为保证光伏虚拟同步发电机(virtual synchronous generators,VSG)并网后的安全稳定运行,全面评估光伏VSG小信号稳定性已成为一项亟需解决的任务.为此,首先建立并网型光伏VSG系统的小信号模型,通过特征根法研究VSG系统的阻尼特性,辨识引入VSG功能后逆变器并网系统产生的新振荡模态,分析并网条件和控制参数对VSG并网系统阻尼特性的影响,并据此提出VSG多参数协调设计原则.研究结果表明,引入VSG功能对逆变器并网系统的稳定性存在明显影响,设计VSG控制参数时需考虑小信号稳定性的安全约束.

With the rapid development of inverter-based generators (IGs), power grid is faced with critical frequency stability challenges because the existing IGs have no inertia. To equip IGs with inertial response, researchers have proposed several virtual inertia control methods, which can be classified into two categories: virtual synchronous generator (VSG) control and droop control based on rate of change of frequency (ROCOF-droop control). In this paper, the comparison between both virtual inertia control methods is conducted from three perspectives: mathematical model, output characteristic and small-signal stability. State-space models are firstly built to analyze the control mechanism of VSG control and ROCOF-droop control methods. Simulation and eigenvalue analysis are conducted to study the transient responses and oscillation characteristics of both methods, which is helpful to understand the advantages and limitations of existing virtual inertia control methods. Finally, the obtained theoretical results are validated through realtime laboratory (RT-LAB) hardware-in-loop simulation platform.

针对张北柔直电网示范工程设备参数,建立了由换流阀、风机和交流耗能装置等设备组成的张北四端柔性直流电网仿真模型.开展了张北柔直电网孤岛方式下换流站稳态和闭锁故障时的仿真研究.针对换流站闭锁时的功率盈余和系统失稳问题,仿真了柔直电网孤岛运行方式下,换流阀闭锁时耗能装置投切对柔直系统的影响.结果表明,配合投切交流耗能装置可以有效平衡孤岛方式下送端换流站闭锁故障时的风功率盈余,极大缓解因闭锁造成的换流站极电压过充和系统失稳问题.

Virtual Synchronous Generators (VSG), as an effective mean to improve the frequency and voltage regulation ability of the photovoltaic power generation, had aroused wide concern. The analysis on whether VSG would cause stability problem became an important issue to ensure the safety of power system. The stability analysis of photovoltaic VSG is an enormous challenge because this VSG not only retains the characteristic of power electronic devices but also emulates the essential behavior of synchronous generators. Existing studies concentrated on the stability of VSG device and the risk of oscillation when VSG applied in microgrid. The damping characteristic of distributed photovoltaic VSG has not been researched. Meanwhile the impact of each VSG control parameter on oscillation model damping has already been studied. However, the interactive influence from several parameters to system stability has not been discussed in existing researches. To address these issues, this paper developed the small signal model of photovoltaic VSG. The oscillation mode of VSG system was calculated based on this model, which was also applied to investigate the stability of whole system with different control parameters and under various system scenarios. The results obtained by small-signal model are validated by digital simulation equipped with physical controller. Brief conclusions were drawn as follows: (1) grid-connected photovoltaic VSG system has three dangerous oscillation modes of poorly damp, which are the mode caused by the power control, the mode introduced by virtual impedance control and the mode associated with LC filter; (2) unstable subsynchronous oscillation will appear if power droop gain or virtual mechanical time constant exceeds the threshold. High-frequency or synchronous oscillation may be excited by decreasing the virtual resistance. Subsynchronous oscillation can be caused by increasing the virtual resistance. Reduction of virtual inductance is likely to result in high-frequency or subsynchronous oscillation and growing of virtual inductance will contribute to a negative damping for subsynchronous and synchronous oscillation mode; (3) the damping of new oscillation mode introduced by power control of VSG is relatively small when photovoltaic VSG is applied in low voltage power grid. The influence from system condition to damping of oscillation mode associated with LC filter is the key factor to decide the stability of VSG system in high voltage grid.

虚拟同步发电机(virtual synchronous generators,VSG)技术可以使新能源具有频率和电压支撑能力,因此得到广泛关注。目前虚拟同步机的技术路线可以分为电压控制型和电流控制型两大类。现有研究多聚焦于电压控制型VSG,而对电流控制型VSG研究较少。主要研究了电流控制型VSG的并网稳定问题,首先建立了电流控制型VSG的小信号模型,计算得到了系统特征根,分析了系统中各模态的阻尼特性。在此基础上,研究了电流控制型VSG控制参数对其稳定性的影响,并对电流控制型VSG在不同电网条件下的稳定性及其并网适应性进行了分析。研究结果表明,VSG控制环节对电流控制型VSG的稳定性存在显著影响,电流控制型VSG并网运行存在高频振荡和次同步振荡风险,需要合理整定其控制参数以保证其安全稳定运行。

In this paper,the factory-composited insulators developed by combining the conventional porcelain or glass insulators with high quality RTV coating is proposed to change the RTV coating as a temporary anti-pollution-flashover measure into a permanent one.A new mature external-insulation selection at the design and construction stage of power transmission project is provided,which breaks through the application limit of porcelain and glass insulators used in heavy pollution area as to eventually improve the comprehensive quality of UHV grid.Based on the application experience and test data,the product parameters and design principles of factory-composited insulators are put forward,and the distrust of RTV is clarified.