Security Assessment
Please Note: the features described below are only available in the current InterPSS Dev release.
Introduction
Power system security assessment is analysis performed to determine whether, and to what extent, a power system is reasonably safe from serious interference to its operation. From power system simulation perspective, security assessment is based on the contingency analysis, which runs in an energy management system to give the operators an indication of what might happen to the power system in the event of an unplanned or un-scheduled equipment outage. Typical contingencies on a power system consist of outages such as loss of generators or transmission lines. We use the security assessment concept and terminology defined in Ref [1] in our discussion.
Real-time on-line security assessment algorithms used in today's power system control center are designed to be run on one computer. Because of the computation speed limitation, most of them are based on the [B]-matrix approximation, introduced by B Stott in 1974.
- Simplified model - Because of the computation speed limitation, on-line analysis normally treats a power network as a "DC" network using [ΔP] = [B']•[Δθ]. Reactive power and voltage are normally not considered in ED (economic dispatch).
- Linearized prediction - Linearized prediction approach is commonly used to predict, for example, what is the power flow increase on branch A, if there is a branch B outage. Again the [B]-matrix is used to calculate the LODF - Line Outage Distribution Factor. LODF assumes that the power network under the contingency condition can be accurately represented by a network of impedance and constant EMF, which is not true for today's "flexible" power network with many control/adjustment equipments, such as DC transmission line, FACTS, SVC ...
- Screening - Contingency analysis requires running thousands study cases in a short period of time, which is not feasible using one computer. Therefore, the common practice today is to use some kind of screening process to select a sub-set of the study cases to run. The selection is normally based on sensitivity analysis using the [B]-matrix.
Section 11.3.3 in Ref [2] discussed the limitation of the above approach and the need for AC Loadflow based security assessment.
Contingency Analysis
Again we are using the IEEE-14 bus system, shown below, as an example, to show InterPSS Grid Computing based approach.
Using InterPSS custom script run, contingency analysis cases can be distributed to remote grid nodes and run in parallel. The following is a sample InterPSS Xml document for contingency analysis:
<ipss:analysisRunType>ContingencyAnalysis</ipss:analysisRunType>
<ipss:contingencyAnalysis>
...
<ipss:modification>
<ipss:branchChangeRec>
<ipss:fromBusId>0001</ipss:fromBusId>
<ipss:toBusId>0005</ipss:toBusId>
<ipss:offLine>true</ipss:offLine>
</ipss:branchChangeRec>
...
InterPSS release has a sample contingency analysis xml document for the IEEE-14Bus sample system. In the sample contingency analysis run, there are total 23 contingencies - 19 branch outages and 4 generator outages.
After a contingency analysis run, the Branch MVA Rating violation report will be displayed. By clicking the SecMargin button, you can access the Security Margin analysis report.
Using the grid computing approach, contingencies are run at remote nodes in parallel. The analysis results are sent back to the master node real-time through the computer network. The master node collects all analysis results and selects the largest branch mva flow for all contingency cases for each branch.
Branch MVA Rating Violation
Branch MVA Rating Violation Report
Branch Id MvaFlow MvaRating Violation Description
===========================================================================
0001->0002(1) 241.1 200.0 21% Open branch 0001->0005
0005->0004(1) 103.2 100.0 3% Open branch 0002->0003
0006->0013(1) 51.3 50.0 3% Open branch 0007->0008
0003->0004(1) 101.2 100.0 1% Open branch 0002->0003
After the contingency analysis, the largest branch mva flow at each branch for all contingencies is compared with the branch MVA rating to product the Branch MVA Rating Violation Report. In the above example, the most severe case is "Open branch 0001-0005", which over loads branch 0001-0002 by 21%. If we look at the on-line diagram, when opening branch from Bus-1 to Bus-5, all power from the swing bus (Bus-1) to the system is through branch Bus-1->Bus-2, which will cause overloading of 21% for a 200 MVA branch rating.
Security Margin
In addition to the violation analysis, security margin based on analysis of ALL contingencies is computed. These reports tell you how much adjustment room are available with regarding to bus voltage upper/low limit and branch thermal rating limit for all concerned contingencies.
