Load Profile

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Introduction

Figure 1. Example of a load profile (using the autonomy method)

The energy load profile (hereafter referred to as simply "load profile") is an estimate of the total energy demanded from a power system or sub-system over a specific period of time (e.g. hours, days, etc). The load profile is essentially a two-dimensional chart showing the instantaneous load (in Volt-Amperes) over time, and represents a convenient way to visualise how the system loads changes with respect to time.

Note that it is distinct from the electrical load schedule - the load profile incorporates a time dimension and therefore estimates the energy demand (in kWh) instead of just the instantaneous load / power (in kW).

Why do the calculation?

Estimating the energy demand is important for the sizing of energy storage devices, e.g. batteries, as the required capacity of such energy storage devices depends on the total amount of energy that will be drawn by the loads. This calculation is also useful for energy efficiency applications, where it is important to make estimates of the total energy use in a system.

When to do the calculation?

A load profile needs to be constructed whenever the sizing of energy storage devices (e.g. batteries) is required. The calculation can be done once preliminary load information is available.

Calculation Methodology

Figure 2. Example of a load profile (using the 24 hour profile method)

There are two distinct methods for constructing a load profile:

1) Autonomy method is the traditional method used for backup power applications, e.g. UPS systems. In this method, the instantaneous loads are displayed over an autonomy time, which is the period of time that the loads need to be supported by a backup power system in the event of a power supply interruption.
2) 24 Hour Profile method displays the average or expected instantaneous loads over a 24 hour period. This method is more commonly associated with standalone power system applications, e.g. solar systems, or energy efficiency applications.

Both methods share the same three general steps, but with some differences in the details:

  • Step 1: Prepare the load list
  • Step 2: Construct the load profile
  • Step 3: Calculate the design load and design energy demand

Step 1: Prepare the Load List

The first step is to transform the collected loads into a load list. It is similar in form to the electrical load schedule, but is a little simplified for the purpose of constructing a load profile. For instance, instead of categorising loads by their load duty (continuous, intermittent or standby), it is assumed that all loads are operating continuously.

However, a key difference of this load list is the time period associated with each load item:

In the autonomy method, the associated time period is called the "autonomy" and is the number of hours that the load needs to be supported during a power supply interruption. Some loads may only be required to ride through brief interruptions or have enough autonomy to shut down safely, while some critical systems may need to operate for as long as possible (up to several days).

In the 24 hour profile method, the associated time period is represented in terms of "ON" and "OFF" times. These are the times in the day (in hours and minutes) that the load is expected to be switched on and then later turned off. For loads that operate continuously, the ON and OFF time would be 0:00 and 23:59 respectively. A load item may need to be entered in twice if it is expected to start and stop more than once a day.

Calculating the Consumed Load VA

For this calculation, we are interested in the consumed apparent power of the loads (in VA). For each load, this can be calculated as follows:

[math] S_{l} = \frac{P_{l}}{\cos \phi \times \eta} \,[/math]

Where [math] S_{l} \, [/math] is the consumed load apparent power (VA)

[math] P_{l} \, [/math] is the consumed load power (W)
[math] \cos \phi \, [/math] is the load power factor (pu)
[math] \eta \, [/math] is the load efficiency (pu)


Step 2: Construct the Load Profile

The load profile is constructed from the load list and is essentially a chart that shows the distribution of the loads over time. The construction of the load profile will be explained by a simple example:

Figure 3. Load profile constructed for this example

Suppose the following loads were identified based on the Autonomy Method:

Description Load (VA) Autonomy (h)
DCS Cabinet 200 4
ESD Cabinet 200 4
Telecommunications Cabinet 150 6
Computer Console 90 2

The load profile is constructed by stacking "energy rectangles" on top of each other. An energy rectangles has the load VA as the height and the autonomy time as the width and its area is a visual representation of the load's total energy. For example, the DCS Cabinet has an energy rectangle of height 200 (VA) and width 4 (hours). The load profile is created by stacking the widest rectangles first, e.g. in this example it is the Telecommunications Cabinet that is stacked first.

For the 24 Hour method, energy rectangles are constructed with the periods of time that a load is energised (i.e. the time difference between the ON and OFF times).

Step 3: Calculate Design Load and Energy Demand

Design Load

The design load is the instantaneous load for which the power conversion, distribution and protection devices should be rated, e.g. rectifiers, inverters, cables, fuses, circuit breakers, etc. The design can be calculated as follows:

[math] S_{d} = S_{p} (1 + k_{g}) (1 + k_{c}) \, [/math]

Where [math]S_{d} \,[/math] is the design load apparent power (VA)

[math]S_{p} \,[/math] is the peak load apparent power, derived from the load profile (VA)
[math]k_{g} \,[/math] is a contingency for future load growth (%)
[math]k_{c} \,[/math] is a design margin (%)

It is common to make considerations for future load growth (typically somewhere between 5 and 20%), to allow future loads to be supported. If no future loads are expected, then this contingency can be ignored. A design margin is used to account for any potential inaccuracies in estimating the loads, less-than-optimum operating conditions due to improper maintenance, etc. Typically, a design margin of 10% to 15% is recommended, but this may also depend on Client preferences.

Example: From our simple example above, the peak load apparent power is 640VA. Given a future growth contingency of 10% and a design margin of 10%, the design load is:

[math] S_{d} = 640 \times (1 + 0.1) (1 + 0.1) = 774.4 \, [/math] VA

Design Energy Demand

The design energy demand is used for sizing energy storage devices. From the load profile, the total energy (in terms of VAh) can be computed by finding the area underneath the load profile curve (i.e. integrating instantaneous power with respect to time over the autonomy or 24h period). The design energy demand (or design VAh) can then be calculated by the following equation:

[math] E_{d} = E_{t} (1 + k_{g}) (1 + k_{c}) \, [/math]

Where [math]E_{d} \,[/math] is the design energy demand (VAh)

[math]E_{t} \,[/math] is the total load energy, which is the area under the load profile (VAh)
[math]k_{g} \,[/math] is a contingency for future load growth as defined above (%)
[math]k_{c} \,[/math] is a design contingency as defined above (%)

Example: From our simple example above, the total load energy from the load profile is 2,680VAh. Given a future growth contingency of 10% and a design margin of 10%, the design energy demand is:

[math] E_{d} = 2,680 \times (1 + 0.1) (1 + 0.1) = 3,242.8 \, [/math] VAh

Computer Software

The load profile is normally done manually with the help of a spreadsheet. Since it's such a simple calculation, it's hard to argue that special software is warranted.

What Next?

The load profile is usually an intermediate step in part of a larger calculation (for example, AC UPS System or Solar Power System calculations). Alternatively, constructing a load profile may be the first step to analysing energy use, for example in energy efficiency applications.