PowerWorld Simulator version 11. Manual

Schedule Subscriptions Page

The Schedule Subscriptions Page of the Input Pages is used to display all the schedule subscriptions and their

properties.

Schedule Subscriptions Page Local Menu Actions

Insert New Subscription: Use this option to define a new subscription through the Schedule Subscription Dialog

Delete: This option deletes the current Subscription.

Show Dialog: This option brings up the Schedule Subscription Dialog with the information of the current Schedule

Subscription.

Schedule Subscriptions Page Fields

The Schedule Subscriptions Page shows the following information:

Object: Object Type: Generator, Load, Line/Transformer, Shunts, Areas, Transactions

Object IDs : Are explained in the following Table:

Object ID1

Object ID2

Object ID3

Object Type

(Numeric)

Numeric

String[2]

Generator

Bus Number

Gen ID

Load

Bus Number

Load ID

Line/Transformer

From Bus Number

To Bus Number

Circuit ID

Shunt

Bus Number

Shunt ID

Area

Area Number

Transaction

From Area Number

To Area Number

Object Field: Field that subscribes to the schedule

Schedule Name: Name part of the schedule

Suffix: Suffix part of the Schedule Name

Active : If not active, then the schedule values are not applied to the power system

Day Shift: Number of days of the schedule time delay

Hour Shift: Number of hours of the schedule time delay

Relative : If not relative the field takes the exact schedule values. If relative the schedule takes the value of the

schedule scaled by a multiplied and added a Value Shift.

Multiplier: Scaling factor used when the subscription is Relative.

Value Shift: Shift value used when the subscription is Relative.

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Time Step Simulation

Running the Simulation

Running a Timed Simulation

The Timed Simulation is set up in the Options Page -> Time Step Simulation Control subsection.

By default, the Do Run button of the Time Step Simulation Dialog will run a Continuous Simulation, i.e., a simulation

in which each time point is solved immediately after the previous time point. In this case the purpose of the Simulation

is to obtain solution for the time points as fast as possible. As the solution progresses, the results for each hour in

every hourly grid are refreshed, showing the user the evolution of the Simulation. Please read the Time Step

Simulation Quick Start section for a quick introduction on how to run a Continuous Simulation.

On the other hand, the Time Step Simulation tool can also be used to run a Timed Simulation. In this case, the

Simulation takes place according to a time scale proportional to the date times of the time points. When you click the

Do Run Button, the Simulation progresses as if it was running in actual time. The simulation can also be paused and

reset at any time by using the Pause and Reset Buttons of the Time Step Simulation Dialog.

The following are some of the things you can do with the Timed Simulation:

You can hide the Time Step Simulation Dialog and control the simulation by using the Time Step

Simulation Toolbar, which contains buttons such as Do Run, Pause, Reset, etc. which mimic the control

buttons of the Dialog.

You can visualize how the quantities vary proportionally to actual time after each time step is applied.

Recall that the Time Scale defined in the Options Page indicates the relationship between the actual time

and the time scale defined for the time points in Seconds per Hour. Thus a Time Scale value of 60 will

indicate that one hour of time span between two time points will occur in 60 seconds of actual time in the

Timed Simulation.

You can also animate the flows of the solution of a time point while you wait for the s olution of the next one.

You can contour the online diagrams. And see how the visualization changes as the quantities vary in time,

and in addition, you can save those contour diagrams as JPEG or bitmaps for each timepoint.

Finally, you can act on the system (by closing capacitors, changing generator outputs, etc) before the next

time point is applied simulate operating actions in response to system conditions.

All these actions can be combined in outstanding presentations to your colleagues or clients on how the system would

evolve in time across different scenarios.

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Running OPF and SCOPF Time Step Simulations

Besides obtaining hourly power flow solutions for multiple time points, users that own the OPF and the SCOPF add-

ons can obtain hourly optimal power flow and security -constrained optimal solutions. In the Hourly Summary Page,

you can specify the following solution types:

Power Flow

Unconstrained Optimal Power Flow , which is equivalent to Economic Dispatch

Optimal Power Flow (OPF)

Security-Constrained Optimal Power Flow.

Note that different timepoints can be solved by any of the previously listed solution methods in the same Time Step

Simulation. However, the solution settings of the previous timepoints are used as initial conditions for the solution of

the next time point.

Simulator OPF and SCOPF tools are among the most advanced optimization packages for power systems. They have

been extended in the latest versions of Simulator with many features, and have become complex analysis system. We

recommend the user unfamiliar with OPF/SCOPF solutions to read the sections on Optimal Power Flow and Security -

Constrained Optimal Power Flow before setting up OPF/SCOPF Time Step Simulations.

Power Flow Time Step Simulation

The hourly power flow simulation allows the user to obtain AC or DC power flow solutions for a set of timepoints.

During the Time Step Simulation all the power flow options defined in the Solution/Environment Page as well as in

other dialogs are used for the solution. A key concept of the Time Step Simulation is that if you select Solve Time

Point from the Hourly Summary Page or from the grids of the Input Page, you would obtain the same solution that if

you would hit the Single Solution Button. This is true, when the input data does not contain schedule data but only

hourly input data. If your Simulation contains schedule data, there may have been scheduled actions that were applied

in previous timepoints that are not being applied when you select the Solve Time Point option.

The power flow solution will observe all the power balance constraints, control limits and area interchange constrained

defined in the power flow settings.

Unconstrained OPF Time Step Simulation

In the unconstrained OPF solution, the Time Step Simulation removes all the constraints that would normally act in the

OPF and optimizes the system to find the minimum operating cost settings. In doing so, Simulator will change the set

points of the specified controls (generators and phase shifters) to minimize the cost of all Areas and Superareas set to

OPF AGC control.

Besides the power flow solution options, the Unconstrained OPF simulation will take all the options that have been

defined for a regular OPF solution. Most of these options are defined in the OPF-Options Dialog, which is accessed

through LP-OPF in the Main Menu. All the settings such as objective function, cost of unenforceable constraints,

control available, prices for controls, etc are defined in this dialog. Other options are defined for each particular object

such as generators, loads, areas, lines, interfaces, etc. in the menu options of LP-OPF in the Main Menu.

OPF Time Step Simulation

When using the OPF solution type, the Time Step Simulation applies the hourly and schedule input data and optimizes

the control areas set to OPF to minimize cost while enforcing normal operation constraints: transmission line thermal

limits, interface limits, generator control limits, and load control limits. In doing so, the OPF algorithm detects the

controls that need to be moved, the constraints that are binding at the solution point, and the unenforceable

constraints, i.e., constraints that cannot be enforced with the available controls.

Some of the quantities that are of interest in the solution of the OPF algorithm are:

Unconstrained Generator MW Output. Displayed in the Hourly Unconstraint Gen MW page of the Input

Page

Final generator MW Output: Optimal generator output at the solution points, displayed in the Hourly Final

Generator MW Page

Change in Generator MW: Difference between the unconstrained and constrained generator optimum

settings, displayed in the Hourly Delta Gen MW Page.

Locational Marginal Prices: These are displayed in the Hourly Final Bus LMP Page. Average LMP prices

and other LMP metrics are also displayed in the Results Page for Areas, Injection Groups, Super Areas,

and Zones.

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Time Step Simulation

Binding Constraints as well as Marginal Cost of Limit Enforcement for lines and interfaces. These

fields can be seen in the Results: Constraints Page and in the Results Page for Lines, Transformers, and

Interfaces.

Unconstrained, Final and Congestion Cost, displayed in the Results Summary Page for the entire

system. These costs are also displayed for displayed for Areas, Owners, Superareas and Zones in the

Results Page.

LMP Profit for Generators, Owners, and Zones are displayed in the corresponding grids of the Results

Page.

SCOPF Time Step Simulation

The SCOPF combines the power of Simulator’s OPF with the Contingency Analysis Tool to optimize a system for

minimum cost while enforcing both normal operation and contingency constraints. At each time point, the SCOPF

solution provides the optimal operation of the system so that if contingencies occur they would not create security

violations. The locational marginal prices created in this manner are security-constrained signals to the market.

