Check the network survives outages

What it tells you. An N-1 analysis takes each line and transformer out of service in turn and runs a load flow. It shows which outages overload a branch or push a voltage outside its band. Use it to see whether the network stays within its limits after losing any one element.

Before you start

Check that the network has the data below. Select an element to see its data in the inspector.

The analysis judges a branch by its loading, which is its current or power as a share of its rating. A line with a Rated current of 0 has no loading, so the analysis never finds it overloaded.

Set up the study

1. Select the Calculate tab.
2. Under Security, select Generator outages to take each machine out in turn too. It is off by default.
3. To skip outages that are clearly safe, select Screen outages first.
4. To change the other settings, press Ctrl+, to open Study case.
5. Scroll to the Contingency analysis heading and change the settings you need.
6. Select Apply.

The settings are in this table.

Screening saves time on large networks. It still solves in full every outage that screening cannot clear.

Most models drawn in PowerStudio have 1 rating for each branch. Leave Time to act on an overload at 0 s unless an imported model carries temporary limits.

Add contingencies of several elements

Use this for elements that fail together, such as both circuits of a double line.

1. On the Calculate tab, under Security, select Contingencies.
2. Under Contingencies, select Add contingency.
3. In Name, enter a name for the contingency.
4. Select Add line, transformer, machine or busbar, then type or pick each element that fails together.
5. Select Apply.

A busbar in the list stands for a fault there. It takes out everything connected to that busbar.

To keep your contingencies, select Export in the dialog. To read them back, select Import….

Add a remedial action

A remedial action is a step an operator takes after an outage, such as shedding load. See remedial action.

1. Select Contingencies under Security.
2. Under Remedial actions, select Add remedial action.
3. In the name box, enter a name for the action.
4. Under After, add the outage the action is for. Leave it empty to apply the action after every contingency.
5. Under When all of these hold, set the condition. For example, choose Loading of, pick a line and enter 100 %.
6. Under Do, choose the action and its element. The actions are Take out of service, Put in service, Set generation of, Set tap position of and Shed load.
7. Select Apply.

After a contingency, an action whose conditions all hold is applied. The case is then solved again, and the results show the state after the action.

Run it

1. On the Calculate tab, select Contingency, or press Alt+N.
2. Wait for the progress bar in the status bar to finish.

The Contingency tab opens in the results panel. The Output tab logs how many contingencies ran and how many have new violations.

To stop the analysis, press Ctrl+.

Read the results

The top of the Contingency tab shows a pill and a summary.

• The pill says how many contingencies have new violations, or Secure under every contingency. It adds the number that are not solvable.
• Contingencies is the number of cases run.
• Screened shows how many outages the quick method cleared, when you use screening.
• Remedial actions on shows how many cases an action changed.
• Loading limit is the limit you set.
• Base case max is the highest loading before any outage.

A violation is new when the base case does not already have it. The pill counts only new violations.

Use the buttons on the right to switch between the tables.

Result says Secure, a number of violations, or Not solvable. Not solvable means the load flow found no solution after that outage. Violations already in the base case are listed too, in amber.

The Remedial actions column appears when an action ran. Hover over a cell to see how many violations there were before the action.

The diagram shows the worst result for each element. See Work with results for the filters and the chart.

Check your results

• Do it by hand. Select a line, press Shift+O to switch it out, then run a load flow. The loading should match Max loading for that outage.
• Neighbours. The Most loaded branch is usually next to the one you took out. If it is far away, look at the network around it.
• Count. The number of contingencies is the lines plus the transformers, plus any machines, busbars and contingencies of your own.
• Base case. If Base case max already exceeds the Loading limit, the network is overloaded before any outage.
• A tighter limit. Lower the Loading limit to 80 % and run it again. On the IEEE 14-bus sample, 13 contingencies then have new violations.

If something goes wrong

Try it on the IEEE 14-bus sample

1. Select the Help tab.
2. Under Samples, select IEEE 14-bus system.
3. Select the Calculate tab, then select Contingency.
4. Read the pill. It says 10 contingencies with new violations. The summary shows 20 Contingencies, a Loading limit of 100 % and a Base case max of 76.4 %.
5. Select the Max loading heading until the highest value is at the top. Line 1-2 is first with 181.4 %. Its Most loaded branch is Line 1-5.
6. Select the Branches button. Line 1-5 has a Worst loading of 181.4 % under the outage of Line 1-2. Line 9-10 reaches 137.9 % under the outage of Transformer 5-6.
7. Select the Busbars button. The lowest voltage is 0.9935 p.u. at Bus 5, under the outage of Line 1-2. No voltage falls below its band.

The 10 contingencies with new violations are these outages:

• Transformer 5-6, Transformer 4-7 and Transformer 7-9
• Line 1-2, Line 1-5, Line 2-3 and Line 4-5
• Line 9-10, Line 9-14 and Line 10-11

Three of them load a branch above 110 %: Line 1-2, Transformer 5-6 and Line 1-5.

Bus 6 and Bus 8 are above their band in the base case. Most rows list them, but they are not new violations.

To check the first row by hand, select Line 1-2 on the diagram and press Shift+O. Run a load flow. Line 1-5 shows 181.4 % loading. Press Ctrl+Z to undo.

Add generator outages

Under Security, select Generator outages and run it again. The count rises to 25 contingencies. The number with new violations stays at 10.

Try a remedial action

1. Select Contingencies under Security.
2. Under Remedial actions, select Add remedial action.
3. Under After, add the outage of Line 1-2.
4. Under When all of these hold, set the condition to Loading of, choose Line 1-5 and enter 100 %.
5. Under Do, choose Shed load, select Load 3 and enter 60 %.
6. Select Apply, then run Contingency again.

Remedial actions on shows 1. For the outage of Line 1-2, Max loading falls from 181.4 % to 125.0 %. That is still above the limit.

Change the share to 100 % and run it again. Max loading falls to 93.2 % and no new violation is left. Shedding all of Load 3 takes 94.2 MW off the network. The example shows how to read the result. It is not a recommendation.

Try it on the Riverside sample

The Riverside sample has 10 contingencies. One has new violations: the outage of the CHP unit transformer. It takes the CHP unit out and leaves Hilltop at 0.948 p.u. and Brook Farm at 0.930 p.u., both below 0.95 p.u.

Select Busbar faults in the study case and run it again. A fault on Riverside 110 kV is Not solvable. It takes out the grid infeed and both primary transformers.

Read more

• Check machines stay in step
• Work with results
• Set up study cases
• The engine description of contingency analysis