Simulator 16 User Guide - PowerWorld

Low Impedances Lines Voltage Profile
When pre-processing the voltage profile, Simulator will look at groupings of buses connected by very
low impedances lines. If a bus in a grouping of energized buses has a zero voltage while other buses
in the group do not, the zero voltage will be changed to the first non-zero voltage found in the
grouping. This provides a much more reliable solution.
Estimate Voltages at Buses that Have Just Been Connected
As part of the pre-processing of voltages, voltages and angles at buses that have just had their
status changed from Disconnected to Connected will be estimated assuming that the voltages and
angles at buses that have not just changed status remain fixed. This will better facilitate power flow
convergence. A message will be written to the message log to indicate this is occurring.
Three Nested Loops
Power Flow (Inner) Loop : Red Loop
The Power Flow Loop is where the traditional power flow matrix equations are solved. There are
several Common and Advanced Solution options which affect this loop. In the message log, a RED
outline will be drawn around the inner power flow loop. Additionally, a PURPLE outline will be
drawn around any generator Mvar limit checking which occurs inside this loop.
Controller (Middle) Loop : Green Loop
Once the Power Flow Loop is solved, control devices check if their control requirements are being
met. Control devices are checked in the following order or precidence.
1. Generator Mvar Limit Checking
2. DC Line Solution
3. Switched Shunt Controls (each switched shunt or switched shunt control group is examined
individually)
4. Load-Tap-Changing (LTC) transformers and Phase Shifter (can be coordinated switching or
examined individually)
If any control devices requiring changes, then these changes are made and the power flow loop is
re-solved. This continues until no more control loop changes are made. There are several Common
and Advanced Solution options which affect this loop. In the message log, a GREEN outline will be
drawn around this loop.
MW Control (Outer) Loop : Blue Loop
After the Control Loop has completed, the MW Control loop is entered and generation (and possibly
load) is moved to meet the MW control options set in the case. Normally, MW control is done by
area control with each area varying generation to meet its own load, losses and interchange.
However, you may also use island-based control to dispatch MWs by island. If any MW control is
needed, then the Control Loop and Power Flow Loop interaction must be repeated and so on. In the
message log, a BLUE outline will be drawn around this loop.
A depiction of these loops and how you will see them represented in the message log is shown in
the following figure.
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