The solution of SCOPF Time Step Simulation depends on the options that have been set up for the following tools:

Power Flow

Optimal Power Flow

Contingency Analysis

Security Constrained Optimal Power Flow

Time Domain OPF Options

The SCOPF Time Step Simulation does the following for each timepoint:

Applies the hourly input data to the power system

Applies scheduled actions determined by the schedule input data.

Solves a power flow

If specified, solves an unconstrained optimal power flow (economic dispatch)

Initializes the base case of the security constrained OPF by solving a power flow or an OPF

For the initialization system conditions, solves the list of contingencies

Solves the SCOPF optimization problem: minimizes operating cost while enforcing normal and contingent

constraints.

Displays the results in all the result grids.

The SCOPF is on its own a complex computation that often requires significant computer resources. This is due

mostly to the solution of the list of contingencies and the calculation of their sensitivities. The size of the problem can

be dimensioned by:

Size of the system, given by the number of buses and the areas to be optimized. This is difficult to reduce

since the optimization problem is normally defined for a certain region.

Number of contingencies, which can be reduced by developing a contingency screening using peak loading

conditions.

Number of constraints (monitored elements), which can be reduced by selecting critical element, e.g.

interfaces and higher voltage transmission lines.

Number of timepoints in the list.

Another mechanism to speed up the computation of the PF/OPF/SCOPF Time Step Simulation is to use DC solutions

in some of the internal routines:

AC or DC power flow

AC or DC contingency analysis. This one will produce the larger time savings.

AC or DC SCOPF

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Storing Input Data and Results

In large cases, the amount of data that can be potentially generated by a Time Step Simulation in significant since

basically a full PF/OPF/SCOPF solution is available for each hour. A convenient method to store large amounts of

data is to do it in a binary file. The input data, both hourly and scheduled data, as well as the results can be stored in a

Time Series Binary File , referred to in this help guide as the .tsb file .

Here is a summary of what will be stored in the .tsb file when you click the Save Data Binary button in the Time Step

Simulation Dialog:

All hourly input data, defined in the Input Pages for hourly Load MW, Load Mvar, Generator MW, Generator

Maximum MW, Area Total MW Load and Hourly Line Status.

All scheduled input data, defined by the combination of Schedules and Schedule Subscriptions specified in

the corresponding Input Pages.

The values displayed on the grids of the Results: Constraints Page for hourly Binding Lines, Binding

Interfaces, Binding Contingencies, and the Binding Line Matrix and List.

The customization settings defined in the pages of the Custom Results Selection Dialog for the hourly

results of Areas, Buses, Generators, Injection Groups, Interfaces, Lines, Ow ner, Superareas,

Transformers, and Zones.

The customized results in the pages of the Results Page for hourly field values of Areas, Buses,

Generators, Injection Groups, Interfaces, Lines, Owner, Superareas, Transformers, and Zones. Recall that

many fields can be defined for each type of object.

The options defined in the Options Page, except the options set up in the Auto Load TSB File Options ,

which are saved with the .pwb case.

The data of the last solution run contained in the Hourly Summary Page: Skip, Processed and Solved

fields.

The .tsb file description

Note: Recall that the data of any Simulator grid can be copied to the clipboard and to Excel by selecting the options on

the grid Local Menu.

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Introduction to Simulator Add-On Tools

Getting Started

General Overview Simulator Add-On Tools

In addition to the features of the base Simulator package, various add-on tools are available. A brief introduction to

each follows:

Voltage Adequacy and Stability Tool (PVQV)

The purpose of the PVQV add-on is to allow the user to analyze the voltage stability characteristics of a system. After

the PVQV simulation is complete, the user can graph various system parameters. For more information, see the

PVQV Overview.

Optimal Power Flow Tool (OPF)

The purpose of an OPF is to minimize an objective (or cost) function . In Simulator OPF the Linear Programming OPF

algorithm (LP OPF) determines the optimal solution by iterating between solving a standard power flow and solving a

linear program to change the system controls thereby removing any limit violations. For more information see the OPF

Overview.

Security Constrained Optimal Power Flow Tool (SCOPF)

The OPF tool minimizes an objective function (usually total operation cost) by changing different system controls while

meeting power balance constraints and enforcing base case operating limits. The SCOPF tool takes it one step

further by considering contingencies that may arise during system operation and ensuring that in addition to

minimizing the objective function, no unmanageable contingency violations occur. For more information see the

SCOPF Overview.

Available Transfer Capability Analysis Tool (ATC)

ATC analysis determines the maximum MW transfer possible between two parts of a power system without violating

any limits. For more information see the ATC Analysis Overview.

PowerWorld Simulator Automation Server (SimAuto)

SimAuto provides PowerWorld customers the ability to access PowerWorld Simulator functionality within a program

written externally by the user. The Simulator Automation Server acts as a COM object, which can be accessed from

various programming languages that have COM compatibility. Examples of programming tools with COM compatibility

are Borland‚ Delphi, Microsoft‚ Visual C++, Microsoft‚ Visual Basic, and Matlab‚ (among others). For more

information on SimAuto, see the SimAuto Overview.

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Voltage Adequacy and Stability Tool (PVQV)

PowerWorld Simulator PV/QV Overview

The PVQV tool is only available if you have purchased the PVQV add-on to the base Simulator package.

Contact PowerWorld Corporation for details about ordering the PVQV version of Simulator.

PVQV, PowerWorld's voltage adequacy and stability assessment tool, is used to analyze the voltage characteristics of

a power system.

PowerWorld Corporation also offers Optimal Power Flow (OPF), Available Transfer Capability ATC, Simulation

Automation Server (SimAuto), and Security Constrained Optimal Power Flow (SCOPF) add-ons. For more information

see PowerWorld Simulator Add-On Tools.

The PowerWorld Simulator (Simulator) is an interactive power system simulation package designed to simulate high

voltage power system operation. In the base package Simulator solves the power flow equations using a Newton-

Raphson power flow algorithm. However, with the voltage adequacy and stability tool (PVQV) add-on the user can

solve multiple power flow solutions in order to generate a PV curve for a particular transfer or a QV curve at a given

bus.

The PVQV functionality is accessed using the Voltage Stability main menu item. The commands available from this

menu are Refine Model, QV Curves, and PV Curves.

The purpose of the PVQV add-on is to allow the user to monitor any system parameter while automatically increasing

a user-defined transfer. The PVQV module uses the Simulator built-in Newton Raphson power flow algorithm to

accomplish this task. After the PVQV simulation is completed, the user can choose to graph any of the Monitored

system parameters, designated in Quantities to Track.

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PV/QV Setup

The first step in the setup process is to define the source and the sink for the study transaction. The PV/QV tool

expects the source and the sink to be injection groups defined by the user. If injection groups have been previously

created, they can be selected by clicking on the drop-down list arrow of the Source and Sink boxes. If the injection

groups have not been previously defined in the current case, they can be created by clicking the View / Define

Groups button, right-clicking on the resulting list display, and choosing Insert from the popup menu. Alternatively, if a

list of injection groups has been previously saved in a Simulator Auxiliary file, they can be loaded into the current case

by again clicking the View / Define Groups button, right-clicking on the injection group list display, and choosing Load

> Auxiliary File from the popup menu.

Once you have the source and sink points defined, the PV study will model an increasing transfer of power from these

source points to the sink points. The transfer process is performed incrementally, based upon user specified options

on how the transfer should vary during the solution process.

Note that options can be saved to or loaded from an auxiliary file using the Save options and Load options buttons,

located at the bottom of the display.

Manage contingency list

Clicking this button will open the contingency analysis dialog for managing or inserting contingencies to be

processed during the PV/QV analysis. Contingencies defined and marked for processing in the contingency

analysis dialog will be included.

If you wish to globally omit the inclusion of contingencies during the PV/QV analysis, you may check the box labeled

Skip Contingencies.

