For the power transmission of photovoltaic base of the desert and gobi area, it is an important technical solution that the islanded PV station is transmitted via voltage source convert based high voltage direct current (VSC-HVDC). However, VSC-HVDC device is the only steady-state AC voltage source instead of the traditional rotational equipment, which will result in the system voltage characteristics being different from that of the traditional power grid. Therefore, the researches on the steady-state and transient voltage characteristics of the large-scale islanded PV station transmitted via VSC-HVDC are conducted. Firstly, the system of the large-scale PV station transmitted via VSC-HVDC is introduced in detail. Secondly, the analysis model of steady-state voltage of the islanded transmission system is built, and a steady-state voltage characteristic and power limit analysis method using power circle as a representation and VSC-HVDC transient and steady-state reactive power capacity is proposed, and the steady-state voltage characteristic analysis is carried out through a numerical example system. After that, the transient voltage characteristics of large-scale islanded photovoltaic power plants under severe AC faults in the VSC-HVDC island transmission system are studied, and a new energy and VSC-HVDC transient coordinated control strategy is proposeded. Finally, measures are proposed to improve voltage stability and transient characteristics, which can provide reference for the actual operation design of photovoltaic power plant island VSC-HVDC transmission systems.

In order to study the influence of AC faults on the DC voltage and active power of large-scale photovoltaic （PV） power transmission system via voltage source converter based high voltage direct current （VSC-HVDC） during the fault ride-through process, a modeling and analysis method based on equivalent controlled current source model is proposed. Firstly, the AC fault ride-through characteristics of the large-scale photovoltaic power transmission system via VSC-HVDC are qualitatively analyzed, and the influence of AC-side fault at the sending-end on the DC voltage of the VSC-HVDC is indirectly translated into the research of the active power characteristics of the system. Then, based on the operation control characteristics of the PV side inverter and the sending-end converter station during the fault, the equivalent controlled current source model of the whole system is established to explore the influence of the current setting value of the PV side inverter and the current limiting value of the sending-end converter station on the active power transmission characteristics, and then the optimal design method of the current parameters of the PV side inverter and the sending-end converter station during the fault period is proposed. Finally, the effectiveness of the proposed method is verified by PSCAD/EMTDC simulation.

In order to solve the synchronous stability problem of bundled transmission of offshore wind power cluster and thermal power in western Guangdong, firstly the synchronous stabilization characteristics and influencing factors of the wind-thermal bundled system are studied in this paper, and then the optimization strategies of operation modes for improving wind power penetration rate of wind-thermal bundled system and giving priority to wind turbines with good synchronous stability characteristics are proposed. Finally, a real-time simulation system of wind farm cluster hard-ware-in-the-loop access is constructed for simulation verification, and the simulation results show that the proposed synchronous stability enhancement strategy can improve the stability limit of the wind-thermal bundled system, which strongly improves the rate of wind power on-grid consumption and the level of grid security and stability.

Large-scale renewable energy transmission from desert, Gobi and wilderness to load centers is critical measure to achieve the goals of "Carbon Peak and Carbon Neutrality". Two large-scale renewable energy ultra-long-distance transmission schemes are introduced, including line commutated converter based high voltage direct current (LCC-HVDC) and voltage source convert based high voltage direct current (VSC-HVDC). The two schemes are compared in terms of their applicable scenarios, overvoltage, voltage and reactive power adjustment capabilities, and economy, to highlight their respective advantages and disadvantages. VSC-HVDC is superior in terms of technology and economy. An integrated design scheme of ±800 kV/10 GW four-terminal VSC-HVDC system for 10 gigawatt renewable energy ultra-long-distance transmission is proposed, which includes the electrical main wiring, main circuit parameters, coordinated control strategy, and the configuration of AC/DC choppers. Key technologies such as bipolar single-valve and high-low valve scheme selection, AC voltage coordinated control of multi-valve at the sending end, and DC voltage balance control of high-low valve are studied, and the configuration scheme and switching strategy of the configuration of AC/DC choppers are proposed. The effectiveness of the HVDC transmission system scheme is verified through electromagnetic transient simulation.

