In recent years, the risk of broadband oscillations in offshore wind power integration systems has continued to increase, particularly in systems employing direct-drive wind turbine generators, and multiple incidents under weak-grid conditions have been reported in China. Existing countermeasures mainly focus on reshaping the converter impedance by tuning control parameters, while system-level and wind-farm-level suppression strategies remain insufficiently investigated. This paper adopts eigenvalue analysis to identify the dominant unstable modes of an offshore wind integration system and to construct the corresponding oscillatory stability boundary, on the basis of which an oscillatory stability margin index is defined and calculated. Guided by this index, two complementary oscillation suppression strategies are developed, combining preventive regulation of system operating conditions prior to contingencies with emergency adjustment of wind power operating capacity after oscillation events. The effectiveness of the proposed strategies in enhancing oscillatory stability and suppressing broadband oscillations is verified through PSCAD/EMTDC time-domain simulations.

In practical engineering, grid-connected photovoltaic systems exhibit two distinct instability scenarios: instability at high output and instability at low output. The reasons behind these different instability scenarios are a worthwhile research question. Based on the typical topology and parameters of photovoltaic power plants, this paper employs eigenvalue analysis to study the dominant oscillation modes and key control links in grid-connected photovoltaic systems. Through root locus analysis, the impact of active power output on the stability of dominant oscillation modes under variations in parameters of different control links is investigated. The study identifies the key factors influencing the correlation between active power output and the stability of dominant oscillation modes, and validates the analysis results using time-domain simulations. The study shows that the stability of sub-synchronous oscillations in grid-connected photovoltaic systems under weak grid conditions is primarily determined by the current control loop, with oscillations more likely to occur under low current loop bandwidth. The correlation between active power output and dominant oscillation modes is jointly determined by the reactive power outer loop, phase-locked loop, and DC voltage outer loop. Different parameter combinations can lead to different instability scenarios.

In recent years, the risk of broadband oscillations in grid-connected direct-drive permanent magnet synchronous generators (D-PMSG) systems has become increasingly prominent. Multiple incidents of broadband oscillations induced by D-PMSG units under weak grid conditions have been reported in China. Existing research suggests that the integration of grid-forming (GFM) devices can effectively enhance the grid-connection stability of D-PMSG units in weak grid environments. However, the interactions between grid-forming static var generator (GFM-SVG) and D-PMSG units have been rarely studied. This paper employs eigenvalue analysis to calculate the eigenvalues of grid-connected D-PMSG systems incorporating GFM-SVG, and investigates the influence of GFM-SVG control and operational parameters on the system dominant modes. The accuracy of the eigenvalue analysis is further validated by PSCAD/EMTDC time-domain simulations. The results demonstrate that GFM-SVG can effectively improve the grid-connection stability of D-PMSG units under weak grid conditions, while the existing coupling interactions can affect the overall system stability.

With the increasing penetration of renewable energy sources (RESs) and flexible loads, both the source and load exhibit significant uncertainty, making it hard to optimize the unit commitment (UC) scheduling appropriately. This paper proposes a chance-constrained unit commitment (CCUC) model addressing multiple uncertainties. The dispatching capability of electric vehicles (EV s) is aggregated through the Minkowski addition. The operating risks considering uncertainties are restricted by chance constraints, where the joint probability distribution of random variables is characterized by Gaussian mixture model (GMM). By using the affine invariance and the approximate cumulative density function of GMM, the chance constraints are able to be transformed accurately and efficiently into deterministic forms. Simulation results validate that the proposed method can handle the uncertainties, while decreasing the total operation cost.

Nowadays, more green alternative fuels are introduced into shipboard microgrids to reduce emissions and improve energy efficiency. In this paper, an energy optimization framework is proposed for a hybrid-powered all-electric ship (AES) using a dual-fuel generator and energy storage system (ESS). First, the topology of the AES with dual-fuel generators is analyzed. Second, a power scheduling model is developed to minimize operational costs, considering the specific fuel oil consumption (SFOC) of the dual-fuel engine. Finally, several cases are carried out, highlighting both the economic and environmental benefits of this strategy, as well as the importance of integrating a dual-fuel generator in the AES framework.

