Frequency Quality in Power Systems

Inhaltsverzeichnis

1. Frontmatter

2. Basic Concepts and Definitions

   1. Frontmatter

   2. Chapter 1. Instantaneous Frequency

      Taulant Kërçi, Federico Milano

      Abstract

      This chapter provides an overview of the definitions and estimation techniques of the fundamental frequency of ac grids. The chapter explains the link between frequency variations and power imbalances and why frequency, similarly to bus voltage magnitude, is different from bus to bus in transient conditions. The chapter also discusses the conceptual challenges of defining the frequency of non-stationary ac voltages and describes relevant frequency estimation methods and their features and limitations. Finally, the chapter defines the RoCoF and its role in power system operation and protection.

   3. Chapter 2. Frequency Control

      Taulant Kërçi, Federico Milano

      Abstract

      Frequency is currently the best measurable quantity that contains information about the overall balance between generation and consumption. For this reason, an efficient and tight tracking of the frequency allows a secure operation of power systems. The chapter revisits the concept of frequency control and provide a comprehensive overview of it. In particular, the chapter presents various terms and concepts related to frequency control that are often utilized with different meanings by different TSOs. Finally, the chapter discusses how to navigate through the current plethora of terms surrounding the topic of frequency control.

   4. Chapter 3. Frequency Quality

      Taulant Kërçi, Federico Milano

      Abstract

      Frequency quality is an emerging topic in power system analysis [1]. In this chapter, we aim at (re)defining frequency quality, describe the various causes of frequency quality problems, and presenting different metrics, particularly those utilized by TSOs, to evaluate and quantify frequency quality in power systems. The chapter also discusses how TSOs monitor frequency quality and provides examples of its recent deterioration.

   5. Chapter 4. Frequency Distribution

      Taulant Kërçi, Federico Milano

      Abstract

      Intuitively, the higher the symmetry in the dynamic response of a dynamical system, the higher the predictability of the system behavior and controllability. Thus, a precise evaluation of the asymmetries present in a system contributes toward increased power system stability and resilience. The devices and controllers that form a power system are for the most part symmetrical. However, it has been recently observed by some TSOs that power systems are becoming increasingly asymmetrical. The aim of this chapter is to study the causes of frequency probability distribution asymmetry in power systems, provide a qualitative theoretical appraisal of this asymmetry; and, finally, show how asymmetry can be compensated through modified control.

3. Control and Stability Analysis

   1. Frontmatter

   2. Chapter 5. Frequency Strength

      Taulant Kërçi, Federico Milano

      Abstract

      In electric power systems, the balance of generation and consumption is ensured through frequency control. This chapter aims at introducing the concept of “frequency control strength” as a novel approach to understanding how different real-world power systems compare to each other in terms of the effectiveness and performance of system-wide frequency control. The chapter presents a comprehensive comparison, based on measurement data, of the frequency control strength of four real-world, renewable-based, synchronous island power systems, namely GB, AIPS, mainland Australia, and Tasmania. The strength is evaluated by means of different short- and long-term frequency quality metrics.

   3. Chapter 6. Frequency Stability

      Taulant Kërçi, Federico Milano

      Abstract

      Retaining frequency stability is one of the most important and, at the same time, challenging problems Transmission System Operators (TSOs) operating low-inertia power systems have to face on a daily basis. This was not necessarily a major problem for the majority of power systems in the past as they were being operated quite far from the stability limits. In this chapter, we provide real-world evidence on how TSOs are dealing with the frequency stability challenge.

   4. Chapter 7. Converter-Based Demand

      Taulant Kërçi, Federico Milano

      Abstract

      In recent years, an increasing number of new kinds converter-interfaced loads has become prominent. Compared to traditional loads such as motor and resistive loads, converter-interfaced loads do not provide inherent automatic grid-stabilization mechanisms, that is, the power consumption does of converter-interfaced loads does not decrease if the frequency or voltage drops [1]. Moreover, newest electronic loads may disconnect from the grid following normally cleared disturbances. This is the case, for example, of data centers. Certain types of new loads, in particular EVs, also tend to introduce a lot of demand variability given their large numbers and the associated (especially uncontrolled) charging infrastructure.

   5. Chapter 8. Flexible Resources

      Taulant Kërçi, Federico Milano

      Abstract

      This chapter studies the role of flexible resources in supporting the system robustness against various disturbances and normal fluctuations of the frequency. In principle, any controllable resource is flexible. However, it is usually considered that flexible resources are distributed small devices which provide frequency control as an auxiliary service, for example, converter-interfaced energy storage systems, switchable loads and MGs.

4. Operation and Planning

   1. Frontmatter

   2. Chapter 9. Automatic Generation Control

      Taulant Kërçi, Federico Milano

      Abstract

      In this chapter, we study the impact of various AGC implementations on frequency stability and long-term performance of system frequency. We provide an analogy between the discrete AGC and a dynamic real-time electric market model (market-based AGC). Such an analogy then allows studying the impact of Market-based Automatic Generation Control (MAGC)-induced imbalances on the dynamic behavior of power systems. Finally, the chapter studies the effect of “memory” of market participants on short-term frequency quality. The memory effect is represented by employing proper mathematical tools from the theory of fractional calculus.

   3. Chapter 10. Sub-Hourly Unit Commitment

      Taulant Kërçi, Federico Milano

      Abstract

      This chapter focuses on market-driven imbalances and their impact on (long-term) frequency quality. The market-driven imbalances are studied by means of different real-time sub-hourly unit commitment models and strategies. As discussed in Chap. 3, these are still a major concern for TSOs. The impact is mostly quantified using the concept of DFDs and the standard deviation of the frequency of the CoI, \(\sigma _{\textrm{CoI}}\).

   4. Chapter 11. Wind and Solar Generation

      Taulant Kërçi, Federico Milano

      Abstract

      This chapter studies the impact of wind and solar generation on frequency quality, particularly long-term frequency quality. It does so based on operational data from the AIPS and through a correlation analysis to determine whether the penetration of wind and solar generation and the magnitude of system frequency fluctuations show any statistical correlation. This analysis is important for using TSOs as it is not yet clear, especially in large interconnected power systems, if wind and solar PV negatively impact the quality of system frequency. To answer these questions, we quantify, using Pearson correlation coefficient and p-value, the correlation between wind (Sect. 11.1) and solar PV (Sect. 11.2) penetration and frequency fluctuations.

   5. Chapter 12. New Reserves and Volume Forecasting

      Taulant Kërçi, Federico Milano

      Abstract

      This last chapter summarizes a review conducted by EirGrid and SONI of the reserve services for the AIPS. The chapter also presents the Volume Forecasting Methodology (VFM) the TSOs have developed to forecast reserve volumes over different time frames namely long-term (10 years ahead), medium-term (1–2 years ahead), and short-term (week to day-ahead). The review and VFM aim at ensuring reserve services can help to achieve 2030 RES targets and, at the same time, addressing several trends and evolving risks such as OF events, faster-happening frequency nadir/zenith, and higher RoCoF.

5. Backmatter