Due to the demand of green and low-carbon development, China is vigorously promoting the construction of large-scale renewable energy bases. Renewable energy transmission system through voltage source converter-voltage direct current(VSC-HVDC )working as island mode is receiving more and more attention. For the system, renewable energy devices usually adopt grid-following control, and the VSC-HVDC adopts grid-forming control. However, the synchronous stability mechanism for such a high-proportion or even 100% power electronic system is still unclear. Focusing on the role of phase-locked loops and virtual synchronous generator loops in the synchronization, a synchronization model for grid-following and grid-forming devices is established. Then, the small signal stability of the system is analyzed, and the operating area that ensures stable operation is derived. The operating states with instability risks are respectively proposed both for renewable energy and VSC-HVDC devices. Finally, the conclusions are verified in PSCAD simulation.

When a regional power grid with a high proportion of small hydropower and new energy operates in a passive isolated grid, the inner stability characteristics of the isolated grids is directly affected by the small hydropower governor parameters and new energy control protection configuration. Based on the typical cases of the actual isolated grids, the dominant factors affecting the frequency stability of each stage of the isolated grids are analyzed, and a typical example simulation model adapted to multi-type isolated grids is constructed. Through multi-dimensional quantitative simulation comparison analysis, the general principles of coordination of small hydropower to grid and optimization of new energy grid-related parameters in the operation of regional isolated grids are proposed. The research results support the development of safety and stability control plans for isolated grid operation, and assist in the construction of new power systems and the reliable power supply of regional power grids.

Electromagnetic loop network can improve the reliability of power supply in the power grid, but it can also cause problems such as increased short-circuit current and line overload after faults. At present, grid operators mainly adopt measures such as loop breaking operation and mode pre-control to reduce the operational risks of electromagnetic loop networks in China Southern Power Grid. However, loop breaking operation is not applicable to all scenarios, and mode pre-control may cause power outputs limited. Therefore, this article proposes to apply the small and medium capacity VSC-HVDC to the low-voltage power grid with electromagnetic loop network, utilizing the flexible and controllable characteristics of VSC-HVDC to reduce the operational risk of the electromagnetic loop network. Firstly, comprehensively considering the requirements of the "Guidelines for Power System Safety and Stability" as well as the economy, the interconnected location and capacity of VSC-HVDC are determined in the electromagnetic loop network. Subsequently, the principles for determining the DC voltage, topology structure, and equipment parameters of VSC-HVDC are provided. Secondly, a control strategy for VSC-HVDC in the electromagnetic loop network is put forward, which can reduce network losses and improve the stability of the electromagnetic loop network. Finally, the effectiveness of the proposed small and medium capacity VSC-HVDC scheme is verified in reducing the operation risks of the electromagnetic ring network through example simulation.

In order to meet the accurate and efficient simulation requirements of large-scale AC/DC power grid multi-time scale dynamic processes, research has deeply discussed multi-time scale partition simulation methods combining different simulation modeling theories. For multi-time scale partition simulation, the simulation interface between different partitions is the key to determining the success or failure of the simulation. Firstly, the multi-time scale characteristics of the AC/DC power grid and the classification of partitioned multi-time scale simulations are analyzed. Then, the key issues in the design of the electromechanical-electromagnetic transient offline simulation interface are summarized. Furthermore, the research status of multi-rate electromagnetic transient simulation interfaces and frequency shifted conventional electromagnetic transient hybrid simulation interfaces is summarized. Finally, future research directions for the design of multi-time scale partition collaborative simulation interfaces are discussed.

A reliable bus type switching method based on holomorphic embedding is proposed to address the non-operational solution issues that may arise from bus type switching in power flow calculations. Firstly, a power flow equation considering the reactive power and voltage constraints of generator buses is constructed, and the non-operational solution problems that may arise from commonly used bus type switching methods are analyzed. Then the holomorphic embedding form of power flow equation is designed based on complex analysis theory and holomorphic embedding method. The recursive equations of voltage and generator reactive power series are derived. Based on the power series of generator reactive power, the sequence of generator reactive power of limit is judged and the framework of bus type switching is designed. Finally, IEEE standard calculation examples are tested to indicate that the proposed method can reliably obtain operational solutions for power flow equations.

Traditional electromagnetic transient simulations use a detailed model to model the voltage source converter （VSC） and use the double interpolation method （DIM） to handle switching events within a step. Due to the time-varying conductor array of the detailed model and the complex DIM algorithm process, the computational efficiency is seriously affected when performing large grid simulations. To address this issue, the semi-implicit delay decoupling principle is applied to model VSC and the handling of switching events within a step is investigated to improve the simulation efficiency by equating the duty cycle of the switching function within a step instead of DIM. Firstly, starting from the state equation of VSC, the decoupling model of VSC is obtained to realise the parallel computation of the converter and improve the computational efficiency. Secondly, the mechanism for generating the accumulated error when switching events are not processed is analyzed, and the computational time scale and the calculation of the equivalent duty cycle of the switching function is also given to ensure the simulation accuracy. Finally, the accuracy and effectiveness of the proposed method are verified by the simulation waveforms of PSCAD as a benchmark, and the methodological features of the proposed method are discussed.

