Studies On Effect Of Load Characteristics On Voltage Stability Of Power System

Large power grid,extra-high voltage,long distance and AC/DC hybrid interconnect are the direction of the modern power system,the safe and stable operation of power system suffers from more and more tests.Power system stability includes two basic aspects that are the angle stability and the voltage stability.Now the theory of the angle stability is relatively mature,however the study of the voltage stability still has some disputes and deficiencies.The voltage stability is one of main factors affecting the power transmission,whose study has suffered from more and more attention.Load impedance modulus margin evaluation index based on Thevenin equivalent method has theoretical defects.The analysis shows that the injection power of PQ nodes do not have the continuous driving ability in power system.And the load branch is tightly coupled with the power system.When the internal disturbance of the power system causes the node voltage fluctuation,dynamic equivalent impedance of power system is always equal to the load static equivalent impedance.At this time,the voltage stability of the node is analyzed by the impedance modulus margin index will get the wrong conclusion.In order to analyze the influence of the load voltage static characteristics on the voltage stability,the load power components related to the voltage must be decomposed and merged into the power system,so it is not meaning that the load branch is regarded as whole element.The reference axis of online phasor measurement shift form the traditional Thevenin equivalent potential to injection node current,the relative comprehensive dynamic equivalent method is proposed.The resistance margin index that evaluates the static angle stability of PV node is defined,it is only necessary to measure the dynamic relationship between the voltage phasor and injection current phasor of the node to evaluate the static angle stability of the node.Because PV node has voltage support ability,the injection active power of the node has no connection with the voltage amplitude,the power supply or load branch of the load can be regarded as a whole element,which is the essential difference between the resistance margin index and the impedance modulus margin index.For the case that voltage fluctuation of PV node is larger,the generator branch is equivalent to the model that the dynamic potential E' after the dynamic reactance X',which is similar to the local linearization dynamic parameters in nonlinear DC circuit.The regulating characteristic of the reactive power-voltage of the generator is described by the dynamic reactance and potential.According to this,the resistance margin is proposed that evaluates the static angle stability of the generator internal potential.The voltage stability of PQ node with no voltage support is further studied by using the dynamic analysis method,which the regulating characteristic of the active power-voltage of the load is described by cascading the additional resistance and the load dynamic reactance,however to strong static characteristic of reactive power-voltage can be described by identifying dynamic reactance and potential.The general resistance margin index evaluating the voltage stability of load node is proposed,which can describe voltage static characteristic of the load.The simulation results verify the influence of the ZIP load model and the induction motor load model on the voltage stability of the power system.