Analytical Modeling of Modular Multilevel Converter Under Pole-to-Pole DC Fault and Application to System Design and Protection

An analytical model for the half-bridge submodule based modular multilevel converter (MMC) under pole-to-pole fault is formulated, implemented, and validated. The events after the fault occurrence are categorized into four stages, and the equivalent circuit of each stage is derived. A generic equivalent circuit is developed respectively for the analytical calculation of the AC grid currents and the circulating currents during the fault, taking into account the conducting states of the switching devices. With knowledge of those currents, further analytical expressions of the fault currents of the converter arms and the DC grid are derived. Since the analytical expressions of the fault currents are functions of the main circuit parameters and the prefault operating point, the analytical model of the MMC under the pole-to-pole DC fault offers an answer to an entire set of problems. Enhanced insight into the parameters influencing the fault currents is available with the help of the analytical model. As a promising application, the proposed analytical models are utilized for the parameter design of the converter and for the selection of the interrupting capability of the circuit breakers. In the performed validation involving the CIGRE B4 DC test system, the analytical results are shown to be highly consistent with those of computationally more expensive solutions based on numerical simulation.