CIGRE Reference Paper : Power system restoration – World practices & future trends

01 June 2019
By S. Almeida de Graaff (NL) – SC C2 Chair

 


 

Complete or partial blackout of the electric power grid does occur from time to time, despite prudent planning and operations, due to disturbances that either exceed the basic design criteria, or due to various causes such as natural disasters, multiple equipment failure, protection relay miscoordination or malfunctioning, and human errors. Restoration of the power system, following such disturbances, is an extremely important aspect of the System Operator’s role in managing the bulk power system and has as objectives to enable the power system to return to normal conditions securely and rapidly, minimizing restoration time and associated losses, and diminishing adverse impacts on society.

 

State of the art

 

In general, there are two basic strategies for power system restoration, namely the bottom-up and the topdown strategy.

The bottom-up restoration strategy is based on the use of blackstart generators (those able to re-energise the system without relying on the external electric power transmission network), and applies in case of total system blackout and non-existent interconnection assistance. On the contrary, the top-down restoration strategy is based on neighbouring interconnections. These are used to energize the bulk power transmission system first, after which loads and other generators are energized. Both approaches have their advantages and disadvantages, and many system operators choose a hybrid approach to restoration (see Table 1 - Examples of implemented blackstart strategies).

 

 

Restoration in the future

 

A common practice for System Operators is to use conventional power plants for system restoration, making it a stable and predictable process. In a future where less or no synchronous generators will be available, it is important to rethink restoration strategies. Due to the fact that the share of renewable energy sources (RES) in distribution networks is nowadays significant (with the tendency to grow even further), there is a need for TSO/DSO integrated restoration plans, which will involve increased coordination, information exchange, joint operator training, and most likely common tools. Depending on the amount as well as controllability of DSO-connected RES, the responsibilities and contribution of each entity in the restoration process will differ.

With the increasing RES and other power electronics devices in the power system, their capabilities need to be utilised as much as possible. Whereas HVDC links are not commonly used for providing restoration service, their participation is expected to increase in the future. The functionalities of these links can be utilised in order to aid the system restoration, including providing active and reactive support during blackstart and building of the cranking path. Furthermore, Battery Energy Storage Systems (BESS) can be used in several ways for supporting the restoration process. One example is the participation of BESS in load restoration. Another example is the use of BESS as blackstart source for providing the required power to non-blackstart generators.

The role of Wide Area Monitoring Systems based on Phasor Measurement Units (PMUs) for restoration purposes is expected to increase in the future. When compared to traditional SCADA measurements, synchrophasors have an added value of synchronised voltage phase angle information between areas that have to be re-energized and/or re-connected, which can significantly benefit the restoration process. In the preparation phase of the restoration process, and when complemented with state estimation data, the synchrophasors provide precise information of the remaining system, its division in islands and available components in the system. This information helps to construct the restoration strategy. From a restoration viewpoint, the restoration stage can be enhanced with critical data such as synchrophasor measurements from generating units and critical load.

 

Concluding remarks

 

System Operators have predominantly been using conventional synchronous generators in the restoration process. With the rapidly developing power system, changes in the power system restoration strategy become necessary to adequately address the future challenges.

With increasing distributed energy resources, the role of the distribution system operator in power system restoration will become more important, where coordination between different stakeholders will be key. Furthermore, this increasing generation in distribution networks demands an improved observability and increased information exchange. The use of WAMS can greatly help to achieve this, especially during restoration activities where situational awareness in the control room is of utmost important. With increasing integration of power electronics interfaced devices in the power system, it is also worth investigating how these can support the system operator in enabling an effective and efficient restoration process. The use of available HVDC links and battery energy storage systems in the restoration process is expected to increase in the future. This is tackled in the newly established working group C2.26 “Power system restoration accounting for a rapidly changing power system and generation mix”.

 

Further reading

 

This article is a summary of a Reference Paper prepared by a small task force of Study Committee C2 – System Operation and Control. The full paper elaborates in more detail also on the currently used restoration strategies throughout the world, the importance of operator training for restoration and addresses the future, providing examples of innovative solutions. Readers are encouraged to reach out and read the full paper in the CIGRE Science & Engineering Journal’s Volume No 14, June 2019 issue.

 

Download this Reference Paper : Reference RP_304_1

 

CIGRE Reference Paper : Defining power system resilience

01 October 2019
By E. CIAPESSONI (IT), D. CIRIO (IT), A. PITTO (IT), M. PANTELI (UK), M. VAN HARTE (SA), C. MAK (CA) on behalf of C4.47 WG Members

 


 

The term resilience has been used in very different fields of knowledge for many decades. In the electricity sector, the adverse impact of natural and man-made hazards on critical infrastructures has resulted in governments, regulators, utilities, and other interested stakeholders seeking to formalise a framework to oversee and enhance resilience. In essence, such formalisation aims to define strategies to improve the ability of a critical infrastructure to anticipate and prepare for critical situations, to absorb impacts of hazards, prevent deterioration in service to the point of failure, to respond to and recover rapidly from disruptions, and to make adaptations that strive to provide continued essential services under a new condition.

