Application of N-1 Contingency Analysis

The Philippine Grid Code (PGC) regulates that transmission systems must be N-1 compliant. Any single component in outage must not result to any thermal overloading of lines, which means above 100% of MVA rating, and no voltage limits will be violated whether overvoltage or undervoltage. Voltage limits are 0.95 per-unit voltage and 1.05 per unit voltage.

In this post, we demonstrate how to apply this application to the Luzon Approximate Network model though we will focus only on the thermal loading of lines. I am assuming any voltage violations can be eliminated by transformer tap change or capacitor switching at the substations of power plant switchyards.

In Powerworld, there are two ways, I can think of, to do this. Open the case and manually switch of any branch then solve the case. Or use the automated way to apply contingency analysis.

Before we go further, here are initial evaluations needed to undergo: (1) no existing overloads in the pre-contingency, (2) no existing voltage violations, (3) generating machines are not over dispatched.

In the Philippines, the substation configuration is usually a breaker and a half arrangement. With this assumption, the single branch contingency or N-1 is adaptable. In other power systems abroad, there are design contingencies which will trip several lines, not only one branch, for an N-1 set-up. These contingencies are called design contingencies. Also, any bus (outage of 1 or more lines) or breaker fail (outage of one or more lines) contingency are not considered in the N-1 analysis at least in the Philippines.

Here are the steps for N-1 analysis:

1.    Open the case in Powerworld
2.    At Edit Mode or Run Mode, in Case Information palettes go to Limit Monitoring to check existing pre-contingency thermal overloads. Identify overloads at the column of % of Limit Used. Redispatch generation to correct pre-contingency overloads, if possible. If no pre-contingency overloads are present, proceed to N-1 analysis (Figure 1).

Figure 1.

3.    At Run Mode, go to Tools. Click on Contingency Analysis, a new window will open (Figure 2). In Contingencies, click Auto-Insert to create contingency to be applied to the power network. Another window will open then click Do Insert Contingencies, notice the options of contingency combinations (Figure 3).

Figure 2.

Figure 3.

4.    Program will ask you to confirm 92 contingencies to be created (Figure 4). Click Yes. This will create the contingencies and will input the contingencies in the window (Figure 5). Notice Status as Initialized.

Figure 4.

Figure 5.

5.    Click Start Run to begin N-1 contingency analysis.
6.    When the simulation is finished, at the left bottom of the window, there is a status update for the finished contingency calculations (Figure 6).

Figure 6.

Figure 7.

7.    To view results, click Combined Tables, then to Legacy Tables ' click Contingency Definition-Violation Table' Copy to Clipboard. When finished, paste in notepad or any MS Word type program. You can also view result per contingency when you click at any one of the contingency at the window (Figure 7).

If I am working on this I will analyze the results and even apply the contingency manually and observed the resulting overloads. This means, I don't depend on the software's results but "analyze" the results. Why do the overloads happen? What were the pre-contingency loadings at the overloaded elements? Why is the contingency credible enough to produced those thermal violations? If in operations planning horizon, what are the actions needed to mitigate the thermal overloads?

In doing N-1 analysis, don't let the power system engineer be an "N". Meaning, he must not let himself be taken away by the software and he must not depend fully on the software's results but he is to gain more understanding on the network when examining the results.