The mechanisms of broadband oscillations in renewable energy power systems and their suppression technologies are discussed. Starting from the dilemma faced in analyzing AC grid resonance stability based on impedance models, it is demonstrated that the "negative resistance" mechanism of broadband oscillations is untenable. A general method is proposed to describe power electronic devices using broadband voltage source converter models and broadband current source converter models, thereby representing the nonlinear characteristics of power electronic devices with harmonic voltage sources and harmonic current sources. This enables renewable energy power systems to still be described as linear systems, and interprets the mechanism of broadband oscillations as a "harmonic amplification" phenomenon caused by network resonance, thus establishing a new framework for explaining the mechanisms of broadband oscillations in renewable energy power systems. Through analysis of two basic resonant circuits, the relationship between resonant mode damping ratio and harmonic amplification factor is derived, laying the theoretical foundation for analyzing and suppressing broadband oscillations based on the s-domain nodal admittance matrix method and C-type damping filters. Based on the maximum damping criterion, a design method for C-type damping filters is proposed, and the designed C-type damping filters exhibit strong broadband damping effects.

In order to address the oscillation issues induced by the increase in system transmission capacity and the reduction of grid-forming wind turbine capacity share in grid-forming and grid-following hybrid wind farms delivered via VSC DC transmission systems, a overall state-space model of the system is established. Based on this model, root locus and modal analysis methods are employed to identify the dominant contributing factors to system oscillations under different operating conditions. The analysis results indicate that system oscillations are mainly influenced by the control variables associated with the voltage outer loops of the grid-forming wind turbines and the VSC DC converter stations. On this basis, a control strategy introducing virtual impedance into the voltage outer loops of the grid-forming wind turbines and the VSC DC converter stations is proposed to improve the system impedance characteristics and suppress oscillations. Furthermore, the feasible parameter range for the virtual impedance is determined through root locus analysis. Finally, simulation results validate that the proposed virtual impedance control method can effectively enhance system performance, thereby confirming the feasibility and effectiveness of the proposed strategy.

Onshore wind power full DC power generation system can effectively solve the problems of harmonic resonance and reactive power transmission in the process of grid-connected large-scale wind power. But the transient stability mechanism under large disturbances is different from the traditional AC system due to the many types of power electronic devices and the complex control coupling of each device in the system. In order to investigate the large disturbance stability mechanism of onshore wind power full DC power generation system, firstly, the system equivalent model is established based on the operation characteristics and control strategy of key equipment in the full DC system for the scheme of "DC convergence-AC output". Then, based on the Brayton-Moser mixed potential function theory, the large disturbance stability mechanism of the onshore wind power full DC power generation system is analyzed. By solving stability criterion, the influence of key parameters on the system large disturbance stability is analyzed. The results show that the stability of the system is related to both the system parameters and control parameters of each converter. Finally, a simulation model is built based on MATLAB/Simulink to verify the correctness of the theoretical analysis.

Full DC wind farms have good active support capabilities and can solve problems such as reactive power and overvoltage in traditional AC transmission systems, which have attracted widespread attention from academia and industry. Due to the high degree of power electronicization in wind power full DC systems, faults develop rapidly and exhibit strong time-varying nonlinearity in the event of large disturbances. Transient stability is a key issue affecting their stable operation. To address the issue of large disturbance stability in wind power full DC systems, considering the conservatism of stability, the Takagi Sugeno （TS） fuzzy model method is firstly used to establish an average model of the main circuit and controller parameters of the wind power full DC system. Secondly, the Lyapunov second method is used to construct the system energy function and an estimation of the system's stability domain, in order to intuitively analyze the large disturbance stability of the wind power full DC system. Then, based on the linear matrix inequality tool, the relationship between the electrical and control parameters of the system and the large disturbance stability domain of the system is determined. The system stability domain is plotted using the Lyapunov function, and the impact of each system parameter on the wind power DC system is analyzed and compared to achieve system stability analysis under large disturbances. Finally, the effectiveness of the proposed method is verified through simulation results.

Aiming at the unipolar ground fault problem of offshore wind power grid-connected system via flexible HVDC transmission with the background of special geographic environment of remote islands, a fault-tolerant operation strategy to achieve half rated power transmission during faults is proposed to reduce the loss of outage, and to improve the security and stability of the power grid at the receiving end. Firstly, the transmission topologies of half-bridge modular multilevel converter （MMC） on the island wind power side and hybrid MMC on the onshore grid-connected side are adopted, and their DC line fault characteristics are analyzed; the series-connectable standby transformer networking scheme of the converter station on the wind power side is utilized to avoid wind farms going off-grid due to low-voltage, and to realize the continuous DC voltage reduction operation during the fault period. Further, a wind power side DC energy dissipation device is configured on the island to solve the energy surplus problem of the wind power side converter station under long time load shedding of the wind farm, enable the system to enter a fault-tolerant operation mode with half of its rated power transmission. Finally, based on the PSCAD/EMTDC simulation platform, the effectiveness of the proposed fault-tolerant operation strategy for offshore wind power grid-connected systems with flexible HVDC link under unipolar grounding faults in remote islands is verified.

