LCC-HVDC Bipole System with MMC-Based DC Tapping Stations
Creators: Persand, Manish
Publisher: name: University of Alberta Library
Year: 2022

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