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

Abstract

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.