Modeling of the DC Fault Response of Modular Multilevel Converters With Fault Control Capability Considering Converter Saturation

Abstract

Modular multilevel converters (MMC) with fault control capability, such as full-bridge and hybrid MMC, can maintain the operation of DC system by actively limiting the fault current. However, converters with these capabilities present complex fault characteristics, complicating the DC protection design. Although fault features can be analyzed by simulations, short-circuit current calculations are desirable as they give analytical expressions, which simplify the analysis of factors affecting the fault response and are suitable for large-scale applications. However, calculations become challenging when MMC arm voltage reaches the limitation sum sub-module capacitor voltage, a condition known as converter saturation, which is hard to represent with transfer functions. To clarify the fault characteristics during converter saturation, a DC fault current calculation method for converters with fault control capability is proposed. The features under fault control are analyzed and divided into different stages. Then the basic analytical equation based on control and fault circuit is deduced and the saturation impact is considered by adopting a short-time numerical calculation. The proposed method is validated in Simulink and a statistical similarity check shows 91% similarity between the proposed model and simulation results, versus 23% similarity when using a transfer function model that does not consider saturation effects.