A novel contingency screening and ranking index for grid-forming converter-based sources following generator outage

With the current trend, the power generation in the future power grids is expected to consist of a large number of converter-interfaced generations (CIGs) with a small percentage of synchronous generators (SGs) producing power from hydro, solar thermal, or even nuclear resources. These notable changes pose significant challenges to the secure operation of the power grid. One such issue is new cascading failure mechanisms introduced by CIGs, especially due to the dc-link voltage collapse in a dc-current limited grid-forming converter (GFC) following a generation outage. Therefore, in the online dynamic security assessment (DSA), the system operators would be interested in knowing the critical outages that may trigger this cascading mechanism in the system. To address this issue, this paper presents a contingency screening and ranking approach (which can be a part of a DSA) for generator outages in a modern grid that consists of SGs and GFCs. Our specific focus is to identify the critical outages that may lead to a cascading failure in the system due to a dc-voltage collapse in the GFCs. To that end, first, a novel contingency screening and ranking index for an (N-1) contingency scenario (i.e., outage of a single generator) is derived based on the traditional generator power tracing algorithm. Then, it is shown that the proposed ranking index can be extended to any (N-k) contingency scenarios. Finally, the effectiveness of the proposed approach following generator outages (up to (N-2) contingencies) is demonstrated using detailed time-domain simulations in two different modified test configurations of a 16-machine 68-bus New York-New England test system with GFCs using the MATLAB/Simulink platform.