Study On Optimal Control Strategy For VSC-HVDC System Considering Stability Improvement

With the development of power electronics and control technology, voltage source converter based high voltage direct current (VSC-HVDC) has been widely used in many fields, which has various advantages including supplying power to the weak AC power grid and even passive networks, requiring no communication between converters, easy to establish multi-terminal HVDC transmission networks and controlling the active power and reactive power of the converters rapidly and independently. These advantages make VSC-HVDC widely used in many fields such as large-scale wind power integration, power grid interconnection, urban distribution network capacity increase and power quality improvement. while some problems still exist: 1) the inner current control is difficult to be completely decoupled in the traditional control methods; 2) power system transient instability is easily caused when the system is subjected to large disturbances, hence the system transient characteristics still need to be improved. Thus this paper conducted the following work:Firstly, this paper apply the idea of synchronverter to VSC-HVDC, which is defined as, synchronverter based HVDC (SHVDC). In SHVDC system, the rectifier and the inverter are respectively regarded as an equivalent synchronous motor (SM)and an equivalent synchronous generator (SG), which are connected together by DC lines. With reference to the existing models of synchronous generator and motor,this paper establishes a two machine equivalent model for SHVDC system.Secondly, in order to avoid the problem that inner current control is difficult to completely decouple in the process of d-q coordinate transformation and take advertage of the moment of inertia and frequency and voltage modulation characteristics of synchronous generator to improve the transient stability of power system meanwhile,a controller with droop characteristics similar to synchronous machine is designed in order to replace the classic nested-loop d-q vector controller.Futher, for successfully joined into the grid, the optimization design of the controller is carried out.Finally, considering the Bang - Bang control superior than the conventional PID control under large disturbance and the quick power regulation ability of VSC-HVDC, in order to further improve the transient stability of power system with VSC-HVDC, an additional control strategy based on Bang-Bang principle is proposed,which adopts hybrid modulation signals generated by power angle variation of generators in the system.