The electric power system is currently undergoing a period of unprecedented changes. Environmental and sustainability concerns lead to replacement of a significant share of conventional fossil fuel-based power plants with renewable energy resources. This transition involves the major challenge of substituting synchronous machines and their well-known dynamics and controllers with power electronics-interfaced generation whose regulation and interaction with the rest of the system is yet to be fully understood. In this article, we review the challenges of such low-inertia power systems, and survey the solutions that have been put forward thus far. We strive to concisely summarize the laid-out scientific foundations as well as the practical experiences of industrial and academic demonstration projects. We touch upon the topics of power system stability, modeling, and control, and we particularly focus on the role of frequency, inertia, as well as control of power converters and from the demand-side.

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Foundations and Challenges of Low-Inertia Systems (Invited Paper)

Renewable energy sources (RESs), e.g., wind and solar photovoltaics, have been increasingly used to meet worldwide growing energy demands and reduce greenhouse gas emissions. However, RESs are normally coupled to the power grid through fast-response power converters without any inertia, leading to decreased power system inertia. As a result, the grid frequency may easily go beyond the acceptable range under severe frequency events, resulting in undesirable load-shedding, cascading failures, or even large-scale blackouts. To address the ever-decreasing inertia issue, this paper proposes the concept of distributed power system virtual inertia, which can be implemented by grid-connected power converters. Without modifications of system hardware, power system inertia can be emulated by the energy stored in the dc-link capacitors of grid-connected power converters. By regulating the dc-link voltages in proportional to the grid frequency, the dc-link capacitors are aggregated into an extremely large equivalent capacitor serving as an energy buffer for frequency support. Furthermore, the limitation of virtual inertia, together with its design parameters, is identified. Finally, the feasibility of the proposed concept is validated through simulation and experimental results, which indicate that 12.5% and 50% improvements of the frequency nadir and rate of change of frequency can be achieved.

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Distributed Power System Virtual Inertia Implemented by Grid-Connected Power Converters

Inertia plays a vital role in maintaining the frequency stability of power systems. However, the increase of power electronics-based renewable generation can dramatically reduce the inertia levels of modern power systems. This issue has already challenged the control and stability of small-scale power systems. It will soon be faced by larger power systems as the trend of large-scale renewable integration continues. In view of the urgent demand for addressing the inertia concern, this paper presents a comprehensive review of inertia enhancement methods covering both proven techniques and emerging ones and also studies the effect of inertia on frequency control. Among those proven techniques, the inertia emulation by wind turbines has successfully demonstrated its effectiveness and will receive widespread adoptions. For the emerging techniques, the virtual inertia generated by the dc-link capacitors of power converters has a great potential due to its low cost. The same concept of inertia emulation can also be applied to ultracapacitors. In addition, batteries will serve as an alternative inertia supplier, and the relevant technical challenges as well as the solutions are discussed in this paper. In future power systems where most of the generators and loads are connected via power electronics, virtual synchronous machines will gradually take over the responsibility of inertia support. In general, it is concluded that advances in semiconductors and control promise to make power electronics an enabling technology for inertia control in future power systems.

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On the Inertia of Future More-Electronics Power Systems

Integration of inverter-interfaced distributed generators (DGs) via microgrids into traditional power systems reduces the total inertia and damping properties, while increasing uncertainty and sensitivity to the fault in the system. The concept of the virtual synchronous generator (VSG) has been recently introduced in the literature as a solution, which mimics the behavior of conventional synchronous generators in large power systems. Parameters of the VSGs are normally determined comparing the conventional synchronous generators. This paper introduces an extended VSG for microgrids by combining the concept of virtual rotor, virtual primary, and virtual secondary control as a virtual controller to stabilize/regulate the system frequency. An $H_{\infty }$ robust control method is proposed for robust\optimal tuning of the virtual parameters. The effectiveness of the proposed control methodology is verified via a microgrid test bench.

Robust Frequency Control of Microgrids Using an Extended Virtual Synchronous Generator

This paper presents robust virtual inertia control of an islanded microgrid considering high penetration of renewable energy sources (RESs). In such microgrids, the lack of system inertia due to the replacement of traditional generating units with a large amount of RESs causes undesirable influence to microgrid frequency stability, leading to weakening of the microgrid. In order to handle this challenge, the $H_{\mathbf {\infty }}$ robust control method is implemented to the virtual inertial control loop, taking into account the high penetration of RESs, thus enhancing the robust performance and stability of the microgrid during contingencies. The controller’s robustness and performance are determined along with numerous disturbances and parametric uncertainties. The comparative study between $H_{\mathbf {\infty }}$ and optimal proportional-integral (PI)-based virtual inertia controller is also presented. The results show the superior robustness and control effect of the proposed $H_{\mathbf {\infty }}$ controller in terms of precise reference frequency tracking and disturbance attenuation over the optimal PI controller. It is validated that the proposed $H_{\mathbf {\infty }}$ -based virtual inertia controller successfully provides desired robust frequency support to a low-inertia islanded microgrid against high RESs penetration.

Robust Virtual Inertia Control of an Islanded Microgrid Considering High Penetration of Renewable Energy

In the virtual synchronous generator (VSG) field, the traditional methods, such as the constant parameters control method and the self-adaptive inertia control method, always neglect the effect of the damping factor. This letter proposes a self-adaptive inertia and damping combination control method to improve the frequency stability with an interleaving control technique. Tests on the MATLAB/Simulink VSG model demonstrate the effectiveness of the proposed method.

A Self-Adaptive Inertia and Damping Combination Control of VSG to Support Frequency Stability

In this study, waste concrete fine aggregates and cement are applied to sludge solidification and resource recycling. The unconfined compressive strength (UCS) test is performed to investigate the variation in the strength and stress–strain properties of the solidified sludge with the content and particle size of waste concrete fine aggregate, cement content, and curing time. The results show that incorporating waste concrete fine aggregates can improve the UCS of cement-solidified sludge, which can achieve the optimum effect when the fine aggregate content ranges from 12% to 15%. However, compared with the fine aggregate content, the cement content and curing time are the main factors in improving the strength of waste concrete fine aggregate and cement-solidified sludge (WCSS). The stress–strain curves of WCSS comprise four stages. The failure strain εf of WCSS with four fine aggregate contents decreases in a power function with the increase in qu, and εf is mostly distributed when the content is 1.1–2.1%. A linear relationship is observed between E50 and qu. This study attempts to promote the recycling of waste concrete fine aggregates and obtain solidified sludge with excellent mechanical properties, providing some reference for practical engineering applications.

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X. Y. Bian

Papers

A Self-Adaptive Inertia and Damping Combination Control of VSG to Support Frequency Stability

Experimental Study on the Strength and Stress–Strain Properties of Waste Concrete Fine Aggregate and Cement-Solidified Sludge