Thesis

Control of an H-bridge modular multilevel converter for reliable operation of DC transmission systems

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Awarding institution
  • University of Strathclyde
Date of award
  • 2014
Thesis identifier
  • T13768
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Abstract
  • This thesis presents improved modulation and control schemes for an H-bridge modular multilevel converter (MMC) that can be used to enhance the transient response of DC transmission systems. The schemes enable H-bridge MMC cell capacitor voltages to be regulated independent of the DC link voltage in a DC transmission system, and also permit operation with variable DC link voltage, down to zero voltage, with full control over active and reactive power exchange. The proposed schemes also offer protection functionality during a pole-to-pole DC fault by restraining the DC fault current magnitude in the converter arms to a level compatible with the current rating of the converter switching devices. The modulation and control schemes use the perturbations in the cell capacitor voltages and common mode currents of an individual phase to eliminate the second-order harmonics from each converter arm. This is achieved without a dedicated controller for suppression of the second-order harmonics. The validity of the proposed modulation and control schemes is confirmed using simulations and experimentation in open and close loop using a scaled down H-bridge MMC. Their viability in DC transmission systems is assessed using simulation of point-to-point and multi-terminal DC networks; this includes power transmission with reduced DC link voltage and survival from permanent and temporary DC faults with DC link controlled recharging following fault clearance. The major practical implication of the proposed modulation and control schemes is that they offer the possibility for voltage source converter based DC transmission systems to ride-through DC faults without the need for expensive and fast DC circuit breakers, as is being pursued by HVDC manufacturers. This thesis demonstrates the possibility of operation without converter blocking, without risk of converter damage from excessive current stresses. In this manner, a converter station of the DC transmission system can be used during a DC fault to provide voltage support to an AC grid.
Resource Type
DOI
Date Created
  • 2014
Former identifier
  • 1033038

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