For the combined renewable power generation and thermal plant transmission system, the integration of renewable power would affect the rotor angle characteristics of thermal power units. When the output of renewable power and thermal plant is high, the risk of rotor angle instability increases. In the actual operation, configuring reactive power compensation device on the collector bus can effectively enhance the rotor angle stability margin. However, the impact mechanism of these device on rotor angle stability and the assessment method for reactive power capacity requirement still need to be clarified. In this paper, a rotor angle characteristics mathematical model of thermal plant is firstly established, considering the configuration of reactive power compensation device. On this basis, the impact of reactive power configuration on the rotor angle stability is analyzed. At the same time, an assessment method is proposed for determining the reactive power capacity requirement under rotor angle constraints. Cases studies validated the effectiveness of the analysis results, which would be referred for practical engineering applications.

More flexible ramping service is required due to the increase of renewable power generation in power systems. Electric vehicles (EVs) could provide such flexible ramping products (FRPs) at low cost while participating in the electricity market through aggregation. However, EVs’ dispatching capability cannot be fully utilized without the right incentives. This paper addresses a distributed optimal model developed between EV aggregators (EVAs) and the independent system operator (ISO). To make such concept, a cloud-edge collaborated market structure is adopted. At the edge level, EVAs assess the dispatching capability and solve the market bidding subproblem. At the cloud level, ISO solves the market clearing subproblem considering system’ economy and security. The overall problem is solved by the analytical target cascading (ATC) method. Heuristic constraints are also introduced into the model to improve convergence performance. The model is tested on a modified IEEE 30-bus system. Results demonstrate that the proposed method can incentivize EVAs with different owners to shift load and provide FRPs accurately, meanwhile reducing the cost and increasing the consumption of renewable energy effectively.

With large scale offshore wind power integration in recent years the grid operation department put forward need for risk assessment of oscillation for offshore wind power integration system. Based on analysis of typical oscillation scenario of offshore wind power integration system using eigenvalue method, this paper first suggest that stability boundary should be constructed for each specified mode according to its relevant oscillation scenario. Using sub and super synchronous oscillation mode dominated by current control of PMSG as example it was proposed to construct stability boundary using wing power output and short circuit ratio as parameters. The boundary could be generated by offline eigenvalue analysis of the system. 10 machines and 39 buses case system was applied for validation of the proposed method. Results showed that the proposed method was feasible for engineering application.

Distributed photovoltaic (DPV), represented by household rooftop PV, has been developing rapidly under the promotion of county-wide rooftop PV policy in China. However, its large-scale integration has brought serious challenges to the reliability of the distribution network. It is of great significance to scientifically evaluate the hosting capacity of DPV and provide hosting capacity improvement strategies. By constructing distribution network operational reliability evaluation indexes, the maximum hosting capacity of DPV is evaluated by considering several factors, such as power flow, bus voltage, new energy consumption, etc. The DPV capacity is dispersed evenly to each distribution network bus, which more accurately simulates the dense distribution scenario of household PV and improves the accuracy of the hosting capacity results. A demand response (DR) model based on a two-layer game between load aggregators and distribution network operators is constructed, and the results of the calculation show that the maximum hosting capacity of DPV can be effectively improved based on guaranteeing the maximum interests of both parties. This research provides a scientific basis for the rational planning of the construction scale of DPV.

In the transmission system of combined renewable power generation and thermal units, there exists risk of the phase-locked instability when the renewable power generation output is high. To deal with this problem, this paper firstly establishes the mathematical expression of the PLL phase angle motion equation to analyze the mechanism for phase-locked instability. Based on this, the method of renewable power phase-locked stability evaluation is proposed. The output combination of synchronous machine and renewable power, under the constraint of renewable power phase-locked stability, can be calculated accurately according to the method. Case study validates the effectiveness of analysis results and computation performance of the proposed method.

