Research On Multi-level Key Prevention Technologies For Commutation Failure Of LCC-HVDC

Power industry is the key industry for China to achieve the goal of“Carbon Peaking and Carbon Neutrality”,therefore,the Central Committee of the Communist Party of China with Comrade Xi Jinping at its core proposes to build a"New Power System"with new energy as the main body.So,new energy will certainly play a leading role in the future.However,60%of China’s new energy installations are in the“Three North”areas,while electricity consumption is mainly concentrated in the central and eastern regions.Therefore,in order to achieve the goal of“Double Carbon”,it is necessary to accelerate the construction of ultra-high voltage backbone power grid to provide channels for the consumption of clean energy in the“Three North”area.Due to its advantages in transmission capacity,power loss,asynchronous networking capacity and investment cost,the line commutated converter based high voltage direct current（LCC-HVDC）has been rapidly developed and wildly used in China.However,in recent years,with the formation of large capacity AC/DC hybrid power grid and the increasing proportion of DC transmission capacity,the characteristic of“strong HVDC but weak AC system”becomes prominent as well as the problem of commutation failure（CF）.CF will cause short circuit of inverter valves,provoke power shock,and then lead to power oscillation,frequency fluctuation of AC system at the sending and receiving ends,and even lead to many new energies off grid,which seriously threatens the safe and stable operation of power grid.Therefore,it is urgent to develop multi-level prevention methods for CF in LCC-HVDC system.This article is funded by the National Key Research and Development Program Project and other projects.In this article,the research works on CF suppression methods are carried out from the aspects of improving DC control system,installing dynamic reactive power compensation equipment,retrofitting converter topology and enhancing the security and stability of AC/DC hybrid power grid.A multi-level comprehensive prevention and treatment technologies for CF including control and protection layer,equipment layer and system security layer is formed,which can provide theoretical basis and practical guidance for the prevention and treatment of CF in practical projects.The main research works include:（1）At the control and protection layer,aiming at the problem that it is difficult to determine the firing angle advance during the first CF suppression,the relationship between the suppression effect of the first CF and the firing angle advance is explored.Firstly,through the extraction and analysis of the three action paths from the firing angle advance to the extinction angle,the mechanism of advancing firing control is revealed.It is found that advancing firing control plays a positive role in suppressing the first CF through one direct path,but plays a negative role through two indirect paths.The influence of short circuit ratio,DC current dynamics and fault detection delay on the effect of advancing firing control is verified by using the system quasi-steady-state model and electromagnetic transient simulation analysis.In addition,aiming at the problem that the subsequent CF involves many factors and complicated mechanism,firstly,through the analysis of the interaction characteristics between control quantities and electrical quantities in the development process of subsequent CF,it is revealed that the subsequent CF is the result of the extinction angle overshoot effect after the recovery of the first CF.Then,the influencing factors and mechanism of the overshoot effect are analyzed.Based on the idea of suppressing the overshoot effect of extinction angle,an extinction angle error control with the capability of both subsequent CF suppression and power level recovery is proposed,and its effectiveness is verified by simulation.（2）At the equipment level,in order to fully develop and utilize the CF suppression capability of dynamic reactive power compensation equipment,the CF prevention strategies of STATCOM in different scenarios are studied.Firstly,aiming at the problem of the limited effect of advancing firing control caused by the increase reactive power consumption of inverter,a coordinated control strategy between LCC and STATCOM is proposed,and its effectiveness is verified by simulations.Then,aiming at the problem that how STATCOM should compensate under single-phase fault condition,based on the system quasi-steady-state equation and electromagnetic transient simulation results,the relationship between the proportion of STATCOM negative-sequence reactive current and the suppression effect of the first CF is explored and verified,and it is found that there is a negative correlation between them.Thirdly,aiming at the compensation strategy for dynamic compensation equipment under the participation of grid-side energy storage,the influence of reactive and active power compensation proportion of energy storage converter on CF suppression and power recovery is explored through simulation analysis and mechanism description.Also,a control strategy that considers the first and subsequent CF suppression and AC power recovery is proposed.Finally,based on the HVDC physical simulation platform,the effect of energy storage converter to suppress CF is verified.（3）At the equipment level,a commutation failure suppression method based on controllable series resistor（CSR）is proposed.Firstly,the main circuit structure and control scheme of CSR are presented.If the voltage stress is the same,the CSR has halfless powers semiconductor devices than the bridge type controllable capacitor（BTCC）Then,based on quasi steady-state equations,the CF mitigating mechanism and control characteristics of CSR are analyzed.The analytical results demonstrate that the series resistor at AC outgoing line can support the commutation voltage amplitude and suppress the phase leading jump,and thus facilitates the commutation process.Next,the current/voltage stress and the loss of CSR are discussed.Then,the effectiveness of the proposed scheme is verified based on the CIGRE benchmark model in PSCAD/EMTDC:the CF mitigation ability of CSR is 6～7.2 times higher than that of the benchmark model,and 80%～90%higher than that of BTCC.Finally,the methods proposed in chapters 2-4 are compared and analyzed from the aspects of first and subsequent CF suppression ability,power recovery promotion ability and economy.（4）At system security layer,aiming at the commutation failure caused by small-signal instability in AC/DC hybrid power grid,the interaction paths between LCC and the receiving power grid and their damping characteristics are studied,and the stability enhancement method is proposed.Firstly,the structured small-signal model of the LCC-HVDC system is derived.Then,based on the small-signal model,using the concept of motion equation model,the main interaction paths between the LCC and the AC grid and their damping characteristics are briefly analyzed.The results show that the amplitude direct path provides positive damping while the phase paths provide negative damping.Next,the influences of two correlation coefficients corresponding to the extinction angle,namely tracking error of PLL-extinction angle coefficient k_δγand firing angle order-extinction angle coefficient k_αoγ,on the system stability are analyzed.The results show that increasing k_δγor decreasing k_αoγcan enhance the system damping.Then,the damping characteristics of six interaction paths between LCC and AC grid are analyzed in detail by using positive and negative feedback analysis method,and the corresponding physical mechanism is explained Next,according to the conclusion of mechanism analysis,two stability-enhancing methods that can equivalently modify k_δγand k_αoγrespectively are proposed.Finally,based on PSCAD/EMTDC electromagnetic transient simulation model,the correctness of theoretical analysis and the effectiveness of the proposed method are verified.