Researsh On Power Grid Stability Of Ultra High Voltage DC Hierarchical Connection

With the development and widespread use of ultra high-voltage alternating current (UHVAC) and ultra high-voltage direct current (UHVDC) transmission technology, a novel UHVDC hierarchical connection mode is proposed. UHVDC hierarchical connection mode is helpful to increase the voltage support capability of multi-infeed HVDC system, and it is beneficial to enable the rational power flow distribution between the 1000kV layer and 500kV layer in the multi-infeed HVDC system.Due to massive UHVDC power, receiving-end grid is faced with lots of problems and challenges, including:how to build grid structure to ensure safe and stable operation of DC power transmission line near UHVDC inverter stations; which UHVDC control modes and which UHVDC terminal location should be chosen to maximize reactive voltage support ability; mutaual influences between alternatingand direct current failures after UHVDC hierarchical connection system is equipped; whether UHVDC hierarchical connection system is a threat to safe and stable operation of receiving-end grid; how to take full advantage of units leading phase ability. In response to aforementioned problems, this paper carries out following research:Firstly, the paper studies power flow evacuation problem in the vicinity of UHVDC inverter stations based on BPA data of actual power grid. This includes load condition of lines in the vincinty of UHVDC inverter stations and the effect of UHVDC power distribution and terminal location on power flow distribution.Secondly, the paper deduces the model of UHVDC hierarchical connection system based on traditional single layer UHVDC model, develops a voltage stability factor calculation method of UHVDC inverter side bus, proposes a static voltage stability index for UHVDC hierarchical connection system and analyzes the effect of UHVDC control modes and terminal location on voltage stability on inverter bus.Then the paper analyzes interaction of AC and DC faults at receiving-end in UHVDC hierarchical connection system, introduces a hierarchical connection interaction factor (HCIF), verifies HCIF validity with PSCAD model of UHVDC hierarchical connection system and applies the factor in interaction analysis.Finally the paper expounds the importance of units leading phase operation to UHVDC hierarchical connection system, proposes a simulation method of units leading phase operation considering receiving-end grid voltage, unit operation condition and auxiliary power load.