Theory- 44 :- UPS - Introduction, Types, Advantages- Disadvantages,Applications

An uninterruptible power supply (UPS) is a device that allows a computer to keep running for at least a short time when the primary power source is lost. UPS devices also provide protection from power surges.

A UPS contains a battery that "kicks in" when the device senses a loss of power from the primary source. If an end user is working on the computer when the UPS notifies of the power loss, they have time to save any data they are working on and exit before the secondary power source (the battery) runs out. When all power runs out, any data in your computer's random access memory (RAM) is erased. When power surges occur, a UPS intercepts the surge so that it does not damage the computer.

Types of Uninterruptable Power Supply Systems

UPS systems can generally be classified as being one of these five types:

• Standby UPS
• Line-interactive UPS
• Standby-ferro UPS
• Double conversion online UPS
• Delta conversion online UPS

Note that these types are based on a demand for an AC power backup for the load.

Standby UPS

A standby UPS is a configuration in which a battery backup is charged by the line voltage and is fed through an inverter to a transfer switch. When the prime power is lost, the transfer switch brings the standby power path online (represented in figure 1 below as the lower path with the dashed line). The inverter is generally not active until there is a power failure, and hence the term ‘standby” is used to describe this type of UPS. The need to actively switch the power path does mean that there is a brief outage that will occur from the time that primary power is lost until the switchover is complete. The primary power path shown in Figure 1 includes an LC filter and surge protection circuitry for noise isolation.

The laptop example that was presented earlier might be considered a simplified type of standby UPS where the desired output is DC instead of AC and no transfer switch is needed.

 

Line-interactive UPS

One of the most commonly used designs for an uninterruptable power supply is the line-interactive UPS, presented in Figure 2 below. With the line interactive design, prime power is fed through a transfer switch to an inverter and then out to the load. The inverter in this design is always active and when prime power is on, it operates in reverse to convert incoming AC power to DC which is used to keep the backup battery charged. If the line power goes out the transfer switch opens and the inverter works in the normal direction, taking the DC power from the battery and converting it to AC to supply to the load.

This design which keeps the inverter active provides improved filtering and reduces the switching transients that could be present with the standby UPS configuration. Depending on the inverter design, this configuration can provide two independent power paths for the load and eliminates the inverter as a single point of failure. So even if the inverter were to fail, AC power can still flow to the output. This type of UPS offers low cost, high reliability, and high efficiency, and can support low or high voltage applications.

Standby-ferro UPS

The standby-ferro UPS uses a three winding transformer to couple the load to the power source, as shown below in Figure 3. Prime power flows through a transfer switch that is normally closed to the coils in the transformer where it couples to the secondary coil of the transformer and then supplies the power to the output load. The backup power path takes line voltage to a battery charger and maintains the backup battery, which then connects to an inverter that joins the third coil of the transformer.

When the prime power fails, the transfer switch opens and the inverter supplies power to the load from the backup battery. In this design configuration, the inverter is in standby and becomes active when the prime power fails, and the transfer switch is opened.

The transformer, while providing isolation of the load from line voltage transients, can create output voltage distortion and transients of its own possibly worse than those from a poor AC connection. Additionally, the inefficiencies of the ferro transformer may result in the generation of a significant amount of heat, on top of which they are quite large and heavy, making standby-ferro UPS systems also bulky as a result.

This type of UPS is less used nowadays in powering modern computer system loads because they can become unstable. The power supplies used to power servers and routers are “power factor corrected” so as to draw only sinusoidal power from the utilities. This is accomplished by using capacitors that have a reactance such as to lead the applied AC voltage. The transformer output of the standby-ferro UPS contains coils whose inductance results in a lag in voltage compared with the AC power. This combination of an inductive and capacitive circuit results in resonance or ringing that may produce high currents and can potentially damage the load equipment.

Double-conversion online UPS

For applications above 10kVA, the double conversion online UPS is often the configuration of choice. Diagramed in Figure 4 below, the double conversion online UPS is similar to that of the standby UPS except that the inverter output represents the primary power path whereas in the standby UPS this was the secondary or backup path. The AC prime power main feeds a rectifier (AC to DC converter) and then is fed right back to the inverter that regenerates AC power from the DC power. A backup battery ties into the DC line and is charged by the rectifier.

A static bypass switch is available but is not activated in the event of failure of the AC prime power. The battery power will seamlessly feed the inverter should the AC mains fail, resulting in a design that results in no transfer time in the event of a power loss. Because the inverter and rectifier are continuously active in this design, there is reduced reliability of the electrical components versus other designs. But from the perspective of the electrical power, this type of UPS delivers ideal power output performance.

Delta conversion online UPS

The delta conversion online UPS is a relatively new design that was introduced to address some of the drawbacks associated with the double conversion online UPS discussed previously. As with the double-conversion design, the delta conversion online UPS has the inverter supplying the output power to the load and hence is always operating. Figure 5 below shows the design of this type of UPS.

A delta transformer couples the AC mains to the delta converter, which generates a DC power output. As with the double-conversion design, the DC output serves to maintain the charge on a backup battery and also feed the inverter, which then produces an AC output that is transmitted to the load. The prime power also has a feed that meets up with the inverter output.

The delta converter functions to minimize any harmonics that might be reflected back to the utility grid or to a connected generator system, making this UPS design compatible with generator sets and eliminating the need to oversize wiring or generators. From the perspective of output power characteristics, the delta conversion online UPS is identical to that of the double-conversion online UPS, but with a substantial reduction in energy losses or higher efficiency.

Advantages and disadvantages of UPS

• Advantages to using uninterruptable power supplies include:
• No delay between switching from the primary power source to the UPS.
• Can better support critical instruments compared to generators.
• Consumers can choose the type and size of UPS, depending on the amount of power they need to supply to a device.
• UPSs are silent.
• Maintenance of UPS systems is cheaper compared to generators.

Disadvantages to using uninterruptable power supplies include:

• The inability to run heavy appliances- because UPSs are run off of batteries.
• If substandard batteries are used, users may end up replacing the batteries often.
• UPSs may need professional installations.

UPS Applications

    Applications of a UPS include:

• Data Centers
• Industries
• Telecommunications
• Hospitals
• Banks and insurance
• Some special projects (events)