Accelerated switching function model of hybrid MMCs for HVDC system simulation

Abstract

An accelerated switching function model (SFM) of the hybrid modular multilevel converter comprising both fullbridge (FB) and half-bridge (HB) submodules (SMs) in each arm is presented for HVDC system simulation, where auxiliary circuits are adopted to represent all possible current paths during normal and fault conditions. The proposed SFM can represent the negative voltage generating capability of the FB SMs and the equivalent switching functions in the blocking states of the FB and HB SMs are also introduced in the proposed model to accurately replicate the potential charging of the SM capacitors, yielding improved simulation accuracy compared to other alternatives. In addition to the faster simulation speed, the proposed model accurately reproduces the converter behaviour during various operating conditions, including normal operation, AC fault, and DC fault, etc. The proposed SFMs are assessed in MATLAB/Simulink environment using both down-and full-scale HVDC links and the simulation results confirm the validity of the proposed model in terms of model accuracy and improved simulation speed. IntroductionAs modular multilevel converters (MMCs) typically use hundreds of submodules (SMs) per arm in HVDC application, it is extremely time consuming to simulate the whole system using detailed switching models (DSMs). To reduce computation time and accelerate the simulation speed, the average model (AM) [1,2] and the accelerated SM level model [3,4] have been researched extensively to evaluate MMC performance in normal operation and during faults.In [1,5], each arm of the MMC is represented by a controllable voltage source, coupled with a series connection of a controlled current source and a DC capacitor. Such an average model provides adequate representation of the MMC behaviour seen by the external circuits under normal operation. However, the current paths during DC faults are not provided. In order to represent the response of the MMC during faults, ideal switches and a thyristor are introduced into the average value model in [6]. All the converter SM capacitors are equivalent to one DC capacitor, which is disconnected from the DC side during a DC fault. As a result, the charging of the DC capacitors in the arms by the fault current following the blocking of the MMC cannot be represented, resulting in significant errors.In [2], additional switches and diodes are added to the average model to consider the influence of the lumped DC capacitor and the unidirectional characteristics of the freewheeling arm currents during a fault. However, such model is only valid when the SM capacitance is large enough to maintain near constant SM capacitor voltages. In [7], a modified average model using six capacitors (one for each arm) and additional auxiliary circuit was developed to provide an improved representation of the MMC behaviour during various operating conditions, including AC and DC fault scenarios.To represent the detailed behaviour of individual SM, SM level models are proposed to reproduce the capacito...

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