Research On Oscillation Mechanism And Suppression Strategy Of LCC-HVDC System Considering Influence Of Flexible AC/DC Transmission Equipment

Vigorously developing new energy and constructing a new power system with new energy as the main body is an important way for China to realize the"Carbon Peaking,Carbon Neutrality" goals,and is significant for promoting the low-carbon development of Chinese energy and economy.However,since the climate and geographical conditions are limited,China’s new energy power generation bases are mainly distributed in the "three north" areas and coastal waters,while the power load centers are located in the central and eastern regions.Line commutated converter based high voltage direct current(LCCHVDC)transmission develops rapidly in our country with its long-distance and large-capacity power transmission capability.At present,19 UHVDC transmission projects have been put into operation in China.In addition,as the development and utilization of offshore wind resources,more and more flexible DC transmission projects are fed into the central and eastern regions,which makes the receiving-end grid show the characteristics of "Strong DC and Weak AC",and then leads to frequent commutation failures of LCC-HVDC system.At the same time,some flexible AC transmission systems equipment has a significant effect on mitigating the commutation failure of DC transmission system,and has been applied in the receiving-end grid.However,with the commissioning of a large number of flexible AC/DC transmission equipment,the stable operation characteristics of the LCC-HVDC system have been profoundly changed,making the system show strong coupling,nonlinear and other characteristics.These characteristics are easy to cause oscillation of the LCC-HVDC receiving-end AC system,thus causing equipment damage.In serious cases,the LCC-HVDC system can even lock up,which brings challenges to the stable operation of the receiving-end grid.Therefore,it is urgent to study the oscillation mechanism and suppression strategy of LCC-HVDC system considering the influence of flexible AC/DC transmission equipment.This paper is supported by the National Key Research and Development Program,etc.This article focuses on the impedance modeling of LCC-HVDC system and flexible AC/DC transmission equipment,the general impedance criterion for grid-connected stability of LCC-HVDC system considering the influence of multiple types of flexible AC/DC transmission equipment,and the oscillation mechanism analysis and oscillation suppression strategy of LCCHVDC system considering the influence of flexible AC/DC transmission equipment.The complex impedance modeling and oscillation suppression problems of LCC-HVDC system and flexible AC/DC transmission equipment are solved.It provides theoretical guidance and technical support to prevent the oscillation problem in the UHVDC receiving-end grid and ensure the stable operation of HVDC system and flexible AC/DC transmission equipment.The main work and innovation achievements are as follows:(1)To solve the problems of the LCC-HVDC system and the flexible AC/DC transmission equipment,such as the large number of states,rich harmonic components,complex coupling characteristics,and complex commutation process,the multi-input multi-output(MIMO)sequence impedance models of the LCC-HVDC system,static synchronous compensator(STATCOM),and modular multilevel converter based high voltage direct current(MMC-HVDC)are firstly established.In order to deal with the complex linearization of LCC commutation process,an improved calculation method for LCC AC current and DC voltage is proposed.Then,for the convenience of impedance model verification and stability analysis,the MIMO sequence impedance model is equivalent to the single-input single-output(SISO)sequence impedance model.Finally,the correctness of the theoretical modeling is verified through the simulation results of impedance measurement.(2)Aiming at the problem that the existing impedance-based stability criterion for the interconnection system between a single converter and the grid is difficult to be applied to the the oscillation mechanism analysis of the LCCHVDC system considering the influence of flexible AC/DC transmission equipment,a impedance-based stability criterion for the grid-connected system with multiple current-controlled converters and voltage-controlled converters based on MIMO sequence impedance model is firstly proposed.Then,based on the general impedance stability criterion,the stability analysis equivalence between the MIMO sequence impedance model with harmonic truncation order n and its converted SISO sequence impedance model is proved.Finally,taking the receiving-end AC system including delta-connected cascaded STATCOM,MMC-HVDC and LCC-HVDC as an example,the effectiveness of proposed general impedance stability criterion and stability equivalence proof are verified by stability analysis and electromagnetic transient simulation.(3)According to the derived sequence impedance models,the oscillation mechanism of system is analyzed respectively when LCC-HVDC system operates independently,delta-connected cascaded STATCOM is connected to LCC-HVDC receiving-end grid,and MMC-HVDC is fed into LCC-HVDC receiving-end grid.The positive-sequence impedance of LCC-HVDC system shows negative-resistive and capacitive characteristics in some middle and low frequency bands,which makes the system prone to oscillation when the grid strength becomes weak.But the increase of its flat wave reactor can improve the stability.When delta-connected cascaded STATCOM is connected to LCCHVDC system,the positive-sequence impedance characteristic of parallel system is improved,but its negative-sequence impedance phase is reduced to negative-re si stive and capacitive characteristic in some middle and low frequency bands,which makes the system prone to negative-sequence oscillation under the weak grid.After the MMC-HVDC is fed into the LCCHVDC receiving-end grid,when the MMC-HVDC adopts dq current decoupling control,the impedance characteristics of the grid-connected system cannot be improved,and the stability of the system is still poor under the weak grid.When the MMC-HVDC adopts virtual synchronous machine control,the impedance characteristics of grid-connected system can be improved,and the gridconnected stability of the system is enhanced.Finally,the simulation results verify the effectiveness of the oscillation mechanism analysis.(4)For the problem that LCC-HVDC system is prone to oscillation in weak grid,a virtual-inductance-based oscillation suppression method for LCC-HVDC system is proposed,which can improve the phase margin of positive-sequence impedance of LCC-HVDC system in the middle and low frequency bands.To solve the problem that the system is prone to oscillation when the deltaconnected cascaded STATCOM is connected to the LCC-HVDC receiving-end grid,an oscillation suppression method by improving the AC voltage amplitude control of STATCOM is proposed,which can improve the positive-and negative-sequence impedance characteristics in the easy oscillation frequency band.To solve the problem that the system is easy to oscillate when the MMCHVDC with dq current decoupling control is fed into the LCC-HVDC receivingend grid,an oscillation suppression method by improving the DC bus voltage control of MMC-HVDC is proposed.The virtual inertia is introduced into the dq current decoupling control to improve the impedance characteristics.The proposed stability improvement method is effective for addressing the oscillation problem of LCC-HVDC system considering the influence of flexible AC/DC transmission equipment.Finally,simulation results verify the effectiveness of the proposed oscillation suppression strategy and related parameter design.