Simulator 16 User Guide - PowerWorld

Equivalents
An equivalent power system is a power system model of smaller dimension than the original system
that approximates the behavior of the original system reasonably well. In reality, most power system
models are actually an "equivalent" of a much larger interconnected network. When performing power
system studies, it may be desirable to reduce the size of the system model even further so that it may
be solved more quickly.
To bring up the Equivalents Display, while in Edit Mode, go to the Tools ribbon tab, and choose
Equivalencing... from the Edit Mode ribbon group.
The most important part of constructing an equivalent is determining which buses should be explicitly
retained in the equivalent, and which buses should be equivalenced, or removed from the case.
Several definitions are useful here:
Study System
The buses that are to be retained.
External System
The buses that are to be equivalenced.
Boundary Buses
Any buses in the study system that are connected to buses in the external system.
How well the equivalent system approximates the behavior of the original system depends upon which
buses are retained in the study system. Retaining more buses yields results that more closely match
those of the original case, but at the expense of greater computation time. The number of buses to
retain in the study system depends upon how the equivalenced system will be used. Building system
equivalents is as much an art as it is a science, with few solid rules of thumb. However, to improve
accuracy, you should retain as many generator units as possible.
The actual equivalent is constructed by performing a matrix reduction on the bus admittance matrix. A
result of this process is the creation of "equivalent" transmission lines that join boundary buses
equipped with equivalent shunts or loads. Equivalent lines typically have a circuit identifier of 99, but
have also been seen to have other numerical values between 90 and 99, or an alphanumeric identifier
of EQ. Since many of the equivalent lines created during the matrix reduction have very high
impedance values, an option is provided to ignore equivalent lines with impedances exceeding a
specified threshold value. Additionally, an option is provided to convert the equivalent shunts added at
the boundary buses to constant PQ loads. These PQ loads will be given circuit identifiers similar to
those given to equivalent transmission lines.
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