Research On Operation Characteristics And Coping Strategies Of HVDC System Under Disturbance Of Receiving-end Grid

The energy production and demand of China are characterized by reverse distribution.Energy resources are mainly distributed in the north,west and southwest regions.The central and eastern regions have huge demand for electric energy.However,due to the impact of environmental protection and land cost factors in recent years,and saturation of hydropowe in the middle and lower reaches of China's major river basins,t,it is difficult to build new power plants on a large scale.The above-mentioned status quo determines that China must implement the “National Networking,West-East Power Transmission,and Nortel South Transmission” cross-regional power resource optimization allocation strategy.The high-voltage direct current(HVDC)transmission mode has the advantages of high long-distance transmission efficiency,the ability to realize asynchronous connection between regional AC power grids,saving transmission corridors and flexible investment modes.HVDC has become an important means for long-distance cross-regional transmission.The various disturbances of the receiving-end power grid,including voltage distortion,various faults and transient voltage instability,will have an impact on the stable operation of the HVDC system.In view of the huge development prospects and technical advantages of HVDC transmission engineering,it is necessary to study the operating characteristics of HVDC systems under various disturbances of the receiving-end power grid,and propose improvement measures to provide technical support for the safe operation of the receiving-end power grid.This thesis focuses on the analysis of the operating characteristics of the HVDC system when it is disturbed by the receiving-end power grid,and the countermeasures of HVDC system,which provides support for the operation of the AC-DC hybrid power system.This paper mainly includes 4 points.1.Research on the operating characteristics of HVDC system under the converter bus voltage distortion.2.Research on the immunity of the commutation failure of the HVDC system for single-phase fault;3.Research on HVDC system commutation failure detection method basing on the local waveform characteristics of the direct current;4,Research on HVDC system emergency reactive power control method based on global orthogonal configuration-nonlinear model prediction control.(1)In need of the analysis of the operating characteristics of the HVDC system under the harmonic distortion of the converter bus voltage,this paper deduces the output characteristics of the phase-locked loop and the valve commutation characteristics when the inverter-side AC bus voltage is distorted.The mechanism of the influence of harmonic voltage on the constant extinction angle(CEA)controller of the inverter side is explained.It is explained that the existence of harmonic voltage will increase the value of ? order in the inverter side and reduce the transmission capacity of the HVDC system.In view of the harmonic distortion of AC bus voltage,the CEA controller can not track its setting value very well.In this paper,by reducing the gain of current error(CE)controller,the steady-state error amount of the CEA controller in AC voltage distortion is effectively reduced,thus reducing the loss of transfer capacity.Finally,in order to cope with the problem that the HVDC quasi-steady state model can not accurately describe the operating characteristics of the HVDC system under the influence of harmonics,a new calculation method of the steady-state operating point of the HVDC system under the influence of harmonics is proposed.Through the PSCAD/ EMTDC simulation,the correctness of the theoretical analysis,the effectiveness of the improvement measures and the accuracy of the calculation method are verified.(2)For the single-phase faults,which is the most common fault in power systems,index of local single-phase fault immunity(ILSFI)and index of multi-infeed system single phase fault immunity(IMSFI)of the multi-feed system are proposed.This method has considerable accuracy compared to the simulation method,while reducing the workload.A New short-circuit voltage calculation method based on a new type of HVDC quasi-steadystate model is proposed,as the traditioinal impedance method is insufficient in describing characteristics of HVDC converter station in AC fault,to obtaine the critical impedance more accurately.Based on the Cigre-Benchmark model,a three-feed system is constructed.The effectiveness and advancement of the proposed algorithm are proved by comparing the results of time domain simulation,impedance-method calculation and the results of proposed method.(3)The protection of the HVDC system in the simulation platform of the new generation control system needs to obtain accurate starting criteria based on the simulation results.Although there are some practices in the engineering of commutation failure detection,the criteria are more complicated and do not meet the needs of rapidity in simulation work.The method of commutation failure detection needs further study.Based on the distortion characteristics of direct current waveform before and after commutation failure,this paper proposes a commutation failure detection method based on the second-order vanishing moment Daubechies wavelet.Once it is detected that the wavelet coefficient reflecting the second-order and above derivatives of the direct current exceeds the threshold,and the direct current has a tendency to become large and no reverse occurs,the commutation failure is considered to occur.The method of this paper is based on the actual situation of direct current when the commutation failure occurs.It is more objective and accurate than the existing commutation failure detection method.It is more targeted to the commutation failure fault,and can quickly provide accurate information to the AC/DC grid stability control system.This method can also provide accurate criteria for the startup of the system protection for the next generation control system simulation platform.The time domain simulation results based on PSCAD/EMTDC prove the validity and accuracy of the proposed method.(4)When there is a rotating loadin the receiving-end power grid,the load bus voltage may continue to decrease after the AC fault,which may affect the commutation bus of the nearby converter station,thereby affecting the HVDC transmission capacity.The HVDC system is a huge reactive load for the receiving system,but at the same time,its fast modulation characteristics can provide reactive support for the receiving system in a short period of time.The current HVDC inverter station's strategy for emergency reactive power support for the receiving system is mainly based on qualitative analysis or empirical judgment,lacking quantitative calculation.At the same time,for the rapidity of voltage recovery,the generation of emergency reactive power support strategy needs to be accurate and fast.Based on the differential-algebraic equations of power systems,this paper studies the medium and long-term transient voltage instability problems in the HVDC receiving-end power grid with asynchronous motors as the main load.By reducing the direct current,the converter station is used as an emergency reactive power source to prevent the medium and long-term transient voltage instability problems.In the quantitative calculation of direct current control quantity,this paper adopts the nonlinear model predictive control method based on global orthogonal configuration,and establishes the nonlinear optimization with the minimum of HVDC transmission energy loss and the motor slip recovery path as the constraint.The model obtains the optimal direct current control scheme by the interior point method,which can avoid the occurrence of medium and long-term transient voltage instability and minimize the loss of HVDC system transmission energy.The simulation results demonstrate the effectiveness,accuracy and rapidity of the method.