Contingency Options: DC and Screening Options

 

These options are all available on the Contingency Analysis dialog under the Contingency Options tab under the Modeling grouping.

DC Method Options Moved from Modeling Basics page in Version 20

The following options are available if using one of the Linearized Lossless DCCalculation Methods.

For DC methods, allow amp limits by assuming a constant voltage magnitude

If checked, Simulator will allow converting MVA limits to Amp limits by assuming constant voltage magnitudes based on the state operating point just prior to the contingency calculations. Converting to amp limits is only enforced if the Limit Monitoring Settings Option of Treat Transmission Line Limits as Equivalent Amps is checked. When this option is checked, the contingency loading of the line will be calculated from the post-contingency current on the line and the calculated amp limit.

Model reactive power for DC methods by…

When you choose to use one of the Lossless DC calculation methods, you can also specify how to handle changes in reactive power during the calculations. The lossless DC methods are based on the real power MW in the system, thus an assumption needs to be made about the reaction of the Mvar flows during the linear calculations.

The choices are:

Ignoring reactive power

Reactive power is completely ignored. This results in the MW flow only being compared to the limits of the elements in the system. It is important to recognize this fact, as branch and transformer limits are usually given in complex power (MVA) ratings, thus, ignoring reactive power results in comparing active power flow (MW) to total complex power limits (MVA.).

Assuming constant voltage magnitude

One way to include reactive power in the linearized DC results is to assume the voltage magnitudes remain constant during the linearized DC contingency analysis. Thus, the MW flows are determined from the linearized calculations, and the MVAR flows are calculated from the resulting flows and constant voltage magnitudes. Thus you will still receive an approximate complex power flow on each element (MVA), which can then be directly compared to the complex power limit of the element.

Assuming reactive power does not change

Another way to include reactive power in the linearized DC results is to assume that the reactive power magnitudes remain constant during the linearized analysis. Thus, the MW flows are determined from the linearized calculations, and the complex power flow of each element can be approximated using the calculated MW flows and the assumed constant Mvar flows from the base case. This again allows the approximate complex flow on each element to be compared to the complex power limit of the element.

 

The following option is applicable when using one of the Linearized Lossless DCCalculation Methods or using contingency screening:

Iterate on Action Status (Do NOT treat all Statuses as CHECK)

This option is intended to model conditional actions more accurately during linear analysis. See the Contingency Iterated Linear Analysis topic for details.

Full AC Power Flow Screening Options Added in Version 20

The contingency screening process uses linear contingency analysis to screen contingencies prior to running full AC analysis on contingencies that pass the screening. This attempts to speed up the contingency process by only running full AC analysis on a subset of contingencies.

Screening followed by a full ac power flow solution of contingencies can only be done when using the contingency Calculation Method of Full Power Flow and the power flow solution options are using AC power flow mode. If these two conditions are not met, screening can still be done if the option to Screen Only is used.

When using screening and not choosing to only screen, the contingency solution process is done in two parts. First the screening is done on all contingencies that are not being skipped and Screen Allow = YES. Second any contingencies that are not being skipped and pass the screening conditions or have Screen Allow = NO will be solved with the full AC power flow.

Screening is also only done when running all contingencies. If using the option to Solve Selected Contingency screening is not done.

Use Screening

This option determines if screening should be used. The following settings are possible:

No

No screening will be done. All contingencies will be run using the selected Calculation Method set on the Basics page.

Yes

Screening will be done for contingencies where Screen Allow = YES and the contingency is not being skipped. The Screening Method will determine how the screened contingencies are processed. Screening will only be done if the Calculation Method is set to Full Power Flow and the power flow solution is in AC power flow mode. Contingencies that are not being skipped and have Screen Allow = NO will only be solved using the full AC power flow and no screening will be done.

Screen Only

Screening will be done for contingencies where Screen Allow = YES and the contingency is not being skipped. The Screening Method will determine how the screened contingencies are processed.

Include Voltage

Check this box to include bus voltage screening when screening is being used with either the Yes or Screen Only option. Because voltage screening is not done by using linear sensitivities, the screening process can take significantly longer when using this option compared to not using this option. Voltage screening will only be done if the power flow solution options are using AC power flow mode. The process used during voltage screening is described below in the Voltage Screening Process section.

Maximum number to select

These options determine how many contingencies will be run using the full AC power flow method following screening. The screening process will determine a ranking of each type of violation for each contingency as described in the Screening Rank section below. The ranking is determined from highest impact to lowest. If a contingency has a ranking for a particular violation type that falls within the top number to select for that violation type, that contingency will be run using the full AC solution.

Screening Method

Linear methods are used during the screening. The methods available are Linearized Lossless DC and Linearized Lossless DC With Phase Shifters. These calculation methods are the same as the options described for the contingency Calculation Method.

 

Screening Rank

The screening determines the rank of each contingency for each type of violation that is screened. The rank for each contingency for each violation type can be found in the Contingency Records Display. These fields are not shown in this table by default, but they can be found in the list of available fields in the Results\Screening folder.

Determining the rank for each type of object requires screening tolerances and multipliers that are specified with the limit group of the object being monitored. These values can be found on the Limit Group Dialog for the object's limit group. The calculation of the rank for each type of object is as follows:

Branch

A branch will be included in the ranking for branches if it is being monitored and its MW flow is greater than its limit adjusted by the screening percent of limit tolerance specified with the branch's limit group.