To address the issue of DC voltage rise in large-scale wind power DC transmission systems based on modular multilevel converters （MMC） during AC voltage dips at the receiving end, a fault ride-through strategy utilizing supercapacitor energy storage MMC is proposed. The strategy involves utilizing the supercapacitor energy storage system to absorb surplus power during faults, thereby stabilizing the DC bus voltage, while optimizing the active and reactive current distribution of the receiving-end MMC to prioritize reactive power support for the AC grid. After the fault, the stored energy in the supercapacitor is released back into the grid, improving the utilization efficiency of wind power. Additionally, a parameter design method is presented for the supercapacitor energy storage system, including the filter inductance of the bidirectional Buck/Boost converter, the series and parallel numbers of supercapacitors, and the voltage reference value during steady-state operation. Simulation results in PSCAD/EMTDC demonstrate that the proposed strategy effectively suppresses overvoltage on the DC side and enhances the system's fault ride-through capability. In addition, the simulation results also verify that the proposed scheme has advantages in terms of DC voltage stability and energy utilization efficiency compared with the fault ride-through strategy based on DC unloading circuit.

The single-phase split-source inverter （SSI） is a new and highly compact boost inverter with a topology consisting of a DC-link capacitor, a boost inductor, two diodes, and a conventional voltage-source inverter （VSI）. However, the conventional space vector pulse-width modulation （SVPWM） strategy leads to a large ripple in the passive element. In addition, the ripple of the passive element under the conventional sliding mode control （SMC） strategy has large jitter and the output voltage quality of the SSI system is low. In this paper, a finite control set⁃sliding mode control （FCS-SMC） strategy based on multi-objective cost function （MOCF） is proposed to address the problems of complex modulation, large jitter of sliding mode control and low quality of output voltage in SSI. The control of inductor current and capacitor voltage in the DC side, as well as the control of inductor voltage and capacitor current in the AC side of the SSI is essential, and the proposed control strategy based on MOCF FCS-SMC control method can accomplish the control of both DC-side and AC-side variables due to the use of MOCF functions. In addition, the use of finite control set eliminates the requirement of modulation scheme and the use of multi-objective cost function with FCS simplifies the design of the controller. The robustness of the proposed control strategy to parameter variations is investigated and compared with the conventional SMC based approach. Simulation and experimental results show that the proposed FCS-SMC strategy can effectively suppress ripple and reduce jitter to improve the output power quality of SSI.

The low inertia power system with a high proportion of asynchronous power sources has insufficient anti-interference ability. Before the primary frequency response, the frequency drop caused by disturbance may trigger low-frequency load shedding risk, and it is urgent to allocate sufficient fast frequency response resources. A fast frequency response reserve allocation method based on the coordinated optimization of DC frequency limiting controller （FLC） and energy storage is proposed to address issues such as the lack of a domestic fast frequency response reserve system, limited optimization space for FLC reserve, and insufficient utilization of energy storage multiple functions. Firstly, based on the operational characteristics of the DC interconnection line in the sending-end system, a fast frequency response economic allocation method for reserve capacity is proposed, which combines DC FLC with energy storage. Secondly, based on the multiple time scales regulation characteristics of energy storage, a energy storage utilization mode considering multifunctional regulation capacity stacking and reuse is proposed. On this basis, a regional spot market clearing model is constructed for the coordinated optimization of DC FLC and energy storage, achieving rapid frequency response and day-ahead full time optimization of the system's reserve. Finally, the effectiveness of the configuration method and clearing model proposed in this paper is verified through a simulation case of clearing the spot market in the southern region of China.

To mitigate the negative impact of modular multilevel converter （MMC） based high voltage direct current （HVDC） systems partitioning on frequency stability, a method for frequency coupling control of the sending and receiving ends is proposed, and the system stability after adopting this method is analyzed. Firstly, for the receiving-end MMC station, a coupling additional control of DC voltage and AC frequency is proposed, mapping changes in AC frequency to the DC voltage signal. Secondly, for the sending-end MMC station, active power additional control is proposed, adjusting the reference value based on changes in sending-end frequency and DC voltage. Then, under the application of the aforementioned control method, a state-space model of the AC/DC system is established to analyze the impact of frequency coupling control on system stability. Finally, tests are conducted in a modified IEEE 11-bus system to validate the effectiveness of the proposed frequency coupling control method and the correctness of stability analysis results.

The multi-terminal flexible DC system offers high efficiency and economic benefits, making it particularly suitable for large-scale integration of renewable energy from multiple sites. However, compared to AC systems, DC systems have lower inertia and are more susceptible to disturbances, which can lead to system instability in severe cases. To address this issue, this paper proposes two inertia control methods: the inertia control method based on the equivalent sub-module number of the modular multilevel converter （MMC） and the inertia control method using virtual energy storage elements. The first method increases the equivalent capacitance of the MMC DC side by adjusting the number of sub-modules contributed by the upper and lower arms during the modulation process of the MMC. The second method introduces virtual energy storage on the MMC DC side by adding disturbance compensation to the outer loop power control. Building upon these approaches, this paper presents an inertia integrated control strategy for multi-terminal flexible DC systems, achieving better disturbance suppression by combining both control methods. Finally, a three-terminal flexible DC system model based on Nan’Ao Three-Terminal MTDC System is developed in PSCAD/EMTDC to validate the effectiveness of the proposed control strategy. The results show that, compared to systems without any inertia control strategy or those employing a single inertia control method, the system with the proposed inertia control strategy demonstrates the best suppression of DC voltage and active power fluctuations following a disturbance.

