Roger Dugan - 2014-08-29

What is Unique About OpenDSS?

A common question we receive is "What is unique about OpenDSS?" This is a fair question since there are many other distribution system analysis programs available. Here are some things that are generally different than most typical electric power distribution system analysis tools in common use today.

  • Quasi-static solution mode (i.e., Sequential-time simulations). In 1997, this was a key distinguishing feature of the OpenDSS. Now, there are several other tools with similar capabilities. OpenDSS was designed from the beginning to perform yearly, daily, and duty cycle simulations. Each load and generator may have a unique load curve, which is an important feature because modern energy meters can provide demand interval data for every customer. The ease at which OpenDSS handles this task is perhaps still unique in the industry. The Monitor and EnergyMeter objects can capture time-series results from lengthy simulations. For many analyses with renewable resources, storage, electric vehicles, etc., it is difficult to get the right answer without being able to model system behavior as a function of time.
  • The source code is provided so that smart grid researchers needing advanced simulation capabilities that have not already been implemented can modify the code to develop new capabilities. At the least, researchers can simply see how something has been done and reproduce it in their own tools. The data and the API are public.
  • Easy to convert data sources to OpenDSS script. The scripting language was designed to be reasonably close to common text data formats used in distribution system analysis tools. The program was developed for a consulting and research environment in which model data is received from utilities in a variety for formats. Since OpenDSS can model transmission networks as well as distribution circuit, data for a single model often come from more than one source.
  • Ability to Script the simulator behavior. It is impossible to anticipate what everyone will want to do with Smart Grid simulations, etc. OpenDSS provides the basic characteristics of a distribution system, in great detail. Several often-used solution processes are built into it. Scripting is accomplished by creating scripts in files and by driving the OpenDSS from another program using the COM interface. The COM interface allows you to write some code in Excel VBA, Matlab, Python, R, etc. and make the OpenDSS do what you want it to do.
  • Dynamics mode enables the simulation of Generator swings during disturbances. A simple model is provided, but users may create special models with DLLs.
  • Ability to model n-phase lines of arbitrary configuration, not just 3-phase or 1-phase lines. This enables the modeling of some difficult problems such as stray voltage from multiple feeders sharing a common neutral, transmission overbuild falling on distribution lines, and many other coupled-conductor problems.
  • Ability to model n-phase, m-winding transformers rather than simple, traditional 2-winding or 3-winding models supplied with traditional power system analysis tools. Of course, the appropriate impedance matrix data must be supplied, which for higher-order transformers may have to be obtained from the manufacturer or lab tests. However, if the data are obtained, this model allows for some extraordinary simulations.
  • Controllers are modeled separately from circuit elements. They are designed to operate exactly like their real physical counterparts when simulating in small time steps. Users may create their own controller models in a variety of ways.
  • Controllers can be used to develop and test distribution automation algorithms.
  • An extensive energy meter model. The model has numerous registers for tabulating different kinds of losses during simulations.
  • The solution engine could be put underneath a GIS user interface to provide a power distribution system analysis tool if none already exists in the GIS package.
  • Harmonics analysis is a native capability of OpenDSS; it is not an add-on. The heritage of the simulation methods can be traced to harmonics simulation programs dating to the late 1970's in the Systems Engineering department of McGraw-Edison Power Systems, now a part of Cooper Power Systems. Therefore, harmonics simulation on complicated circuits can be performed quite easily. OpenDSS has more of a planning mentality than other power system harmonics tools. For example, all loads have a default spectrum and one may obtain a reasonable harmonic flow solution after solving the base power flow simply by issuing the command "Solve Mode=Harmonics". With Monitor objects distributed appropriately, this will generally expose any potential harmonic resonances the present circuit model might yield.
  • The program is designed with an Object oriented structure. This enables new models of power-carrying equipment and controls to be added with less concern for breaking other parts of the program.
  • A Load has a Bus. This may seem a trite statement -- and the subtleties lost on the reader -- but it is one of the fundamental differences between OpenDSS and many familiar power system analysis tools for power flow and related analyses. A traditional power flow formulation would use the statement that A Bus has a Load and relatively simple ZIP model characteristics would be assigned to the bus. In the OpenDSS, a Load object is simply another instance of a power conversion element that may be connected to a bus. This concept allows for the connection of many different types of loads to the same bus, each with its own loadshape, growthshape, voltage characteristic, etc. This enables the simulation of many issues related to Smart Grid implementation that would otherwise be quite difficult.
    • A Fault is simply another circuit element – a simple resistive element. Many distribution system analysis tools have a completely separate tool for short circuit analysis. There are many complex algorithms for performing short circuit studies in these tools. In OpenDSS, one simply defines a Fault object at one of the buses. The power flow solution will often converge with a fault on the system, which is unimaginable in other more traditional tools. And there is nothing special about multiple simultaneous faults: simple define multiple Fault objects and execute the Solve command.
    Per units and symmetrical components are not used in the solution algorithm. Voltages, currents, and impedances are modeled in actual volts, amps, and ohms, respectively. While this is not necessarily unique among distribution system analysis programs in North America, the absence of symmetrical components in the solution algorithm still makes OpenDSS fairly unique. Some circuit elements can be defined using sequence impedances, but those are immediately converted into phase domain matrices. Also, the results can be presented in sequence values, at least for the first 3 phases of a bus (symmetrical components are undefined if the number of phases is not 3).

29 August 2014

Last edit: Roger Dugan 2014-08-29