See Also
To access the Circulating Mvar and MW Flow Cycle Dialog go to the Tools ribbon tab, and chooseConnections > Find Circulating MW or Mvar Flows... from the Other Tools ribbon group.
Circulating power flows are possible in power systems and can lead to unnecessary power losses and voltage drops in the transmission system. The most common example is circulating Mvars caused by transformers with unbalanced tap ratios. This dialog provides a tool to detect and highlight the circulating real and reactive flows in a power system and also provides a metric to rank the severity of these circulating flows.
A Flow Cycle is defined as a series of transmission branches with positive flows that can be traversed to form a loop. Normally these types of cycles are not possible in a power system, but certain configurations of transformer tap ratios or phase shifts can results in circulating Mvar or MW flows. The detailed theory behind this tool is completely described in the follow paper: C. Davis, James Weber, and Kyle Johnson, “Circulating MW and Mvar flows in large systems,” in North American Power Symposium, Oct. 2009. You may also email PowerWorld for more details.
Dialog Options
Find Cycles
Click this button to recalculate the list of Flow Cycles after making changes in the other options.
Cycle Type to Find
Choose whether to locate circulating Mvars (much more common) or circulating MWs. Changing this option will automatically recalculate the list of Flow Cycle
Flow Threshold
When searching for flow cycles, enter a threshold below which flows will be ignored. This is needed to remove trivial flow cycles which are below the solution tolerance and are just generally of little interest.
Maximum Related Cycles
It is possible (and quite common) for many related cycles to be found in one power system. A group of related cycles is defined as a group of cycles which can all be mutually reached by from one another by traversing a series of positive branch flows. (Mathematicians would call this grouping a Strongly Connection Component of a directed graph). Groups of cycles which can be reach from one another can be combined in many ways to form many new cycles. To prevent the results in Simulator from displaying millions of cycles, this value is entered limit the number of related cycles which will be returned by entering this value. Simulator will return the cycles which have the largest minimum flow.
After finding the Flow Cycles, the lists at the bottom of the dialog show the flow cycle results. The top list shows the list of Flow Cycles. As you click on a particular flow cycle, the bottom list will show the Branches in Selected Flow Cycle (or Buses). There are also tabs which show Branches in any Cycle and Buses in Any Cycle. You can use these case information displays to mark all the branches that are in a cycle and then use a visualization tool such as Dynamic Formatting or Contouring to show visually show the Flow Cycles on a one-line diagram. Example pictures of this are shown at the bottom of this topic.
Flow Cycle List Display Results
The Flow Cycle List has the following columns by default.
Related Cycle Group
Related Cycle Group Number will be the same for that which can be reached from one another. These cycles are likely related to one another.
Area Names
Comma delimited list of area names which are traversed by the cycle.
Loss Mvar Reduction, Loss MW Reduction
Estimate of the reduction in Mvar (or MW) losses in the entire cycle if the circulating flow is removed. The larger this number is, the more severe the circulating flow is. The list is sorted by the Loss Mvar Reduction by default.
Cycle X-Weighted Average
Shows the weighted average flow in the cycle weighted by the series reactance (X) values.
Cycle Min Flow
Shows the minimum flow on any branch in the cycle. The larger this number is, the more severe the circulating flow is.
Lines in Cycle
Number of lines (branches) in the cycle.
Min Bus Num, Max Bus Num
Shows the Minimum and Maximum bus number traversed in the cycle
Min Tap Ratio, Max Tap Ratio
Show the minimum and maximum tap ratio on any branch in the cycle.
Max Percentage MVA Flow
Shows the maximum percent MVA flow on any branch in the cycle.
Branches in Selected Flow Cycle
The Branches in Selected Flow Cycle list has the following columns by default.
Cycle Flow
Magnitude of branch's flow for the cycle (the minimum of the from and to flow).
Cycle Flow Direction
Direction of flow in cycle relative to line's direction
Loss Mvar Reduction, Loss MW Reduction
Estimate of the reduction in MW and Mvar losses on this branch if the circulating flow is removed
Tap Ratio
Tap Ratio of the branch
Phase (Deg)
Phase Shift of the branch
Example Visualizations show circulating Mvar Flows
