UPS SIZING: LOAD PROFILE

Monday, May 30, 2011

The Load Profile is basically a graph showing how the load demand changes with respect to time. It is the subsequent step after choosing the critical and essential loads for uninterrupted power protection. Through the load profile, the design load and energy demand values, which are prerequisites for UPS Sizing calculation, can be derived.

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There are two techniques for creating a load profile:

1.  Autonomy Method

This is the conventional way used for emergency and backup power applications, such as in UPS systems. The instantaneous loads are displayed over a so-called autonomy time, which is the period of time that the loads need to be supported by a backup power system in the event of an interruption. IEEE 446 has published typical autonomy times for some of the critical and essential devices.

2. 24-Hour Profile

This method is more commonly associated with stand-alone applications, like in solar power systems. It displays the average instantaneous loads over a 24-hour period.

However, this post will only focus on the autonomy method in order to make a load profile for UPS Sizing.

Basic Steps

Preparation of Load List

1.      List all the selected critical and essential equipment to be protected by the UPS in one column.
2.      Read the nameplate of each load and write down the voltage (V) and current (A).
3.      Multiply the voltage and current in order to get the Volt-Ampere (VA) or apparent power for each load and enter these values in another column. Sometimes, the nameplate will only indicate the real power or load in Watts. In this case, divide the wattage by its nameplate power factor (PF) to derive the load VA. If PF is unknown, use typical values of 0.6 to 0.7.
4.     Decide on the autonomy time for each equipment. Some loads may only be required to ride through short interruptions or to have enough time for proper shut down. Meanwhile, other critical equipment may need to operate for as long as possible.
5.      Multiply the load VA and Autonomy time for each load and write the results (VAh) in the last column.
6.      Get the total for VA and VAh.

UPS Sizing Load List
Sample Load List for the Critical and Essential Equipment
Graph the Load Profile

Using the autonomy method, the load profile is constructed by piling the energy or VAh rectangles on top of each other. An energy rectangle has the autonomy time as its width and the load VA as height. The load profile is created by piling the widest rectangles first (i.e. equipment with the largest autonomy time).

UPS Sizing Load Profile
Load Profile for the Selected Loads
Design Load and Energy Demand

Prior to the calculation of the design load and energy demand, there is a prerequisite to set the design margin factor (DMF) and the load growth factor (LGF).

The design margin factor is used to account for any inaccuracies in estimating the load that may lead to UPS overloading. A 1.25 factor is usually recommended. On the other hand, the future load growth factor is typically in the range between 1.1 to 1.3. If certain that no future loads are expected, then this allowance can be omitted (not advisable).

Design Load

It is the total load VA of all the critical and essential equipment that should be protected from interruptions. This value is the basis for the UPS size, as well as for the inverter and rectifier.

Design Load = Total Load VA X (DMF) X (LGF)

Design Energy Demand

Computing for the energy demand is important for the sizing of energy storage devices such UPS batteries. It is calculated by finding the area under the load profile curve, which in this case is the total area of the rectangles. In other words, the energy demand is simply the total VAh of the loads multiplied by the margin and load growth factors.

Design Energy Demand = Total Load VAh X (DMF) X (LGF)

Example:

The total peak load apparent power is 640 VA and the total load energy is 2680 VAh. Using a margin factor of 1.25 and 1.2 growth factor, the following are calculated:

Design Load = 640 VA X (1.25) X (1.2)
Design Load = 960 VA

Design Energy Demand = 2680 VAh X (1.25) X (1.2)
Design Energy Demand = 4020 VAh


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.