ONLINE UPS – THE TRUE UNINTERRUPTIBLE POWER SUPPLY

Monday, May 2, 2011

Online UPS (Uninterruptible Power Supply) is a static UPS, which is designed such that under normal operating conditions, the primary power flow is through the battery-inverter circuit instead of the mains supply. Thus, the main power source is the online UPS and the utility is the backup. In other words, during normal operation, the load is always fed from the PQ device while the mains supply charges the bank of batteries. When there is an interruption, the battery charging is halted and the stored energy in the battery will supply power to the inverter. This is exactly the opposite of a standby type, where the primary source is the utility and the UPS as the alternative.

The online UPS is commonly referred to as the “true” UPS. It is also known as voltage and frequency independent (VFI) system.
Online UPS Schematic Diagram
Online UPS Schematic Diagram
Online UPS is the most suitable type for data centers/servers and other vital computer applications, as it is independent from variations in both the supply frequency and voltage. Therefore, it is usually applied for critical and large equipment from 5 kVA and above. In addition, it is designed to be robust as its components are always active in contrast to the standby type that only provides output during power failure. Also, the online UPS is the most common system because it is the only true provider of power without any interruption.

Types of Online UPS

Double Conversion

The double conversion online UPS is the most common design above 10 kVA. First, it uses a rectifier to convert AC power to DC in order to charge the batteries and feed the inverter. Second, it uses the inverter to convert the DC power back to AC to energize the critical and sensitive loads - hence, the name double conversion.

Delta Conversion

The delta conversion online UPS is a modern patented design. It eliminates the major weakness of the double conversion type – low efficiency.
Online UPS - Delta Conversion
Delta Conversion Online UPS (Courtesy of American Power Conversion)
The delta conversion online UPS design works similarly as the double conversion with the inverter always ON to power the critical load during normal conditions and with the battery supplying the inverter during an interruption. However, the main difference is that the delta conversion online UPS does not require two conversions of the total power flow, thus, improving its efficiency


Advantages of Online UPS

The online UPS, aside from giving ride-through for power interruptions, has the ability to provide very high isolation of the critical and sensitive electronic loads from all power line disturbances.

Another significant advantage of the online UPS is the fact that there is no transfer or switching time delay involved during an interruption. Thus, the momentary break in power (1/4 cycle to a little over 20 milliseconds), which is the weakness of the offline UPS, is eliminated. The operation can be compared to a charged laptop. Typically, the laptop is powered by the charger through its batteries. When the AC supply is out, the laptop still continues to operate because of the stored energy in the batteries. 

Disadvantages of Online UPS 

Some of the disadvantages associated with an online UPS are the following: 

Ø  Relatively expensive
Ø  Has a lower efficiency compared to standby and line interactive UPS systems
Ø  Prone to wear (shorter service life) as its components are continuously active
Ø  Higher operating temperatures tend to decrease reliability 


References:
Carbon Trust. (2010). Uninterruptible Power Supply. A Guide to Equipment Eligible for Enhanced Capital Allowances. 
Rasmussen, N. (2010). The Different Types of UPS Systems.

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I am an Electrical Engineer with a Masters Degree in Business Administration. My interest is in Power Quality, Diagnostic Testing and Protective Relaying. I have been working in an electric distribution utility for more than a decade. I handle PQ studies, power system analysis, diagnostic testing, protective relaying and capital budgeting for company projects.