Research On Load Control Of Static Voltage Stability Based On Impedance Modulus Margin

With the development of economy,the scale of power system is continually expanding,the voltage level of transmission is continuously improving,the regional transmission capacity is continuously enhanced,and nationwide interconnection pattern is formed basically.Now,power system is playing a more and more important role in economic and society field.A safe,reliable,high quality,economic power supply has become an important support for China's energy security.Although the interconnection of large power systems could improve the efficiency of optimal allocation of resources,but the operation and control of the system need higher requirements.Nowadays,many voltage instability accidents occur,which promotes power experts and scholars pay more and more attention to voltage stability problems.Therefore,it is significant to study the voltage stability and measures to prevent voltage collapse.This paper firstly introduces the background and significance of voltage stability,the definition of voltage stability,the history and current situation of voltage stability research.In the field of voltage stability,many researchers focus on the mechanism of voltage instability,voltage stability analysis method,evaluation index and control measures,which are summarized in the paper.As we all know,the power system is the non analytical complex power system.This paper uses Thevenin equivalent principle as the theoretical basis,defines power system comprehensive dynamic equivalent impedance.Then the paper proves the necessary condition of the maximum power transfer in power system is that the system of comprehensive dynamic equivalent impedance module is equal to the load static equivalent impedance model.Then the paper puts forward the impedance modulus margin index to evaluate system voltage stability degree,which could identify weak node in the system.The impedance modulus margin index can indicate the distance from the point of voltage collapse to the impedance modulus margin.The minimum value determines the overall level of system voltage.IEEE 14 node system and IEEE30 node system test the correctness of the method.When the voltage of the power system is unstable,we must take measures to improve the voltage stability.In this paper,the voltage stability of the system is improved by using the load shedding method at a low voltage,and a set of load control scheme is provided,which could define the location and quantity.We can use the impedance modulus to monitor voltage weak point in the system.When the impedance modulus margin value is lower than the threshold value,the lower location is given priority for load shedding.As to the quantity,by tracking the comprehensive dynamic equivalent impedance to calculate,we can gradually approach the optimal load shedding amount,which can effectively prevent the over cutting.This scheme can control flexibility and selectivity in the site selection.When load disturbance form and the operating mode of power system change,the scheme still has good adaptability.In the end,the IEEE 14 node system and the IEEE30 node system are simulated to verify the practicability and correctness of the method.