Stability Analysis Of High Voltage Direct Current System

To keep HVDC transmission in stable and reliable operation is the most basic andimportant requirement. Transformer's dc bias by HVDC's monopole operation mode, thebackground harmonic and various disturbances in power system will make HVDC operate inunrealistic circumstance, which can lead to the breakdown of HVDC's stability. The paperfocuses on the mutual relationship between ac and dc systems, aims at the HVDC's stabilityand the following problems are studied detailedly.(1) The influence of inverter's positive slope characteristic on HVDC stabilityThe original purpose to use positive slope characteristic is to eliminate the multiplecross-point between the rectifier and inverter voltage-ampere characteristic. This paperproves that from the viewpoint of small signal inverter's positive slope characteristic makesthe control system satisfy the stability requirement unconditionally compared with negativeslope characteristic, thus no need to demand any requirement on the parameters of controlsystem and primary circuits.(2) Small signal stability analysis of HVDC based on quasi-steady modelOn the basis of HVDC quasi-steady model, different state equation of small signalmodels of common-used control system are elaborated. Then the method of eigenvalueanalysis is used to analyse the small signal stability of the system. The influence of controlsystem parameters is presented. The model and analysis method can be helpful to theoptimization of control system parameters.(3) The impedance-frequency characteristic of HVDC converter and harmonic instabilityHarmonic instability has already led to several HVDC project shutdown in the pastdecades, while the impedance–frequency characteristic is important for analysis and solve thisproblem. The paper analyses the working course of converter and presents a method tocalculate the impedance-frequency characteristic on the basis of switching function. Themethod can determine the converter's impedance-frequency characteristic seen from the dcand ac side, then the ac and dc circuits are converted to the same frame, and thus the harmonicinstability problem can be studied easily. Simulation results show that the method is quiteeffective.(4) The impact of ac system single-phase earth fault on HVDCWhen ac system fault occurs, the stable operation of HVDC can be destroyed.Single-phase earth fault is very common in ac system and the paper analysis the dynamiccourse of converter under this fault. To calculate the non-characteristic harmonic, the paperpresented a so-called promoted switching function which solved the problem that switchingfunction cannot be used to unsymmetrical operation condition of valve. The method can beused as auxiliary means for harmonic protection in HVDC control and protection system.(5) Optimized grounding resistance configuration against dc bias of transformerThe amplification of harmonic and bus voltage distortion in commutation bus bytransformer dc bias will deteriorate the operation condition and easily lead to harmonicinstability. The traditional cure measures for transformer dc bias are to install suppressiondevice at the happening place only. Aimed at this, the paper presented an optimized configurationmethod of grounding resistance to make all the currents through the transformer neutral point inthe grid within its limits on the basis of adjoint network sensitivity analysis. The proposed methodis verified by the data from Southern China Power Grid, the verification results show that allneutral currents are limited within the specified value, thus the dc bias of transformer is avoided.