@akshay_shah.pdf
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Contingency Analysis of Power System using Power Flow
Akshay Shah (ars2212) Abstract
Contingency analysis (CA) is an important tool in the Energy Management System both for design and operation of a power network. It analyses the system security and the need for a remedy in case of a fault in the network.
This paper performs (N-‐1), (N-‐2) and (N-‐3) contingency analysis on two standard IEEE test cases and shows the difficulty of scaling this analysis for a higher (N-‐x) given the exponential increase in time take between different levels. Keywords -‐ Contingency Analysis, Power System Stability, Power Flow I. Introduction
Contingency analysis (CA) is a key component of operating today’s energy management system (EMS). It is used to tell operators what might happen in case of an equipment outage. It analyses whether the system can still operates within limits upon facing an outage or whether the under voltages and overloads require a remedy. It is key part of what the operator does to avoid equipment damage and to minimize customer outages. i II. Motivation
The security of a power grid has come into increased focus given the
emerging concerns around the threat to a power network from terrorism. A recent report widely circulated in the national media concluded that the U.S. could suffer a national blackout, if only nine out of fifty five substations were knocked out. ii This underscores the importance of contingency analysis, which calculates power system stability upon disabling some parts of the network. III. Typical Causes of Faults The most common cases of contingencies on a power network involve equipment failure, either a loss of a generating unit, or a loss of a transmission component including a transmission line, transformer, substation bus or switching gear. Furthermore failure can be single or multiple if relay failures are taken into account. The other common causes of faults are weather related and the distribution of causes varies by utility (Figure 1.0).
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Figure 1.0 – Typical Causes of Faults
iii Source: Richard Brown. Electric Power Distribution Reliability IV. Use Cases of Contingency Analysis CA is used in the planning stage of designing a network to evaluate feasibility and security, which is based on the ability of the network to resist failures. CA is also used on a live network at regular intervals to evaluate new system conditions. Weak elements are defined as those that have overloads or under voltages under the conditions of contingency. The basic methodology is to disable a particular part (branch) and evaluate network stability using power flow equation. The number of branches disabled determines the (N-‐x) contingency analysis. If only one branch is disabled we have (N-‐1) contingency analysis. Based on this analysis operators can device a ranking to prioritize transmission planning.iv V. Methodology for Simulation CA was performed using the Matlab software and the MatPower 4.1 library (PSERC, Cornell University) to perform power flow calculations for a given power network. The MatPower library’s power flow function searches for convergence using Newton’s method. If convergence is not found in 10 iterations, we infer that the network is unstable and requires a remedy, as a solution to the power flow equation was not found. The success or failure of the convergence was noted, along with the information of which bus was enabled or not. An (N-‐1), (N-‐2) and (N-‐3) was performed on different power network test cases. The power networks used for the simulation were the standard IEEE test casesv. CA was done for IEEE Case 30 (Figure 2.0), which had 30 buses and 41 branches and IEEE Case 300, which had 300 buses and 411 branches. The IEEE 30 case was chosen because of its relative
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simplicity for initial analysis. The IEEE 300 was chosen to see how the analysis would translate to larger networks, with more complexity. The example codes for the simulations can be found in the appendix. VI. Results for IEEE Case 30
This case represents a real network that was a portion of the American Electric Power System (in the Midwestern US) as of December 1961. It has 30 buses and 41 branches. A drawback of this case is that it doesn’t have line limits. A bus diagram representation can be seen in Figure 2.0.
(N-‐1) Contingency analysis of this network was performed by iteratively switching off one bus for each of the 41 buses and checking for convergence. The analysis revealed that convergence was not found upon switching off the 13th, 16th or 34th branches. These branches correspond to the connections from bus 9 to 11, 12 to 13 and 25 to 26 respectively (see Figure 2.0). This intuitively makes sense as the first two cases disconnect the synchronous generator and the last case is connected to a load with a single line and no redundancy.
Figure 2.0 – IEEE Test Case 30, with the failure nodes for N-‐1 CA circled
Source: University of Washington, EE Department
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(N-‐2) Contingency analysis of this network was performed by iteratively disabling two branches of the network at a time and looking for convergence, the results of which are seen in Figure 3.0. The figure shows are 41X41 matrix with each cell representing the success or failure in finding convergence. Yellow represents failure, while red represents success. As expected (N-‐2) convergence will fail for every branch in which the (N-‐1) convergence failed. This gives us our yellow bands, which are seen for rows and column number 13,16, and 34. The other yellow boxes are the ones where the network would not have failed if either of the branches were enabled. The matrix is also completely symmetric as expected.
Figure 3.0 – Result of (N-‐2) CA on IEEE Test Case 30
(N-‐3) Contingency analysis of this network was performed by iteratively disabling three branches of the network at a time and looking for convergence, the results of which are seen in Figure 4.0. The figure shows are 41X41X41 matrix with each cell representing the success or failure of convergence. Yellow represents success, while red represents failure (Note opposite coloring scheme from the N-‐2 case). The matrix is again completely symmetric, as expected. To get a more intuitive understanding of what is happening we can sum over the number of failures on the 3rd dimension. The results of this are seen in Figure 5.0. We can see that we have an (N-‐3) failure whenever we have an (N-‐2) failure. The situations in which we would not have had a failure if one of the branches were active can be seen in the undulations next to the origin where we have about 6-‐7 failures for a particular entry of the matrix.
