N-1 and N-2 Contingency Analysis of the Sfax Electrical Zone of the Tunisian High voltage transmission network

• Dual N-1 and N-2 security assessment under minimum and peak load scenarios for the Sfax high voltage region of Electrical Tunisian transmission grid • Identification of critical minimum and peak-load stress on 150 kV corridors and autotransformers • Extreme overloads and voltage-limit violations under N-2 conditions • Mitigation of N-1 and N-2 stresses through local 120-MW gas-turbines dispatch. • Short-term operational support and medium-term reinforcement needs • N-1/N-2 case study for the Sfax Zone of Tunisia using real operator data. This paper presents a comprehensive N-1 and N-2 contingency analysis of the high voltage transmission network supplying Tunisia’s Sfax zone. PSSE software and the Newton–Raphson approach were used to evaluate grid reliability under single and double outage scenarios. The modeled system comprises eleven buses, ten transmission lines, three 225/150 kV autotransformers, four load centers and three 120 MW gas turbine units. Real network data provided by the Tunisian Electricity and Gas Company (STEG) were used to construct representative minimum and peak-load scenarios. For each operating point, the post-contingency performance was assessed in terms of bus voltages, line and transformer loadings and compliance with normal and emergency limits. Results show that the Sfax network operates securely in the base case and withstands most N-1 contingencies, however, peak-load conditions significantly reduce security margins. Under N-1 peak-load scenarios, autotransformers ATR1 and ATR2 reach critical loading levels of 142% and 137%, respectively; particularly under N-2 conditions where certain double outages lead to extreme overloads exceeding 200%, voltage drops as low as 0.46 p.u., and a non-convergent AC power-flow solution, indicating a high risk of voltage instability and cascading failures. Additional analyses demonstrate that dispatching a single local 120 MW gas turbine eliminates all N-1 violations and mitigates the majority of critical N-2 scenarios, restoring acceptable voltage levels and significantly reducing transformer overloads. These findings provide a concrete short-term solution for system operators to enhance the security and resilience of the Sfax transmission network under stressed conditions.