contengencies analysis user guide

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N-1-1 Contingency Analysis User Guide

1.0 IntroductionThe Category C contingencies in NERCs Transmission System Planning PerformanceRequirements are multiple contingencies and defined as loss of one component, followed bysystem adjustments, and loss of another component. The contingencies are well known as N-1-1contingencies. At PSSE 32, a Python module n11analysis is provided to perform N-1-1contingency analysis. Note that this is a prototype developed in Python and uses various ACCCAPIs available in PSSE. User comments and inputs are greatly appreciated to improve thisprototype.

Terms Used

N-0 Base Case: the original power flow case under normal conditions,

Primary Contingency or N-1 Contingency: the contingency occurs at first,

Secondary Contingency or N-1-1 Contingency: the contingency that occurs after the primarycontingency and system adjustments,

System Adjustments: refer to the control adjustments that are available in corrective actionswhich include generation re-dispatch, phase shifter angle adjustment and load curtailment.

N-1 Base Case: it is the solution after solving a primary contingency and createdautomatically by N-1-1 contingency analysis,

N-1 Contingency Analysis: perform contingency analysis with a base case and a pre-definedcontingency list,

N-1-1 Contingency Analysis: The whole procedure to evaluate Category C contingencies,

Performance Criteria: thermal limits are considered only in the N-1-1 contingency analysis;bus voltage range and deviation limits are ignored.

Base Case Performance Criteria: rating set and percent of flow rating for a base case,

Contingency Case Performance Criteria: rating set and percent of flow rating for acontingency case

Clean Case: a case that does not result in any violations to specified performance criteria.

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Siemens Energy, Inc.N-1-1 Contingency Analysis User Guide Power Technologies International

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This N-1-1 analysis prototype can be used for the following studies:

Check base case conditions and assist the operator to set up a clean base case with thecorrective action function if necessary

Perform N-1 contingency analysis for N-0 base case with predefined primary contingency

cases. Adjustments to the base case should be made if the system does not satisfy thesebase case and contingency case performance criteria.

Set up primary contingency cases (N-1 base cases) identified in the first step. Correctiveactions will be determined and performed to make the primary contingency cases are basecase criteria compatible.

Perform N-1 contingency analysis for each N-1 base case. The list of secondarycontingency cases may be different than that of the primary contingency cases. A primarycontingency case satisfies contingency case performance criteria if none of its secondarycontingency cases cause overloads or voltage violations for N-1 applicable limits. In thecase of contingency case performance criteria failures, corrective actions will be identified to

adjust the primary contingency case to meet contingency case performance criteria. If thereis no feasible set of adjustments by which a primary contingency can pass N-1 contingencyanalysis, the limiting facilities and secondary contingency cases that cause failures arereported.

2.0 Running N-1-1 Contingency Analysis

N-1-1 analysis is performed by Python module n11analysis added to PSSE in version 32. N-1-1analysis can be launched from inside of PSSE only. If PSSE is run from Python shell like IDLE,N-1-1 analysis cannot be performed from there. From inside of PSSE, N-1-1 analysis can belaunched from its GUI or Python automation scripts. A valid Power Flow case must be openedbefore running N-1-1 analysis.

In PSSE 32, launch N-1-1 contingency analysis with:

Power FlowSolutionsN-1-1 Analysis or

Toolbar button N-1-1 or

Run following Python commands in CLI:

import n11analysis

n11analysis.n11(n11optns_file.py)

where, n11optns_file.py is previously created N-1-1 analysisoptions file.

Launching N-1-1 analysis opens dialog as shown in Figure 1.


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Action Buttons

Open N-1-1 Options File button: Open an existing options file.

o An Options File is a Python file containing all the inputs required to run N-1-1analysis. This file has a specific format. Use blank options file (see below) to create

a new options file.

o A blank options file can be generated by saving it using Save N-1-1 Options Filebutton.

Save N-1-1 Options File button: Save current options into an options file.

Run N-1-1 button: Run the N-1-1 contingency analysis. It will be enabled if all requiredinputs are entered and are valid.

