Electromagnetic Transient Equivalent Modeling Method of MMC with Supercapacitor-based Energy Storage System

MMC-ESS(modular multilevel converter with energy storage system) has broad prospects on engineering application in the field of renewable energy consumption. However, MMC with higher levels has the problem of low efficiency in EMT(electromagnetic transient) simulation on offline simulation platforms such as PSCAD/EMTDC and Simulink, which may further decrease due to the addition of ESS. To address this problem, derived from Thevenin equivalent model and NFSSA(nested fast and simultaneous solution algorithm), an EMT equivalent modeling method of MMC-SESS(modular multilevel converter with supercapacitor-based energy storage system) is proposed. Specifically, a MMC submodule model containing a SESS is first established, followed by the construction of the corresponding companion circuit. During the process of simplifying companion circuit, information of internal nodes in submodules is transferred to external nodes to eliminate internal nodes, achieving matrix order reduction and obtaining the Thevenin equivalent circuit of the submodules. After completing one timestep of solution for submodule equivalent circuit, inverse solution can be conducted to obtain information of internal nodes in submodules. Thevenin equivalent circuit of bridge arms can be obtained by incorporating equivalent circuits of all submodules within the bridge arm. Due to the fact that the equivalent circuit of the bridge arm only contains two nodes, the increase in the number of levels has almost no influence on the computational complexity. A MMC-HVDC system consisting of MMC-SESS is built on PSCAD/EMTDC. The comparison with DM(detailed model) on simulation results shows that the proposed EM(equivalent model) can accurately simulate the characteristics of MMC-SESS, and has significant simulation acceleration effect.