Sub-synchronous oscillation （SSO） has always been a widely concerned stability issue in power systems. Currently, there are multiple methods for analyzing SSO, such as eigenvalue analysis method and frequency-domain analysis method （such as impedance analysis method）, but each method has its own advantages and disadvantages. Combing the advantages of frequency-domain analysis easy to quantify and eigenvalue analysis methods easy to model, a new method for directly estimating the SSO eigenvalues is proposed based on the admittance frequency response characteristics. The method can directly estimate the SSO eigenvalues based solely on the extreme features （including extreme point frequency and slope information） of the frequency response curves of the system aggregated admittance and admittance integral, making the calculation simple and fast. The effectiveness of the proposed method is verified by comparing it with the results of eigenvalue analysis tool in MATLAB. The method is suitable for quantitative analysis of different types of SSO, such as sub-synchronous torsional oscillation in the conventional power generation, sub/super-synchronous oscillation in the renewable energy generation, and composite sub-synchronous oscillation in the conventional and renewable energy power bundling delivery. The analysis results have all been verified by PSCAD/EMTDC simulation.

There are multiple converter stations in a multi-terminal voltage source converter-high voltage direct current(VSC-HVDC )transmission system, and the complex coupling relationship among converter stations makes the calculation of DC short-circuit current a challenge. For the scenarios of a multi-terminal VSC-HVDC transmission system containing hybrid modular multi-level converter（MMC）, firstly the fault equivalent model of the hybrid MMC before and after DC fault ride-through is established, and the influence of DC fault ride-through on the short-circuit current of the multi-terminal VSC-HVDC transmission system is analyzed from the perspective of the change in circuit structure during DC fault ride-through of a hybrid MMC. Based on the superposition theorem, the concept of DC fault ride-through additional network is proposed. Then, a short-circuit current calculation method of the multi-terminal system containing hybrid MMC is proposed during DC fault ride-through, combining the approximate decoupling analytic calculation method of multi-terminal system and the concept of DC fault ride-through additional network. Finally, the electromagnetic transient simulation results based on PSCAD/EMTDC show that the maximum error of the short-circuit current values calculated within 6 ms of the fault occurrence is within ±6%, and the proposed calculation method has good applicability. It provides a reference for the planning, design, and protection setting of multi-terminal VSC-HVDC transmission systems containing hybrid MMC.

The partition asynchronous interconnection of power systems and the replacement of traditional synchronous machines by large-scale new energy sources reduce the frequency strength of the power system and increase the risk of frequency instability. At this time, online evaluation of system frequency strength and monitoring system frequency stability margin are of great significance to the safe and stable operation of the system. Therefore, an online evaluation method of frequency strength suitable for the actual provincial asynchronous power grid is proposed, including three parts: data acquisition and preprocessing, simplification of system frequency modulation model and system frequency strength evaluation. Firstly, based on the operation control system, the operating condition information such as the startup and output of each device in the system is obtained, and the dynamic model library of the frequency regulation of devices in the system is established through the BPA data and data in the wide area measurement system. Secondly, the order of device model is reduced by the unified structural approximation method, then the frequency strength indexes such as frequency drop depth coefficient and slope coefficient are calculated to evaluate the system frequency strength. Finally, the developed frequency strength online evaluation system based on the proposed evaluation method and its application in the actual power grid are introduced.

The asynchronous operation of power grid partitions is becoming the development trend of China Southern Power Grid in the future. Aiming at the problem that the frequency of the partitioned asynchronous grids lose synchronization and consistency, the research on the control strategy of asynchronous grid frequency synchronous operation is carried out, and the structure of the asynchronous grid frequency synchronous operation controller is proposed. The key control parameters of the operation are optimized and determined and the effectiveness is verified through simulation. The Luxi back-to-back (voltage soulce converter-high voltage direct crreut,VSC-HVDC) is used as a pilot to implement the frequency synchronous operation of the partitioned asynchronous grid. On site implementation results indicate that the frequency synchronous operation control of the asynchronous grid can effectively ensure the frequency consistency and synchronization between the partitioned asynchronous operation grids in actual operation, and is consistent with the AGC strategy of the grid. There is no power oscillation and frequency oscillation divergence on both sides of the power grid. The frequency stability of partitioned power grids is improved and the flexibility and support performance of power between partitioned power grids are enhanced effectively. At the same time, it also realizes real-time and automatic sharing of frequency modulation resources across the entire network, and promotes the construction of a unified frequency modulation auxiliary service market across the entire network.