Pre-contingency solution options

Clicking this button will open the solution / environment options, allowing you to customize the solution options prior

to processing the PV/QV analysis.

Run base case to completion

Check this box if you wish to find the critical transfer point of the base case condition, in addition to the specified

number of critical cases for the defined contingency scenarios. By default, the PV analysis will process until the

number of critical cases (or scenarios) as specified on the PV Results page have been found, and will halt the

process at that point. If the base case transfer scenario is not one of the critical cases, then checking this option

indicates that you want the PV analysis to continue incrementing transfers for the base case condition to find the

critical point of the base case, IN ADDITION to the number of critical cases specified.

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PV/QV Options to Control the Transfer Increase

These options are located on the Setup tab of the PV Study Form.

Source

Use this dropdown box to identify the source injection group. To model an increase in transfer, generator points in

the source injection group will increase their output, and load points will decrease their magnitude in amounts

proportional to their participation factors.

Sink

Use this dropdown box to identify the sink injection group. To model an increase in transfer, generator points in the

sink injection group will decrease their output, and load points will increase their magnitude in amounts proportional

to their participation factors.

Areas

Both the source and sink dropdown boxes have a button labeled Areas next to them. To differentiate the outputs of

points in the source group or the sink group according to the control areas in which they reside (so that some

control areas contribute more to the group’s transfer than others), click the corresponding Areas button. A new

dialog box will open that features a grid containing area names and percentages. The area names identify all the

areas to which points of the injection group belong, and the percentages describe the portion of the transfer that

points from each control area will contribute to the injection group’s power shift. When this dialog first opens, the

percentages are calculated based on the participation factors of each point in the injection group. Changing the

percentages overrides the individual participation factors so that the actual contribution of each point is weighted by

the proportion of the shift the point’s control area is supposed to provide. This feature is useful if you need to model

a shift according to control area responsibilities (perhaps defined contractually), but the injection groups you are

using come from multiple control areas.

Initial Step Size (MW)

This option indicates the initial rate at which the transfer will be increased following each successful iteration. For

this example, set its value to 100. This tells Simulator to begin studying the transaction in 100 MW increments.

Minimum Step Size (MW)

Whenever Simulator fails to solve the system at a given transfer level, it will return to the previously solved transfer

level, reduce the step size by specified factor, and then try to solve the system with the transfer incremented by the

newly reduced step size. The Minimum Step Size option specifies the minimum size this increment can be. Once

the system fails to solve when the step size is at this value, Simulator will conclude that we have come very close to

the voltage collapse point and terminate the analysis. So, the minimum step size essentially functions as a

tolerance for computing the voltage collapse point. For this example, set the Minimum Step Size to 10.

When convergence fails, reduce step by a factor of…

Whenever Simulator fails to solve the system at a given transfer level, it will reduce the transfer step size by the

value specified for this option. For this example, set this value to 2. Therefore, Simulator will start incrementing the

transfer in 100 MW steps. When it reaches a transfer level that it cannot solve, it will return to the last solved

transfer level, reduce the step size to 50 MW, increment the transfer by 50 MW, and attempt to solve the case

again. The next time it fails to solve, it will reduce the step size to 25 MW, and then to 12.5 MW, and finally to 6.25

MW. Since 6.25 MW is less than the Minimum Step Size value of 10 MW, it will instead use a final step size of 10

MW. Once the system fails to converge with this step size, the analysis will terminate, since it will conclude that it

has arrived at the voltage collapse point, within the specified tolerance.

Stop when transfer exceeds

Provide a MW transfer limit between the source and the sink. When the PV analysis reaches a transfer amount

equal to this value, the PV analysis will terminate.

How should reactive power load change as real power load is ramped?

This option controls how reactive load should vary as real load is changed during the analysis. You can either have

Simulator maintain the same ratio of real to reactive power load by checking "Keep the ratio between ..." checkbox,

or you can specify a power factor at which real and reactive load should change by specifying a value for "As MW

changes, change MVR at a power factor of…." If, for example, you specify 100 for this option, that suggests that the

power factor for the load change should be 100%, meaning that only the real power will change. If you specify

70.7%, the real and reactive load will be ramped by equal amounts.

Allow only AGC units to vary

Control areas in Simulator may practice one of three types of automatic generation control. Then, all generators in

the area that are, in fact, AGC-able will participate in the area's automatic generation control program. Thus,

Simulator distinguishes individual generating units according to whether they do or do not participate in their area's

AGC effort. By checking this option, you instruct Simulator to allow only those generators that are eligible to

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participate in AGC to contribute to the power transfer being studied. For this example, we will assume that all of the

units listed in the source injection group, regardless of their AGC status, are to be used in providing power for the

transfer. Thus, we will leave this option unchecked.

Enforce unit MW limits

If this option is checked, the output of any participating generating unit will be kept within its designed operating

range of MinMW < Output < MaxMW. When a unit is pegged at one of its limits, participation factors of the other

points in the limited generator's injection group will be adjusted to pick up the difference. For this example, leave

this option unchecked. This will allow us to analyze the capacity of the interface to support the transfer, regardless

of the amount of reserves available.

Do not allow negative loads

This is the analog of the previous option for loads. If a load is used as a source point, it will be decreased to make

power available for the transfer. Checking this option will instruct Simulator to keep loads from falling below 0 MW.

If a particular load is capped at 0 MW, participation factors for the remaining points in its injection group will be

recalculated to make up the difference. If you aren't using loads as source points this option is irrelevant. Thus, it

can be left unchecked.

Dispatch generators in merit order

When choosing to dispatch in merit order, injection groups with generators will be dispatched by moving individual

generators to their maximum/minimum outputs in succession.

Skip contingencies

The PV Curve tool computes PV curves for both the base topology and for any contingencies that have been

defined, unless you check the "Skip contingencies" checkbox. If the Skip contingencies checkbox is checked, a PV

curve will be computed only for the model in its present topology.

Manage contingency list…

Click the Manage contingency list… button to open the contingency analysis dialog. This will enable you to create,

modify, and remove contingencies from the list of configurations the PV curve tool will process.

Run base case to completion

The PV curve tool is designed to ramp a transfer until the prescribed number of unsolvable cases, including both

unsolvable contingencies and an unsolvable base topology, have been found. If the requested number of

unsolvable cases have all been identified as being associated with contingencies, the tool will not reveal how much

a transfer can be ramped for the base topology, unless you check the "Run base case to completion" checkbox.

Checking this checkbox forces the tool to continue to ramp the transfer until the base case can no longer be solved,

regardless of whether the requested number of unsolvable transfer level / topology combinations have been found.

Pre-contingency solution options …

Click this button to bring up the Solution/Environment Options Dialog. This will allow you to specify the solution

options to use for solving pre-contingency cases.

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PV/QV Quantities to Track

This section of the PV/QV dialog allows you to define what quantities to be monitored (stored) as the transfer is

increased. Any unselected system parameters will not be saved. The Quantities to Track page contains several sub-

pages that allow you to monitor different types of objects including: buses, generators, injection groups, branches

(transmission lines and transformers), shunts and interfaces.

Buses

For buses, we can monitor voltages, angles, MW load, Mvar load, shunts and the sensitivity of the voltage to

changes in reactive power (dV/dQ) during the PV analysis. In addition, you can specify if the QV curve should be

generated for the bus. The default value of all bus quantities is 'NO', indicating that the analysis will not monitor any

bus-related quantities. To monitor a particular quantity, double-click the corresponding value to toggle it to 'YES'.

Generators

For generators, we can monitor MW output, MVAR output, and MVAR reserve. The display operates exactly like

the bus display.

Groups

The Groups sub-page allows you to monitor the total generator MW and MVAR for the group, as well as the total

load MW and MVAR.