Close button: Exit N-1-1 contingency analysis.


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Input Files Tab

Specify files required by various ACCC APIs used in N-1-1 analysis. Two sets of files are required N-1 files and N-1-1 files. Each set has following files:

Required Files (input field highlighted in yellow):

o Subsystem Description Data File (.sub)

o Monitored Element Data File (.mon)

o Contingency Description Data File (.con)

If the contingency labels provided are not unique and are longer than 12 characters, those labelswill be made unique and truncated to 12 characters. A new contingency file with unique and 12character long label will be created and used.

For example, in the N-1 file set, a .con file defines the primary contingency list.

Optional Files:

o Load Throwover Data File (.thr)

o Unit Inertia and Governor Data File (.inl)

o Tripping Element Data File (.trp)

Figure 1. N-1-1 Contingency Analysis Dialog Input Files Tab


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FDNS Options Tab

The power flow solution method used in N-1-1 is the Fast Decoupled Newton Solution. RequiredFDNS options are provided here. Four sets of FDNS options are provided for N-0 base case, N-1contingency, N-1 base case and N-1-1 contingency solutions.

Figure 2. N-1-1 Contingency Analysis Dialog FDNS Options Tab


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Control Subsystems Tab

Specify various subsystems used to perform N-1-1 contingency analysis. Four sets of controlsubsystems are provided for N-0 base case, N-1 contingency, N-1 base case and N-1-1contingency solutions. N-0, N-1 and N-1 Base solution subsystems must exist in the SubsystemDescription Data File for N-1. N-1-1 solution subsystems must exist in Subsystem Description Data

File for N-1-1. For example, when generation re-dispatch is specified to correct the N-0 base case,the generators in the N-0 generator control subsystem are selected to participate in dispatch.

Figure 3. N-1-1 Contingency Analysis Dialog Control Subsystems Tab

Control Options Tab

Flow Ratings: These are the rating used for calculating monitored elements violations. Foursets of ratings are provided for N-0, N-1, N-1 Base and N-1-1 solutions.

Generation Dispatch Mode: This is used to calculate generation dispatch corrective actions.Four sets of ratings are provided for N-0, N-1, N-1 Base and N-1-1 solutions.

N-1 Contingencies to perform N-1-1 analysis: N-1 analysis is done using all thecontingencies defined in N-1 contingency file. However, if desired N-1-1 analysis can beperformed for selected N-1 contingencies. Provide here list of those selected N-1contingencies. When not provided, N-1-1 analysis is performed for all N-1 contingencies.

Voltage Violations Report: If voltage violations report is to be printed, select this option.


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Saving N-1 base cases: If N-1-1 analysis is not successful; by default such N-1 base casesare saved which can be used for further investigation. Successful N-1 cases are not saved,and if desired to save those cases as well, select this option.

N-1-1 analysis produces massive amount of progress and report outputs. It is best practiceto save those reports to file. The output files names are auto generated using date, time and

Saved case file name. For example, if N-0 saved case name is ieee_25bus.sav. The twooutput files produced would be:

20090321_223627_ieee_25bus_progress_n11.txt

20090321_223627_ieee_25bus_progress_psse.txt

Where, 20090321 is a date (yyyymmdd), 20090321 is a time (24 hour clock, hhmmss).

File with suffix _n11.txt is an output written by n11analysis module, and file with suffix_psse.txt is a progress output written various PSSE APIs used in N-1-1 analysis.

All the outputs written in these files are self-explanatory.

Figure 4. N-1-1 Contingency Analysis Dialog Control Options Tab


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3.0 n11analysis - Python Module

The following functions are available in n11analysis module.

check_conlbls(confnam='', newconfnam='')

Check contingency labels to be unique and 12 characters long.

Inputs- confnam: Contingency description file name.

newconfnam: New Contingency description file name. When

not provided, new contingency file created

with "_unqlbl" appended to name of confnam.

