Research On Frequency And Damping Of AC System Sending Through VSC-HVDC

High Voltage Direct Current(HVDC)transmission is recognized for its ability to efficiently transfer large capacity power over long distances.This is of great importance due to the large separation distance between the majority generation source and load center which is located in the " northwest,north,and northeast China,and“east China coastal area,respectively.Rapid development of power electronic device also has a major impact on the promotion of Voltage Source Converter(VSC)HVDC technology as the VSC can support the AC voltage and control the exchange active and reactive power with the AC network.In recent years,there has been an increase in installed capacity rate of the new renewable energy generation in China,AC network with new renewable energy generations integrated with converters has low inertia problems.This is because the new renewable energy generations can not response to the local AC network’s frequency change and the system inertia will be further decreased due to decoupling effect of the HVDC link.In addition,the power electronization of power system will also reduce the affective damping of AC network.As a result,the AC network is susceptible to wide band frequency oscillation.In this thesis,VSC-HVDC will utilize proper designed controllers to provide frequency and damping support for the connected AC network.To increase AC network’s effective inertia such that converter have a better response to frequency change,a source of energy should be found.Under large disturbance the transient period is too long,and no energy storage device in the system is large enough to support the frequency of AC network."p-f droop control"is used to change the power reference of the HVDC system based on the generator’s"p-f curve" such that energy can be extracted from the grid in the other terminal to compensate for energy required by the low inertia system.This thesis uses the Model Predictive Control(MPC)algorithm for the control system design of the rectifier and inverter and the DC system model is built using MPC.MPC algorithm has the advantages of simple structure,fast dynamic response and good robustness.It is easy to realize the frequency fluctuation suppression of low inertia system connected with VSC.This thesis analyzes the suppression effect of VSC-HVDC using MPC based on the "P-f curve".Verification is further conducted via PSCAD simulation platform by comparing the effect of traditional PI double loop control and MPC under system frequency change.The simulation results confirm that VSC-HVDC with MPC has a better suppression effect on the frequency magnitude.The system frequency recovery period is reduced.Two terminal VSC-HVDC transmission system can he extended to VSC multi-terminal direct current(VSC-MTDC)transmission system by adding new landing points.P-f droop control can be used in the converter connected to low inertia AC system to suppress the frequency change.To further reduce the frequency change and fast adjust power balance between terminals,this thesis proposes an adaptive droop control method.The adaptive droop control can utilize the converter capacity to a large extent and reduce DC voltage fluctuation.Verification is conducted via PSCAD simulation platform by comparing the effect on the frequency and DC voltage of the master-slave control,droop control,and adaptive droop control.The verification results confirm P-f droop control can suppress the AC system frequency change,and adaptive droop control can further enhance the suppression effect while also accelerate the response speed of active power regulation and reduce DC voltage fluctuation,so that increase system stability.The wide band frequency oscillations include the low frequency oscillation and sub-synchronous oscillation between 0.1 Hz and 50 Hz.These two kinds of oscillations has lower time constant compared to the frequency change of low inertia system,such that the energy required for compensation is relatively small.Using adequate control method the energy stored in the Modular Multilevel Converter(MMC)submodule(SM)capacitors can be utilized to compensate for the AC network when wide band frequency oscillations occur.Traditional control method of the MMC includes power and voltage control in the upper level control and circulating current control in the lower level control.The energy control of MMC can use DC component of the circulating current to manipulate the energy stored in the SM capacitors flexibly.In the traditional way,the DC system can utilize a power oscillation damping(POD)controller to provide additional damping for its connected AC network.However,the DC power rating will also oscillate according to the POD output.In order to ensure the stability of the DC power,this thesis proposes the energy compensation control based on the energy control and the useable energy from SM capacitor is mathematically analyzed.A two-machine system sending through MMC-HVDC is conducted via PSCAD/EMTDC simulation platform,where the proposed energy compensation control is employed on the rectifier.The simulation results confirm that the POD energy compensation control can successfully decouple the power transfer of the two terminals of DC system transiently.This decoupling process also allows the proposed POD energy compensation control to eliminate the oscillation increasing effect with traditional POD control on a three-machine system with embedded HVDC,and the superiority of POD energy compensation control is proved.Traditional POD control provides damping support to the connected AC system cooperating with the active power controlling converter.As a result,the POD controller can only be implemented on one side of the HVDC link.Unlike the traditional POD controller,the proposed POD energy compensation control can also be implemented at the voltage controlling terminal as the DC system is decoupled,and composes multi-POD energy compensation control with active power controlling terminal,which can provide more damping to the AC system and is more effective for the system with embedded IIVDC.Based on the proposed multi-POD energy compensation control,the root locus of three-machine system with embedded IIVDC is analyzed,and a detailed simulation model is established in PSCAD simulation platform to verify that the multi-POD energy compensation control has better damping characteristics.A common concern in power system stability is the sub-synchronous oscillation.Traditional solution for this problem is the use of a sub-synchronous dalping controller(SSDC)in a HVDC system.The SSDC provides damping for AC system by making modification of the DC power reference.However,oscillation as result of DC power reference change will also be transmitted to the AC system iin the other terminal.In order to compensate the energy need for the oscillation suppression and decouple DC system transiently,proposed SSDC energy compensation control can be employed.This control can utilize the energy stored in MMC SM capacitors to make up for the energy demand of SSDC.This thesis designed the paremeters for SSDC and energy compensation control based on the first standard IEEE sub-synchronous oscillation system.The controllers arc further verified via simulation testing using PSCAD simulation platform.All of the above dmonstrates VSC-HVDC can provide frequency and damping support for the AC system.As a result,systm stability is improved.