You can compare the powerflow data and branch flows of the current (open) case in PSAT with another saved powerflow data. For this, use the Compare Powerflow Data command of the File menu. The Open File dialog appears where you can choose an existing PFB (or PSF) file to be opened and compared to the current case.
The Workspace changes to show only the “Compare case 1 to case 2” group, where “case 1” is the name of powerflow file of the current case and “case 2” is the name of second powerflow file opened for comparison. Click on the + button beside “Compare case 1 to case 2.” To expand its list, which includes AC Bus, Generator, etc., similar to the list under Powerflow Data previously shown in the Workspace. Double click on each item to open its comparison table.
You may choose to match the two cases by bus number, name or both. This selection must be made in the
Comparison Data Matching Dialog, accessed from the Settings menu, before you open the second powerflow
data. For example, when you choose the compare by number option, if the number of a bus in case 2 is the same as the number of a bus in case 1, these buses match and their data from the two cases will be compared (they may have the same name or different names). Otherwise, they will be listed as two separate buses, each belonging to only one of the cases.
The columns of the comparison tables are similar to those in the powerflow data tables and show the data in case 1 and case 2 and their differences. For example, if you double click on AC Bus, its comparison table opens, showing three columns for Bus Number, three columns for Bus Name, three columns for Bus Type, etc. The three columns of each data item have headings “1”, “2” and “Diff”, which correspond to the data in case 1, data in case 2 and their difference, respectively. If, for example, the type of a bus in case 1 is the same as that in case 2, the “Bus Type Diff” column remains blank for this bus. For non-numerical items, e.g., Bus Type, Status, etc., the “Diff” column shows “Diff” if the data of columns “1” and “2” are different. For un-matched data, e.g., buses that exist in one case but not in the other, the “1” or “2” column remains blank and the “Diff” column shows “N/A”.
You may limit each comparison table to a subsystem by selecting one from the Subsystem menu or the pull-down list, similar to that for Powerflow Data tables. You can also customize the comparison tables similar to other tables. The Customize table command of the Settings menu (or Customize columns command from right click on the table heading row) opens the Table Customization Dialog where you can choose the columns to be shown, their order, etc. Note that each item in the customization dialog corresponds to a set of “1”, “2” and “Diff” columns for that item.
You can specify a threshold for comparison of each data item in the customization dialog. For example, if for the Voltage p.u. column of the AC Bus table, you set this threshold to 0.001, then the bus voltage differences (voltages in case 1 minus voltages in case 2) that are smaller than 0.001 p.u. will not be shown in the table and the “Diff” column remains blank for these voltages. Check the comparison thresholds to make sure you don‟t miss the differences that are smaller than the previously specified thresholds.
You can reduce the rows of a comparison table by hiding the rows that don‟t show any difference in the “Diff” columns. For example, if you show only the Bus Type and Voltage p.u. columns of the AC Bus table (hide other columns) and set the threshold for voltage comparison to 0.001, only those buses that have a different type or have a voltage difference larger than 0.001 in the two cases will appear in the table. The selection to hide or show the rows with no difference values (the same data in case 1 and case 2) is made in the customization dialog of each table. If you click on a “Diff” header, the rows of the table are sorted based on the absolute value of the differences in that column. This is more convenient than sorting by algebraic values when you want to find the large differences between two cases. Another click on the same header toggles the sorting between ascending and descending, but still based on the absolute values.
13 Reduction
The static network reduction feature of PSAT allows you to reduce the current (opened) powerflow case to a smaller case by keeping a specified part of the network and replacing the rest of it with equivalent branches, shunts and loads at the boundary buses. Boundary bus is a bus in the retained part of the network that was connected to a bus in the rest (reduced part) of the network.
The changes that are made to the network by the reduction cannot be undone. Therefore, it is recommended to save the powerflow file before reduction, so you can return to the unreduced case if necessary. For accurate reduction, powerflow must be solved before reducing the network.
13.1 How to Reduce a Network
By selecting Reduction / Purge from the Solution menu of PSAT window, the Reduction window appears where you can specify the reduction options and parameters, reduce the network and view the messages and results. The Specify the subnetwork to be retained box tells PSAT which part of the system is to be retained. Select a simple or composite subsystem from the pull-down list of this box. This list includes the subsystems that you had defined previously in PSAT (by default, it specifies the previously selected areas). If you need to define a new subsystem or change the definition of a subsystem, use the buttons beside this box to open the subsystem definition dialogs. See section 8.10 and 8.11 on how to define or modify the subsystems.
