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In operational practice power system security analyses are usually conducted with respect to "credible" contingencies, such as almost all N-l contingencies and some N-2 contingencies suggested by operational experience. On the contrary, extreme contingencies are generally not evaluated, although they can be responsible for major system disturbances and seriously jeopardize system security; within an in-depth analysis they should be analyzed. The paper presents an extensive analysis performed on a realistic network representative of the Italian High Voltage (HV) grid by means of a Dynamic Security Assessment (DSA) application tool integrated in a power system simulator. Besides large sets of N-l and N-2 contingencies, the aforementioned analysis includes also severe and extreme contingencies (in particular N-k contingencies at HV system nodes) and it also evaluates the effects of protection malfunctioning on system security. The contingency list has been created by an ad hoc automatic external procedure linked to the power system simulator. Results highlight the importance of the above extensive analyses also in view of risk- based approaches to security evaluation.