Fractional frequency transmission system (FFTS) is an offshore wind power energy transmission scheme, the AC/AC converter is the most important equipment in FFTS system, and modular multilevel matrix converter (MMMC) is an AC/AC converter with high-voltage and high-power. For the current control of MMMC input side and output side, the traditional proportional integral (PI) control is generally used, which has many control parameters and unsatisfactory control effect, while Lyapunov function control strategy is superior to PI control in the number of controllers, control complexity and control effect. Therefore, the Lyapunov control strategy of MMMC currents on input side and output side is proposed. According to the topology of MMMC, the current decoupling models of MMMC input side and output side are derived，and combined with Lyapunov control theory, the current mathematical model of MMMC input side and output side Lyapunov function control is established ultimately, and the global asymptotic stability of the proposed Lyapunov control is proved, the imprecision of Lyapunov function control and the parameter selection of Lyapunov control is discussed. Finally, the proposed control strategy and the traditional control strategy are applied to current control of MMMC input side and output side on MATLAB respectively to conduct experimental comparison under different working conditions, and the simulation results verify the correctness and superiority of the proposed Lyapunov control method.

Single-core submarine cables have more advantages in production and construction compared to three-core submarine cables, but the submarine cable faults caused by abnormal grounding circulating current in single-core submarine cables occur frequently. The operation mechanism and detection strategy of grounding circulating current of single-core submarine cable need to be further studied. A mathematical model for single-core submarine cables is established based on the impedance matrix, and the influences of factors such as armor resistivity, armor magnetic permeability, grounding resistance, and short-circuit resistance on the parameters within the impedance matrix and grounding circulating current are revealed. Combined with the practical engineering cases, the mechanism of uneven distribution of grounding circulating currents in submarine cables is analyzed, which provides reference bases for the evaluation of the operational status and maintenance of submarine cables.

Under the background of "carbon peak and carbon neutrality" goals, the power system has formed a "double-high"（high proportion of renewable energy and high proportion power electronic equipment） development trend, resulting in a weak grid characteristic of the power grid. Real-time changes in grid impedance under weak grid lead to stability issues such as resonant frequency offset of photovoltaic （PV） grid-connected inverters, and multiple resonant peaks in inverter clusters. Aiming at the fact that the latest resonance suppression methods for PV grid-connected inverters in weak grid have not been systematically summarized, this article summarizes the hot spot control methods in the field of PV grid-connected inverter resonance suppression in "double-high" power systems from the perspectives of inverter unit resonance suppression methods and inverter cluster resonance suppression methods under weak grid. The development direction of improving the stability of PV grid-connected inverters is pointed out.

Aiming at the problem of small disturbance stability of grid-connected doubly-fed wind farms dynamically dominated by phase-locked loop, a method for determining small disturbance instability of doubly-fed wind farms is proposed. Firstly, a second-order linearization model of a single doubly-fed wind turbine is established. Secondly, the derived second-order linearization model is extended to grid-connected doubly-fed wind farms based on the single-machine equivalence model. Finally, the small disturbance stability limit of grid-connected doubly-fed wind farms dynamically dominated by phase-locked loop is derived based on the Rausch-Hurwitz theorem which explains the influence law and the intrinsic mechanism of active output of wind turbines, grid connection strength, phase-locked loop control parameters and the number of wind turbines on the small disturbance stability of grid-connected doubly-fed wind farms dynamically dominated by phase-locked loop. The results show that the small disturbance stability of the system is adversely affected by the increase of active output, the weakening of system connection strength and the inappropriate setting of the control parameters. A grid-connected doubly-fed wind farm consisted of 15 doubly-fed wind turbines is used to verify the correctness and effectiveness of the small disturbance stability limit proposed, and it is not necessary to set up a full-order model of the wind farm, which reduces the amount of calculation.