Bus Voltage Security Margin
Bus Voltage Limit: [1.10, 0.90]
Bus Id HighVolt UpperMargin LowVolt LowMargin Description
===========================================================================
0002 1.0500 5.0% 1.0247 12.5% Generator outage at bus 0002
0001 1.0600 4.0% 1.0600 16.0% Open branch 0001->0005
0014 1.0426 5.7% 0.9970 9.7% Open branch 0009->0014
0013 1.0552 4.5% 0.9980 9.8% Open branch 0006>0013
0012 1.0590 4.1% 1.0000 10.0% Open branch 0007->0008
0011 1.0635 3.7% 1.0000 10.0% Open branch 0007->0008
0009 1.0689 3.1% 1.0000 10.0% Open branch 0007->0008
0010 1.0620 3.8% 1.0000 10.0% Open branch 0007->0008
0008 1.0900 1.0% 1.0366 13.7% Generator outage at bus 0008
0007 1.0680 3.2% 1.0000 10.0% Open branch 0007->0008
0006 1.0700 3.0% 1.0451 14.5% Generator outage at bus 0006
0005 1.0288 7.1% 1.0000 10.0% Open branch 0007->0008
0004 1.0210 7.9% 0.9983 9.8% Open branch 0002->0003
0003 1.0100 9.0% 0.9537 5.4% Open branch 0002->0003
For all 23 contingencies, the lowest voltage at bus 0003 is 0.9537, which happens when "Open branch 0002->0003".
Branch Rating Security Margin
Branch Id MvaFlow MvaRating P + jQ Margin Description
===========================================================================================
0002->0003(1) 98.4 100.0 ( 98.4+j 2.4) 2% Open branch 0003->0004
0006->0012(1) 26.4 50.0 ( 11.4+j 23.8) 47% Open branch 0007->0008
0009->0014(1) 27.4 50.0 ( 27.3+j -1.8) 45% Open branch 0005->0006
0002->0004(1) 93.8 100.0 ( 93.7+j 3.5) 6% Open branch 0002->0003
0006->0013(1) 51.3 50.0 ( 23.2+j 45.7) -3% Open branch 0007->0008
0010->0009(1) 33.7 50.0 ( 33.5+j -3.8) 33% Open branch 0005->0006
0002->0005(1) 78.0 100.0 ( 77.9+j -1.9) 22% Open branch 0001->0005
0013->0014(1) 16.4 50.0 ( 15.3+j 5.9) 67% Open branch 0009->0014
0005->0004(1) 103.2 100.0 ( 102.3+j -13.4) -3% Open branch 0002->0003
0004->0007(1) 59.8 100.0 ( -57.4+j 16.5) 40% Open branch 0005->0006
0001->0002(1) 241.1 200.0 ( 240.1+j -21.8) -21% Open branch 0001->0005
0003->0004(1) 101.2 100.0 ( 101.1+j -4.7) -1% Open branch 0002->0003
0005->0006(1) 62.9 100.0 ( 47.3+j 13.5) 37% Open branch 0010->0009
0004->0009(1) 33.5 100.0 ( -32.9+j 6.4) 66% Open branch 0005->0006
0007->0008(1) 23.3 100.0 ( 0.0+j 23.3) 77% Open branch 0002->0003
0012->0013(1) 13.6 50.0 ( 13.1+j 3.8) 73% Open branch 0006>0013
0001->0005(1) 95.5 100.0 ( 95.0+j 9.1) 5% Open branch 0002->0003
0007->0009(1) 57.6 100.0 ( -57.4+j 4.2) 42% Open branch 0005->0006
0006->0011(1) 34.0 50.0 ( 14.6+j 30.7) 32% Open branch 0007->0008
0011->0010(1) 26.4 50.0 ( -23.7+j 11.6) 47% Open branch 0005->0006
Branch 0002->0003, although with no rating violation, only has 2% room for increasing its mva flow before hitting its security limit, while branch 0006-0012 has 47% room (margin) for scheduling more power transfer.
The security margin is obtained by running 29 AC Loadload runs using the Grid Computing approach. Therefore, there is no simplification or approximation in calculating the margin.