The renewable energy sources (RESs) dominated power grid is an envisaged infrastructure of the future power system, where the commonly used grid following (GFL) control for grid-tied converters suffers from lacking grid support capability, low stability, etc. Recently, emerging grid forming (GFM) control methods have been proposed to improve the dynamic performance and stability of grid-tied converters This paper takes two common grid-connected converters as an example, firstly introduces their control strategies and mathematical models, then analyses the influence of grid-connected control technology on the oscillation characteristics of the power system, and finally elucidates the mechanism of grid-connected control technology on the improvement of system stability margin from the perspective of impedance characteristics.

As the large-scale integration of the off-shore wind generation, the grid strength of the wind-thermal-bundled transmission system has become much more weaker and phase-locked instability problems are becoming more easily to encounter. As the phase-locked stability is mainly affected by the terminal voltage dynamic of the renewable generation, the reactive power compensation device which influence the terminal voltage dynamic will play a great role in phase-locked stability. However, the phase-locked stability interactions between the reactive power compensator and renewable generation are still not well researched. In this paper, PLL phase angle motion characteristic considering the influence of reactive power compensation device has been proposed and the influence of reactive power compensation device has been analyzied. Then, the calculation method of reactive power compensation device capacity is proposed, which can accurately calculate the capacity of reactive power compensation device under the constraint of phase-locked stability. At last, case studies have been carried out to verify the correctness and effectiveness.

To deal with the day-ahead dispatch problem of energy and reserve resources in multi-area AC/DC hybrid systems, this paper proposes a hierarchical distributed dispatch method for AC/DC hybrid system considering wind power uncertainty. Firstly, a decomposition method for AC/DC hybrid systems is designed, and the two-stage regional robust dispatch model considering wind power output uncertainty is constructed. Then, analytical target cascading (ATC) is adopted to achieve distributed solution for multi-area unit commitment and tie-line scheduling, and column and constraint generation algorithm is employed to solve the two-stage robust optimization problem. Case study validates the effectiveness of dispatch schedules and computation performance of the proposed method.

Most of the existing literature studies the small disturbance oscillation problem of offshore wind farm VSC-HVDC transmission system basing on simplified models. In order to clarify the influence of equivalent modeling on oscillation analysis, this paper first reveals the decoupling characteristics among machine side (MS) offshore station, grid side (GS)-sending end (SE) onshore station, and RE (receiving end) onshore station, and proves the rationality of local subsystem analysis approach. Furthermore, for the GS-SE offshore station and RE onshore station, the feasibility of different modeling schemes is evaluated by calculating eigenvalue deviation. The result shows that when analyzing the oscillation mode of the GS-SE offshore station, MS can be simulated by constant power source, and RE can be simulated by constant voltage source. When analyzing the oscillation mode of the RE onshore station, the SE system can be simulated by constant power source under the premise of retaining the dynamic of the DC submarine cable. The theoretical analysis results are verified by time domain simulation.

随着我国新能源装机规模日益扩大，电网对并网风电机组的性能要求逐渐从被动跟随电网转变为主动支撑电网，由此使得基于虚拟同步控制的电压源型风电机组具有广阔的应用前景。但是，目前对于此类新型风电机组的宽频动态特性的研究尚有不足，鉴于此，该文通过建立电压源型双馈机组的类Phillips-Heffron模型，从电气阻尼的角度研究其由虚拟同步控制环节主导的低频模态的振荡特性及失稳机理，并探究不同控制参数及电网条件对电压源型双馈机组电气阻尼特性的影响。研究结果表明，当系统其他动态部分引入的电气负阻尼大于虚拟同步控制环节固有的正阻尼时，电压源型双馈机组将面临低频振荡失稳的风险。最后讨论了可提高电压源型双馈机组低频段电气阻尼的应对措施，设计了类电力系统静态稳定器（PSS），可有效抑制机组低频振荡问题。研究结果可指导实际工程中电压源型风电机组的控制器方案设计及参数整定优化。

