Power systems with high renewable energy sources: A review of inertia and frequency control strategies over time (2004.02951v1)

Abstract: 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.

• The paper reviews the evolution of inertia and frequency control methods, highlighting synthetic and virtual inertia to counter reduced synchronous generation.
• It presents a comparative analysis showing a significant decrease in system inertia with higher RES penetration, affecting ROCOF and transient frequency deviations.
• The findings underscore the potential of advanced power electronics and decentralized control strategies to enhance grid stability in renewable-rich environments.

Power Systems with High Renewable Energy Sources: Inertia and Frequency Control Strategies

The increasing penetration of renewable energy sources (RES), such as photovoltaic (PV) and wind power plants, is provoking a fundamental shift in the dynamics of power systems. This paper conducts a comprehensive review of inertia and frequency control strategies that have evolved in response to these changes. The integration of RES into traditional power systems, which historically relied on synchronous machines, has necessitated innovative approaches to maintain stability and reliability.

Inertia in Transition

In conventional power systems, inertia is inherently provided by the rotating masses of synchronous generators. However, the dominance of power electronics in renewable generation units detaches these installations from the grid, thereby diminishing the overall system inertia. The paper introduces terms like 'hidden inertia,' 'synthetic inertia,' and 'virtual inertia' to describe artificial inertia introduced through advanced converter control strategies.

A comparative analysis is presented, outlining the inertia values associated with various generation technologies as well as the damping factors over time. Notably, the paper highlights a significant decrease in system inertia, especially in regions like the EU, where renewable integration has increased from 14% in 1996 to 31% in 2016. This shift has implications on grid stability, particularly concerning Rate Of Change Of Frequency (ROCOF) and transient frequency deviations.

Frequency Control Mechanisms

Frequency control is traditionally implemented via a hierarchical structure encompassing primary, secondary, and tertiary controls. RES integration complicates this framework due to their non-synchronous nature. Wind and PV power plants can participate in frequency regulation through innovative control techniques. For instance, wind turbines can utilize 'hidden inertia' from their rotor inertia, which, although not naturally coupled with the grid, can be emulated through control strategies. The paper extensively discusses droop control and fast power reserve techniques among other strategies to enhance system stability and response.

PV power plants, typically devoid of physical inertia, rely on methods like energy storage systems (ESS) and de-loading techniques to provide grid support. De-loading involves operating below the maximum power point to maintain reserve capacity, contributing to frequency stabilization during contingencies.

Future Implications

The paper suggests that, despite the inherent challenges posed by RES, ongoing developments in control strategies can mitigate potential instability risks. As the energy sector continues to transition towards greater RES integration, there is a necessity for adaptive and flexible frequency control methods. The notion of emulated inertia, particularly in wind generation, suggests that with appropriate technology, renewable installations can mimic traditional inertia, thus supporting grid robustness.

Looking forward, it is critical to further explore the role of decentralized generation and the potential for peer-to-peer control systems. The ongoing evolution of grid standards and operational protocols will play a pivotal role in facilitating higher RES penetration while maintaining system reliability and efficiency.

In conclusion, this paper provides a detailed overview of the current landscape of inertia and frequency control within high-RES power systems, presenting both the challenges and the strategies being deployed to address these issues. As the field progresses, further research into advanced control techniques and the integration of distributed generation will be essential to further enhance grid stability in renewable-rich environments.