LINE INTERACTIVE UPS

Wednesday, May 4, 2011

Line Interactive UPS (Uninterruptible Power Supply) is another type of static UPS, which conditions and regulates the utility supply using only one main power converter. In other words, the inverter, battery charger (rectifier) and the transfer switch are replaced by a combination inverter/converter in the line interactive design. Nonetheless, the primary power source is still the utility supply while the battery is the alternative just like the standby type. 
Line Interactive UPS Schematic Diagram 
Generally, the line interactive UPS provides technical performance in between that of a standby and online designs. It responds faster than a standby UPS during an interruption since the main power converter is always connected to the critical or sensitive load. However, it does not provide the same protection as the online UPS, because the switching or transfer time is not completely eliminated - but only reduced.

The line interactive UPS is commonly used for medium-sized load applications from 500 VA to 5 kVA such as in the business and office units. Also, the cost per VA of a line interactive design ranges in between that of a standby (low cost) and online (high cost) types.

Combination Inverter/Converter

The primary difference of a line interactive UPS to other static UPSes is the use of a single unit for converter/inverter. The said device charges the battery during normal operating conditions and operates in reverse by converting the stored energy back from DC to AC in cases of interruptions. The modern inverter/converter is also capable of regulating the output voltage and filtering out noise, giving additional protection during undesirable voltage variations. However, such conditioning capabilties are just added benefit and should not be compared to the performance of other power quality devices mainly intended for such disturbances. It must be noted, that the primary function of the UPS is to provide equipment ride-through during interruptions.

Moreover, the inverter/converter distributes a portion of the AC input power in order to keep the batteries fully charged during normal conditions.  Usually, this requires less than 10% of the UPS power rating, subjecting the components at a lower temperature while in this operation.  As an example, the main power converter in a 2.5 kW line interactive UPS operates at about 250 W while charging its batteries. 

Advantages

Ø  Relatively high efficiency because of lesser power conversion
Ø  High reliability due to lesser components
Ø  Lower operating temperatures - less heat load on the facility

Disadvantages

Ø    Transfer or switching time delay is not completely removed.
Ø    Battery will be always used when power supply is unstable, which could reduce available capacity during sustained interruptions.
Ø     It does not have a built-in power factor correction capability that is required by some types of loads.

References:
Carbon Trust. (2010). Uninterruptible Power Supply. A guide to equipment eligible for Enhanced Capital Allowances. 
Samstad, J. and Hoff, M. (2004). Technical Comparison of Online and Line Interactive UPS Designs. American Power Conversion

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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.