There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

Traditionally, inertia in power systems has been determined by considering all the rotating masses directly connected to the grid. During the last decade, the integration of renewable energy sources, mainly photovoltaic installations and wind power plants, has led to a significant dynamic characteristic change in power systems. This change is mainly due to the fact that most renewables have power electronics at the grid interface. The overall impact on stability and reliability analysis of power systems is very significant. The power systems become more dynamic and require a new set of strategies modifying traditional generation control algorithms. Indeed, renewable generation units are decoupled from the grid by electronic converters, decreasing the overall inertia of the grid. ‘Hidden inertia’, ‘synthetic inertia’ or ‘virtual inertia’ are terms currently used to represent artificial inertia created by converter control of the renewable sources. Alternative spinning reserves are then needed in the new power system with high penetration renewables, where the lack of rotating masses directly connected to the grid must be emulated to maintain an acceptable power system reliability. This paper reviews the inertia concept in terms of values and their evolution in the last decades, as well as the damping factor values. A comparison of the rotational grid inertia for traditional and current averaged generation mix scenarios is also carried out. In addition, an extensive discussion on wind and photovoltaic power plants and their contributions to inertia in terms of frequency control strategies is included in the paper.

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Power systems with high renewable energy sources: A review of inertia and frequency control strategies over time

Coordination between transmission and distribution system operators in the electricity sector: A conceptual framework

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

Summary form only given. The usage of communication channels introduces time delays into load frequency control (LFC) schemes. Those delays may degrade dynamic performance, and even cause instability, of a closed-loop LFC scheme. In this paper, a delay-dependent robust method is proposed for analysis/synthesis of a PID-type LFC scheme considering time delays. The effect of the disturbance on the controlled output is defined as a robust performance index (RPI) of the closed-loop system. At first, for a preset delay upper bound, controller gains are determined by minimizing the RPI. Secondly, calculation of the RPIs of the closed-loop system under different delays provides a new way to assess robustness against delays and estimate delay margins. Case studies are based on three-area LFC schemes under traditional and deregulated environments, respectively. The results show that the PID-type controller obtained can guarantee the tolerance for delays less than the preset upper bound and provide a bigger delay margin than the existing controllers do. Moreover, its robustness against load variations and parameter uncertainties is verified via simulation studies.

Delay-dependent robust load frequency control for time delay power systems

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Core functions of the Web-of-Cells control scheme

In order to systematically plan, specify, and execute system-level test for smart grids in a cyber-physical and multi-domain manner, the ERIGrid holistic testing approach is introduced. Also, the corresponding test description and templates are discussed.

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Test Procedure and Description for System Testing

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Experimental validation of Adaptive FCC and BRC functionalities for the proposed ELECTRA Web-of-Cells

Selected validation scenarios and corresponding test cases in the context of the ERIGrid project are presented in this chapter. Furthermore, the benefits of using the ERIGrid validation method and testing tools are discussed on the realized system validation examples.

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From Scenarios to Use Cases, Test Cases and Validation Examples

Ever increasing shares of intermittent RES in present and future power systems pose new challenges with regard to operation, particularly balance, frequency and voltage stability. Towards effective solutions, the ELECTRA IRP project has developed a novel structure for future power systems operation, by dividing them in a number of Cells, constituting so a Web-of-Cells, and equipped with controllers addressing operation objectives. This paper deals with the Frequency Containment Control use case and, in particular, its implementation in the context of operation constraints imposed by different system conditions. To this end, a design method based on fuzzy logic for avoiding conflicts caused from these conditions or multiple control loops implemented on the same resource is proposed. Simulation results for various selected scenarios and controllers show the effectiveness of the proposed approach.

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Evangelos Rikos

Papers

Core functions of the Web-of-Cells control scheme

Test Procedure and Description for System Testing

Experimental validation of Adaptive FCC and BRC functionalities for the proposed ELECTRA Web-of-Cells

From Scenarios to Use Cases, Test Cases and Validation Examples

Implementation of fuzzy logic for mitigating conflicts of frequency containment