Research On Sending-end Frequency Control Of Hybrid Cascaded UHVDC System With Large-scale Renewable Energy Access

In view of the unbalanced distribution of energy bases and load centers in China,the UHVDC transmission technology that can realize large-scale and long-distance power transmission came into being.The hybrid cascaded DC system with line commutated converter(LCC)and two modular multilevel converters(MMC)in series at the DC side fully combines the advantages of LCC and MMC,has both flexibility and economy,and has a certain DC fault clearing ability,which is a feasible scheme to send wind and thermal power in the western region in a long distance.However,in the sending end system of DC transmission project,the increasing penetration rate of "zero inertia" power electronic equipment such as renewable energy generation units and DC system has led to a continuous weakening of the system dynamic frequency response ability,and challenge the security and stability of the system frequency greatly.Firstly,aiming at the scenario of large-scale wind power and thermal power transmission through hybrid DC system in the northwest of China,the electromagnetic transient model and mathematical model of thermal power unit,wind power unit and hybrid DC system are established respectively,and the parameters of hybrid DC system are designed;The operation strategy of the hybrid cascaded DC system is proposed,and the technical and economic analysis is conducted,and its advantages in the large-scale long-distance transmission scenario are clarified;Finally,the accuracy of the model is proved by the simulation.Secondly,the frequency characteristics of a new type of power system characterized by high renewable energy permeability and high proportion of power transmitted through DC system is studied.The paper introduces the frequency response model of the power system considering the renewable energy permeability and proportion coefficient of DC transmission power.The steady-state and dynamic frequency characteristics of the power system are described by combining the frequency domain analysis with the time domain analysis.Three indicators reflecting the system frequency characteristics are proposed,and the upper limit of the ratio between the renewable energy permeability and the DC system under typical parameters is given.The influence of renewable energy and DC system participating in power system frequency regulation on system frequency characteristics is analyzed.The results make known that the access of high penetration of renewable energy aggravates the system frequency characteristics,and the participation of renewable energy and DC system in frequency regulation can increase the upper limit of power electronic equipment penetration and improve frequency characteristics of power system.Finally,in view of the problem that the frequency of the transmission end power network of the hybrid DC system is easy to exceed the limit due to the high proportion of wind power access,the frequency control method of the sending end grid with the coordinated participation of wind power and hybrid DC system is given.The additional frequency regulation strategy considering primary frequency regulation characteristics and inertia characteristics is introduced into the wind turbine and LCC station respectively,and the virtual inertia control strategy is introduced into the MMC station;The time sequence action rules of wind power and hybrid DC system stations participating in the transmission end frequency regulation and the LCC-MMC voltage coordination strategy between the converter stations are designed;Meanwhile,the design method of relevant parameters of control strategy is proposed.Finally,the model is established in PSCAD/EMTDC for validation discussion.The analysis results reveal that the proposed frequency coordination control strategy can improve the inertia and primary frequency modulation ability of the sending system,and effectively improve the frequency stability of the AC system under different operating conditions.