The limit for a branch is determined based on the limit set chosen for its limit group. The percentage of limit is not applied because the screening percentage is used instead. The Model reactive power for DC methods by... options are used when determining the MW flow.

For any monitored branch the rank is then non-zero if (MW > Limit*ScreenBranchPercent) and rank = (MW - Limit*ScreenBranchPercent). The total branch rank for a contingency is the sum of the rank for all monitored branches.

Interface

An interface will be included in the ranking for interfaces if it is being monitored and its MW flow is greater than its limit adjusted by the screening percent of limit tolerance specified with the interface's limit group. Interfaces can have negative limits. If the flow is negative it will be compared against the negative limit.

The limit for an interface is determined based on the limit set chosen for its limit group. The percentage of limit is not applied because the screening percentage is used instead.

For any monitored interface the rank is then non-zero if (Abs(MW) > Abs(Limit*ScreenInterfacePercent)) and rank = Abs(MW - Limit*ScreenInterfacePercent). The total interface rank for a contingency is the sum of the rank for all monitored interfaces.

Bus

A bus will be included in the ranking for buses if it is being monitored and its voltage is either above its high voltage limit adjusted by the screening high bus voltage tolerance, below its low voltage limit adjusted by the screening low bus voltage tolerance, or voltage changes are being monitored and its change is greater than the change tolerance adjusted by the screening change bus voltage tolerance.

The adjusted high voltage limit becomes: ScreenHighLim = HighLimit - ScreenTolHighVolt.

The adjust low voltage limit becomes: ScreenLowLim = LowLimit + ScreenTolLowVolt.

Change violations are included if the Always report as a violation if the... options are in use with the Advanced Limit Monitoring contingency options or with the Limit Group options for the limit group to which the bus belongs. These options specify that change violations be reported if the change in bus voltage from the contingency reference state either increases by a specified amount or decreases by a specified amount. For use with the screening rank, these values are modified by the screening change bus voltage tolerance.

The adjusted change tolerances for either an increase or decrease becomes: ScreenChangeLim = ChangeLim - ScreenChangeTolVolt. If ScreenChangeTolVolt > ChangeLim then ScreenChangeLim = 0.

For any monitored bus the rank is then non-zero for the following conditions:

If (Volt > ScreenHighLim) then rank = Volt - ScreenHighLim

else if (Volt < ScreenLowLim) then rank = (ScreenLowLim - Volt)*ScreenLowVoltMult

else if (Volt < InitialVolt) and ((InitialVolt - Volt) > ScreenChangeLim) then rank = (InitialVolt - Volt) - ScreenChangeLim

else if (Volt > InitialVolt) and ((Volt - InitialVolt) > ScreenChangeLim) then rank = (Volt - InitialVolt) - ScreenChangeLim

The total bus rank for a contingency is the sum of the rank for all monitored buses.

Bus Pair

A bus pair will be included in the ranking for bus pairs if it is being monitored and the absolute value of its angle difference is greater than its limit adjusted by the screening percent of limit tolerance specified with the bus pair's limit group.

The limit for a bus pair is determined based on the limit set chosen for its limit group. The percentage of limit is not applied because the screening percentage is used instead.

For any monitored bus pair the rank is then non-zero if (Abs(AngleDiff) > Limit*ScreenBusPairPercent) and rank = (Abs(AngleDiff) - Limit*ScreenBusPairPercent). The total bus pair rank for a contingency is the sum of the rank for all monitored bus pairs.

 

Voltage Screening Process

During the screening process, bus voltages are screened using an AC power flow method. Voltage screening is only done if the power flow solution options are set to be in AC power flow mode. Contingencies will be solved using linear solution techniques first. This process will determine which contingency elements and remedial action elements will actually be implemented. The actions are implemented and the AC power flow is solved for two iterations to get an estimate of voltage changes caused by implementing the contingency. The power flow is solved without automatic control enabled for switched shunts, SVCs, LTC transformers, phase shifters, and DFACTs. Generator Mvar limits will NOT be enforced in the inner power flow loop. Generator make up power will come from all online AGCable generators. Generator line drop compensation will be set based on any PostCTG settings. The bus screening rank is based on the system voltages after the power flow is solved using these settings.

Calculation Method Field

When screening is used, contingency violations will be reported. To determine if these violations were calculated using the linear method used with the screening or the AC method used with the contingency calculation method, a Calculation Method field is available with contingency records in the Contingency Records Display. This field can be found in the Results folder in the list of available fields. Possible entries for this field are: AC, DC, DCPS, FullDC, ScreenDC, ScreenDCPS, IteratedDC, IteratedDCPS, IteratedScreenDC, and IteratedScreenDCPS. AC is the full power flow solution method. Any entry containing DC used the Linearized Lossless DC method. The exception to this is the FullDC method, which used linear techniques while the power flow solution method was in dc mode. Any entry containing PS used the Linearized Lossless DC With Phase Shifters method. Any entry containing Screen shows results from the screening without running any additional analysis. Any entry containing Iterated used the option to Iterate on Action Status.