Active control of medium-voltage cascaded three-level neutral-point-clamped converters

Chen, Jinlei 2022. Active control of medium-voltage cascaded three-level neutral-point-clamped converters. PhD Thesis, Cardiff University. Item availability restricted.

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Abstract

Three-Level Neutral-Point-Clamped (3L-NPC) converters have been widely used in the high-power motor drives. In recent years, a novel cascaded 3L-NPC converter has been developed and adopted in the ANGLE-DC project − a 30 MVA MVDC link demonstration project in North Wales, UK. This cascaded configuration provides exceptional waveform quality, modular design and a cost-effective solution to MVDC applications. Although the control strategy for a single 3L-NPC converter has been well established, control of the cascaded 3L-NPC converter is still under-researched. The potential challenges to control strategy design arising from their cascaded connections need to be specifically explored. In particular, due to the series DC connection, the voltage imbalance across 3L-NPC submodules (SMs) may occur and influence the system stability. This issue may occur in converter stations where power is controlled in either point-to-point or multi-terminal systems. Beyond the electric characteristic, thermal characteristic is also vital to the performance of system. Thermal imbalance of 3L-NPC SMs may occur in a cascaded 3L-NPC converter even the voltage and power are equally shared, which poses great challenges to the system reliability. To address aforementioned challenges, this thesis developed suitable control schemes for the cascaded 3L-NPC converter system and demonstrated their operation using a 30 kVA MVDC testbed based on the real ANGLE-DC project. The DC voltage imbalance was analysed through a small-signal model-based approach. Two DC voltage balancing methods with and without communications were presented. The PI-based method can automatically switch to the droop-based method upon failures of communication. The DC voltage imbalance of the cascaded 3L-NPC converter is further investigated in a three-terminal MVDC network in consideration together with the interactions of control characteristics between different converter stations and the power control accuracy. Then suitable control scheme was proposed. Multiple crossovers due to the interactions are avoided while DC voltage balance and power control accuracy are achieved as well. To mitigate the thermal imbalance, a thermal sharing controller was superposed on the DC voltage balancing controller to regulate the active and reactive power of each SM according to their individual junction temperatures. The thermal stresses are hence equally shared in presence of mismatched component parameters and cooling system failures. The effectiveness of presented methods in the thesis has been verified in MATLAB/Simulink simulation and experimentally validated.

Item Type:	Thesis (PhD)
Date Type:	Completion
Status:	Unpublished
Schools:	Engineering
Uncontrolled Keywords:	1)._______medium-voltage direct-current_____________________ 2). _____cascaded three-level neutral-point-clamped converters_________ 3)._________________active control___________________________________ 4).__________________distribution networks___________________________ 5)._________________stability________________________________ 6).__________________thermal control_________________________________
Date of First Compliant Deposit:	30 May 2023
Last Modified:	31 May 2023 10:00
URI:	https://orca.cardiff.ac.uk/id/eprint/160038

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