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Identifying Clusters: An Electricity Example

  • Selwyn McCracken
  • Friday, Jul 24, 2020
  • Tags: Analysis

Introduction

Cluster analysis is a powerful technique when groups of similar (or dissimilar) behaviour needs to be identified within a population of people / items / assets / sensors etc. Knowledge of common or unusual patterns of behaviour allows interventions to be targeted where, when and how they will be most effective. For example, knowing normal electricity consumption patterns by building type or usage category allows unusually high behaviour to be pinpointed and energy saving activities to be directed.

To illustrate, we cluster the typical weekly electricity patterns for a selection of meters from an anonymised building energy usage dataset provided by Schneider Electric.

Average hourly energy consumption (kWh) and temperature

The average hourly electricity consumption across a week is presented in Figure 1 below for 24 power meters. These power meters are installed at two different facilities - a laboratory and an office, and the outdoor temperature readings are also shown as a useful comparison.

Figure 1: Average hourly electricity consumption and temperature across the week

The main observations from Figure 1 are:

  • Most meters record what would be expected for a standard working week, with a regular diurnal profile for Monday to Friday during working hours, and much lower consumption during the weekend.
  • Similar patterns for every day are seen at two meters (the lab guardhouse - top row; office other - bottom row) and the outside temperature (bottom right).
  • An irregular usage pattern is noted for the office elevators (second row, middle).

While these main patterns can be discerned visually in this instance, this method does not scale much beyond the small number of meters presented now. The next section shows how this can be improved upon.

Hierarichal Clustering of Weekly Patterns

Hierarchical clustering is one of many multi-variate clustering techniques. As the name suggests, groups or clusters are defined in a hierarchical or tree-like manner, with group members that are placed in the same branch of the tree sharing the greatest degree of similarity. Because we are examining time series or trend data, the variant used here employs dynamic time warping. This means each time series is stretched when making the similarity comparisons, to allow some leeway for identifying correlation patterns that might be slightly out of sync with one another.

The hierarchical clustering dendrogram (i.e. tree) of our meters of interest is presented on the left side of Figure 2, along with their respective weekly consumption patterns at right.

Figure 2: Cluster membership and average weekly profile by meter

The following can be seen in Figure 2:

  • The time series plots at right provide a nice visual confirmation of how effectively the clustering algorithm works, with markedly different trends being put into their own branches, and similar patterns being colocated in the same parts of the dendrogram.
  • To split the dendrogram into discrete groups (as is often required in practice), we assigned four cluster groups that correspond to the first 4 branching points in the tree (and indicated with different colours). Four groups were chosen as this seemed to capture the main types of patterns that could be seen visually, and aligned with expected behaviour.
  • The office elevator consumption (purple line, at bottom) was the most noticably different pattern from the rest, and was treated as such by the clustering algorithm, being the first branch split off on its own.
  • The largest cluster were those meters that broadly followed a standard working week pattern with low weekend activity (indicated in red). Those positioned towards the top of this group have a smoother daily profile than those towards the bottom of the group, which exhibited greater variance or spikiness in activity throughout the working day.
  • Meters with similar profiles for every day of the week can be seen in the cluster group coloured green. Within this group it is notable that peak electricity consumption for the ‘lab guardhouse’ and ‘office other’ is out of phase with the temperature peak, indicating that electricity consumption for those two locations is probably for heating. It is also interesting to note that these negatively correlated patterns are placed into the same cluster, showing the ‘stretching’ effect that the dynamic time warping method can have when determing cluster membership.

Clustering weeks for a single meter

The same method can be applied to a single meter, creating clusters of weeks with similar electricity profiles (Figure 3).

Figure 3: Weekly electricity profiles for a single meter by cluster group

Mouse over lines to highlight individual trends. Red line shows cluster average.