If contingency labels are non-unique and longer than 12

characters, it creates "newconfnam" file with new contingency

labels to be unique and 12 characters long.

Otherwise, "newconfnam" file will not be created.

n11(n11optnfile)

Run N-1-1 analysis.

Input- n11optnfile: N-1-1 analysis options py file name.

File must exist. No default allowed.

n11optnfile(infnam='')

Create empty N-1-1 options file (template).

Input- infnam: File name to which options file template will be

written to. When not provided, it will be written

to the progress window.


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4.0 Methodology of N-1-1 Contingency Analysis

The contingency analysis in PSSE features automatic AC contingency analysis, tripping simulationand corrective actions. The N-1-1 contingency analysis python module uses these contingencyanalysis features in PSSE as a basic computation engine to evaluate each contingency.

The corrective actions described here only include generation dispatch, phase shifter angleadjustment and load curtailment. These actions, e.g. dispatch adjustments and changes tophase shifter set points, are closely related to correcting thermal limit violations. Voltageproblems are neglected from the analysis, though they are printed in the report.

A set of corrective actions is specified for each secondary contingency with violations. Thealgorithm can identify a global set of corrective actions by which the system under primarycontingency case condition can meet N-1 criteria.

The procedure to perform N-1-1 contingency analysis is shown in Figure 5. It consists offollowing steps:

1. Set up the base case, contingency definitions, monitored elements, etc. to preparefor contingency analysis.

2. Perform N-1 contingency analysis on the base case. If the base case results incontingencies that fail contingency case performance criteria, appropriateadjustments to the base case condition must be made. If fails, stop.

3. Set up a primary contingency case (N-1 base case) with power flow solution optionsand adjustments as used for a base case for each of the primary contingencies.Facilities will be checked to see if they are within base case performance criteria.Likely, some elements are not within base case performance criteria; for example,lines that may be between normal and LTE ratings. Corrective actions will be

determined and performed to eliminate violations and thus to bring the primarycontingency case to meet base case performance criteria.

4. Perform N-1 contingency analysis for each primary contingency case. If noviolations are found in the post contingency solution, go on to the next contingency.Corrective actions will be calculated for each secondary contingency (as performedon the primary contingency case) that does not meet contingency case performancecriteria.

5. When finished with step 4, check the contingency analysis results. Produce a reportof each secondary contingency with criteria failures, the respective correctiveactions, and violations, if any, following the corrective actions.

6. In steps 2 and 4, when an N-0 or N-1 base case failed the contingency caseperformance criteria, adjustments to the base case condition are made based on thefeedback information from contingency analysis results in terms of violations andcorrective actions. These adjustments are performed on an N-0 or N-1 base caseand a new corrected base is created. This case is then subjected to the sameprocess (step 2 for N-0 base case and steps 4 through 6 for N-1 base caserespectively). This is repeated until:


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either no contingency case performance criteria violations exist, that is,adjustments have been found that allow the base case to satisfy base caseperformance criteria and contingency case performance criteria,

the allowed number of iterations has been reached and no set of correctiveactions have been found that make base case meet base case performance

criteria and contingency case performance criteria.

7. Continue with the next primary contingency case, that is, go to step 3 or stop if allprimary cases are tested by N-1 analysis.


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Meets N-0

criteria?

Get Correct

N-0 Base Case

N-0 Base Case

Meets N-1criteria?

No

Yes

Get CorrectBase Case

No

Select

ith Outage

i = 1 to number ofcontingencies

Yes

Apply ith N-1 Outage

ith N-1 Base Case

# Perform Correcive Action Adjustments on ith

N-1 Base Base to Meet N-0 Criteria if necessary# List/Store Corrective Actions if any

