UPS SIZING: GUIDE FOR UNINTERRUPTED POWER

Sunday, May 29, 2011

UPS Sizing is an important process in order to closely match the UPS capacity to its load. This is done to come up with a sufficient overall uninterruptible power supply, so that it will operate efficiently at the lowest possible cost. An oversized UPS system will lead to increased operating expenses, while an undersized unit will be susceptible to overloading and has a small load capability for long-term growth. Also, UPS sizing aims to determine the practical ratings of its main components – battery, rectifier and the inverter. 

The UPS Sizing process involves the following five basic steps:

1.    Identify and select the prospective UPS loads
2.    Create a load profile and derive the UPS design load (VA) and energy (VAh)
3.    Compute the battery size (number of cells in series and Ah capacity)
4.    Determine the overall UPS Size
5.    Determine the size of the UPS rectifier and inverter

The following discussions include the details for each basic step in UPS sizing plus an example in order to guide through the entire process.

Identify and Select the UPS Loads

In UPS Sizing, the first step is to identify and select the critical and essential equipment that need to be protected from the damaging effects of voltage sags and interruptions. Listed below are some of the possible loads, which may require uninterruptible power supply.

Critical Loads: Data Center, DCS and ESD processor, telecommunications equipment, blade file servers and other sensitive electronics.

Essential Loads: Lighting, Heating and Ventilation.

As an example, the following loads are chosen for UPS protection: Telecommunications, Computer Console, DCS cabinet and ESD cabinet.

Create the Load Profile

Refer to the UPS Sizing: Load Profile post for details on how to create a load profile and subsequently derive the design load and energy demand. The resulting load profile, design load VA and energy demand VAh of the selected equipment are shown below:

UPS Sizing Load List
Sample Load List 
Design Load: 960 VA
Design Energy Demand: 4020 VAh

Battery Capacity 

Refer to the  UPS Sizing: Battery Capacity post for details on how to size the battery for UPS applications. Meanwhile, additional information regarding battery sizing for UPS applications are discussed below:

Nominal Battery or DC Link Voltage

This is often selected by the UPS supplier. However, if required to be selected, the following factors should be considered:

·         DC output voltage range of the rectifier – the rectifier must be able to output the specified DC link voltage
·    DC input voltage range of the inverter – the DC link voltage must be within the input voltage tolerances of the inverter. Note that the battery end of discharge voltage should be within these tolerances.
·        Total DC link current at full load – affects the sizing of DC cables and inter-cell battery links.
·       Number of battery cells required in series – this will affect the overall dimensions and size of the battery rack. If physical space is a constraint, then fewer batteries in series would be preferable.

In general, the DC link voltage is usually selected to be close to the nominal output voltage.

Overall UPS Sizing

The overall UPS Size is to be based on the computed design load VA from the load profile. The idea is to simply select the next standard UPS rating that exceeds the design load VA.

Design load = 960 VA
Use a 1000 VA UPS (Next standard UPS rating)

Rectifier and Inverter Sizing

Refer to the UPS Sizing: Rectifier and Inverter post for details on how to size the corresponding rectifier and inverter. In the example, for a selected UPS rating of 1000 VA and battery capacity of 60 Ah, the sizes are:

Rectifier

Idc = 37.7 A; Use a 40-Ampere DC rectifier.

Inverter Size (Single-phase Inverter and 120 V output)

Iac = 8.33 A; Use a 10-Ampere Inverter.

Reference:
IEEE 446-1995. Recommended Practice for Emergency and Standby Power Systems for Industrial and Commercial Applications

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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.