3.2 Current Distribution Planning and Operational Procedures

3.2.3 Distribution Planning Tools

Over the last two decades, distribution system planning has been conducted by utilities primarily by estimating new or increased loads expected to be served in each of their feeder areas over the next 3 to 5 years.   Most completed by calculating the maximum demand during peak periods for each of these areas, and then designing and building the distribution substations and feeders to meet those maximum demand requirements. 

As part of the analysis involved, distribution planners determine new feeder extensions or routes for new feeders and laterals, assess the voltage profiles for each and where to place voltage regulators and capacitor banks along the feeder, and develop fusing, sectionalizing and recloser outage switching processes for each type or location of potential faults.

Generally each feeder is studied separately, with assessments of multiple feeders only where they might impact each other either at the substation or for improved reliability options if used during reconfigurations due to emergencies or maintenance activities.

Capital costs are determined for these modifications, and then presented to regulators in utility rate cases as necessary to meet the projected loads. These costs are generally approved, since the assumptions can be clearly presented, and, in the larger scheme of utility rates, these costs have been relatively small.

In the past, these distribution planning calculations were based on spreadsheets and paper maps. Recently, Geographical Information Systems (GIS) and other automated tools are starting to be used, but the results are still focused on steady-state distribution system design based on maximum demand (peak load). These tools do not yet support advanced distribution automation and DER capabilities, nor do they model dynamic management of the distribution system.   For example, the use of CymeDist 4 years ago only had one very basic inverter that could be modeled in that software.

Some discussions have suggested the use of dynamic transmission power flow tools for distribution studies, but transmission models are quite different from distribution models. For example, transmission models legitimately assume that all phases are balanced; however, distribution systems rarely can assume balanced phases.   This is due to the presence of single phase residential loads that are randomly allocated to different phases in an attempt to balance, but depending on actual usage, one phase may see heavier loads creating the unbalanced measurements across the three phases.

Transmission models are completely networked, while distribution systems are radial (even if seemingly networked). Transmission models assume fixed kVA ratings and fixed impedance of various equipment, while distribution systems have variable values depending upon customer load characteristics, customer DER characteristics, demand response reactions, weather impacts, and other configuration issues. And distribution systems simply have far more circuits and equipment than transmission systems, making the collection of data more unwieldy and the modeling more cumbersome.

The existing distribution planning tools do not yet have good models for distribution systems with high penetrations of DER or where dynamic distribution system operations are possible through autonomous advanced DER functions and/or more direct control by the distribution operational center. In the future, distribution analyses will therefore need to include:

Net Zero Construction on Distribution Feeders

An emerging trend in new building construction is the concept of “Net-Zero” buildings.  Building designers are taking advantage of numerous techniques to design and build a building with zero net energy consumption, meaning the total amount of energy used by the building on an annual basis is roughly equal to the amount of renewable energy created on the site. These buildings consequently do not increase the amount of greenhouse gases in the atmosphere. They do at times consume non-renewable energy and produce greenhouse gases, but at other times reduce energy consumption and greenhouse gas production elsewhere by the same amount.

Most zero net energy buildings get half or more of their energy from the grid, and return the same amount at other times. Buildings that produce a surplus of energy over the year may be called "energy-plus buildings" and buildings that consume slightly more energy than they produce are called "near-zero energy buildings" or "ultra-low energy houses".

The challenge for distribution planners is the old formulas for estimating new connected load for buildings is changing.  Now, seasonal differences can change load and voltage characteristics in the planning process.  Further, if DER systems fail, the distribution system becomes a back-up system for net-zero buildings, changing the load profile markedly on a daily basis.

This trend can also change the way line extension costs are determined and charged to customers.

Distribution Planning Tools

Although none of the existing tools can yet do all of the functions needed for DER planning, some of the more commonly used commercially-available distribution analysis tools include features that help:

These tools are being upgrading to handle some of the issues identified above. See Appendix 4.2 for more information about the functions that these planning tools may provide.

Distribution Planning Process

An area where regulators can facilitate the inclusion of DER and net-zero buildings is to add more transparency and alternative advocacy in the distribution planning process during regulatory proceedings.  Typically growth projections and utility one-year and five-plans were reviewed for prudency of planned construction activities, now adding additional public input to plans may allow new concepts and issues to be raised.  For example as shown in Figure 7, the distribution planning process of a utility in Australia includes a public review process where proponents of non-network solutions have the opportunity to review the Distribution Planning Report and provide input and suggestions.

Figure 7 : Australian utility distribution planning process