Research On Steady Frequency Prediction And Automatic Load Shedding Control In AC/DC Hybrid Transmission Systems After Disturbnces

Affected by the distribution of energy and load,geographical features and many other reasons in China,HVDC transmission technology plays a crucial role in transmitting large capacity power to a long distance.With the gradual implementation of the energy strategy,the emergence of large power supply bases,the increase of HVDC transmission power,the safe and steady operation of power system becomes increasingly complicated and the problems related to AC/DC system safe and steady research are getting more and more attention.Frequency stability is an important part in AC/DC system safe and steady research.Using the capability of short time overload and fast adjustable power of HVDC system,the stability of AC system can be improved.The main contents in this thesis are focused on the steady frequency prediction and automatic load shedding optimal control considering emergency DC power support after disturbances.The main contents of research are as follows:Firstly,some basic concepts related to power system frequency characteristics including static characteristic and dynamic characteristic are introduced,and then several methods of the power system dynamic frequency analysis are reviewed,which are dynamic power flow,frequency stability analysis direct method and frequency prediction algorithm based on wide-area measurements respectively.Secondly,a steady frequency prediction algorithm for AC/DC hybrid power grid is proposed,which takes into consideration of the DC power support as well as the frequency response of the AC systems at both sides of the HCDC link after disturbances.With the establishment of generator electromagnetic power increment equation,load power increment equation,DC transmission system power increment equation and the system network equation between the moment right and steadily after disturbances as well as considering the emergency DC power support,the steady frequency prediction equation of two-area AC/DC hybrid transmission systems after disturbances is derived,which can calculate the steady frequency of every area in the two-area AC/DC hybrid transmission systems after disturbances quickly and directly.A typical two-area AC/DC parallel hybrid transmission system is simulated and the simulation results show that the proposed algorithm has high accuracy and rapid computation speed,which proves the applicability and effectiveness of it.The algorithm can be used in the field of power system frequency security and stability on-line assessment.Finally,a control strategy of frequency stability considering the DC power support for two-area AC/DC hybrid power grid is proposed.When frequency of one area in the two-area AC/DC hybrid transmission system decreases after disturbances,the control strategy will firstly predict the steady frequency and if the steady frequency is lower than the set value,the amount of DC power support to make the steady frequency restored to the set value will be calculated.If the amount of DC power support is larger than the maximum amount of power that the another area can provide,the linear optimization method will be used to calculate the least amount of loads that the area needs to shed so as to make the system frequency controlled to the set value.With the analysis of examples,the control strategy can calculate the amount of DC power support and the loads to be shed quickly and accurately,which satisfies the requirement of system frequency stability control and economic operation.