Despite several attempts by organisations worldwide in the power and energy engineering communities to define resilience, there is not as yet a universally accepted definition because resilience is a multi-dimensional and dynamic concept. Resilience is more than simply “the ability to bounce back” after a failure; an organisation seeking to be highly resilient also needs to continuously focus on aspects related to the potential for multiple failures at all levels of the organization, to find opportunities to improve its emergency preparedness and operational practices prior to, during, and following major disturbances, and service interruptions, as well as improvements based on lessons learnt from past events.

 

CIGRE WG C4.47 – Power System Resilience

Given these challenges facing the electricity sector, CIGRE SC C4, in 2017, has established a Working Group to provide guidance on these challenges and attempt to set a standardised approach to resilience thinking and practices in the electricity sector. WG C4.47 – Power System Resilience comprises a large number of international experts from 19 countries. This worldwide perspective has formed the expertise foundation for the development of an industry-accepted resilience definition and framework in the electricity sector.

The need for a standardised approach is further confirmed by an international survey conducted by the WG in 2018 with results highlighting the pressing need and elevated interest of utilities worldwide in evaluating the impact of extreme events that could potentially cause widespread disruptions of critical infrastructures. The survey suggested that utilities require measures to contain and/or respond to the effects of such extreme events.

The purpose of this reference paper is to present the CIGRE WG C4.47 definition of power system resilience in the electricity sector.

This will assist utilities to better understand the concept of resilience and how it differs from the well-established concept of reliability. The WG conducted a comprehensive review of resilience literature leading to the final definition of power system resilience that is discussed in this paper.

 

From reliability to resilience

The concept of reliability was introduced in order to assess the performance of the power system in providing energy to users even in the case of disturbances. This property has been defined by several well-recognised institutions, such as CIGRE, IEEE, IEC, NERC, and ENTSO-E, in terms of adequacy and security. The definition of reliability has recently been updated in TB 715 on the “Future of reliability” and in the corresponding article in Electra No 296 (February 2018).

All these definitions agree that reliability refers to the probability of the satisfactory provision of power and energy to meet load demands and ability to withstand disturbances. The performance and degree of reliability of a power system can be generally measured and benchmarked through the frequency, duration, and intensity of service degradation due to grid disturbances.

Resilience, as a concept, adds a new dimension to system management and reliability. The concept discussed below is intended to assist utilities and regulators to encourage prudent investments to enhance resilience capabilities of the interconnected power system in case of extreme events that are characterised by their low frequency of occurrence but with significant consequences. These extreme and disruptive events are normally initiated by multiple contingencies resulting in significantly deteriorated operational capabilities, possibly leading to widespread cascading impacts that could also affect interdependent critical infrastructures with catastrophic consequences.

Therefore, resilience assessments may require a multi-dimensional evaluation of the response of an interconnected power system to these extreme and disruptive events. Furthermore, achieving resilience may require multiple strategies with due consideration of utility response objectives for planning and/or response efforts. These undertakings can be very complex and challenging due to the interdependence and relationship with essential services and mission-critical loads.

 

Definition within the electricity sector

Ecologist CS Holling is considered by many to be the first to provide a foundational definition of resilience, in 1973. This definition of resilience has been adopted by numerous researchers from different disciplinary perspectives and evolved into different resilience definitions. The key capabilities in the definition of resilience can be tailored to support particular applications for enhancing utility strategies against extreme events.

To adapt the definition of resilience to the electricity sector, CIGRE WG C4.47 performed a comprehensive review of the applicable resilience definitions, provided by different stakeholders (academia, government, engineering societies, regulators, infrastructure operators), some of whom are generic on critical infrastructures while others are specific on electricity infrastructures. The goal of the WG was to compare their merits and appropriateness so that the key features can be incorporated into a comprehensive resilience definition that is suitable for power system application.
 

The review of the WG has culminated in the following concept of resilience that:

  • requires a comprehensive evaluation of system response to disturbances, including not only the system degradation but also the system behaviour during the restoration phase, as well as all the measures taken to preventively improve system performance;
  • supports the characterization and design of actionable measures aimed at improving the performances of the power system response following extreme events triggered by adverse weather conditions, malicious acts, cyber-attacks, etc. with due consideration to past extreme events.

 

CIGRE WG C4.47 definition for power system resilience

The new definition is intended to be different from the existing definitions in separating the resilience properties (or abilities) from the key actionable measures that collectively contribute to the achievement of enhanced power system resilience.

WG C4.47 defines power system resilience as follows:

Power system resilience is the ability to limit the extent, severity, and duration of system degradation following an extreme event.