Perform N-1-1

Contingency Analysis

Violation

# Find Worst Contingency Based on

Performance Index

# List/Store Violations for Each Contingency

Choose Worst Contingency

# Apply Worst Contingency and Find Correction Actions

# List/Store Correctve Action

# Apply Corrective Action Adjustments to ith N-1 Base Case

# This is ith N-1-1 Base Case

# Check ith N-1-1 Case to Ensure it Meets N-0 Criteria# Find/Apply Corrective Actions If Necessary

2MVA Loading

2MVA RatingAll monitored elements

=

Yes

No

Perform N-1Contingency Analysis

Meets N-1-1

Figure 5. Flowchart of N-1-1 Contingency Analysis


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5.0 Analyzing N-1-1 Contingency Analysis Results with a Test Case

The N-1-1 Contingency Analysis creates two report files: one file is used for the destination ofprogress outputs, another contains the N-1-1 contingency analysis results, whose name consists ofdate and time when the N-1-1 contingency analysis is start, base case name followed by N11, i.e.20090320_135508_ieee_25bus_progress_n11.txt.

In viewing reports, note that conventions in contingency analysis and corrective actions are appliedto N-1-1 contingency analysis, since the N-1-1 contingency analysis uses Contingency Analysisand Corrective Action function as its basic engines. The percent unit program option setting for non-transformers and transformers are used to tabulate percentage loadings of monitored elements.

Strictly speaking, for a system that stratifies N-1-1 contingency criteria, both its N-0 base case andN-1 base case should meet base case and contingency case performance criteria.

Because the analysis is very complicated, in order to understand the results, the N-1-1 contingencyanalysis function presents and tabulates calculation and analysis results at each step during thesolution, besides the summary results at the end of reports.

A report of N-1-1 contingency analysis is organized in three major parts, N-1 contingency analysisreport with respect to steps 1 and 2, N-1-1 contingency analysis report with respect to steps 3 to 6and summary report.

N-1 Contingency Analysis Report

Initial violations

Figure 6. Base Case Report

The report can be further divided into Base case report (step 1) as shown in Figure 6, N-1contingency analysis report.


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If a base case satisfies base case performance criteria, a message is presented to indicate that thebase case is clean. Otherwise the base report lists for each overloaded monitored element: therating, MVA flow, Current flow and percent loading; followed by corrective actions specified toremove these violations and. In the each group of Generation dispatch, phase shifter angleadjustment and load curtailment controls, the specified controls initial, new values are tabulated.

Performance Index

Figure 7a. N-1 Contingency Analysis Report

Figure 7b. N-1 Contingency Analysis Report


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Figure 7c. N-1 Contingency Analysis Report

The base case report is followed by the N-1 contingency analysis results. The N-1 contingencyanalysis report includes a summary report on the initial N-1 contingency analysis and analysisresults upon adjusting of base case and verification of the adjustments when the base case doesnot satisfy base case performance criteria. It is an iterative process to correct the base case, ateach iteration corrective actions applied to the base case and N-1 contingency analysis results arereported.

The initial N-1 contingency analysis report as shown in Figure 7a presents first the number of

converged cases and the number of cases with violations and the number of clean cases. For eachcase where the thermal limit violations are detected, its performance index is reported. For eachflow overloading violations in the case, its rating, MVA flow, current flow and percentage loading arelisted. The performance index of each case with violations is calculated in the form:

=

Si iP

PiPI

2)

max(

Where S is the set of all overloaded monitored elements, Pi is the power flow on monitored elementi, Pmaxi is the rating of monitored element i.


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The analysis result report first lists N-1 contingency analysis results obtained from ContingencyAnalysis function with corrective action enabled as shown Figure 7b. Contingency cases areclassified into converged cases, converged cases with thermal limit violations, clean cases,contingency cases which a set of corrective actions is specified, contingency cases which a set ofcorrective actions is specified and which are clean after the corrective actions are applied. For eachclass, the number of contingency in the class and contingency labels are listed. The set of

corrective actions from the worst case is selected to apply to the base case and an N-1 contingencyanalysis is re-performed on the corrected base case. The worst case is the case that has thermallimit violations and for which a set of corrective actions can be found to remove the violations.