Large-scale renewable energy bases in deserts, Gobi areas, and wastelands face challenges in their AC collection grid frameworks, such as lacking conventional power source support and AC main grid connections. Grid-forming converter stations configured for centralized power delivery can provide voltage support to the AC collection grid. Optimizing the placement of these grid-forming converter stations can maximize the voltage strength of the AC grid. To address this, this paper proposes a voltage strength evaluation method for renewable energy collection grids supported by grid-forming voltage-sourced converters （VSCs）. Furthermore, it introduces a multi-objective particle swarm optimization （MOPSO） algorithm to optimize the placement of grid-forming converter stations in such scenarios, thereby enhancing the overall voltage strength of the renewable energy collection grid. Firstly, based on the electrical distance between devices and the voltage fluctuations caused by their grid integration, a voltage strength evaluation method is derived for radial AC collection grids supported by grid-forming converter stations. Secondly, the proposed grid strength evaluation method is integrated into the MOPSO algorithm to construct an optimization model for the placement of grid-forming converter stations, facilitating the planning of renewable energy AC collection grids. Finally, case studies verify the accuracy of the proposed grid voltage strength evaluation method and demonstrate the rationality of optimizing converter station placement for improving overall grid voltage strength.

The fault characteristics of large-scale photovoltaic （PV） sending-end grid under VSC-HVDC are significantly affected by the control strategy and system topology form of double-end power electronic equipment, and the adaptability of traditional relay protection is challenged. Considering the fault control strategy of the converter, the topology of the sending-end grid, the transformer wiring mode and the fault location, a fault-equivalent model of the AC transmission line of the PV sending-end grid in terms of both equipment modeling and system modeling is established. By analyzing the composite sequential network of the system, the differential current characteristics of asymmetrical faults and fault distance impedance characteristics are extracted. Based on the fault characteristics of the VSC-HVDC side, asymmetric fault distance protection schemes for PV sending-end grid are proposed. For the distance protection scheme of grounded short-circuit fault, the fault distance impedance is calculated by using the sequential component characteristics, which is not affected by the transition resistance in principle. For the distance protection scheme of the ungrounded short-circuit fault, the fault distance is calculated based on the impedance complex-plane method by utilizing the fault-phase differential-current characteristics. Based on the simulation results of PSCAD/EMTDC, the proposed schemes can accurately calculate the fault distance impedance by utilizing only the single-ended quantity information of the VSC-HVDC side, reliably distinguish between internal and external faults, and improve the ability of the distance protection to tolerate the transition resistance in the double-ended weak-feeding scenario.

Modular multilevel converter （MMC）-based DC transmission systems have gradually become an important way for integrating offshore wind power due to its advantages such as high efficiency, flexible control, and easy expansion to multi-terminal systems. The active power of offshore wind farms is random and fluctuating, and large-scale power fluctuations may cause problems at the receiving end converter stations, such as power limit violation, DC voltage limit violation, and power circulation between AC and DC systems. To solve these problems, two power coordination control strategies for the receiving-end of large-scale offshore wind power via flexible HVDC transmission systems are proposed. Firstly, the basic structure of the transmission system and the MMC control strategy are introduced. Then, the active power self-balancing control strategy at the receiving-end and the common-outer-control-loop-based DC voltage control strategy at the receiving-end are designed. These two strategies feature distinct technical characteristics, yet both enable rational active power distribution among multiple receiving-end converter stations, thereby preventing unnecessary power limit violations, voltage limit violations, and power circulation issues. Finally, the effectiveness of the two control strategies is verified by PSCAD/EMTDC simulation.

The large-scale development of offshore wind power is one of the key measures to achieve the "dual carbon" goals. To address the future demand for centralized offshore wind power development and transmission, an analysis of influencing factors and construction principles is conducted for the topology selection of offshore wind power direct current （DC） transmission to load centers. Firstly, against the backdrop of expanding offshore wind power transmission scale, the necessity of transmitting offshore wind power via overhead DC lines directly to load centers after onshore connection is discussed. Then, from five perspectives which are offshore wind power aggregation schemes, multi-terminal DC network topologies, DC fault isolation, offshore converter station topologies, and flexible DC system receiving-end connection methods, the influencing factors for topology selection in large-scale offshore wind power DC grid integration are analyzed. Correspondingly, construction principles for offshore wind power DC transmission to load centers are proposed. Finally, a multi-terminal DC system for offshore wind power is constructed in a specific regional power grid demonstrating the feasibility of the proposed construction principles.