[Objective] The increasing penetration rate of renewable energy has decreased the inertia of power systems. The traditional fixed-parameter frequency control strategy makes it difficult to effectively support system frequency stability requirements. Frequency stability faces severe challenges， especially in renewable energy sending systems. [Methods] To enhance the frequency support performance of VSC-HVDC for sending-end systems with renewable energy， an adaptive dual droop control strategy based on changes in system frequency and DC voltage is proposed. First， a model of renewable energy transmitted by a VSC-HVDC system was established. For frequency droop control， a frequency-adaptive coefficient was designed based on the logistic function of frequency deviation to enable the system to dynamically adjust the droop coefficient and respond flexibly to frequency changes. For the DC voltage droop control， via the coupling relationship between DC voltage and active power using virtual inertia technology， the DC adaptive coefficient was designed as the adaptive virtual inertia coefficient based on the frequency deviation and the rate of change of frequency to enhance the system’s ability to suppress frequency fluctuations. Finally， a simulation model based on the PSCAD/EMTDC platform was established， and a comparison study with the traditional method in different scenarios was performed. [Results] The results show that the proposed strategy reduces the maximum frequency deviation by more than 10% in various scenarios while increasing the maximum utilization of the VSC-HVDC system by over 10%. [Conclusions] Compared with traditional methods， the proposed strategy can significantly reduce the maximum frequency deviation and effectively improve the frequency support performance of VSC-HVDC systems.

[Objective] As renewable power bases gradually expand from load centers to remote areas such as deep offshore， deserts， and barren lands， using grid-forming voltage source converter-based high voltage direct current （VSC-HVDC） transmission technology is a promising solution for integrating renewable power to the grid. Considering power quality and fault current limitations， current grid-forming control usually adopts a dual closed-loop structure of AC voltage and AC current. This control structure relies on the voltage-current integral relationship of the AC-side filter capacitor and grid current feedforward to achieve stable control of the AC voltage. However， the AC side of the modular multilevel converter （MMC） usually lacks a centralized filter capacitor， and obtaining AC-side current feedforward to decouple from the grid is difficult， leading to degraded AC voltage control performance. [Methods] To solve this problem， this study proposes a novel dual-closed-loop AC voltage-control strategy for grid-forming VSC-HVDC converters. The strategy does not rely on the AC capacitance but adjusts the AC voltage of the PCC by controlling the voltage drop on the grid impedance. [Results] Simulation verification based on PSCAD/EMTDC showed that， under the proposed control mode， the grid-forming MMC exhibited good operational stability and active/reactive power dynamic control performance under different grid conditions. [Conclusions] The proposed control strategy can effectively improve the AC voltage-AC current dual-closed-loop control performance， operational stability， and grid adaptability of grid-forming MMC.

[Objective] With the increase in the proportion of new energy connected to the grid， issues with power system stability associated with weak grids are becoming increasingly prominent. Static var compensator （SVC）， an important device for improving the dynamic response of power systems， may trigger sub-synchronous and super-synchronous interaction （S²SI） in weak power grids， causing system oscillation. This paper aims to study the mechanism of sub- and super-synchronous interactions induced by SVC in a weak current network and proposes an effective oscillation suppression strategy. [Methods] First， a frequency-coupled impedance model （FCIM） was used to analyze the frequency coupling characteristics between the SVC and weak grid， revealing a strong coupling relationship between the sub-synchronous and super-synchronous frequencies. Second， based on the stability criterion of impedance crossing， the influence of the controller parameters on the oscillation mode is studied， and a supplementary sub-synchronous damping controller （SSDC） is proposed. Finally， an electromagnetic transient time-domain simulation was performed using the PSCAD/EMTDC platform to verify the correctness of the theoretical analysis and effectiveness of the control strategy. [Results] The results show that SVC can cause S²SI in a weak power network， with the oscillation frequency closely related to the controller parameters. The oscillation phenomenon can be effectively suppressed by optimizing the controller parameters. The simulation results show that the proposed SSDC control strategy can significantly reduce the amplitude of sub-super-synchronous oscillation and improve system stability. [Conclusions] The results reveal the mechanism of S²SI triggered by SVCs in a weak power grid at the frequency-domain impedance level. The proposed control strategy improves the oscillation suppression effect while ensuring the regulation rate of the SVC， thus highlighting significant engineering application value.

