high reliability while at the same time providing superior power protection. High efficiency, low cost, high reliability coupled with the ability to correct low or high line voltage conditions make this the dominant type of UPS in the 0.5-5kVA (Volt-Ampere) power range.
Standby On-Line Hybrid – The Standby On-Line Hybrid is the topology used for most UPS under 10kVA (Volt-Ampere) that are labeled “on-line”. The standby converter from the battery is switched on when an AC power failure is detected, just like in a standby UPS. Additionally the small battery charger resembles a standby type UPS. This UPS will exhibit no transfer time during an AC power failure. The most misunderstood part about this topology is the belief that the primary power path is always “on-line,” when in fact the power path from the battery to the output is only half “on-line” (the inverter), while the other half (the DC to DC converter) is operated in the standby mode. This design is sometimes fitted with an additional transfer switch for bypass during a malfunction or overload.
Standby-Ferro UPS – The Standby-Ferro UPS was once the dominant form of UPS in the 3-15kVA (Volt-Ampere) range. This design depends on a special transformer that has three windings (power connections). The primary power path is from AC input, through a transfer switch, through the transformer, and to the output. In the case of a power failure, the transfer switch is opened, and the inverter picks up the output load. In the Standby-Ferro design, the inverter is in the standby mode, and is energized when the input power fails and the transfer switch is opened. The transformer has a special “Ferro-resonant” capability, which provides limited regulation and output waveform “shaping”. The isolation from AC power transients provided by the ferro transformer is as good or better than any filter available, but the ferro transformer itself creates severe output voltage distortion and transients which can be worse than a poor AC connection. Even though it is inherently a standby UPS, the Standby-Ferro generates a great deal of heat because the ferro-resonant transformer is inherently inefficient. Standby-Ferro UPS systems are frequently represented as On-Line units, even though they have a transfer switch, the inverter operates in the standby mode, and they exhibit a transfer characteristic during an AC power failure. High reliability and excellent line filtering are the strengths of the Standby-Ferro design. However, the design has very low efficiency combined with instability when used with some generators and newer power-factor corrected computers, which has caused the popularity of this design to decrease significantly.
Double Conversion On-Line UPS – This is the most common type of UPS above 10kVA (Volt-Ampere). Double Conversion On-Line UPS is the same as the Standby UPS except that the primary power path is the inverter instead of the AC mains. In the design of Double Conversion On-Line operation, failure of the input AC does not cause activation of the transfer switch, because the input AC is not the primary source, but is rather the backup source. Therefore, during an input AC power failure, on line operation results in no transfer time. The on line mode of operation exhibits a transfer time when the power from the primary battery charger/battery/inverter power path fails. This can occur when any of the blocks in this power path fail. The inverter power can also drop out briefly, causing a transfer, if the inverter is subjected to sudden changes in the load, or if the inverter experiences an internal control “glitch”. Contrary to popular belief, Double Conversion On-Line UPS systems do exhibit a transfer time, and in actual installations may transfer as frequently as standby type UPS systems; however on line UPS transfers are not related to AC input power failures as they are in a standby UPS. Both the battery charger and the inverter convert the entire load power flow in this design, which causes undesirable heat and results in reduced efficiency. Due to practical design constraints, UPS below 10kVA that are represented as Double Conversion On-Line UPS are almost always actually of the Standby On-Line Hybrid type UPS. The Double Conversion On-Line UPS provides nearly ideal electrical output performance. However, the constant wear on the power components can reduce reliability and the energy consumed by the electrical power inefficiency is a significant part of the life-cycle cost of the UPS. Also, the input power drawn by the large battery charger is often non-linear and can interfere with building power wiring.
Delta Conversion On-Line UPS – This UPS design is a relatively new technology that was introduced to eliminate the drawbacks of the Double Conversion On-Line design and is available from 5kVA and greater. Like the Double Conversion On-Line design, the Delta Conversion On-Line UPS always has the inverter supplying the load voltage. However, the additional Delta Converter also contributes power to the inverter output. Under conditions of AC failure or disturbances, this design behaves identically to the Double Conversion On-Line. During steady state conditions the Delta Converter allows the UPS to deliver power to the load with much greater efficiency than the Double Conversion design. A simple way to understand why this works is to consider the energy required to deliver a package from the 4th floor to the 5th floor of a building. It obviously saves energy to carry the package only the difference (delta) between the starting and ending points. The Double Conversion On-Line UPS converts the power to the battery and back again whereas the Delta Converter moves much of the power from input to the output directly. In the Delta Conversion On-Line design the Delta Converter can also charge the battery so the overall design is no more complex than the Double Conversion approach and it provides the same output characteristics. In addition, the Delta Conversion On-Line UPS offers reduction in energy losses and costs by approximately a factor of 4. As a side-effect of the design, the input power quality of the Delta Conversion UPS is also superior, particularly in the large kVA sizes.