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Part III
Computerized Management and Basic Network Applications
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4
Unbalanced Three-Phase Distribution Power Flow
Advanced distribution management systems (DMS) become an extension of SCADA features by applying the software management principles from transmission networks to distribution systems. In addition, the modern DMS is capable of assessing the operational state of the distribution system and evaluating control functions such as reactive dispatch, voltage regulation, āwhat-ifā scenarios, and analysis [78]. Real-time evaluation using a power flow module will provide a close-to-real-world scenario simulation, to predict the best consequences that can be useful for operational purposes [79]. A calculation or analysis method should be developed to meet the requirements of rigorous operational type analyses, system power losses, and hypothetical analyses on a large-scale distribution system that connects multiple substations.
Establishing an unbalanced power flow module is essential. This is because the program will play an important role to determine the unknown variables, such as voltage and angle for all load nodes, and reactive power and angle for the generator nodes. Upon a convergence of power flow calculation, the solution assures complete balance of a feeder. This is a very useful solution to determine the total real and reactive losses at a given time, in which hypothetical scenarios can be simulated, and the solutions with operational constraints [80, 81] will provide a choice so that dispatcher in the control center can decide which tie switch they have to close to supply power to the āhealthyā part of the area.
Visual representation of topology from a geographical map is essential. Fig. 4.1 shows a sample GIS feeder that consists of detailed geographical information. This chapter introduces how to extract topological information, shown as part (a). In graph modeling, part (b) shows the connectivity between lines, nodes, loads, and the switches (remote-controllable only). Part (c) is a simplified one-line diagram with remote-controllable switches (RCSs) and the lump load in between them. Part (c) will be used later to illustrate a specific area of interest for fault location and partial restoration. The lump load between the RCSs is useful because it can show intuitively the connection of RCSs for the entire feeder. Part (d) is similar to part (c), but includes the non-RCSs. Typically, between RCSs, there are a few non-RCSs between boundary RCS in the area.
FIGURE 4.1
Sample feeder from geographic information system (GIS) to two one-line diagram representations (the left two).
4.1 Important Roles of Unbalanced Three-Phase Distribution Power Flow
Two basic ingredients needed to have the best outcome of power flow results are the accurate topology data and real-time measurement where the topology data is from GIS. In distribution systems, the most difficult part is the topology update where some errors incurred, topologically may cause power flow fails to converge [82]. As the measurements are either top-down, that is, pro-rated by allocation factors, or through load profile by the representative of load types, none of these are accurate enough to represent the close-to-real-time state due to the estimation. Real-time measurements from RTUs and FRTUs within a feeder can serve as the reliable live information to be used to project good estimates for each child node (loads) of a feeder.
The decision to make an investment in improving system observability is critical. The tree structure (radial) of a feeder would be an ideally good setup of observation if the āparentā and āchildrenā nodes have meters deployed. However, this is not economically feasible because the ongoing costs of maintenance and emergency conditions are often uncertain. As the system is evolved toward more automation and more sensors around primary and secondary networks, the numerical analysis for power flow calculation is becoming very burdensome. For example, some nodes are often estimated based on offline information, which can be far away from the real state of the system in general [83].
Since the practical design of overhead lines, cables, and transformers in normal conditions has to meet the requirements of a particular situation, the real distribution systems are inherently unbalanced [84]. The asymmetrical phas...