Lines

Note: all branches (whether transmission lines or transformers) appear on the Lines sub-page. The Lines sub-

page allows you to monitor real, reactive, and MVA flow (in either the FROM- TO or the TO-FROM direction), the

MW and MVAR losses, and the PTDF value on any branch. For the flow fields, double-clicking on a particular entry

will toggle its value from 'NO' to 'FROM-TO', and double-clicking again will toggle its value to 'TO-FROM'. Double-

clicking the losses or PTDF value field will toggle the value between 'YES' and 'NO'. If the branch contains a

transformer, you can also monitor the XFMR Tap.

Transformers

Note: all branches (whether transmission lines or transformers) appear on the lines sub-page. See Lines above

for setting up common branch quantities. For branches containing transformers, the Transformers sub-page is

used to set the transformer type (Fixed, LTC, Mvar or Phase) and set monitoring for Regulated Value, Tap Position

and Regulation Error. If no transformers exist in the case, you c annot use this option.

Shunts

The Shunts sub-page allows you to monitor Actual and Nominal Real(P) and Reactive(Q) Power, as well as the

Regulation Error and Regulated Value. Double click the value fields to toggle between 'YES' and 'NO'. If no shunts

exist in the case, you cannot use this option.

Interfaces

Simulator allows you to define groups of branches that together comprise an Interface. The Interfaces sub-page

allows you to monitor real, reactive, and MVA flow, the MW and MVAR losses, and the PTDF value on any interface

that you have defined. If you have not defined any interfaces, you cannot use this option.

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PV/QV Limit Violations

The Limit Violations page allows you to define what should be considered a violation of a monitored voltage.

Identify bus voltages

The Low Voltage and High Voltage options should be checked if you wish to see which of the buses are violating

their voltage limits at the end of the PV/QV analysis. The limits used are those defined using the limit monitoring

settings. To examine the limit monitoring settings, press the Limit Group Definitions button to open the Limit

Monitoring Dialog.

Inadequate voltage level

This option allows you to specify at what value a monitored voltage is determined to be "inadequate". The results of

the PV/QV analysis will report the transfer level at which the first instance of a voltage below the inadequate level

was detected.

You may also choose to have the PV/QV analysis halt when it first detects the inadequate voltage. Checking the

box labeled Stop when voltage becomes inadequate will force the PV/QV analysis to do so.

If you wish for the inadequate voltage feature of the PV/QV analysis to be ignored, uncheck the box labeled Identify

inadequate voltage .

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PV/QV PV Output

Simulator records the value of each monitored quantity at each transfer level. However, unless you tell Simulator

where to write the data, it will be present only in memory. The Output Tab allows you to designate where the data

should be logged.

Click the Save Results to File option to indicate that you want to write results to a file. Then, in the adjacent text box,

supply the complete path for the output file. You may use the Browse button to locate the place where you want the

file to be written.

In addition to recording how the monitored quantities vary with the transfer, you can instruct Simulator to save the

entire system state at regular intervals during the analysis. This can be helpful if you want to analyze particular

transfer levels more closely after the analysis is complete. To archive all system states, click the option labeled 'Save

all states', then, supply the directory where the states should be written, and a prefix to use in naming the state files.

You can use this prefix to distinguish the states of different runs that might have been written to the same directory. If

you do decide to use this option, keep in mind that, depending on the size of your system, archiving states frequently

can require significant disk space and delay the process. However, it can be quite helpful if previous analyses have

shown interesting behavior at a particular transfer level.

You may also save only the base case for each critical contingency, or save no system states at all, by checking the

appropriate option.

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PV/QV PV Results

To start the PV study, switch to the PV Results tab, the last page of the form. From this tab, you can control the

progress of the run by initiating, pausing, or aborting it. You can even reset the process to the beginning. This tab

also gives you a few different views of the run's output, including the ability to plot various quantities.

To begin the PV analysis, click Run. The Status indicator will change from 'Inactive ' to 'Running'. To pause the

process at any time, click the Pause button. Then, to restart the analysis, click Run. To terminate the analysis at any

time, click Abort.

As the analysis continues, the scenario list on the Overview page will keep you informed of its progress. Present

step size identifies the current size of the increment the application is using to increase the transfer. Present

Nominal Shift indicates the size of the transfer that was most recently solved.

The Plot sub-page gives you access to the application's data plotting functions. Any of the values you designated to

monitor can be plotted using this display

To produce a plot, follow this example procedure:

· From the X Axis Quantity dropdown box, select 'Total Shift (MW)'.

· For the X Axis Caption, type 'Total Transfer in MW'.

· From the Y Axis Quantity list box, select the first four monitored bus voltages.

· For the Y Axis Caption, type 'Bus Voltage (pu)'.

· For the Title, type 'Voltage vs. MW Transfer for SOURCE Selling to SINK'.

· To complete the procedure, click the button labeled Plot.

The plot will appear in its own window. You can produce as many plots as the memory on your system will allow.

Right-clicking on a plot will expose a local menu with four options. You can send the plot to y our printer by clicking

Print. You can change the printer configuration by selecting Printer Setup. You can save the plot as a bitmap,

Windows metafile, JPEG, or text file by clicking Save As and selecting the appropriate file type. Finally, to close the

plot window, click Close.

Note that the plot sub-page also has an option for plotting pre-contingency values, for plotting values for specifically

identified scenarios, and for always including the plot of the base case conditions.

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PV/QV QV Setup

The PV curve tool stresses the system by increasing a transfer between injection groups. Eventually, the transfer is

increased so much that the power flow can no longer be solved. Under the assumptions of static voltage stability

assessment, the point at which the system becomes unsolvable is regarded as the point of voltage collapse. Voltage

collapse tends to be a localized phenomenon associated with a lack of reactive voltage support at a bus or group of

connected buses. It is important to identify this group of stressed buses so that you can properly direct efforts to

reinforce the system. QV analysis is often used to identify the buses that are most prone to a voltage collapse because

they are close to having insufficient reactive support.

The design of the PVQV add-on to Simulator acknowledges the important role that QV analysis plays in identifying the

buses that are most heavily stressed by a transfer. The approach is straightforward. First, perform a PV curve

analysis that models a transfer from source to sink for both base and contingency topologies. This will yield a number

of PV curves that terminate at a transfer level that causes the system to become unsolvable for each topology. For

each of these critical transfer level / contingency pairs, perform a QV analysis at a set of buses to try to determine

which area or areas constitute the epicenter of the collapse. The set of buses at which the QV analysis is performed

can be predefined by the user and supplemented by the list of buses that have the lowest voltage magnitudes or the

highest VQ sensitivities.

To specify that Simulator should automatically launch a QV analysis after finishing a PV study, check the box labeled

Automatically launch QV analysis at end of PV computation. After the PV curve has found all the critical cases it

has been asked to identify, it will launch the QV Curve Tool. The buses the QV Curve Tool will analyze will then come

from two groups: those that the user has pre-selected, and those that tool automatically identified because they are

among the lowest-voltage buses or have the highest VQ sensitivities. To pre-select buses for the QV analysis, switch

to the Buses sub-tab of the Quantities to track tab and toggle the "Draw QV?" field of each bus for which you want to

compute a QV curve to YES. Whether other buses are automatically selected in light of their voltage or VQ sensitivity

is governed by settings stored in the QV options file.

We now continue our look at the QV Setup tab of the PV Curve Tool by describing the use of the remaining controls.

QV options file

Specify the name of the file that contains settings for a variety of options associated with the QV study in this text

box. Use the Browse button to locate this file using a file dialog. Among the topics that may be addressed by the

QV options file is whether buses should be automatically selected as buses for which to draw QV curves based on

their voltage or VQ sensitivity. The easiest way to build a QV options file is to use the QV Curve Tool to specify the

settings of the various options and to save these settings in a file.

How should scenarios be handled?

QV curves can be drawn for the selected buses either for all the scenarios that the PV tool has processed, or just

for the scenarios for which the PV Curve Tool found an unsolvable transfer level. Select the option that describes

how you wish the QV Curve Tool to process scenarios from this option group. If you select the Consider just the

unsolved scenarios option, the QV tool will compute QV curves for the selected buses at the last transfer level at

which each scenario was successfully solved. Otherwise, the tool will compute QV curves for the selected buses for

each scenario at either the last transfer level at which the scenario was successfully solved or the last transfer level

that the PV tool attempted to model.

