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You can review concepts of power flow and then contingency analysis (on the net or from books on power systems.)

The power flow run will check if the electric network reaches or not a secure, operable state. Typical mismatch tolerance is 0.1 MW and could be as large as 2 MW for contingency studies but never 200 MW. If the load flow does not converged to the mismatch tolerance during pre-contingency or post-contingency, then it is an indication that the total generation of P & Q does not match total consumption of P & Q and it is recommended to resolve such issues. On the extreme, non-convergence of the network could mean a voltage collapse condition.

Before a contingency is applied, a load flow is performed and only if it converges then overloads and voltage violations are checked to assure an acceptable operable state ("base case"). In post-contingency, the effect of a contingency on the network is checked with a load flow solution.

ACCC is a tool to allow processing many contingencies in an automated way that include an internal step in "solving the load flow" in post-contingency. In the API manual, there are additional python functions (pssarrays) to process the results.

In your case, it seems your base case is not good enough to perform contingency analysis.