Active control of DC fault currents in DC solid-state transformers during ride-through operation of multi-terminal HVDC systems

Abstract

When a pole-to-pole dc fault occurs in a multiterminal HVDC system, it is desirable that the stations and dc solid-state transformers on healthy cables continue contributing to power transfer, rather than blocking. To reduce the fault current of a modular multilevel converter based dc solid-state transformer, active fault current control is proposed, where the dc and ac components of fault arm currents are regulated independently. By dynamically regulating the dc offset of the arm voltage rather than being set at half the rated dc voltage, the dc component in the fault current is reduced significantly. Additionally, reduced ac voltage operation of the dc solid-state transformer during the fault is proposed, where the ac voltage of transformer is actively limited in the controllable range of both converters in the transformer to effectively suppress the ac component of the fault current. The fault arm current peak and the energy absorbed by the surge arrester in the dc circuit breakers are reduced by 31.8% and 4.9% respectively, thereby lowering the capacities of switching devices and circuit breakers. Alternatively, with the same fault current level, the dc-link node inductance can be halved by using the proposed control, yielding lowered cost and volume. The novel active fault current control mechanism and the necessary control strategy are presented and simulation results confirm its feasibility.

DC Fault Detection and Location in Meshed Multiterminal HVDC Systems Based on DC Reactor Voltage Change Rate

“…As MMCs typically use hundreds of submodules per arm in HVDC application, it is extremely time consuming to simulate the whole system using detailed switching models, which considers the switching behavior of the IGBTs/diodes. To reduce computation time and accelerate the simulation, average models are widely adopted to represent the MMC behavior [31][32][33][34][35]. It has demonstrated that such average models provide adequate accuracy for DC fault detection studies [12,13,32,33] and are thus adopted in this study.…”

Recent contributions and future prospects of the modular multilevel converters: A comprehensive review

“…In Li et al, a new active fault current control strategy is developed during pole‐to‐pole DC fault conditions for multiterminal MMC based DC SST application. Control methodology is explained for AC voltage and power control mode.…”

Improved equivalent circuit model of MMC and influence analysis of simulation time step

“…Due to modularity and scalability, modular multilevel converter (MMC) has become the most attractive converter topology for medium/high-voltage applications, especially for voltage-sourced converter high-voltage direct current transmission systems [1]. For large-scale MMC-embedded power systems, it is required to investigate dynamic performance, fault, protection, and stability [2][3][4]. Modelling of the MMC is one of the main challenges associated with the study of the large-scale MMC-based power systems.…”