Trends
The Load trends tab is on the right hand panel.
Load trends tab
Load Trends show time series aggregated smart meter data for any part of the distribution network. The following types of trend displays are available:
- Daily profiles of the top 'n' max/min days,
- Load over time, and
- Load duration.
The load can be expressed in kVA, kW or, for conductors, Amps.
When combined with rating information for assets, it's possible to easily identify overloaded assets anywhere in the network.
How they work
Load trends are generally only available in the Network Explorer if your organisation has complete, or nearly complete, smart meter penetration and 30 minute interval (or less) power or energy data, and this is made available from the meter data management system for ingestion by the Energy Workbench.
The load trends are calculated by summating all smart meter load data downstream of the asset from which the trend was requested, for the number of days needed to satisfy the type of trend requested. For a max/min trend display, this will be all available historical data, unless a From-To date range is selected, which will limit the search to the days in the specified From-To date. For load-over-time and load duration trends, all smart meter data in each day in the specified From-To date range will be summated.
In the Energy Workbench, the interval between smart meter load data samples is 30 minutes, with Max/Min and date range trend displays drawing a straight line between each 30 minute load data point to form a continuous trend line. Each data point represents the summated load at that sample time for all smart meters connected to customers found in the downstream trace. To improve responsiveness, the load data for all customers connected to transformers is pre-aggregated for each 30 minute sample interval, so that load requests in the HV network will use the aggregated load values for the transformer, rather than tracing into the LV network and looking for smart meter load data on individual customers.
This aggregated load can be cached in memory, depending on how the Energy Workbench has been deployed for your organisation. When this is done, the load trends will be very performant - returning most results in a few seconds. If the load data is not cached in memory, the results can take a lot longer to be returned.
When a load trend is requested on any of the LV network, only the load data on that portion of the LV network that is traced will be included. The individual load points cannot be cached in memory as the amount of memory that would be required would be too large in practical terms, and so load trend requests for LV only network can take somewhat longer than the HV load requests that can use the in-memory cache.
Smart meters typically report real energy values for each interval in kilowatt hours (kWh). Energy is expressed in kilowatt hours, so for a 30 minute energy reading (half of one hour) the average power (kW) for that 30 minute interval can be found by multiplying the energy by 2. The Power Factor for individual smart meters is generally not available, and so the real power (kVA) is calculated by multiplying the real power (kW) by a global power factor of 0.9. When dealing with conductors, the current is found by considering the number of phases present on the conductor, and applying the following equation. TODO get the equation.
Invoking a load trend.
Load trends are invoked from an icon on the pop up menu for each asset. When the trend is invoked, the right hand screen panel will open, showing the trend tab. This panel can be resized by dragging the left hand panel edge left or right. The default trend displayed is the "Max/Min" trend. This can be changed to a "Date Range" trend by selecting this option in the "Mode" drop down.
The "Compare Trends" checkbox allows multiple trend sets to be displayed concurrently on the same axis. This is explained further in the following sections.
Max/Min Trends
This shows a set of 'n' trend lines for a 24 hour period. The number of trend lines shown will be the number specified in the "Limit", with the default number being 5. The x axis of the trend represents a 24 hour time period, with 48 intervals - each one representing the average load for the previous 30 minute time slot.
The algorithm to determine the maximum and minimum load days ranks each day in the historical date range being considered according to the highest load value of the aggregated smart meter data for that day - the peak value. The maximum demand days are the days with maximum peak value within the day. The minimum demand days are the days with the lowest peak values within the day.
Each trend line on the "Max Demand" graph represents one of the top 'n' maximum demand load days. Each trend line on the "Min Demand" graph represents one of the 'n' minimum demand load days.
If a "From" and "To" date is not provided, all available days with load data in the past will be considered.
The screen shot below shows a Max/Min load trend that has been invoked from a switch.
Date range trends
As the name suggests, a date range trend provides a continuous load trend downstream of the selected asset, for the specified date range. As with the Max/Min trend, the load can be expressed in kVA or kW for all assets, and for conductors, it can also be expressed in Amps, using the operating voltage of the conductor.
A typical date range trend is provided in the illustration below.
In some cases, there will be gaps in the trend display - this indicates there is no data available for those dates. This can happen because either there was no smart meter data available, or because some days have been removed from the historical record by your system administrator for performance and storage reasons, as they did not have any load that was "of interest" to most people, that is, not a high or low demand day.
Load trends of aggregated smart meter data should in theory be close to the values reported by SCADA or other forms of telemetry - however in practice there may be missing smart meter data for a portion of the network being examined, or there can be mapping issues between the smart meter data and the GIS supply points they connect to. For this reason, its always a good idea to compare the load data from other telemetered sources too ensure the smart meter trends are providing acceptably accurate results. Once that is done, you can be reasonably confident the load trends will give you accurate data anywhere along the feeder.
The aggregated smart meter trends are always provided against the networks normal state. This means if you are comparing, for instance, a load value obtained from SCADA with a load trend in the network explorer obtained from smart meter data, and the network was in an abnormal switched state at the time, you will see differences in the load trends. The smart meter derived trends effectively remove the impact of changing switch states in the network.
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Load duration curves
Immediately below the Date Range trend display is a load duration curve (LDC). An (LDC) is a graphical representation that plots the magnitude of electrical load (y-axis) against the duration (x-axis) for which it is equaled or exceeded, arranged in descending order from highest to lowest. It transforms chronological demand data into a format that highlights capacity requirements and usage. Unlike the date range trend, which shows the chronological order of load, the LDC does not show when a specific load occurs, only how long it lasts.
In many situations, you may notice a date range trend has periods of time where there is reverse power flow, that is, negative load. In these cases, the LDC still provides an indication of how much time the load is negative, and how much time at each negative value, however the percentage figure provided on the x-axis for negative load figures should be calculated as 100 minus the percentage value on the x-axis.
The screen shot below shows a typical load duration curve, immediately below the date range trend is was calculated from. Note the negative values present in the trends, and also the gaps in the continuous trend in the date range, indicating missing data for those time.
Comparing trends
Both the Max/Min and date range trends allow more than one trend to displayed on the same trend panel. This is enabled by checking the compare trends checkbox. When this is done, any new trend created will be added to the existing trend display, on the same set of axis. In the screen shot below, two Max/Min trends are being compared for two separate reclosers. In this example, we have limited the number of days to test for max and min values has been limited to 2. Note how the colour code scheme for the legend has changed - when only looking at one set of trends, each trend line has a different colour. When comparing trends, each max min set has the same colour, but each in a different shade of that colour.
The same approach is taken to the legend for date range trends.