随着风电、光伏占比的不断上升,系统惯量以及常规备用调频容量趋于降低,电力系统频率稳定问题凸显.从频率稳定约束的角度,研究了基于频率响应模型的电网新能源承载能力评估方法.首先,建立了含新能源发电等多种频率调节电源的电力系统频率响应模型,并通过电磁暂态仿真验证了模型的有效性.然后,以频率跌落最低值和最大频率变化率为频率稳定约束,提出基于频率响应模型的电网新能源承载能力评估方法.该方法可以根据新能源占比构建系统的频率稳定域,评估系统的新能源承载能力.最后,通过所提方法在系统层面评估考虑频率稳定约束的电网新能源承载能力以及新型频率控制技术对于电力系统频率稳定性的提升作用研究中的应用,验证了方法的有效性与适用性.

This paper provides a detailed introduction to a modular modeling approach for the transfer function of the VSC grid-connected system. The obtained results can accurately reflect the small-disturbance stability level of VSC grid-connected systems. These results are based on the VSC outer-loop reference signal and the corresponding feedback signal, constituting a two-input two-output system. The modeling process involves three steps. Initially, only the dynamic processes of the VSC inner-loop control and PLL control modules are considered. Subsequently, the relevant modules of the VSC outer-loop control are included. Finally, it is condensed into a concise form. By utilizing the basic property of linearity in systems that adhere to the superposition principle, it can be converted into an equivalent single-input single-output system. This enables the application of the Nyquist stability criterion for stability analysis. All results presented in this paper have been validated through PSCAD/EMTDC simulation.

The offshore subsystem of grid-integrated wind farm via VSC-HVDC (Voltage Source Converter-High Voltage Direct Current) consists of direct-drive type wind turbine generators (WTGs) and sending end converter (SEC) of VSC-HVDC. The SEC regulates AC voltage using voltage/frequency (V/F) control, while the grid-side converters (GSC) of WTGs synchronize to the AC voltage by a phase-locked loop (PLL). Eigenvalue analysis reveals that there exists a pair of low-frequency modes within the offshore subsystem which might be unstable at high level power output of WTGs. Participation factors and eigenvalue sensitivity analysis demonstrate that the low-frequency mode is closely related to the V/F control of SEC and PLL of GSC. According to this knowledge, a reduced-order s-domain model is derived from the full-order state-space model of the offshore subsystem with PLL dynamics of WTGs reserved. The damping of PLL dynamics is then investigated and divided into two parts, i.e., the intrinsic damping of PLL control and additional damping introduced by V/F control of the SEC. Consequently, a damping calculation method is developed. It is found that the damping calculation based on the reduced-order model represents the stability of the low-frequency mode exactly, implying that this reduced-order model reserved the mode dynamics mentioned above. The impact of different PLL and V/F control parameters on the damping characteristics is further studied. Time-domain simulations validate the effectiveness of the proposed damping analysis method, which could be helpful for control design to avoid the risk of low-frequency oscillation in the offshore system of grid-integrated wind farms via VSC-HVDC.

随着风电装机的增加,风电机组在弱电网下的稳定运行面临严峻考验.电压源型风机采用自同步方式运行,因而不受锁相环的影响,并且能够提供频率和电压支撑,更适于在弱电网下运行.为研究基于虚拟同步控制的电压源型直驱风机的并网稳定性,建立了电压源型直驱风机的数学模型及状态空间模型,采用特征模态分析方法,对比了电流源型/电压源型直驱风机的并网稳定性,分析结果表明电压源型直驱风机具有更好的弱电网适应性,但在较高短路比下容易出现低频振荡问题,在此基础上研究了关键参数对于低频模态稳定性的影响.然后进一步研究了电流源型/电压源型直驱风机混合风场的并网稳定性,结果表明:加入电压源型直驱风机可以提高原有电流源型直驱风场在弱电网下的稳定性.最后,通过PSCAD/EMTDC电磁暂态仿真验证了特征模态分析结果的正确性.