Distribution System Capacity Planning and Overload Forecasting

Distribution System Capacity Planning can be viewed as a four-stage process, as depicted in Figure 1 below:

Forecast Load

The load forecasting phase of the capacity planning process addresses those activities that allow planners to predict with a reasonable degree of accuracy the demand for electricity in a given area as well as for each distribution circuit. These forecasts account for both normal increases in customer use as well as known incremental one-time additions of load. In order to accomplish this, there are three steps that need to be accomplished:

• Monitor Latest Peak Load: Typically, utilities use demand metering at their substations and demand ammeters on all circuits and transformers to obtain peak load information, which, in turn, is entered into a database to monitor potential overloads to circuits and transformers. Metering at the substations is also used to monitor VARs to assist in determining locations for the installation of capacitors.
• Forecast Load: Peak transformer and circuit load information is loaded into a load forecast data management system, which uses models (e.g. straight line and exponential) to project future loads. It is important to note that this forecast is net of the expected demand reductions that result from energy efficiency, demand response, distributed resources, and other demand related initiatives.
• Compare with Local Business and Economic Data: Load growth remains a primary factor in the load forecasting process, and tracking new developments and commercial expansion within specific operating areas can provide valuable insight in anticipating this type of growth.

Key factors to track in order to improve planning accuracy include:

• Sudden prosperity or an economic downtown in an area
• Reliability of builders’ plans for growth within their developments
• Feedback from city officials and area developers
• Coordination with a utility’s internal customer support group

Assess Capacity:

This phase of capacity planning consists of the following activities:

• Perform Feeder Analysis on Expected Normal Load: Utilities typically use modeling tools (e.g. Milsoft) to identify potential voltage regulation and conductor overload issues. The analyses performed annually in areas anticipating significant growth, and at a longer frequency for feeders in areas with more stable or declining growth.
• Identify Automatic Load Transfer Schemes: These schemes provide the customers with a separate feeder to provide power in the event of an outage on the main circuit, virtually instantaneously (i.e. within 2 seconds).
• Identify Voltage/Overload Problems: In order to effectively evaluate all design options, analyses of line capacitance at crucial points; and prioritization of pre-identified projects require an assessment of risk (number of customers potentially impacted and time required to restore service). A small number of overloaded feeders is not necessarily indicative of poor planning as not all feeders need to operate at levels less than their normal ratings. Similarly, most equipment will continue to operate past their normal rating for a period of time.
• Iterate for Long-Range Planning: Distribution capacity planning normally takes a near term view (i.e. the next 1-2 peak seasons) becausethe amount of time required to design and build a solution to an overload (e.g. changing out line transformers, reconductoring circuits, or adding feeders, transformers and substations) is generally one year. This is in contrast to substation and transmission related solutions that may take up to a decade to fully implement. In the long run, a series of short-term solutions may well be more expensive than one properly planned long-term solution. However, there needs to be a balance between the two as a series of short-run solutions buys time to gain a clearer view of the future.

Design Options

This phase of capacity planning consists of two steps, aimed at selecting the most cost effective method for designing (and subsequently implementing) capacity improvements.

• Evaluate Alternative Design Options for Line and Substation Problems: Typically, planners will develop a conceptual design to increase capacity or otherwise enhance the infrastructure within an assigned area. Should the cost exceed a prescribed cost threshold, a formal request that evaluates alternative approaches and risks of deferral are prepared.
• Coordinate with Other Areas and Transmission: Distribution Planning is tasked with communicating substation improvement plans, recognizing that increasing substation capacity will have a direct impact on system-wide transmission planning, and similarly, will alter contingency plans for losses of circuits or transformers.

Manage Projects

This phase of capacity planning consists of revising the planned projects database, prioritizing and scheduling each project, designing the project, building the facilities and verifying the accuracy of all records. These activities are commonly organized under the Work Management functions of the utility.