1
Concepts of Resiliency

Sayonsom Chanda1, Anurag K. Srivastava2 and Chen-Ching Liu3

1National Renewable Energy Laboratory, Golden, CO, USA

2Lane Department of Computer Science and Electrical Engineering, West Virginia University, Morgantown, WV, USA

3The Bradley Department of Electrical and Computer Engineering, Virginia Polytechnic Institute and State University, Blacksburg, VA, USA

1.1 Introduction

Resilience is an evolving concept for the power grid. There are many related system concepts - such as reliability, security, and system hardening - which further makes it complex to clearly and uniquely define resilience. This chapter attempts to lay a clear foundation about the various definitions and interpretations of power systems resiliency and a structured taxonomy. All modern critical infrastructure are greatly impacted by poor power quality issues and frequent discontinuity of service. Higher-reliability and better-quality electricity service is indispensable to sustain any strong and progressive modern economy and minimize financial losses. However, electric power infrastructure is encountering unique challenges:

• Climatological Challenges. Global climate change is attributed to number of weather-related significant power outages1 at the US Atlantic coast. The number of significant power outages increased from average of 22.3 between 1990 and 2000 to 76.4 between 2001 and 2015 - which corresponds to an alarming 342% increase. In 2018, power delivery infrastructure in large regions in the states of Texas, Florida, Louisiana, and the territories of Puerto Rico and Virgin Islands were completely devastated by three successive Category 4 and above hurricanes in a span of two months.

  In other countries, devastating tsunamis (Japan, India, and Indonesia), and earthquakes (Nepal and Mexico) caused extensive infrastructure damage and power outages.

• Power Resources. Push for sustainability and the aforementioned ecological concerns led to transition to clean and renewable energy. These distributed energy resources (DERs) are interfaced to the power grid using power electronics causing harmonics. Also, distribution systems have to deal with intermittency. Reduced system inertia is also an associated result.
• Aging Infrastructure. The power grid infrastructure was installed and has been operational (except for occasional repairs) for several decades. Several studies have shown that aging infrastructure, like transformers, are more prone to failures. Transmission and distribution system poles which may have been installed many decades ago, may not conform to modern ASCE standards for withstanding extreme wind speeds.
• Changing Demographics. By 2030, more than 60% of the 9 billion human population will live in urban regions. Urban regions have higher per capita power consumption and will likely to further stress system.
• Cybersecurity. Earlier adoption of digital devices and associated cyber infrastructure typically were not designed with security in mind, but security features was later added (as "patches") to them as an afterthought following many cybersecurity breaches. Smart grid substations with remote monitoring, controllability, and automation are vulnerable to cyber attacks.

These challenges must be addressed in order to meet the surge in demand for higher-reliability, better-quality electricity service.

1.2 Resilience of Complex Systems

It is important to approach this topic by breaking it down to its constituent components. First, a clarification of the definition of complex systems will be presented; then, a brief discourse on its origins and its importance will be discussed, before introducing the concept of resilience, as it applies to such systems.

1.2.1 What are Complex Networks and Complex Systems?

We are all exposed to multiple complex networks every day. Power grids, airplane networks, interstate and road networks, railway freight networks, supply chain networks, and the Internet are complex networks.

1.2.2 Why Is Understanding Resilience of Complex Systems Important?

Complex systems often emerge and are not designed.

Unlike a carefully designed bridge, the complex power system topology grew without consideration for its cascading outages or cyber-attacks, and thus, the smart grid developing upon the legacy power system inherits the complex system flaws that emerge out of ad hoc growth. Since complex systems can contain structural flaws which aid in rapid propagation of disruptive factors. Once individual systems and networks are integrated into the complex system, the effectiveness of each component (or the reason for their existence) can be curtailed by the lack of defense mechanism of the complex system as a whole. That is why understanding the resilience of complex systems is extremely important.

A complex system, characterized by interdependent complex networks and systems, can be susceptible to the following, unlike a single wire connecting a battery to a light bulb:

• Butterfly Effect. Agents for damage, destruction, or disturbance can target a weakly guarded aspect of our lives to cause a widespread damage in another domain.
• Amplification Effect. Flaws or vulnerabilities in one complex network can go unnoticed for long periods of time because other interdependent complex networks often compensate shortcomings of another network. This leads to accumulation and amplification of factors that introduced the vulnerabilities, leading to stronger threats to the overall functioning of the complex networks.
• Domino Effect. Adversities can propagate from one domain of modern society to another across large geographical boundaries.

The abovementioned adverse implications of being reliant on interconnected complex networks can be conveniently remembered using the acronym "BAD" - from the first letters of each of the effects. It is well known that simplicity of networks lead to robustness, but complex systems, can lead to extremely efficient, high-functional societies.

However, the proliferating presence and negative influence of BAD actors in modern society is neither viable nor practical reason to retract from interconnected, interdependent, or inter-operating systems. Instead, it is upon us to deliver a carefully designed, engineering response - by leveraging latest tools and technologies to deploy materials and resources to empower existing infrastructure to minimize adversity and its consequences by responding to any known or unknown threat with resilience.

Figure 1.1 Interdependent complex network infrastructures in the smart grid.

The origin of the word resilience dates back to the early seventeenth century in southeastern Europe, where the present participle form of a Latin word resilire was being used to describe the ability of an object to rebound to its original state after having endured damaging forces. Five centuries later, the meaning of the word has not changed, though it has gained new definitions and contexts across many social and scientific domains, ranging from ecology to economics and engineering.

Figure 1.1 shows a normal interdependence of multiple complex networks - the power system, the communication, and the road transport system - that comprise the distribution system. In case a physical event (e.g. an earthquake) affects the region, it will impact all the three complex networks to various degrees. However, if each of the complex networks can be coordinated effectively, the overall functioning of the distribution system can be maximized despite the damages. That is the goal of resilient distribution systems.

In the next section, a review of closely related terms that partially overlap with the concept of resiliency (and help achieve resilience) is discussed.

1.3 Related Terms and Definitions for Power System

The most common terminologies associated with power system operations has been put forth by North American Reliability Corporation (NERC) [6]. In this section, we summarize the common terminologies, as a staging ground to highlight the uniqueness of the notion of resiliency.

1.3.1 Protection

Since a vast majority of the power system infrastructure, such as transmission and distribution lines, is exposed to the elements of nature and weather phenomena, faults such as line-line, line-ground, and over-current due to short circuits caused by insulation breakdowns are everyday occurrences [7].

Power system protective devices - such as electromechanical or digital relays - primarily protect the healthy parts of the power system from the currents induced by faults through the isolation of faulted parts from the rest of the...