QV accelerator settings

This group of controls enables you to define a voltage window over which each QV curve should be calculated. If

you select Do not use shortcuts; trace the entire QV curve , each QV curve will be computed over the voltage

range specified for the bus in the QV options file. This voltage range may be specific to that bus or may be a

window that has been defined for the entire run. If, on the other hand, you select Trace abbreviated QV curves

using these settings…, the voltage range over which each bus’s QV curve will be computed will be customized for

each bus according to the values you specify for Max voltage increase and Max voltage decrease. For example,

if a bus is currently at 0.9 per-unit voltage for a particular scenario and you specify the max voltage increase and

decrease to be 0.1 and 0.3, respectively, then the QV curve for that bus for that scenario will be traced over the

voltage range 0.6 pu to 1.0 pu.

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PV/QV QV Curve Buses

To designate the buses for which you would like to calculate QV curves, toggle the Selected? field to "YES". If you

would like to specify specific solution parameters (min volt, max volt, and step size) for any bus, simply type the

numbers in the appropriate cells. If you leave any cells blank, default values will be used for those parameters. Only

non-default values will be shown in these cells. If you change a value in these cells to a default setting, the cell w ill be

automatically be set blank.

Alternatively, you may type a range of bus numbers to add them to the list of buses that will have a QV curve

calculated, or you may type the name of a bus. Furthermore, you may load a list of buses, as well as other option

settings, from an auxiliary data file. To create such a file to store your option settings, press the Save Settings button

that appears near the bottom of the display.

To load settings you saved previously in an auxiliary data file, click the Load Settings button. You can search for the

file instead of typing its name. Once the name of the file appears in the text field, press Open to import the settings

stored in the file.

You also can check the box Additionally, automatically draw curves for… and then the number of lowest-voltage

buses and highest dv/dq buses that you require. These buses will be selected from the limit group indicated in the

field from the limit group.

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PV/QV QV Curve Options

The second page of the QV Curve dialog is the Options page. The options page contains three tabs labeled

Solution, Output and Contingencies.

Solution

In the three text fields enclosed in the Default solution parameters group box, specify the values of voltage step

size, minimum voltage, and maximum voltage to use when computing QV curves for buses that don’t have specific

values set for these parameters.

If you don’t understand the meaning of these parameters, consider how a QV curve is computed: a fictitious

generator (i.e. voltage source) is placed at the monitored bus. Its set point voltage is varied in steps of the specified

size between the specified maximum and minimum voltages, and the MVAr injection at the bus is calculated and

recorded at each step.

To make the changes you’ve made register with QVCC, click the Set Options button. To restore the factory-default

settings, click Restore Defaults .

To set power flow solution parameters, click the Global solution options button. Be aware that the changes you

make in the resulting dialog affect all power flow computations in Simulator.

Output

The Output tab allows you to set a location for saving the results of the QV analysis to a file. To save the QV

analysis to a text file, you must first check the box labeled Save data in file . Once this box is checked, you can

then either type in a directory location for saving the data, or click the Browse button and select a file to save as.

You can also customize the file prefix and extension to be used when Simulator names the output files.

Another option on the Output tab is the check box labeled Plot curves as they are computed. When this option is

checked, the QV curve displays will be updated and drawn during each step of the QV analysis process.

Contingencies

The new QVCC can calculate QV curves for the specified buses for both base and contingency conditions. To

analyze a set of contingencies, you must define the contingency set using the separate Contingency Analysis Form,

which you can access from run mode by selecting Options / Tools - Contingency Analysis from the main menu.

To have QVCC analyze these contingencies for each bus, check the box labeled Process each of the currently

defined contingencies. Check the box labeled Skip base case for avoiding the computation of QV curves for base

conditions.

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PV/QV QV Curve Control/Results

To perform the QV curve calculations, switch to the Control/Results tab and press the Run button. QVCC will

respond by calculating QV curves for each of the specified scenarios (i.e. for each bus / contingency pair). When it

finishes the QV curve calculation for a scenario, it will record the critical points for the curve in a case information

display. These critical points include (V0, QV0), (Vmin, QVmin), (Vmax, QVmax), and (VQmin, Qmin).

Each scenario can be identified using the bus number and name fields together with the case name field, which simply

specifies either "BASE CASE" or the name of the contingency. As with all case information displays, the QV Results

case information display has a local menu that offers a number of options. From the local menu, you can plot the QV

curve for a scenario, record the (Q, V) pairs that comprise each curve to a text file or to an Excel spreadsheet, and

clear the results.

The QVCC interface remains active while the curves are calculated. You can terminate the run at any time by clicking

Stop.

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PV/QV Refine Model

Simu lator PVQV has the ability to refine the system model to fix modeling idiosyncrasies that cause premature loss of

convergence during the PV and QV curve studies. This option is available from the main Voltage Stability menu.

The user can refine the case in the following ways:

Fix transformer taps

If there are transformers that have Vmax and Vmin that are very close together, the power flow may have a difficult

time converging. This option allows the user to fix all transformer taps at their present values if their Vmax - Vmin is

less than or equal to the user specified tolerance.

Fix shunts

If there are shunts that have Vmax and Vmin that are very close together, the power flow may have a difficult time

converging. This option allows the user to fix all shunts at their present values if their Vmax-Vmin is less than or

equal to the user specified tolerance.

Take units off AVR control

If there are generators that have Qmax and Qmin that are very close together, the power flow may have a difficult

time converging. This option allows the user to remove these units from AVR control, thus locking their MVAR

output at its present value, if their Qmax - Qmin is less than or equal to the user specified tolerance.

These refinements will only be applied to those areas or zones that have the "Apply?" field set as YES in this dialog

box. This field can be changed by right-clicking on it and selecting Toggle all to YES/NO or by double clicking on the

field.

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Optimal Power Flow (OPF)

PowerWorld Simulator Optimal Power Flow Overview

Note: The OPF option in PowerWorld Simulator is only available if you have purchased the OPF add-on to the

base package . To learn more about the OPF, please feel free to read through the information contained in

these help files. Contact PowerWorld Corporation for details about ordering the OPF version of Simulator.

The PowerWorld Simulator (Simulator) is an interactive power system simulation package designed to simulate high

voltage power system operation. In the standard mode Simulator solves the power flow equations using a Newton-

Raphson power flow algorithm. However with the optimal power flow (OPF) enhancement, Simulator OPF can also

solve these equations using an OPF. In particular, Simulator OPF uses a linear programming (LP) OPF

implementation.

All of the OPF commands and options are accessed using the LP OPF main menu item. Other commands in this

menu are used to specify input options, see results, and store/retrieve OPF specific data into auxiliary files.

The purpose of an OPF is to minimize an objective (or cost) function by changing different system controls taking into

account both equality and inequality constraints which are used to model the power balance constraints and various

operating limits.

In Simulator OPF the LP OPF determines the optimal solution by iterating between solving a standard power and then

solving a linear program to change the system controls to remove any limit violations. See OPF Primal LP for more

details.

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OPF Objective Function

The objective of the OPF algorithm is to minimize the OPF objective function, subject to various equality and inequality

constraints. Since the objective of the OPF is to minimize an objective function, what objective function is used has a

significant impact on the final solution.

Currently two objective functions are available in Simulator OPF: Minimum Cost and Minimum Control Change.

Minimum Cost attempts to minimize the sum of the total generation costs in specified areas or super areas. Minimum

Control Change attempts to minimize the change in the generation in the specified areas or super areas.

To include an area or super area in the OPF objective function, simply change the Area AGC Status field to "OPF" on

the OPF Area Records Display or the Super Area AGC Status field to "OPF" on the OPF Super Area Records Display.

