,
Abstract
Most of PMI’s power quality analyzers have the ability to measure Power. Depending on the model and options, it is often possible to measure to kW, kWh, kVAR, and VA.
Having the ability to measure power allows PMI recorders to act as a demand measurement tool. In this discussion, the Demand graphing feature found in ProVision will be explained.
What is Demand?
Demand can be defined as the highest average power (kW) measured in a sliding 15-minute interval during a billing or recording period. That means the electric utility takes a measurement of the kW over a typical billing cycle to find the max power that the customer used over a 15 minute average of the power recorded. This differs from electric power consumption because Demand is the max of an average period over the billing cycle. The customer could have high Demand but a small amount of total energy consumption or average power due to the peak measurement.
How is Demand Computed?
Demand is a measure of the power used by a customer on an average basis over a sliding fixed time frame. The typical measure of Demand is a 15 min average over a Demand period of a typical billing cycle. The MAX measure is considered the Demand for the customer at that time and the service is rated based on that measurement. In ProVision, the Demand is computed based on the Interval recording of Power measured during the recording session. To compute Demand, it is necessary to enable at least the Real Power Stripchart for recording; the Apparent Power (VA) Stripchart is also recommended (see Figure 1).
Figure 1. To compute demand, it is necessary to enable at least the Real Power Stripchart for recording; the Apparent Power (VA) Stripchart is also recommended
The recorder will measure and store the max, min, and average readings for each enabled stripchart once per stripchart interval. For example, if the interval is one minute, then one cycle max and min, and one-minute average power values will be recorded.
The Demand graph is computed by ProVision from the recorded interval data. A sliding window is used to average the stripchart data points. This window is applied to the average values, not the one cycle max or min values. This window slides at the interval rate and defaults to 15 minutes.
Effects of High Demand
Most electric distribution equipment is ultimately limited by heat. The higher the power transferred, the higher the heating, due to inefficiencies in each component. Since most distribution components are fairly large, the time it takes to reach a high temperature is often long. The goal of Demand measurement is to gauge how the power demand affects the heat rise in distribution equipment, and the 15-minute window is designed to conservatively match the time constant of typical small transformers.
High Demand can cause equipment failures or shortened life for distribution transformers and conductors. Small spikes in power consumption can often be overlooked due to the short period that these loads are energized or encountered. For example, a motor start may consume a very high peak power, but since it only lasts for a few cycles, the transformer temperature is largely unaffected, since its thermal mass is so high. The reason the high Demand loads cause heating is due to the “HIGH Averages” or longer “HIGH Power” periods of power consumed by the customer.
ProVision Demand Graph
As previously discussed, the Demand Graph is computed in ProVision based on the Power Intervals measured with the PMI recorder. Figure 2 is an example of a 15 min sliding window Demand graph which is set by default in ProVision.
Figure 2. Example of a 15 min sliding window demand graph which is set by default in ProVision
This graph is generated by clicking Graph->Power Interval->Demand. It is highly recommended to use a Demand Window that is divided evenly by the interval setting in ProVision (e.g. Interval = 2 Min, Demand = 16 min). Choosing a Demand window that is evenly divided by the interval setting will give evenly spaced averages per Demand period. This setting can be changed in ProVision as shown in Figure 3 under Options -> Preferences.
Figure 3. Demand settings in ProVision
The Demand Interval is set in cycles (15 min = 900 seconds, by default).
Power Consumption vs. Demand
Using ProVision to find sources of high Demand is straightforward with some understanding of how Demand is computed. Figure 4 is an example of the Power Interval vs. 15-minute Demand on a typical power system.
Figure 4. Example of the Power Interval vs. 15-minute demand on a typical power system
Notice that the top plot is Max Apparent Power and the bottom plot is 15-minute Apparent Demand. As revealed in the plot, there is very little correlation between the two graphs which is what make the Demand graph so unique. If a customer were rating a transformer based on the Max Apparent Power of a system, he might oversize it due to the instantaneous levels and not the “Longer Average” Demand of the system. Notice when the Apparent Power Interval graph goes up in magnitude for a lengthy period the Demand goes way up (time = 6:45 to 7:30); however, when the instantaneous Power goes up, the Demand is not affected much. The Demand tends to better reflect the heating of equipment than the one cycle maximum power level, or even a 1 or 2-minute average as shown in Figure 3.
Conclusion
High Demand can greatly decrease the lifetime of distribution equipment due to the underrating of transformers and conductors. High Demand customers can cost the industry in elevated equipment failure, but this is an easy issue to pinpoint using ProVision software and a PMI power quality analyzer. For complete details on recorder options, visit the PMI website here.
D.J. Diehl
Design Engineer
TheEngineers@powermonitors.com
https://www.powermonitors.com
(800) 296-4120