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Figure 4.0 – Result of (N-‐3) CA on IEEE Test Case 30
Figure 5.0 – (N-‐3) CA on IEEE Test Case 30, with 3rd Dimension Summed Over
An important consideration while performing CA is the time taken for the analysis. The summary results and time taken to run the programs are listed in Table 1.0. Note that the time taken increases exponentially when increasing x for a (N-‐x) analysis. This is because the number of iterations increases exponentially. The time taken is also driven by the % of the failures as the newton convergence for failure does 10 iterations, whereas success is typically achieved in less than 4 iterations. The N-‐1 case has constant overheads, but the increase in Time/Iterations from N-‐2 to N-‐3 can be attributed to the increase in number of failures. Note that the increase in % of failures from N-‐2 to N-‐3 is quite significant, it rises from 17% to 29%.
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Table 1.0 – Results of the CA for IEEE Case 30 and time taken for the simulation
IEEE CASE 30 Number of iterations Time(s) Number of Failures Failures/Iterations Time/Iterations N-‐1 41 1.678 3 7.32% 0.040926829 N-‐2 1681 65.233 289 17.19% 0.038806068 N-‐3 68921 3360.781 19797 28.72% 0.048762801
VII. Results for IEEE Case 300
This case was made by Mike Adibi in 1993 for the IEEE Test Systems Task Force. It has 300 buses and 411 branches. Unlike Case30 it has line limits, which is part of the reason we shall a higher failure rate for this system. The motivation for choosing this case was to see how CA scales with an increase in number of buses.
(N-‐1) Contingency analysis was performed by iteratively disabling each of the 411 buses, the results of which can be seen in Figure 6.0. The white bands represent failure and the blue bands represent success. The significant change from Case 30 is that the failure rate is much higher (25%) and the time taken for each iteration is a significant 5 times larger than Case 30. See Table 2.0 for time taken results. Figure 6.0 – Result of (N-‐1) on IEEE Test Case 300, white is failure, blue is success
(N-‐2) Contingency analysis of this network was performed by iteratively disabling two branches of the network at a time and looking for convergence, the results of which are seen in Figure 7.0. The figure shows are 411X411 matrix with each cell representing the success or failure of convergence. Yellow represents success, while red represents failure. As expected, the matrix is symmetric and we
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have failure in (N-‐2) whenever we have failure in (N-‐1). From Table 2.0 we see that number of iterations increases exponentially and (N-‐2) requires 411^2 iterations.
Figure 7.0 – Result of (N-‐2) on IEEE Test Case 300, Red is failure, Yellow is success
An (N-‐3) analysis was not performed, as it would have taken a very large amount of time. (N-‐2) analysis required 41623 seconds (~11.5 hours) and (N-‐3) would have at least taken 17107253.57 s (~4752 hours). Note also the significant increase in Failure % and increase in time per iteration in Table 2.0. Table 2.0 -‐ Results of the CA for IEEE Case 300 and time taken for the simulation
IEEE CASE 300
Number of iterations Time(s) Number of Failures Failures/Iterations Time/Iterations
N-‐1 411 81.664 105 25.55% 0.198695864 N-‐2 168921 41623.488 75603 44.76% 0.246408013 N-‐3 69426531 17107253.57 -‐-‐ > Estimation
Conclusion
This paper performed contingency analysis on different IEEE test cases. The major limitation in continuing this analysis was the amount of time CA takes for larger systems (with more buses) and to perform analysis in greater depth, i.e. (N-‐x) with x>3. This is due to the exponential time it takes for an increase in the depth of the analysis. Finding a better algorithm to perform this is an active area of research with efforts such as particle swarm optimizationvi, contingency screeningvii and many more. There is even an area of study on how unsolvable there problems can beviii. While this paper does not offer a solution to this complex problem, it shows us using standard cases why this is a hard to problem and also an important one.
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Appendix Example code used to perform CA (This one is for N-‐1 and IEEE Case 30)
(This one is for N-‐2 and IEEE Case 300)
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References i R. Bacher, “Graphical Interaction and Visualization for the Analysis and ii Smith, Rebecca. "U.S. Risks National Blackout from Small Scale Attack." The Wall Street Journal. Dow Jones & Company, 12 Mar. 2014. Web. 15 May 2014. iii Brown, Richard E. Electric Power Distribution Reliability. Boca Raton, FL: CRC, 2009. Print. iv G. C. Ejebe and B.F. Wollenberg, “Automatic Contingency Selection”, IEEE Trans. on. PAS-‐98, pp. 97-‐109, Jan/Feb 1979. v Power Systems Test Case Archive. University of Washington, Department of Electrical Engineering, n.d. Web. vi H. Yoshida, K. Kawata, Y. Fukuyama, S. Takayama, and Y. Nakanishi "A particle swarm optimization for reactive power and voltage control considering voltage security assessment", ibid., vol. 15, pp. 1232-‐1239, Nov., 2000 vii G. C. Ejebe, G. D. Irisarri, S. Mokhtari, O. Obadina, P. Ristanovic, and J. Tong, "Methods for contingency screening and ranking for voltage stability analysis of power systems", Proc. PICA Conf., pp.249 -‐255 1995 viii T. J. Overbye, "A power flow solvability measure for unsolvable cases", IEEE Trans. Power Syst., vol. 9, pp.1359 -1365 1994