The first step in creating models of distribution feeders in GridLAB-D is to convert the available distribution feeder models into the GridLAB-D format. Traditional commercial distribution analysis tools, such as CYMDIST or SynerGEE, contain topological and component information about the physical make-up of the distribution system. This includes system topology, conductor sizing and configuration, transformer data, regulator settings, and more. Typically, information is only available from the substation to the primary side of the secondary transformer, or in limited quantities on the secondary system. Nearly all of this information is compatible with the GridLAB-D powerflow simulator, with the appropriate conversion tools. This section will describe the basic methodology for converting from a known database to GridLAB-D format.
The basic steps in creating a powerflow model of a distribution system within GridLAB-D from a known database are as follows:
- Convert available information from the database to a syntax and format compatible with GridLAB-D format.
- Validate the powerflow solutions between the two tools.
- Populate additional components that may be missing from the original database.
In the past, this process was developed using a PERL script due to the open-source nature of PERL, and its ability to easily handle text parsing and numerical functions in a single scripting language. The PERL script was used to convert the commercial database into GridLAB-D syntax, using components such as overhead lines, underground lines, transformers, regulators and capacitors (including control information), switches, fuses, substation transformers and static loads. Recloser and sectionalizer devices were not explicitly modeled, except as basic switches during this process. The scripts can be seen in the GridLAB-D source tree.
Additionally, distributed loads were converted to spot loads via Kersting’s method. Essentially, scripting code for each object type was written, which then printed out all of the database information into a new format. In most cases, this step was completed with few problems. However, a few common errors occurred regularly:
- Secondary transformers and triplex cables are not contained within most commercial packages at this time and are added through average sizing.
- Cable data is often in the form of sequence components. Conversions are made to estimate physical parameters, often from a look-up table.
- Naming conventions when combining multiple circuits on a substation may not be appropriate.
- Topological loops (as opposed to electrical loops) can be used to represent some components in radial solvers. These should be evaluated during the conversion to verify consistency.
- Zero-length or connectors are not allowed in GridLAB-D, but can be converted to a parent-child relationship.
During this process, loads can also be tagged to help determine the load composition. A simple comment line using “//” is placed in each load object for later classification and use.