When reducing the network to a specified subsystem, PSAT can retain a few additional buses in the rest (reduced part) of the network to maintain its sparsity. If you do not want PSAT to retain any additional bus, click the Retain
additional buses to maintain sparsity box to remove its check mark. However, if you can accept retaining a few
additional buses, it is recommended to select this option (click on the box to put a check mark in it).
Some of the equivalent lines generated by the reduction may have very high impedance, amounting almost to an open circuit. By setting the value of Cut-off impedance for equivalent lines you ask PSAT to ignore (reject) the equivalent lines with impedance higher than this value. A smaller impedance cut-off value results in fewer equivalent lines (and a more sparse network), but if this value is set too small, it results in the rejection of too many equivalent lines and inaccurate reduction (mismatches at the boundary buses).
Reduction will be different if you set the value of cut-off impedance to zero. In this case, the eliminated part of the network is completely replaced by equivalent loads and shunts at the boundary buses (equivalent lines and phase shifters are replaced with their injections and there will be no mismatch at the boundary buses).
After specifying the above parameters/options, to start the reduction click the Reduce button (before clicking the Reduce button, you may click the Close button to return to the main PSAT window without reducing the case). The message box shows the progress of reduction process. Before computing the equivalent network, PSAT checks the powerflow solution at the buses marked for elimination by computing the MW/MVAr mismatch at these buses. If PSAT finds buses with mismatch larger than the solution tolerance, it will list them in the message box and abort the reduction. If there are no mismatches, PSAT will reduce the case and display the following information in the message window:
Original number of buses in the network
Number of equivalent lines generated by the reduction
Number of equivalent phase shifters generated by the reduction
Number of rejected equivalent lines (those with impedance higher than the cut-off value)Before closing the Reduction window, you may click the Examine boundary buses and Examine additional
retained buses buttons to view the list of these buses in separate tables. You can customize these tables and
save/copy their contents by using their menus, similar to the data and report tables (see section 8.12 and section 7.6 for description of these menus).
If the reduction completes successfully, the current case in PSAT will be the reduced network, but if the reduction is aborted because of an error or mismatch, PSAT keeps the original network. When you click the Close button PSAT returns to the main window. You may check the validity of the reduced case by checking its mismatches in the Solution Reports (there might be small mismatches caused by the rejection of high-impedance equivalent lines) and solving the powerflow (it should solve in one or two iterations if some controls do not require more iterations as they would in the un-reduced case).
In the reduced case, since equivalent lines, loads and shunts might have replaced the area tie lines of the un-reduced case, the Area Interchange control might not function properly. In this situation, you need to reset the Desired MW Export of the areas to correspond to the reduced case. For example, if Area 1 was exporting 200 MW to Area 5, and now area 5 is replaced by equivalent loads and shunts, the Desired MW Export of Area 1 must be reduced by 200.
13.2 Purging from the Network
Instead of performing a reduction, purging can be used to remove unwanted network components and replace them with equivalent loads and (optionally) generators at the boundaries. By selecting Reduction / Purge from the
Solution menu of PSAT window, you can specify the purge options and parameters and reduce the network.
The Specify the subnetwork to be retained box tells PSAT which part of the system is to be retained. Select a simple or composite subsystem from the pull-down list of this box. This list includes the subsystems that you had defined previously in PSAT (by default, it specifies the previously selected areas). If you need to define a new subsystem or change the definition of a subsystem, use the buttons beside this box to open the subsystem definition dialogs. See section 8.10 and 8.11 on how to define or modify the subsystems.
When purging the parts of the network that are not retained, you have the option to either remove the components completely (Delete) or to retain the network data and simply set the components to be offline (Reserve). Select the appropriate setting from the area marked “External System”.
Buses on the boundary of the retained system can be converted to generator buses and a fictitious generator added to hold the voltage. If you wish to use this option select Change to Gen Bus option in the “Boundary Bus Type” area; otherwise, leave the Unchanged setting selected and the buses will retain their original type and fictitious loads will be used to represent tie line flows at the bus.
When purging the network, it is possible for retained buses to become isolated into islands. Setting the Out of
Service option in the “Simple Islands” area will automatically detect and offline any islands containing a single bus;
otherwise, the Remain in Service option will leave these islands unaltered.
After specifying the above parameters/options, to start the purge click the Purge button (before clicking the Purge button, you may click the Close button to return to the main PSAT window without reducing the case). The message box shows the progress of purge process