With the proposal of the "dual carbon" goal, China's new energy construction is in a new stage of development. However, the randomness and volatility of high permeability new energy also pose challenges to the static voltage stability analysis of the power system. At the same time, the development of power electronics technology has led to the widespread application of flexible AC transmission system (FACTS) in regulating power flow and improving static voltage stability of power systems. A probability assessment method is proposed for static voltage stability of power systems with distributed static series compensator (DSSC) considering the uncertainty of new energy sources. Firstly, the static voltage stability mechanism of the new energy grid connected system is analyzed. Then, the working principle of DSSC is explained, an equivalent power injection model of DSSC is established, and based on this model, the power flow equation containing DSSC is derived. Secondly, based on the power flow calculation equation, a static voltage stability index that can map the uncertainty of new energy sources is proposed, and the Monte Carlo simulation method is used to realize the probability assessment of the static voltage stability index of the system. Finally, through case analysis, it is verified that the uncertainty of high permeability new energy leads to a larger distribution range of static voltage stability indicators in the system and an increase in the probability of system instability. Reasonable configuration of DSSC can effectively improve the static voltage stability of power systems. The proposed method can accurately reflect the impact of new energy uncertainty on the stability of the power grid, and has certain engineering application value.

Due to the large amount of reactive power consumed by conventional DC receiving-ends and the reactive power and voltage is highly relative, the voltage stability problem at the receiving end of DC infeed power grid is serious. At present, there are a large amount of research on a single DC fault, but the research on the interaction among multiple DC faults is insufficient. This article uses the network equivalence method to derive the relationship between bus voltage and voltage of fault DCs after DCs fault simultaneously, summarizes the conditions under which multiple DC faults significantly increase the risk of stability. Meanwhile the selection principles of DC infeed mode at the receiving end based on DC voltage stability characteristics is studied. A method is proposed to determine the areas that require special attention to the simultaneous fault effects of multiple DCs. The proposed method could provide reference for optimizing the structure of the receiving end of DC infeed power grid and improving the reliability of power grid operation.

The characteristics of large-scale stochastic output of renewable energy in the new power system have posed a risk of failure in the current online transient stability assessment results. This issue simultaneously challenges existing methods in terms of component modeling accuracy, system topology adaptability and computational speed. In this study, a novel approach is proposed that combines improved confidence band method based on re-probability and a probability distribution and confidence band evaluation model of stable index based on graph deep learning. The graph deep learning model rapidly evaluates a few sampled points, which are then expanded using the confidence band method based on re-probability. Temporal domain simulations are guided by co-occurrence knowledge from labeled data to enhance accuracy. Finally, the confidence band calculation method is employed to derive the stability probability distribution and assessment conclusion in intervals under stochastic output conditions. The method capitalizes on the topological adaptability and rapid computation inherent to graph deep learning. Moreover, it remains unhindered by limitations in component modeling accuracy. The conclusions drawn from the confidence band calculation are firmly rooted in theory and can evaluate the stable probability distribution. Evaluation accuracy verification on the IEEE-39 and IEEE-300 bus systems demonstrates the efficacy of the proposed method in accurately predicting specified transient stability indices and delivering reliable probability assessments.

With the development of new power systems and the continuous production of new energy generation units, the southern region will form a scenario of multi regional new energy and adjacent conventional power sources jointly connected to the system. The centralized access of renewable generation often shares transmission channels with conventional power sources, which will affect the transient stability characteristics of regional systems and the transmission capacity of transmission channels, posing unprecedented challenges to the safe and stable operation and risk prevention of southern regional systems. An equivalent system model for regional renewable generation transmission system is established to deeply study the stable characteristics of regional new energy transmission. The main mechanism of the impact of renewable generation integration on the transient stability characteristics of regional systems is analyzed. A quantitative analysis is conducted on the acceleration and deceleration areas of the transient process of the regional transmission system based on the equal area rule. A coordinated optimization method for emergency control shutdown strategy is proposed to achieve the minimization of emergency control shutdown quantity and control cost for traditional and renewable generation units in the regional transmission system. Finally, based on the example of the Guangdong regional wind-thermal-combined power transmission system, the correctness of the analysis results is verified.

With the construction of new power systems, the "double-high" characteristics of the power grid have a significant impact on its operation and stability characteristics. A standardized design solution for the stability control system of photovoltaic power station grid-connected is proposed, including network communication architecture, software and hardware functional configuration, export mode design and coordination methods. The solution addresses the current limitation where the export of photovoltaics stability control system can only be manually selected between "complete cut-off" or " complete adjustment." The effectiveness of the solution has been demonstrated through its successful implementation in the first hydro-photovoltaic complementary project in China Southern Power Grid. The results of RTDS simulation tests confirm that the designed stability control system enables precise adjustment of fast power modulation. It exhibits short communication delays and demonstrates high reliability in communication networking. Moreover, the system proves its adaptability to various stability requirements of actual power grids.