Research On The Control Of ±1100kV UHVDC System With Hierarchical Connection Mode

With the voltage classes of AC and HVDC transmission system upgrading, the ±1100kV UHVDC system will adopt the hierarchical connection mode owing to the limitation of insolution requirement and volume restriction. The hierarchical connection mode means that the ±1100kV UHVDC system connected to 500/1000kV AC system hierarchically. Different with the traditional bipolar HVDC project, the characteristic of hierarchical connection mode, which is that the inverter side of DC line is connected to more than one AC system, makes the control of it be more complex. On one hand, the power flow of converters have to be controlled independently, on the other hand, the coordination of current orders from VDCOL becomes more complex. In view of this two aspects, a kind of additional power flow-voltage control and VDCOL coordinating control is proposed in this paper. The former control is designed to enable the converters to adjust the power flow in accordance with the needs of AC systems connected to them. The latter control makes sure the system choose the right current order from VDCOLs in both normal and fault operation conditions. In the last part, a ±1100kV UHVDC system connected to 500/1000kV AC system hierarchically, is built based on PSCAD/EMTDC simulation software. The simulation results show that, on one hand, the additional power flow-voltage control based on converter voltage difference is suitable for the UHVDC system with hierarchical connection mode, on the other hand, the VDCOL coordination control is good for the recovering of system after the failure.