Research On Voltage Stability Evaluation Method Of AC/DC Power Network

With the application of High Voltage Direct Current transmission technology(HVDC),the voltage stability issue between DC system and weak AC system is more widely concerned.Power system voltage stability research faces two problems : one is approaching distance,that is how close is the current state of the system voltage to the unstable state of voltage?Second is the reason,that is why is it that the system has a stable problem?Where the voltage level is lower and where the nodes are weaker?In other words,the voltage stability indicator should be able to answer: once a large change in load causes the system to deteriorate,it should be able to estimate whether the grid can fail in an accident.If you need to prevent the voltage instability in advance,what can be done to improve the level of the voltage.So the voltage stability indicator should be able to give a measure of voltage safety and provide decision basis for improving operation capability of the system and preventing the occurrence of voltage collapse accident.This article is from the perspective of online monitoring,based on the local measurement,it research on a new type of static voltage stability index and the evaluation of the situation of voltage instability mode,and analyze the adaptability of the index when AC and DC conditions change.To reduce the excessive complexity of communication and control networks in the online monitoring process,first of all,this paper discusses how to build a static voltage stability index using load bus local measurements.Based on the analysis of the relation between the maximum power supply capacity of the load bus and the Norton current distribution of the equivalent system,the conclusion is drawn that the maximum power supply capacity is equivalent to the equivalent system where the Norton current is equivalent to the equivalent power supply itself and the external load respectively.Based on this conclusion,a static voltage stability evaluation index is constructed using only load bus voltage and current amplitude measurement information.The index does not need to monitor the phase and does not rely on external network equivalents.It greatly reduces the technical difficulty and the cost of system implementation and control.It is able to monitor the voltage stability of load bus voltage and the line capacity margin to the limit.It is easy to find the path of the grid's weak nodes and their associated transmission and distribution capacity crisis.In order to further study the applicability of the voltage stability assessment index under various variable conditions for AC systems,the paper comprehensively analyzes the variation of the indexes in the way of reactive power compensation,on load tap changer adjustment and generator excitation regulation.Through the simulation of the actual power grid data,it is proved that the evaluation index is valid in various typical variable conditions.On the basis of obtaining the stability index of voltage to reflect thesystem's steady state in real time,the real-time tracking problem of the index is studied when the load is abrupt.In order to study the applicability of the index in AC and DC power grid,a DC control system model is established.By changing the control mode of the DC system,the conditions of different DC transmission power are investigated,and the change of the index of AC load bus(bus station)is studied.The conclusion is that the index is also applicable to monitor AC and DC system voltage stability situation.Finally,through the combination of DC control,reactive power compensation,on load tap changer adjustment and generator excitation regulation,the validity of the voltage stability assessment index is verified by the simulation of the actual power grid data.