Large-Scale Solar Power Station DC Interconnection Topology And Its Simulation Studies

China is rapidly deploying large scale solar power plant national wide, with installed photovoltaic power generation capacity ranked world first in 2015. Coupled with environmental pollution, especially the need to control wide range haze, clean and emission-free photovoltaic power, becomes a national strategy for sustainability, with many photovoltaic power stations planned in the pipe line. Large-scale PV power stations are generally remote; transmission lines are required to connect the solar power stations to the electricity grid. It is challenging to integrate all these intermittent, weather dependent photovoltaic power into the grid, while maintain the security and stability of the grid.At present, voltage source based high voltage direct current (VSC-HVDC) transmission is increasingly used to integrate large scale renewable power plants into the grid. The technology demonstrates less grid spinning reserve needs and better stability. This thesis employs VSC-HVDC technology to integrate large-scale photovoltaic power plants, investigates various topologies, and simulates the schematics in PSCAD/EMTDC. The main achievements are given as below:1) The mathematical models of the photovoltaic power stations and VSC-HVDC system are analyzed. Simulation models are constructed based on grid-connected photovoltaic power station. The characteristics of PV station output power are modeled and compared, and the influence on PV station output power by changes caused variation of sunlight intensity are analyzed. A simulation case of the DC grid topology for a PV power station using double conversion is given that contains a DC-AC-DC dual-conversion process. Control algorithms over active power and DC voltage are applied and studied, simulation results show that DC voltage fluctuation is significantly reduced with reactive power compensation control.2) An integration plan for connecting a cascaded photovoltaic power station to grid via VSC-HVDC link is proposed, and is simulated in PSCAD/EMTDC. A control strategy for DC voltage-power control is presented in the thesis, with DC power control dead band and slop control to combat the PV module output fluctuation challenges. The proposed control strategy adjusts the dead band and control slop according to power fluctuations of the photovoltaic power station. This avoids unnecessary controls due to DC voltage fluctuation caused by white noise, or temperately random changes, and is proven be able to maintain grid stability during simulation studies.3) A topology of connecting photovoltaic power station to grid via DC/DC converter is designed, and its simulation models are established. By combining voltage-power coordinated control of the VSC-HVDC system, together with controls of each PV sub-system in the station; it is able to combat with the voltage fluctuation at the interconnection point. It must be noted that it is necessary to reduce the DC Terminal voltage deviation of each PV sub-system, so that the PV station can maintain voltage within security range.