Preventive/Corrective Action Evaluation
When there are violations in contingency situation, sometimes, one might want to evaluate preventive or corrective actions, such as adjusting transformer turn ratio or load shedding. InterPSS allows you to define a rule set to evaluate the preventive/corrective actions. For example, the following rule base says, under contingency conditions, if branch 0001->0002 has RatingMav1Violation, cut Bus 0014 load first, since it has the highest priority = 1. If the condition still exits, cut Bus 0013 load with priority = 2. The priority field controls the order in which the corrective rules are applied.
<ipss:ruleBase>
...
<ipss:priority>1</ipss:priority>
<ipss:condition>
<ipss:fromBusId>0001</ipss:fromBusId>
<ipss:toBusId>0002</ipss:toBusId>
<ipss:branchCondition>RatingMva1Violation</ipss:branchCondition>
</ipss:condition>
<ipss:busAction>
<ipss:busId>0014</ipss:busId>
<ipss:loadShedding>true</ipss:loadShedding>
</ipss:busAction>
<ipss:priority>2</ipss:priority>
<ipss:condition>
<ipss:fromBusId>0001</ipss:fromBusId>
<ipss:toBusId>0002</ipss:toBusId>
<ipss:branchCondition>RatingMva1Violation</ipss:branchCondition>
</ipss:condition>
<ipss:busAction>
<ipss:busId>0013</ipss:busId>
<ipss:loadShedding>true</ipss:loadShedding>
</ipss:busAction>
After applying the preventive/corrective actions, cut load at bus 0014 and then 0013 in the event of Branch 0001->0002 overloading, the branch violation reduced from 21% to 4%.
Branch MVA Rating Violation Report
Branch Id MvaFlow MvaRating Violation Description
===========================================================================
0001->0002(1) 207.8 200.0 4% Open branch 0001->0005
0005->0004(1) 103.2 100.0 3% Open branch 0002->0003
0006->0013(1) 51.3 50.0 3% Open branch 0007->0008
0003->0004(1) 101.2 100.0 1% Open branch 0002->0003
PSS/E File Format
In most power system control center here in North America, network data is normally represented in PSS/E *.raw data format. Contingency cases are described by a control file. The following is a sample control file:
CONTINGENCY LOSE60-70
OPEN LINE FROM BUS 60 TO BUS 70 CIRCUIT 1
END
....
END
InterPSS has been extended to work with PSS/S *.raw data format for contingency analysis. A custom Xml script run plugin has been developed to load the contingency control file into InterPSS and run the contingency analysis. As shown below, a PSS/E raw file is first loaded into InterPSS. Then click the “J” button – Custom Script Run and select the Custom radio button. Copy/Paste the contingency control file to the textarea. Clicking the Run button will allow you to run the contingency analysis.
By clicking the XMl button, you can examine how InterPSS interprets PSS/E contingency control file.
Sample Contingency Analysis
InterPSS release has a sample PSS/E file psse_format/CR113Bus.ipss with 113 buses. The contingencies are listed in the following control file:
CONTINGENCY LOSE60-70
OPEN LINE FROM BUS 60 TO BUS 70 CIRCUIT 1
END
CONTINGENCY LOSE70-90
OPEN LINE FROM BUS 70 TO BUS 90 CIRCUIT 1
END
CONTINGENCY LOSE60-80
OPEN LINE FROM BUS 60 TO BUS 80 CIRCUIT 1
END
CONTINGENCY LOSE80-151
OPEN LINE FROM BUS 80 TO BUS 151 CIRCUIT 1
END
END
Run InterPSS contingency analysis, the following violations are found:
Branch MVA Rating Violation Report
Branch Id MvaFlow MvaRating Violation Description
===========================================================================
90->151(1) 137.6 110.0 25% LOSE60-70
111->110(1) 153.8 150.0 3% LOSE80-151
60->80(1) 111.2 110.0 1% LOSE80-151
References
[1] Neil Balu, et al, “On-line Power System Security Analysis”, Proc of the IEEE, Vol. 80, No. 2, Feb 1992.
[2] A. J. Wood, B. F. Wollenberg, "Power Generation Operation and Control", John Wiley & Sons, Inc, 1984