This gives you great flexibility in defining the OPF study. For example you can set the OPF to minimize costs for the

entire system, or just selected areas or super areas.

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OPF Equality and Inequality Constraints

In solving a constrained optimization problem, such as the OPF, there are two general classes of constraints, equality

and inequality. Equality constraints are constraints that always have to be enforced. That is, they are always

"binding". For example in the OPF the real and reactive power balance equations at system buses must always be

satisfied (at least to within a user specified tolerance); likewise the area MW interchange constraints. In contrast,

inequality constraints may or may not be binding. For example, a line MVA flow may or may not be at its limit, or a

generator real power output may or may not be at its maximum limit.

An important point to note is because the OPF is solved by iterating between a power flow solution and an LP solution,

some of the constraints are enforced during the power flow solution and some constraints are enforced during the LP

solution. The constraints enforced during the power flow are, for the most part, the constraints that are enforced

during any pow er flow solution. These include the bus power balance equations, the generator voltage set point

constraints, and the reactive power limits on the generators. What differentiate the LP OPF from a standard power

flow are the constraints that are explicitly enforced by the LP. These include the following constraints:

Equality Constraints

Inequality Constraints

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OPF Equality Constraints

Area MW Interchange

The area MW interchange constraints are enforced during the LP for those areas that have an AGC Status equal to

"OPF" provided the area is not part of a super area that is also set on AGC. The AGC Status field for an area can

be set using the OPF Area Records display, while the AGC Status field for the super area (if any) is set using the

OPF Super Area Records display. Areas whose interchange is enforced during the LP do not have their

interchange enforced during the power flow solution; during the power flow these areas are treated as though they

were off of AGC (and hence the output of generators in that area is not varied during the power flow).

It is perfectly acceptable to have some areas on "OPF" AGC control and to have other areas on the more traditional

power flow area AGC such as "ED" or "Part. AGC." The interchange for such areas is controlled during the power

flow solution.

Following a successful solution, marginal costs are calculated for the area interchange constraints; these values are

displayed on the OPF Area Records display and can be contoured. See OPF Marginal Costs for details.

Bus MW and Mvar power balance

Enforced during the power flow solution. Following a successful solution, marginal costs are calculated for the bus

MW (real power) balance constraint; these values are displayed on the OPF Bus Records display and can be

contoured.

Generator Voltage Setpoint

Enforced during the power flow solution. Following a successful solution, marginal costs are calculated for the

voltage setpoint constraint; these values are displayed on the OPF Bus Records display .

Super Area MW Interchange

Super area interchange constraints are enforced similar to the area constraints. That is, super area interchange

constraints are enforced during the LP only for those super areas that have an AGC Status equal to "OPF." The

AGC Status field can be set using the OPF Super Area Records display. During the power flow solution such

super areas are treated as though they were off of AGC.

Interface MW limits when treated as Equality

Interface MW limits are enforced during the LP solution. Interface MW limits are normally treated as inequality

constraints (see Inequality Constraints ), however they can optionally be treated as equality constraints. See the

Interface Dialog for information on how to treat the limit as an equality constraint.

Transmission Line and Transformer (Branch) MVA limits

Branch MVA limits are enforced during the LP solution. Branch MVA limits are normally treated as inequality

constraints (see Inequality Constraints ), however they can optionally be treated as equality constraints. See the

Line Transformer Dialog for information on how to treat the limit as an equality constraint.

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OPF Inequality Constraints

The following classes of inequality constraints are enforced during the OPF solution.

Generator real power limits

Generator real power limits are enforced during the LP solution.

Generator reactive power limits

Generator reactive power limits are enforced during the power flow solution.

Interface MW limits

Interface MW limits are enforced during the LP solution. In short, interface records are used to represent the

aggregate flow through a number of different devices (see Interface Records<+> for details). During the LP the MW

flow through the interface is constrained to be less than or equal to a user specified percentage of its limit, provided

the interface is active for enforcement. For an interface to be active for enforcement the following three conditions

must be met:

· Interface enforcement must not be disabled for the case. This field can be set from either the OPF Options

dialog or the OPF Interface Records display. The default is that case interface enforcement is not disabled.

Also note that interface flow is limited to a percent of its limit as specified by the interface's Limit Monitoring

Settings.

· Interface enforcement must be active for at least one of the interface's areas. Note, an interface is assumed to

be in each area that contains at least one of its components. This field can be set from the OPF Area Records

display. Note: the default is that interface enforcement is not active, so be sure to activate this if you want these

constraints enforced.

· Enforcement must be active for each individual interface. This field can be set from the OPF Interface Records

display or in the Limit Monitoring Settings Dialog. The default is active.

Each interface that is ac tive for enforcement is modeled as an inequality constraint, which may be either binding or

not binding. If the constraint is not binding then it does not impact the solution. If a constraint is binding then it has

an associated marginal cost of enforcement, which is shown on the OPF Interface Records display.

Transmission Line and Transformer (Branch) MVA Limits

Transmission line and transformer (branch) MVA limits are enforced during the LP solution. During the LP the

branch line flow is constrained to be less than or equal to a user specified percentage of its limit, provided the

branch is active for enforcement. For a branch to be active for enforcement the following three conditions must be

met:

· Line/Transformer enforcement must not be disabled for the case. This field can be set from either the OPF

Options dialog or the OPF Line/Transformer Records display. The default is that case line/transformer

enforcement is not disabled. Also note that the branch flow is limited to a percent of its limit as specified by the

branche's Limit Monitoring Settings.

· Branch enforcement must be active for the branch's area. For tie-lines enforcement must be active for either

area. This field can be set from the OPF Line/Transformer Records display. The default is that branch

enforcement is not active , so be sure to activate this if you want these constraints enforced.

· Enforcement must be active for each individual branch. This field can be set from the OPF Line/Transformer

Records display or in the Limit Monitoring Settings Dialog. The default is active.

Each branch that is active for enforcement is modeled as an inequality constraint, which may be either binding or

not binding. If the constraint is not binding then it does not impact the solution. If a constraint is binding then it has

an associated marginal cost of enforcement, which is shown on the OPF Line/Transformer Records display.

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Determining Set of Active Inequality Constraints

A key issue in quickly solving the OPF is for the LP to effectively determine the set of active inequality constraints.

Currently this includes the line MVA limits and the interface MW limits. Because the speed of the LP varies as the

cube of the number of constraints active in the LP basis, it is extremely important to keep this number as small as

possible. Therefore it would be very computationally prohibitive to setup an inequality constraint for each transmission

line and interface (except in very small systems.)

The solution of setting up constraints only for those inequality constraints that are actually violating their limits is a

step in the right direction, but suffers from the problem that during a solution a line may initially be violating its limit and

then after the first iteration it is no longer violating. However if it is not subsequently included as a constraint during

the next iteration the solution may simply oscillate between enforcing/unenforcing this constraint. This problem can be

resolved by keeping that constraint in the basis even though it is no longer binding.

However this raises a question about how to handle these constraints during future OPF solutions. For example what

would happen if a user solved the OPF, and then immediately resolved the OPF. Following the first solution the

constraint would be enforced so that it may actually be less than its limit. However if this constraint is not included in

the LP basis during the next solution the constraint may immediately violate during the first iteration, requiring a

number of iterations just to return to the original initial solution.

Simulator solves this issue by keeping track of the enforced constraints from one solution to the next. Constraints are

only removed from the basis if they the fall below a specified percentage of their limit. This percentage is enterable on

the Constraint Options page of the OPF Options Dialog. This prevents the set of constraints in the basis from building

up over time as a number of different system conditions (and hence constraints) are studied. Also, at any time this set

of constraints can be cleared using the Initialize OPF Button on the OPF Options Dialog.

Also, the user is free to specify that a particular constraint always be included in the basis. This is done by toggling

the Constraint field to "Always" on the OPF Line/Transformer Records or OPF Interface Records displays.

