| Due to its unique advantages,the line commutated converter based high voltage direct current(HVDC)are currently widely used in the interconnection of cross-regional grids and large-capacity long-distance transmission.However,with the operation of the actual project,the harmonic problem of the HVDC transmission system is gradually rising,which has become one of the biggest challenges for the safe and stable operation of the AC and DC system.This article focuses on the harmonic problem of the HVDC transmission system,and establishes a harmonic analysis model covering the HVDC primary and secondary system.From the two dimensions of the harmonics transmission and coupling,it is comprehensively studying from several views: the transmission characteristics of the HVDC line,the saturation effect of the converter,the response characteristics of the control system and the influence of harmonics on the commutation failure assessment.The main contents listed as follows:(1)A complete two-port model of the HVDC system is established,considering the characteristics of the distributed parameters of the DC line.Based on the analysis of converter switching characteristics and line characteristics,a model that can reflect the interaction of harmonics between AC and DC systems is constructed.Using the model for harmonics calculation and comparing with the results in PSCAD/EMTDC electromagnetic transient simulation,the results show that the model is simple in calculation and reliable in accuracy.Considered the distribution parameters of the line,it is much easier to analyze the transmission mechanism and amplification characteristics of the harmonics in the DC line,which can be in application to the evaluation of the harmonics in the early stage of engineering plan,providing a theoretical basis for the formulation of harmonic protection strategies.(2)A criterion for harmonic instability of AC and DC systems is constructed that takes into account the effect of commutation variable saturation.Aiming at the harmonic instability phenomenon of AC and DC system caused by the combined effect of complementary resonance and commutation variable saturation characteristics,a comprehensive consideration of the harmonic instability criterion of the complete system is established.The criterion is clear in theory and simple in expression.The parameters are easy to obtain based on the two-port model.The accuracy and validity of the criterion are verified by the PSCAD/EMTDC electromagnetic transient simulation results.Using this criterion to analyze the influence of different parameters on the harmonic stability can provide a basis for avoiding harmonic instability phenomena in the design as well as in the operation phase of HVDC transmission systems.(3)A double second-order generalized integrator phase-locking method with anti-harmonic disturbance is proposed.Aiming at the problem that the phase-locked loop of the HVDC transmission system,which susceptible to voltage asymmetry and harmonics,cannot accurately track the synchronization signal.A phase-sequence decoupling controller and quadrature harmonic elimination module are proposed to improve the phase-locked loop to strengthen its anti-interference ability.The PSCAD/EMTDC electromagnetic transient simulation shows that the improved phase-locked loop still has strong robustness when the grid voltage is asymmetric and the harmonic content is high.Applying the improved phase-locked loop to the HVDC transmission system,it is found that the phase-locked loop can not only reduce the number of commutation failures,but also reduce the harmonic content of the system.(4)An improved control strategy for the HVDC transmission system is proposed.Aiming at the problem that the output firing angle of the pole control system is easily affected by harmonics.This article proposed to improve the performance of the pole control system based on the combination of the notch filter,the improved voltage dependent current order limiter link with the action sequence of the fault.The improved control strategy is applied to the HVDC transmission system.Through the comparative analysis of PSCAD/EMTDC electromagnetic transient simulation,it is found that the strategy can weaken the influence of harmonics on the response of the HVDC system during a fault.This strategy not only improves the transient characteristics and recovery characteristics of the DC transmission system,but also reduces the harmonic content of the system,which can provide a reference for improving the performance of the HVDC transmission system in actual projects.(5)A calculation method of the extinction angle is proposed which takes into account the influence of the harmonic voltage,phase shift and DC current fluctuation of the converter bus at the same time.In view of the influence of harmonics on the commutation of HVDC transmission system,the DC current and harmonic voltage are fitted based on the analysis of its influence law;And the calculation method of phase shift is constructed.Based on the above analysis,it is proposed to improve the traditional extinction angle calculation formula.The improved formula can be used to evaluate the commutation failure.The accuracy and effectiveness of the proposed evaluation method are verified by comparison with PSCAD/EMTDC electromagnetic transient simulation results.This method improves the quantitative analysis of the influence of harmonics on the commutation failure of the HVDC transmission system after the AC system failure on the inverter side.The research work of this paper was supported by the Joint Fund Program of the National Nature Science Fund Program of China,"Research on Commutation Failure Evaluation Theory and Flexible Control Technology of Multi-infeed DC System(Project number: U1766213)" and the National Science Foundation of China,"System attributes of fault and HVDC Protection strategy optimization Based on Modal Switching Mechanism in AC/DC Hybrid grid(Project number: 51677073)".Some of the research results have been applied to the operation of actual HVDC systems,obtaining great economic and social benefit. |
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