Estimating frequency stability margin for flexible under-frequency relay operation

https://doi.org/10.1016/j.epsr.2021.107116Get rights and content

Highlights

  • Online calculation of frequency stability margin based on RoCoF.

  • Additional shedding criterion based on frequency stability margin.

  • Robust, transparent and adaptable UFLS protection.

  • Inertia-independent UFLS with no wide-area communication.

Abstract

SmartGrid solutions are often misinterpreted as complex approaches requiring large amounts of (online) data and computational power for delivering high-quality results. With today's technology, this is surely the most obvious and technically achievable approach for dealing with complex problems. However, by taking this road in the power system field, providing the required high-level reliability becomes a challenging task. Therefore, a thorough consideration of whether increased complexity justifies the gains is surely warranted, especially when it comes to power system protection, where the applied procedures are all about transparency, simplicity and robustness – which is often in contrast to the above interpretation of SmartGrids. Therefore, the motivation of the presented research was to design an effective SmartGrid solution for avoiding power system frequency instability, appropriate for real-life application. Under-frequency load-shedding protection is the subject of many research activities that often result in complex and, consequently, unfeasible suggestions. In contrast, this paper presents an effective RoCoF-based upgrade to the conventional approach, eliminating its most important shortcoming: inflexibility. By estimating frequency stability margin in real time, shedding decisions remain local, whereas the overall efficiency of this system protection scheme is brought to the level of the most sophisticated solutions available in the literature.

Introduction

One of the main reasons for the extreme level of efficiency of modern electrical power systems (EPSs) lies in how emerging technologies are treated and implemented by power system utilities. A very conventional approach in the process of harvesting capabilities of new technologies often seems rather unnecessary and difficult to comprehend. However, throughout the years, EPSs have become the leading infrastructure in supporting modern society's existence and so indispensable in a variety of ways. They became the most notable representatives of the so-called critical infrastructure. Many areas of human activities are so dependent on an EPS's operation that conditions emerging from blackouts are often unimaginable (social unrest, food shortage, increased crime rate, etc.) [1]. For this reason, system integrity protection schemes (SIPS) were introduced to prevent system operating conditions leading to blackouts. Major system imbalances between active power generation and consumption are being dealt with by under-frequency load-shedding (UFLS) protection. The conventional UFLS setting ([2,3]) appeared satisfactory up until the last decade or so when the penetration level of intermittent converter-based generation units began to seriously interfere with EPS inertia. Consequently, EPS frequency responses to active power imbalances became more turbulent in terms of the increased Rate of Change of Frequency (RoCoF). This led to the need for the under-frequency protection devices to enable RoCoF calculation in real time. In order for the measured RoCoF to be highly accurate, a time window of at least 100 ms is required. Nevertheless, RoCoF obtained from a shorter time window can also give some rough indications of a potentially dangerous situation, however, one should handle its value with care. Despite the above, many algorithms that extract appropriate information from RoCoF are still under research.

Several approaches to using RoCoF for UFLS purposes can be found in the existing literature. The large majority of them rely on knowing the relation between the active power imbalance and RoCoF that corresponds to a single synchronous machine (e.g. [4]) – the swing equation [5]. Substituting the entire set of individual machines with an equivalent generator enables one to study the average EPS frequency response, usually referred to as the Center of Inertia (COI) [6]. However, such solutions have a tendency to increase the UFLS complexity with the involvement of wide-area communication and the estimation of (mostly unknown) EPS inertia (such as [7], [8], [9], [10]). In contrast to such suggestions, this paper formulates the technology of a RoCoF-based modification of conventional UFLS based entirely on locally-obtained measurements and the estimated value of the frequency stability margin, which helps to recognize the need for immediate UFLS activation. A brief preliminary and elementary explanation of the concept in terms of basic principles and initial observations can be found in [11], whereas this paper presents its extensive in-depth formulation and analysis. The modification introduces an additional shedding criterion which significantly increases the scheme's flexibility to changes in operating conditions. This criterion is developed from an innovative representation of operating conditions in a newly-defined plane, which differs from commonly used frequency versus RoCoF planes ([12,13]). Apart from this, several further possibilities for user-defined UFLS settings emerge from the proposal that might open up a wide array of additional variations in the future. The concept was proven in [14] with real-time digital simulator in a hardware-in-the-loop setup that also served for improving RoCoF filtering technique and supporting the applicability of the approach.

Section snippets

Basic facts

Currently, most of UFLS schemes used in practice are of the conventional type, encompassing several stages ([5, 15,16]). The setting of each stage includes its size (amount of disconnected consumption/load, in practice achieved by tripping appropriate feeders) and the corresponding frequency threshold at which it is being activated. However, load feeders (assigned to an individual stage) are subjected to different power flows depending on different factors, such as the season within a year, day

Background

The idea for the presented RoCoF-based criterion arose from the existing set of predictive UFLS suggestions. The feasibility of the initial attempts of applying the second time derivative of the COI frequency [21] for short-term prediction of the frequency response was indeed questionable in terms of practical applications. However, they inspired several publications that followed, each of them eliminating a few questionable elements. As a result, a WAMS-based predictive UFLS was suggested in

Power system model

For a case study of the operation and efficiency of the presented approach, we used an existing EPS model. It encompasses a part of the 110 kV network of the Slovenian EPS. Since this model was successfully validated against PMU measurements before [20], the authors consider it appropriate. It consists of twelve hydro units with synchronous generators having an equivalent inertia constant of 6 s (exciter, governor) and nine substations. We modeled each out of the nine substations as having ten

Conclusions

In this paper, a transparent and effective modification of a conventional UFLS relay setting is presented. It employs RoCoF for real-time frequency stability margin calculation, which is further monitored against the potential violation of newly-proposed frequency stability margin thresholds. The authors suggest implementing this new criterion in existing under-frequency relays alongside the already existing one. As a result, conventional UFLS gains the required flexibility with minimum

CRediT Author Statement

Urban Rudez: Conceptualization, Methodology, Software, Validation, Resources, Writing - Original Draft, Writing - Review & Editing, Visualization, Supervision, Funding acquisition.

Denis Sodin: Formal analysis, Investigation, Data Curation.

Rafael Mihalic: Project administration, Funding acquisition.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This work was funded by the Slovenian Research Agency through the Electric Power Systems No. P2-0356 research program and the Resource management for low latency reliable communications in smart grids - LoLaG, J2-9232 project. The authors would like to thank the Slovenian Research Agency for the financial support. This work is subject to a pending International Patent Application No. PCT/EP2018/059048 filed on April 9, 2018.

References (24)

  • P.M. Anderson et al.

    An adaptive method for setting underfrequency load shedding relays

    IEEE Trans. Power Syst.

    (1992)
  • M.S. Pasand et al.

    New centralized adaptive under frequency load shedding algorithms

  • Cited by (0)

    View full text