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PowerWorld Simulator Add-on Tools

OPF Unenforceable Constraints

The goal of the LP OPF is to minimize the objective function subject to the user specified constraints. However there

is no guarantee that it is even possible to simultaneously satisfy all of the specified constraints. In fact, it is quite easy

to create a system in which all of the constraints cannot be enforced. A simple example is a two bus system

consisting of a single generator supplying a single load through a transmission line. If the transmission line MVA

rating is below the MVA of the load then it is impossible to supply this load while simultaneously satisfying the

transmission line constraint. In Simulator OPF such a situation is known as an unenforceable constraint. In studying

large systems, such as the U.S FERC 715 cases, such situations actually appear to be quite common. Seemingly

unenforceable constraints are often due to a lack of controls available to the LP OPF or due to faulty limits entered in

the case. In such cases unenforceable constraints can be corrected by making more controls available to the LP OPF

or correcting the limits.

Simulator OPF allows you to solve systems with unenforceable constraints by only enforcing those constraints that

have a marginal cost below a user specified tolerance. These tolerances are specified on the OPF Options Constraint

Options Page. Any constraints that have marginal costs above these values are not enforced, including any

unenforceable constraints. This functionality is implemented in Simulator OPF through the use of slack variables.

Slack variables are artificial variables introduc ed during the LP solution in order to satisfy the constraints with the slack

variable costs equal to the user specified values. Then, during the LP solution the slack variables are usually removed

from the LP basis. The only time this does not occur is if the constraint can not be enforced with a marginal cost less

than the specified value. The number of unenforceable constraints are shown on the OPF Option Solution Results

Page.

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OPF Marginal Costs

During any constrained minimization there is practically always a cost associated with enforcing the equality

constraints and the binding inequality constraints. These costs are known as the marginal costs.

In Simulator OPF marginal costs are calculated for the following record types:

Bus MW Equality Constraints

The Bus MW marginal costs tell the incremental cost to supply one additional MW of load at the specified bus.

These values can be viewed on the OPF Bus Records display; they can also be contoured or viewed on the one-

lines using bus fields.

In the absence of any binding inequality constraints (such was Line MVA constraints) all of the bus marginal costs in

an area should be identical. Bus marginal costs can only be determined for buses that are in areas or super areas

on OPF control.

Area MW Equality Constraints

The Area MW marginal costs tell the incremental cost for the specified area to import one additional MW of load

from the system slack bus. These values can be viewed on the OPF Area Records display; they can also be

contoured or viewed on the one-lines using area fields. In the absence of any binding inequality constraints the

area MW marginal cost is identical to the bus MW marginal costs for all the buses in the area. When there are

binding inequality constraints this is no longer the case.

Super Area MW Equality Constraints

The Super Area MW marginal costs are identical to the area marginal costs except they apply to super areas rather

than areas.

Interface MW Constraints

The Interface MW marginal costs tell the incremental cost of enforcing the interface MW constraints. These values

are only nonzero if the interface constraint is actually active (binding); they can be viewed using the OPF Interface

Records display.

Line/Transformer MVA Constraints

The Line/Transformer marginal costs tell the incremental cost of enforcing the line or transformer MVA constraint.

These values are only nonzero if the line or transformer constraint is actually active; they can be viewed using the

OPF Line/Transformer Records display.

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OPF Primal LP

Select LP OPF > Primal LP to solve the OPF using the primal LP algorithm.

In Simulator OPF the LP OPF determines the optimal solution by iterating between solving a standard power and then

solving a linear program to change the system controls to remove any limit violations. The basic steps in the LP OPF

algorithm are

· Solve the power flow

· Linearize the power system about the current power flow solution. Both constraints and controls are linearized.

· Solve the linearly-constrained OPF problem using a primal LP algorithm, computing the incremental change in the

control variables. Slack variables are introduced to make the problem initially feasible. That is, the slack variables

are used to satisfy the equality and inequality constraints. The slack variables typically have high costs so that

during the iteration the slack variables change to satisfy the constraints. The LP then determines the optimal,

feasible solution for the linear problem.

· Update the control variables and resolve the power flow.

· If the changes in the control variables are below a tolerance then the solution has been reached; otherwise go to

step 2.

· Finish by res olving the power flow.

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PowerWorld Simulator Add-on Tools

OPF Future Enhancements

In this first version of Simulator OPF we have implemented the OPF functionality that will, we believe, be most useful

to the largest number of users. While we certainly plan on introducing additional functionality in future releases, we do

want to be as clear as possible about what functionality is not currently provided.

In short the current version of Simulator OPF allows users to calculate the optimal solution to a power system using

generator real power MW outputs and phase shifters as controls, while enforcing area, super area, interface MW and

line/transformer MVA constraints. Marginal losses can also be included in the OPF calculation.

Some functionality that is not included in the current version of Simulator OPF, and which we hope to include in future

versions, include the following:

· Enforcing bus low/high voltage magnitudes as limits

· Including additional devices as controls, such as generator voltage setpoints, LTC transformers, switched shunts.

· Allowing the optimization of different cost functions, such as maximization of social welfare.

· Additional functionality as suggested by customers.

We hope you have found the current version of Simulator OPF useful, and look forward to interacting with customers

to help determine additional functionality for future versions.

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Options

OPF Options

The OPF Options dialog allows you to customize the OPF solution. To display this dialog, select LP OPF > Options .

The dialog consists of three general pages; Options, Results and LP Solution Details.

The Options page has three tabs as well, Common Options, Constraint Options, and Control Options.

The Results page has four tabs, Bus MW Marginal Price Details, Bus Mvar Marginal Price Details, Bus Marginal

Controls, and Solution Summary.

The LP Solutions Page has five tabs, All LP Variables, LP Basic Variables, LP Basis Matrix, Inverse of LP Basis, and

Trace Solution.

The dialog also has several buttons at the bottom of the display:

OK, Cancel

Select to close the dialog. Selecting OK saves your changes while Cancel does not. Note that changes are also

saved anytime you select Solve LP OPF or Single Outer Loop.

Solve LP OPF

Solves the OPF using the Primal LP algorithm. Equivalent to selecting LP OPF, Primal LP.

Single Outer Loop

Does a single outer loop of the Primal LP algorithm. Equivalent to selecting LP OPF, Single Primal LP Outer Loop.

Initialize LP OPF

Returns the LP OPF variables to their original states .

Prints the selected page of the dialog.

Help

Displays this help page. To view help for a particular page place the cursor on the page and press the F1 key.

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PowerWorld Simulator Add-on Tools

OPF Options: Common Options

The OPF Dialog, Common Options page displays general options associated with the OPF solution. The display

contains the following fields:

Objective Function

Allows a choice of solving the LP using either a minimum cost or a minimum control change objective function.

Controls

Disable All Phase Shifter Controls

Prevents phase shifters from attempting to control devices during the OPF solution.

Disable All Generator MW Controls

Prevents generators from shifting MW output during the OPF solution.

Disable All Load MW Controls

Prevents loads from shifting MW demand during the OPF solution.

Disable Area-toArea MW Transaction Controls

Prevents MW transactions between areas from being dispatched during the OPF solution.

LP Options

Maximum Number of LP Iterations

Maximum number of allowable iterations for the LP portion of the LP OPF. How many iterations are required to

obtain a solution depends, among other things, upon the number of breakpoints in the control cost models. Since

each LP iteration can only move from one breakpoint to the next, the finer the model the more iterations required.

However the LP is quite fast so a large number of iterations can be performed quite quickly. Default = 1000.

Phase Shifter Cost ($ / Degree)

Specifies the assumed cost for moving phase shifting transformer taps away from their initial values. The purpose

for this fictitious cost is approximate the cost of actually changing the angle of a phase shifting transformer, and to

avoid large changes in phase shifter angles that have very little impact on the system. This field may be zero.

Default = $ 0.10 / Degree.

Calculate Bus Marginal Cost of Reactive Power

When this option is checked, the OPF algorithm will also calculate the marginal cost of reactive power at each bus.