Aiming at the problems of system inertia reduction and frequency regulation capacity reduction at the sending-end of large-scale wind power via the ultra-high voltage direct current (UHVDC) transmission system, a multi-time scale coordinated control strategy of doubly-fed wind turbines and traditional synchronous units jointly supporting the sending-end system frequency is proposed, so as to effectively improve the stability of system frequency. According to the frequency regulation reserve capacity of synchronous units and disturbance power, the sending-end system frequency regulation demand in different scenarios is analyzed, and the multi-time scale frequency regulation control strategy based on frequency active support of wind turbines considering the frequency regulation dead zone is proposed, and the coordination and cooperation between variable speed control and propeller distance angle control are realized through the reasonable design of the limiting link and switching logic. And based on the reserve capacity of traditional synchronous units, the frequency regulation dead zone value of doubly-fed wind turbines is designed, so that it can adaptively switch the inertia support and primary frequency regulation for different scenarios, and realize the optimal utilization of frequency regulation resources. The dynamic response process of doubly-fed wind turbine and synchronous machine participating in frequency regulation is analyzed, which further proves its superiority. Finally, the large-scale wind power via the UHVDC transmission system simulation model is built based on RTLAB real-time digital simulation platform, which verifies the effectiveness of the proposed control strategy.

Long-term operation of neutral-point-clamped(NPC) direct-mounted energy storage converter may cause the bridge arm open-circuit fault, which causes the shutdown of equipment and affects the low-voltage fault ride-through. In this paper, a virtual bridge arm is formed by directly connecting the faulty phase and the midpoint of the DC bus. The fault-tolerant topology is reconstructed, and a fault-tolerant low voltage ride-through(LVRT)control strategy based on model predictive control is proposed. Considering that the reconstructed fault-tolerant topology structure reduces the utilization rate of the DC side voltage, the maximum output current setting method of the fault-tolerant structure is studied and analyzed. Due to the change of the output characteristics of the fault-tolerant topology during the LVRT process, the model prediction vector analysis is carried out on the current and output voltage of the direct-mounted fault-tolerant structure of the energy storage converter, and the relationship between the voltage drop and the maximum output current is deduced. In the reconstructed space voltage vector, a vector state that can support LVRT is selected, and the energy storage converter is controlled to allocate active and reactive power according to the LVRT technical specifications to support the power grid voltage. The simulation results verify that the proposed control strategy ensures that the fault-tolerant topology structure can operate during bridge arm faults and the reliability of reactive power support during low voltage ride-through.

Kunliulong hybrid multi-terminal UHVDC project(KBL) is the first one of this kind project in the world. Its operation is more flexible than traditional two-terminal HVDC projects. Meanwhile it also increases the complexity of control and protection strategy. Abnormal action has occurred several times since KBL was put into operation. So the reliability of this "first set" control and protection system needs to be analyzed and improved. This paper first illustrates the "6·9" valve-controlled triggering abnormal events of KBL by comprehensively sorting out the control and protection configuration and DC response. The valve group triggering abnormal detection and protection function is proposed and put into application. This strategy can detect the valve control pulse loss or pulse delay fault accurately and fast and improve the ability of risk identification and resistance for DC control and protection system to abnormal valve group triggering in abnormal working conditions. It can also effectively improve the reliability and stability of KBL operation, and the research results can provide reference and guidance for the functional design of control and protection system of subsequent hybrid multi-terminal UHVDC project.

Modular multilevel converter （MMC） contains many power devices and large bridge arm current, which leads to large VSC unit operation loss and directly affects the system economy. To solve this problem, this paper proposes a strategy to reduce the switching loss of power modules, which reduces the switching loss by reducing the switching times in the area with higt bridge arm current, and suppresses the voltage fluctuation of modules by increasing the switching times in the area with low bridge arm current. Finally, taking the VSC unit in Liubei Station of Kunliulong Project as an example, the effectiveness of the loss reduction strategy is verified through PSCAD simulation and RTDS test, which significantly reduces the switching loss of power modules, and the system efficiency is improved.

Modular multilevel converter （MMC） has many advantages such as stable operation and diversified control，precise modeling of which has become a current research hotspot. A complex vector-based impedance modeling method for MMC AC/DC side is proposed. According to the basic principle of complex vector modeling, the positive and negative sequence conversion relationship of converter is analyzed. Under the premise of considering the three-phase coupling, the modeling points of the three-phase MMC are briefly described from the perspective of differential common mode, and the complete time domain model of the three-phase system is established. The theory of harmonic state space （HSS） is introduced to process the model to improve the modeling accuracy, and the basic principle of interface matrix and impedance solution is introduced to complete the establishment of AC/DC impedance model of MMC. Finally, a comparison experiment is carried out on the simulation platform, which proves the feasibility of the modeling method.