[Objective] Constructing renewable energy transmission systems is a key approach for addressing the uneven spatial distribution of resources. However， integrating a high proportion of renewable energy poses significant challenges to the frequency security and control of power systems at the sending-end base. [Methods] To address these challenges， this study proposes a frequency control and parameter optimization method tailored for the coordinated operation of heterogeneous multi-source systems in a new high-proportion energy-sending-end base. First， for active frequency support in a heterogeneous multi-source sending-end base， a coupled model of system parameters and security indicators was developed for parameter optimization. Second， a multi-objective model-predictive automatic generation control method was designed， considering system frequency deviations， generation costs， and emission costs. This method enables the rational allocation of frequency regulation commands， reduces the overall system costs， and enhances new energy utilization. [Results] The theoretical analysis and simulation results demonstrated that the proposed method effectively stabilized the system frequency response indicators within a safe range. Moreover， compared with traditional frequency control methods， the proposed strategy exhibits significant advantages in terms of frequency deviation suppression， dynamic response speed， and economic efficiency. [Conclusions] The proposed collaborative control and parameter optimization approach offers an innovative solution for frequency stability control in high-penetration renewable energy sending-end systems. While ensuring the secure operation of the system， this approach also considers economic efficiency and frequency regulation performance， thus providing theoretical support for the design and optimization of the energy-sending-end system.

[Objective] Power grids face challenges in managing steady-state automatic and collaborative prevention and control measures， particularly owing to issues such as DC blocking， large unit tripping， and reliance on manual scheduling for the rapid response from resources including pumped storage and electrochemical energy storage. These have resulted in challenges in ensuring the frequency safety， regional control deviations， and minimizing the risk of load shedding. To address these issues， an intelligent processing method is proposed for high-power shortage in receiving-end power grids comprising multiple DC inputs. [Methods] This method automatically detects power grid shortage faults， calculates the total regulation demand， and allocates the demand based on prioritized criteria， including resource availability， operating conditions， and unit capabilities， considering the characteristics of the control object and various safety constraints. Control instructions are generated by employing strategies that incorporate available capacity statistics， shortage startup judgment， regulation demand allocation， control of pumped and energy storage systems， safety constraints， and control recovery. This approach facilitates the remote activation of multiple types of fast regulation resources， enabling rapid stabilization of the grid during steady-state power shortage. [Results] Using this method， the first domestic high-power-shortage intelligent processing system for power grids was developed and successfully implemented in a large-scale DC receiving-end provincial power grid. The control effect and reliability satisfied the actual usage requirements. [Conclusions] The application of the proposed methodology in handling steady-state high-power shortages in power grids represents a significant shift from manual telephone scheduling to automatic intelligent processing. This transition reduces decision-making time from minutes to seconds， thereby substantially improving fault-handling efficiency， and ensuring the stability of the power grid frequency and interconnection line power. Operational testing demonstrated the practicality and high reliability of the system， underscoring its potential for broader adoption and implementation in other power grid systems.

[Objective] Grid-forming control （GFM） energy storage converters based on virtual synchronous generator （VSG） control can effectively provide voltage and frequency support for the system； however， current research has rarely considered the support capability of VSG energy storage converters in the scenario of new energy devices connected to weak power grids. [Methods] This study analyzed the stability laws between photovoltaic systems， VSG energy storage systems， and weak electricity grids. First， small signal models for photovoltaic and VSG energy storage systems under weak AC grids were established. Second， the eigenvalue analysis method was used to comprehensively analyze the impact of changes in photovoltaic output and SCR on the system's eigenvalues. Third， we identified the high-sensitivity control parameters that affect system stability by studying the oscillation modes and participation factors. Finally， a method for adjusting control parameters to improve system stability was proposed by studying the root locus of the dominant mode at the critical steady state. Finally， the accuracy of the stability impact law of the system was verified through a MATLAB/Simulink simulation under different operating conditions. The time-domain simulation model constructed using MATLAB/Simulink verified the effectiveness of the theoretical analysis. [Results] The simulation results showed that the small signal model established in this paper can accurately identify the control parameters that significantly impact system stability under various operating conditions， such as photovoltaic output fluctuations （0.5~2.0 p.u.） and changes in grid short-circuit ratio （SCR=0.6~2.0）. By implementing the proposed parameter adjustment scheme， the photovoltaic output of the system increased from 1.2 p.u. to 2.0 p.u. and remained stable at SCR=0.6. [Conclusions] When the output of the photovoltaic system changes， the q-axis pi parameters of the VSG energy storage system and the photovoltaic system have high sensitivity； when SCR changes， the internal and external q-axis pi parameters of the VSG energy storage system have high sensitivity. Orderly adjustment of the control parameters with different sensitivities improved the static stability of the system， thereby providing an effective solution for improving the stability of a high proportion of new energy grids.