Distribution analysis in a GIS environment
Analytical functions and optimizations
The load flow (a.k.a. power flow) is the most basic of all analytical calculations performed on an
electric distribution network. The load flow provides a snapshot of a network operating state. Given
a set of loads, the load flow calculation determines voltage at all points of a network and current,
power and loss for each branch in the network. Instances of high or low voltage and/or equipment
overloading can be readily determined from load flow results, as can losses in the network.
A state-of-the-art load flow algorithm must accommodate a variety of load types including constant
power, constant current and constant impedance loads in addition to induction and synchronous
machines. The ability to handle looped, or even meshed, networks is important since
subtransmission (looped) and/or urban secondary systems (meshed) are often modeled. While
distribution networks are typically operated in a radial configuration with a single source of power at
the substation, the ability to include multiple sources is important as distributed generation plays an
ever-increasing role in the distribution system. Finally, the load flow algorithm must faithfully handle
unbalanced systems, that is, unbalanced loads, unbalanced topology, and unbalanced devices
such as open-delta autoregulators.
Two variations supplement the load flow: the short circuit calculation and the motor starting
calculation. The short circuit calculation considers abnormal (faulted) situations. Motor starting
considers the impact of starting large motors. Like the load flow, both short circuit and motor
starting calculations determine voltage at all points of a network and current and power for each
branch in the network.
An additional set of analysis functions deals with various optimizations of the network. Most of the
optimization functions in turn make use of the load flow algorithm. Optimizations include:
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How best to operate transformers, regulators and capacitors
Where best to install new capacitors-
Where best to install new regulators
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How best to configure or reconfigure the network
Finally, it is important to predict the reliability of the network to properly assess the reliability, or
change in reliability, of the network as changes are proposed or implemented. Therefore, an
analytical module that predicts reliability is also important.
The PSS/Engines
The PSS/Engines are a unique suite of power system simulation libraries. The PSS/Engines
encapsulate the algorithms for load flow, short circuit, network optimizations, and many other
functions that are needed for engineering analysis of distribution networks. Since they are libraries
and not stand-alone programs, the PSS/Engines must be embedded in another system and the
GIS is the logical target. The PSS/Engines are the building blocks from which a truly integrated
analysis capability, or simply an embedded tool, is built.
Direct users of the PSS/Engines are programmers who must integrate analysis capabilities into a
GIS. In the case of the GIS implemented at SRP both Smallworld and GeoData were involved in
the integration. The PSS/Engines provide state-or-the-art algorithms and optimizations without
requiring that the programmers themselves become experts in the algorithms and optimizations.
SRP, the user of the GIS and the one who benefits from the integrated analysis, is only an indirect
user of the PSS/Engines. Indeed SRP need not be aware of the existence of the PSS/Engines or
the particular algorithms employed to solve problems. SRP also benefits from robust and
thoroughly tested algorithms and optimizations but only to the extent that their problems are solved
faster and more easily.
