N-1, N-1-1, and N-2 Contingencies Explained

🔹 What is Contingency Analysis?

Contingency analysis is a critical part of power system planning and operations. It simulates the impact of potential equipment failures (like lines, transformers, or generators) to ensure the grid remains stable and secure under realistic fault conditions.

✅ N-1 Contingency

This simulates the loss of any one single element (line, transformer, etc.). The system must still operate securely without violating thermal or voltage limits.

Example: If there are 100 lines, 100 contingency cases are evaluated where each line is taken out one at a time.

🔁 N-1-1 Contingency

This simulates the loss of one element, allows the system to re-dispatch or stabilize, and then simulates the loss of a second element. More realistic for operational security planning.

Example: Lose line L1, wait for reconfiguration, then lose transformer T2.

⚠️ N-2 Contingency

This assumes any two elements can fail simultaneously — more severe and used for high-security planning or post-blackout studies. Typically modeled for critical corridors.

Example: Evaluate failure of Line L1 and Transformer T3 at the same time.

📌 Summary

N-1: One failure, no operator action between
N-1-1: Two sequential failures with intermediate recovery
N-2: Two simultaneous failures — worst-case