It may take several iterations to correct the base case so it satisfies contingency case performancecriteria. At each iteration the applied corrective actions are tabulated in groups of generation re-dispatch, phase shifter angle adjustment, and load curtailment. Each controls initial, change andnew values are listed, in MW for generation re-dispatch, in degree for phase shifter angleadjustment, in MW for load curtail. An N-1 contingency analysis is then re-performed on thecorrected base case and the summary report of the N-1 contingency analysis lists the numbers ofconverged contingency cases and clean cases, as well as contingency case labels in each group.

For example, in the Figure 7c the contingency case N1OVRLOD 2 has the largest PI and can becleaned by corrective action analysis, and is then picked as the worst case. AC corrective actionanalysis is carried out on the contingency case and corrective actions specified for the caseN1OVRLOD 2 are used to adjust the base case and N-1 contingency analysis is re-conductionwith the modified base case. The results show all 14 contingency cases do not result in thermallimit violations; therefore the base case satisfies base case and contingency case performancecriteria.

N-1-1 Contingency Analysis Report

For each N-1 contingency case, the N-1-1 contingency analysis results are organized in the sameformat as defined in the N-1 contingency analysis report. Note that the base case is now the N-1

base case and an N-1 contingency analysis is performed with the pre-defined contingency list ofsecondary contingencies. Similar to N-1 contingency analysis report, an N-1-1 contingency analysisreport is divided into two parts: N-1 base case and N-1-1 contingency analysis reports.

If an N-1 base case satisfies base case performance criteria, a message is presented to indicatethat the base case is clean. Otherwise the base report lists for each overloaded monitored element:the rating, MVA flow, Current flow and percent loading; followed by corrective actions as shown inFigure 8.


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Figure 8. N-1 Base Case Report

Following is an N-1-1 contingency analysis report. If an N-1 base case does not satisfy contingencycase performance criteria, the iterative process is started to correct the base case and verify thatthe corrected N-1 base case satisfies contingency case performance criteria.

Summary Report

The Figure 9 shows a sample of summary report. The summary report tabulates the N-1-1contingency analysis results. The first column lists the primary contingencies; the second columnlists secondary contingency cases of each primary contingency. The rest of the columns list resultsat each iteration; each of them is headed by the iteration number. To meet N-1-1 criteria, resultsshown at the last iteration must be all YC.


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Figure 9. Summary Report


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6.0 Application Notes

Each primary and secondary contingency must have a unique contingency label. Function

check_conlblscan be used to pre-process contingency labels to make sure they are

unique.

A primary contingency (N-1 contingency) can not be used as an N-1 base case if thecontingency involves generation dispatch specified by contingency specification commandDISPATCH.

The N-1-1 contingency analysis function only evaluates user specified contingency cases, inother words the contingencies that are defined in N-1 and N-1-1 Contingency DescriptionData File. Contingency ranking feature is disabled for N-1-1 contingency analysis.

N-1-1 contingency analysis is essentially based on AC power flow solutions. N-1contingency analysis must be performance at least once for the N-0 base case and each N-1 base case. The number of power flows solved could be huge if the lists of primary andsecondary contingency are long.

Under normal condition, a system should satisfy base case and contingency caseperformance criteria. Although N-1-1 contingency analysis function has the capability tocorrect a base case in case the base case fails to the criteria, the N-1-1 contingencyanalysis will be greatly accelerated when run with a clean base case.

_____________________________

Copyright 1990-2009 Siemens Energy, Inc., Siemens Power Technologies International (Siemens PTI)

Information in this manual and any software described herein is confidential and subject to change without notice and does notrepresent a commitment on the part of Siemens PTI. The software described in this manual is furnished under a license

agreement or nondisclosure agreement and may be used or copied only in accordance with the terms of the agreement. Nopart of this manual may be reproduced or transmitted in any form or by any means, electronic or mechanical, includingphotocopying, for any purpose other than the purchasers personal use, without the express written permission of SiemensPTI.

PSSE high-performance transmission planning software is a registered trademark of Siemens PTI in the United States and

other countries.

The Python programming language is a trademark of the Python Software Foundation.

Other names may be trademarks of their respective owners.

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