Reserch On Static Voltage Stability Analysis Of Wind Farm Grid-Connected DC System

The wind power has many advantages, such as its renewable, non-pollution, abundant, wide distribution and so on. The increasing depletion and global environmental degradation caused by traditional energy sources make the wind energy favored by many countries. But China’s major wind resource is far away from load centers, so the power is transported via the network to load centers in a long distance. Compared traditional AC transmission and DC transmission, Voltage Source Converter of High Voltage Direct Current transmission(VSC-HVDC) has many advantages, such as having independent control of active and reactive power, supplying power for passive network, having small footprint, issuing reactive power, having economic benefits in merging the dispersion of small renewable energy power generation into network and so on. VSC-HVDC is more suitable for wind power. AS the increasing scale of the wind farm has more impact on the system voltage stability, so it is necessary to analyze voltage stability of the wind power system based on VSC-HVDC.Firstly, aiming at the existing AC-DC Unified iterative method has poor inheritance in AC program, heavy workload in programming as well as alternative and iterative algorithm does not consider the coupling relationship between DC and AC leading to the poor convergence, low-precision defect, this paper combined DC network equations and VSC control equations to make AC, DC subsystem decoupled completely and solved independently, thus forming an AC/VSC-MTDC hybrid flow decoupling fast algorithm.Secondly, aiming at Hybrid HVDC system which is the combination of the line commutated converter(LCC) and the voltage source converter(VSC) is particularly suitable for multiple wind farms connected to the grid network simultaneously, thus forming a AC/Hybrid-MTDC system. Based on the realization of decoupling between AC and DC subsystem, solved the flow of AC/DC subsystem independently and alternately through adding the coupling terms of LCC and VSC to AC subsystem Jacobi matrix, The algorithm has a good convergence and fast calculation speed.Then, the paper analyzes the static voltage stability of the AC/VSC-MTDC hybrid system. Local parameterization continuous flow algorithm which is based on AC/VSC-HVDC Hybrid Power Flow Decoupling fast algorithm is used to analyze AC/VSC-MTDC hybrid system voltage stability. The Algorithm has a fast calculation speed and takes VSC-MTDC system operation mode conversion into consideration.Finally, when the DC system communication nodes are removed, the traditional continuous power flow algorithm fails to calculate isolated wind farm communication node. Aiming at the defect, this paper proposed an improved continuous power flow algorithm containing VSC-HVDC, in which the wind farm is equivalent to PQ node, the reactive power is the function of the active power and node voltage in the wind power node, thus using the preceding AC/VSC-MTDC hybrid system static voltage stability analysis method to analyze the static voltage stability of wind farm grid-connected system based on VSC-HVDC.The proposed methodology has a good reference value for AC/VSC-MTDC hybrid flow calculation, static voltage stability analysis, AC/Hybrid-MTDC power flow calculation and static voltage stability analysis of wind farm grid-connected system based on VSC-HVDC.