 

As an integral part of the definition, it includes the following key actionable measures:

Power system resilience is achieved through a set of key actionable measures to be taken before, during, and after extreme events, such as:

  • anticipation
  • preparation
  • absorption
  • sustainment of critical system operations
  • rapid recovery; and
  • adaptation

including the application of lessons learnt.

 

Resilience properties of new definition:

  • Almost all of the definitions describe resilience as an “ability” of the power system or system or infrastructure. However, most of them are “operationally oriented definitions,” that is, they define resilience by using those measures (such as fast recovery, shock absorption) that make the system resilient. Some of the definitions also describe resilience as a contingency-withstanding capability, which does not help clarify the salient characteristics of resilience in response to extreme events resulting in multiple contingencies on the system.
  • The terms “extent and severity” in the WG definition respectively refer to the geographical extent and the intensity of the effects of the event on the interconnected power system. This assures a more focused characterization of the dimensions of system degradation while keeping the definition concise and informative. Note that the term “severity” of system degradation must be kept separate from the “severity of the event,” which in general does not imply any system degradation. “Severity” also depends on the (inter)dependence between essential or mission-critical loads and the disrupted and/or impaired system.
  • The term “duration” refers to the time period of the negative effects on system performance with respect to the normal situation.
  • The term “degradation” is intended as a deviation from specified target performances. This term refers to the criteria used to apply the resilience concept in system planning and operation and it also refers to both infrastructural and operational resilience. As is commonly known, the costs to assure power system reliability in case of multiple contingencies can be unacceptably high and unsustainable; thus, the rationale is to provide a resilience-centric criterion of not exceeding maximum specified deviations of system performances (degradation) in case of extreme events.
  • The term “extreme event” refers to an event with a large impact in terms of degraded system performance, damaged components, and reduction of component operational capabilities, as well as unsupplied customers. With this specification, WG C4.47 intends to link the definition of resilience properties with the application criteria (that is, extreme events). Due to the physical nature of large synchronously interconnected transmission systems, extreme events can be accompanied by the loss of multiple components, cascading outages, or loss of stability followed by widespread interruption to electricity users and, in the worst-case scenario, a total system blackout.

 

Key measures of the WG definition

The new definition clearly separates the definition of the properties from the key actionable measures that can be deployed [Before (B), During (D) and After (A) events] to achieve or enhance resilience, considering the utility’s objectives and the lack of an international standardised framework to support decision-making for resilience enhancement investments:

  • The process of “anticipation(B) refers to evaluating and/or monitoring the onset of foreseeable scenarios that could have disastrous outcomes. It assists power system engineers to enumerate plausible disaster scenarios and proposed mitigation plans and allows decision-makers to envisage the “multiple” future states and strategies required to contain, avoid, and/or respond to an emergent threat to the power system.
  • Preparation(B) is the process required by decision-makers to advance the knowledge gained during the anticipation phase from the resilience strategies to clear objectives to guide the deployment of measures considering tolerance to the possible adverse consequences, with emphasis on maintaining mission-critical loads and the minimum system load level to sustain a reduced but acceptable functioning of everyday life and importantly orderly functioning of a modern society.
  • The process of “absorption(D) is to meet defined objectives by means of which a system can absorb the impacts of extreme events and can minimise or avoid consequences. The outcomes are represented by the slope and the amount of the power system performance degradation after the shock has occurred or been avoided.
  • The “sustainment of critical system operations(D, A) refers to the process of maintaining the operational capability of the impaired power system to supply the mission-critical loads and a minimum system load level to maintain a reduced but acceptable functioning of everyday life and, importantly, orderly functioning of a modern society that are dependent on so many critical and interdependent infrastructures driven by electricity. This may require the deployment of additional components (for example, mobile generator), systems (for example, uninterruptible power supplies), and distributed energy resources to sustain operations until the power system is restored to a normal or near-normal state.
  • The “rapid recovery(D, A) process requires the operational response to the initial shock to contain or limit the consequence to the disruptive events, by focusing on mission-critical or essential loads that are required to support the restoration efforts. This requires integrated planning to develop efficient and effective response plans in a co-ordinated manner to recover the system operation to a normal or near-normal state.
  • In the “adaptation(A) process, changes are carried out in the power system management, defence and operational regimes, on the basis of past disruptions, in order to contain and/or limit the undesirable situations. This process includes the upgrades of prevention barriers, operational regimes, and maintenance procedures on the basis of lessons learnt from past disruptive events.

 

Concluding remarks

This reference paper is a summary of the outcomes of CIGRE WG C4.47 activities on the definition of resilience within the electricity sector. It should be read in conjunction with the technical papers and/or brochures to be published by the WG.

In consideration of the scope and technical complexity of the topic, resilience assessments and enhancements require analyses on the interactions between humans, environment, power systems, and other interdependent critical infrastructures evolving over the planning and operation horizons, with due consideration to lessons learnt from past events and projected future scenarios.

In this context, the present paper attempts to emphasize the foundational definition for power system resilience.

 

Download this Reference Paper here

 

 

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