LCC-HVDC Bipole System with MMC-Based DC Tapping Stations

• Author / Creator

  Persand, Manish

• Line Commutated Converter based High Voltage DC (LCC-HVDC) is the
  dominant technology of HVDC power transmission worldwide. LCC-HVDC
  has lower cost of implementation and lower power losses, accompanied by
  higher voltage capabilities and higher power levels, in comparison to Voltage
  Source Converter based HVDC (VSC-HVDC) technology. However, VSCHVDC
  has technical superiorities such as independent control of active and
  reactive powers, elimination of the risk of commutation failures, and drastically
  reduced size of harmonic filters. Combining the advantages of both the
  existing LCC-HVDC systems and newer VSC-HVDC technology, hybrid LCCVSC
  HVDC transmission systems are being developed. As LCC-HVDC lines
  span very long distances, one particular hybrid system of interest is HVDC
  line power tapping where a small amount of power is tapped using VSC technology.
  To date, most systems level research studies on HVDC tapping have
  focused on using DC-AC VSC stations implemented on monopole systems with
  simplified controls Moreover, few works explore HVDC tapping using DC-DC
  converters that can create intermediate medium-voltage DC (MVDC) output
  buses, which offer increased flexibility for connection to downstream DC-AC
  VSCs or even for renewable energy integration.
  This thesis develops a comprehensive tapping study system in RSCAD
  on an RTDS Novacor simulator, which consists of a ±500 kV 3 GW LCCHVDC
  bipole system, designed based on the existing 3-Gorges HVDC system,
  and includes two DC-DC tapping converters using modern Modular Multilevel
  ii
  Converter (MMC) technology, one connected at the middle of the HVDC line
  on each pole. The LCC-HVDC bipole system is modeled on the processor
  cores of the simulator. The two DC-DC MMCs are implemented using both
  processor and GT-FPGA based valve models: averaged MMC5 and detailed
  U5-MMC models for positive and negative pole tapping stations, respectively.
  The firing controls for the DC-DC MMCs are also modeled using the GTFPGA
  units.
  The LCC-HVDC bipole is rated at 500 kV, 1500 MW per pole and each
  DC-DC MMC is rated at 75 MW, designed with a 500/40 DC step ratio to
  create a bipolar ±40 kV MVDC output bus. Controls are provided to operate
  each tap independently. The resulting hybrid LCC-VSC system therefore
  offers significant flexibility for systems level tapping studies, owing both to its
  independent pole design but also the realistic LCC-HVDC controls and modes
  of operation. Simulations are carried out to study independent pole power
  tapping feasibility as well as bidirectional power flow scenarios involving the
  tapping stations and their effects on existing LCC systems. Fault Studies were
  also carried out. Different types of AC and DC line-to-ground faults were triggered
  on the rectifier and inverter AC networks and HVDC links respectively.
  In all fault scenarios, the whole hybrid LCC-VSC HVDC system recovers to
  its pre-fault operating modes once they are cleared.

• Subjects / Keywords

  • Power Flow Studies
  • MMC
  • Power Tapping
  • Hybrid VSC-LCC HVDC Bipole System
  • Fault Studies

• Graduation date

  Fall 2022

• Type of Item

  Thesis

• Degree

  Master of Science

• DOI

  https://doi.org/10.7939/r3-qz7n-5t73

• License

  This thesis is made available by the University of Alberta Library with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.