When the modular multilevel converter (MMC) is put into the solid state transformer (SST) system, the proportional integral derivative (PID) control method is usually selected. However, this strategy has the disadvantages of complex parameters selection and poor dynamic performance. In order to improve the dynamic performance of the system and simplify parameter selection, a feedback linearized sliding-mode control strategy for MMC-SST system is proposed. Firstly, the overall simulation model of MMC-SST is established. Then, a MMC-SST control model using feedback linearization sliding-mode control is established. Finally, the proposed control method is compared with the conventional PID control strategy through simulation using MATLAB/Simulink platform, verifying that the proposed feedback linearization sliding-mode control strategy has the advantages of easy parameter selection and excellent dynamic performance.

With the proposal and implementation of the "emission peak and carbon neutrality" strategy, China will focus on building a new power system based on renewable energy. However, large-scale integration of renewable energy such as wind power will bring threats and challenges to the new power system with the characteristics of AC/DC hybrid connection. Based on the analysis of Jiangsu Power Grid at the end of the 14th Five-Year Plan, it is noticed that under the background of large-scale wind power integration, the problem of 500 kV AC transmission lines overload after N-1 faults need to be solved urgently. Therefore, based on the comprehensive sensitivity calculation method, an active power coordination control method is proposed, in which the modular multilevel converters(MMCS) of DC systems and the unified power flow controller （UPFC） are used to optimize the power flow distribution and eliminate the potential line overload. Finally, taking Jiangsu Power Grid as an example, simulation analysis is conducted to perform the active power coordinated control method on Suzhou 500 kV UPFC, Baihetan-Jiangsu DC system and a planned three-terminal DC system based on MMC. The simulation results show that the proposed coordinated control method can effectively eliminate the line overload after N-1 faults when applied to Jiangsu Power Grid. In addition, the proposed method has advantages over typical optimization methods in terms of computational speed.

The traditional droop control strategy involved in power grid frequency regulation can take advantage of frequency deviation and DC voltage to achieve mutual support between the frequencies of voltage source converter based multi-terminal flexible direct current transmission systems(VSC-MTDC). However, the droop coefficient of traditional DC voltage is fixed. The operation status of power grid and converter station capacity are neglected when allocating the unbalanced power which results in the converter station overload and weak grid frequency fluctuate greatly when bearing power. To solve this problem, an improved droop control strategy participating in power grid frequency regulation is proposed. Firstly, the power energy allocation relationship of the traditional droop control strategy involved in power grid frequency regulation is analyzed. Secondly, the voltage droop coefficient is redesigned considering the frequency margin of the power grid and the capacity of the converter station itself. The control strategy proposed can ensure the converter station not overloaded and the stable operation of weak power grid when the system is disturbed. Finally, simulation verification is conducted through MATLAB/SIMULINK, the results show that the proposed strategy can improve the frequency adjustment capability of multi-terminal flexible DC transmission systems, and operating ability of system is effectiely improved.

The traditional droop control strategy involved in power grid frequency regulation can take advantage of frequency deviation and DC voltage to achieve mutual support between the frequencies of voltage source converter based multi-terminal flexible direct current transmission systems(VSC-MTDC). However, the droop coefficient of traditional DC voltage is fixed. The operation status of power grid and converter station capacity are neglected when allocating the unbalanced power which results in the converter station overload and weak grid frequency fluctuate greatly when bearing power. To solve this problem, an improved droop control strategy participating in power grid frequency regulation is proposed. Firstly, the power energy allocation relationship of the traditional droop control strategy involved in power grid frequency regulation is analyzed. Secondly, the voltage droop coefficient is redesigned considering the frequency margin of the power grid and the capacity of the converter station itself. The control strategy proposed can ensure the converter station not overloaded and the stable operation of weak power grid when the system is disturbed. Finally, simulation verification is conducted through MATLAB/SIMULINK, the results show that the proposed strategy can improve the frequency adjustment capability of multi-terminal flexible DC transmission systems, and operating ability of system is effectiely improved.