Typically the result of interest from the OPF algorithm is the MW marginal cost of each bus (the LMP), but the

MVAR marginal cost can be determined as well.

Save Full OPF Results in PWB File

When checked, Simulator will store the full set of results, including the LP matrix, in the PowerWorld Binary case

file.

Do Detailed LP Logging

When checked, Simulator will write details on the LP algorithm solution during each pivot of the LP matrix. This is

useful for debugging LP solution issueswhen running a LP OPF solution.

Power Flow Recalculation

Resolve Power Flow

Choose one of the options to determine how often the power flow is resolved. The three options are, "When total

generator MW change > than tolerance"; "After each LP solution"; and "Only at end of LP OPF".

Total Generator Change Tolerance (MW)

Specifies the total generator change tolerance. The default is 500 MW.

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OPF Options: Constraint Options

The OPF Dialog, Constraint Options page displays options associated w ith the enforcement of the constraints by the

OPF. The display contains the following fields:

Line/Transformer Constraints

Disable Line/Transformer MVA Limit Enforcement

Select to disable enforcement of Line/Transformer MVA constraints for the entire case.

Percent Correction Tolerance

Specifies a tolerance for the enforcement of line/transformer MVA flows. The tolerance is necessary to prevent

solution oscillations due to the non-linear nature of the actual constraints.

Violated elements are always enforced to their limits multiplied by the MVA Enforcement Percentage. If power

systems were completely linear then following the LP solution the constraint would actually be equal to this value.

However because of nonlinearities, the constraint is close to this value but usually not identical to the value. The

Percent Correction Tolerance is used to tell the OPF how close is close enough. Provided all the constraints are

violating their limits by less than the correction tolerance percentage the optimal solution is assumed to have been

found. You may set this value as low as you like, but setting it too close to zero may result in convergence

difficulties. The default is 2 percent.

MVA Auto Release Percentage

Specifies a MVA level at which transmission lines can be released as an OPF constraint equation if the branch MVA

flow falls below the level specified.

Maximum Violation Cost ($/MWhr)

If a branch MVA limit cannot be enforced during an OPF solution, the branch will be assigned a fictitious cost of

enforcement equal to this value. This value is usually rather large in order to easily determine where the

unenforceable constraint is occurring. The default value is 1000

$/MWhr.

Enforce Line/Transformer MW Flow Limits (Not MVA)

Checking this box will cause Simulator to treat the limits of the transmission elements as MW limits instead of MVA

limits. Thus Simulator will report violations on these elements in the OPF based on the MW flow of the element

versus the elements rating.

Interface Constraints

Disable Interface MW Limit Enforcement

Select to disable enforcement of Interface MW constraints for the entire case.

Percent Correction Tolerance, MW Auto Release Percentage, Maximum Violation Cost ($/MWhr)

These fields are equivalent to the entries described above for Line/Transformer MVA Constraints except that they

apply to Interface MW constraints.

Monitor/Enforce Contingent Interface Limits

This option allows you to choose if contingency elements in interfaces should be enforced during the OPF solution.

Even if they are not enforced, the flows on the remaining elements in the interface will be monitored. The choices

you have for enforcing contingency elements in interfaces during the OPF are Never, Power Flow/OPF but not

CA/SCOPF, or All Applications including CA/SCOPF. The reason CA (contingency analysis) and SCOPF (Security

Constrained OPF) are singled out is because those two tools are already looking at contingency actions separately,

outside of the interface definitions. Therefore it may be desired to ignore the inclusion of contingency elements

within an interface definition when using these two tools.

Limit Monitoring Settings…

This button opens the Limit Monitoring Settings dialog, which allows you to change the enforcement percentages for

monitored elements.

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PowerWorld Simulator Add-on Tools

OPF Options: Control Options

The LP OPF Dialog, Control Options page displays options for generator control and power flow solution. The display

contains the following options:

Generator Control Options

Allow Commitment of Fast Start Generators

If this option is checked, then generators designated as Fast Start generators can be turned on or "committed" if the

OPF routine determines that doing so would reduce the overall generating costs of the system.

See the Fast Start description in the help on OPF Generator Records for a more detailed description of the Fast

Start option of generators.

Allow Decommitment of Fast Start Generators

If this option is checked, then generators designated as Fast Start generators can be turned off or "de-committed" if

the OPF routine determines that doing so would reduce the overall generating costs of the system.

See the Fast Start description in the help on OPF Generator Records for a more detailed description of the Fast

Start option of generators.

Modeling Generators without Piecewise Linear Cost Curves

The following fields specify how the OPF should handle generators that are specified as having a cubic cost model.

Because the OPF is based upon an LP implementation, all control costs must be modeled using piecewise linear cost

curves. These options permit an automatic conversion of cubic models to piecewise linear models. Alternatively, you

can very easily convert the cubic models manually using the # Cost Curve Points field on the OPF Generator

Records display or using the generator dialog.

Generators Cost Models

This field specifies how generators with cubic cost models should be handled in the OPF. The field has three

values

Ignore Them -

Generators with cubic cost models are Ignored during the OPF solution. That

is, they are considered as though their AGC status was off.

Change to Specified Points per Curve - A piecewise linear cost model is automatically inserted for the generator

with a fixed number of points specified in the Total Points Per Cost Curve

field described below. This curve will approximate the generator's cubic cost

model as closely as possible; the existing cubic model is not modified. This is

the default value.

Change to Specified MWs per Segment - A piecewise linear cost model is automatically inserted for the generator

such that each segment in the cost model covers the amount of MWs specified

in the MWs per Cost Curve Segment field described below. This curve will

approximate the generator's cubic cost model as closely as possible; the

existing cubic model is not modified.

Total Points Per Cost Curve

Specifies the total number of segments that should be automatically inserted into the piecewise linear cost models

for those generators that are modeled using cubic cost functions. This is only done if the Generator Cost

Modeling field is Change to Specified Points per Curve. Default = 5.

MWs per Cost Curve Segment

Specifies the number of MWs for each segment of the piecewise linear cost models that are automatically inserted

for those generators that are modeled using cubic cost functions. This is only done if the Generator Cost

Modeling field is Change to Specified MWs per Segment. Default = 10 MW.

Save Existing Piecewise Linear Cost Curves

Generators that are modeled with cubic cost curves may have existing piecewise linear cost curves which may

have been manually entered by the user. These curves may or may not resemble the cubic cost function. During

the OPF solution the existing piecewise linear cost curves are replaced with the auto-created cost curves. If this

option is checked then the existing piecewise linear cost curves are restored at the end of the OPF. The default

and recommended option is false since this allows one to view the actual cost curves used by the OPF.

If you would like to use a particular piecewise linear cost function simply make sure that the generator is modeled

using the piecewise linear model, which can be set on the OPF Generator Records display.

Case OPF Options File

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This optional field is used to specify a default file name for storing OPF specific data. The OPF specific data is

always stored with the pwb file. Additionally, in order to make it easy to transfer the OPF specific data between

cases, this data may be stored in a pwo auxiliary file as well using the LP OPF, Store LP OPF Data command. The

Case OPF Options File field specifies the default name for this file.

Modeling of OPF Areas/Superareas

During the Initial OPF Power Flow Solution

Choose what manner of generation control you wish to be employed in the FIRST power flow solution the OPF will

perform, which will establish the base case load flow condition for performing the subsequent OPF generation

dispatch.

During Stand-Alone Power Flow Solutions

Choose what manner of generation control you wish to be employed in all load flow solutions FOLLOWING the

initial load flow solution. In other words, after the LP OPF routine has determined the new generation dispatch,

what type generation dispatch should be used during the normal load flow solution.

NOTE: it is NOT recommended that you use Economic Dispatch in this case, although it is an available option. The

reason it is not recommended is that you will remove the optimal dispatch (including constraints) just determined by

the OPF in favor of lowest cost economic dispatch, which will likely result in the re-introduction of overloaded

elements that were corrected by the OPF dispatch in the first place.

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PowerWorld Simulator version 11. Manual - page 17