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



CIGRE Reference Paper : Overhead transmission lines, gas insulated lines and underground cables

02 December 2019


There is a considerable amount of highly technical information, specifications and guides available on the transmission of bulk electrical power from one area to another.  This information is available from bodies such as CIGRE, IEC (International Electrotechnical Commission), and many National-based organizations. There is, however, very little information that would explain the fundamentals of the technologies in such a manner that it could be understood by a non-technical person or a person not involved on a day-to-day basis in that industry. This paper will attempt to fulfil that need by providing basic information in hopefully a readily understandable manner.

This paper refers to transmission lines exceeding 170kV alternating current (AC). Direct current (DC) connections and subsea cables are not a part of the scope of this paper (for those, other criteria apply to compare).  


Technical Basics


Some of the fundamentals of power transmission are the voltage and current levels used to transmit the power from one area to another. Roughly speaking the voltage multiplied by the current is equal to the power.  If one thinks of electricity in terms of water flow then voltage is like pressure i.e. it drives the current through the conductor in the same that pressure drives water through a pipe. Current is the flow of electricity through the conductor.

In AC transmission the power is transmitted utilizing a three phase system with three metallic conductors; the size of the conductors govern their thermal capability to carry current i.e. the larger the conductor the more current it can carry.  The conductors must be insulated from the ground and from each other in order to be able to withstand the voltage applied; again, the more insulation the higher the voltage that can be used in the transmission circuit.

The three conductors may be assembled in an overhead line circuit (OHL), an underground cable circuit (UGC) or a gas insulated lines (GIL) circuit. Each one will be described below.


One basic technical aspect to be considered is related to routing a transmission line, including:

  • route availability: it must be possible to construct the line
  • urbanisation: if the line is to be routed through an urban area, then future developments may have an impact on the route and design of the line.
  • route topography: if the terrain is very uneven or hilly, the technical challenges and costs increase



OHL: Overhead Lines


An overhead line circuit is typically composed of lattice steel towers which support the three conductors that make up the circuit. In some lines, tubular poles (pylons) are used instead of lattice structures. The conductors are insulated from the structures by means of insulators, which are made of toughened glass, porcelain or of composite materials.


Typical examples of OHL designs in current use are shown in several Cigre technical brochures and the Cigre Green Book “Overhead Lines”.

Depending on voltage and terrain, towers are typically 200 - 500m apart from each other. Many OHL designs are fitted with one or several earth wires at the top of the tower. These earth wires have two functions, firstly to protect the conductors from a lightning strike which might cause an outage and secondly in the event of a fault: the fault current will be mainly contained within the conductor/ earth wire loop and returned to earth. Earth wires are often fitted with fiber optic elements for communication purpose. It should be noted that in many cases OHLs have two electric circuits or more on the same tower. Each circuit may have 1-4 conductors in a bundle for each phase (and even more at Ultra High Voltage Lines). These OHLs can carry many times the power of a single circuit line with single conductors.




The design of an OHL depends on many factors including:

  • conductor size: the size of the conductor is dependent on the current to be carried. Of course, the size of the conductor also has an impact on the weight the tower must support – currently the standard maximum conductor size used is about 800 mm2.
  • ground clearance: the conductor must have a safe clearance from the ground and any buildings that may be located underneath it i.e. there must be no possibility of flashover from the conductors to the ground, persons or obstacles.
  • impact of weather: very strong winds may exert considerable mechanical loadings on the conductors and the towers; in addition, large ice loadings on conductors can impact on the towers. Of course, the worst loading is the potential combination of wind and ice. The lines are designed for such loadings.
  • electrostatic/charging effects: the impacts on metal structures in the proximity of the OHL are eliminated by earthing of such metallic facilities. In some countries, national regulations may apply in addition to ICNIRP values (International Committee for Non-Ionising Radiation Protection).
  • magnetic effects: The current in the conductor produces a magnetic field, and the voltage produces an electric field, both must be considered during the design of the line. There are non-binding, but recommended limit values provided by ICNIRP 1998/2010. The ICNIRP recommendation must be considered taking into account of the costs and benefits and where the time of exposure is significant.



UGC: Underground Cables


An underground cable circuit is composed of three power cables (three phases) and normally one communication cable installed in the ground to form one electric circuit. A typical design of a power cable is shown in figure below:



If the power to be transmitted is beyond the capability of one circuit, more parallel cable circuits (or more cables per phase) must be installed. The larger the conductor the more current it can carry and the thicker the insulation the more voltage it can withstand. The cables are manufactured in highly specialised factories and they are normally delivered in drum lengths varying from 500 - 1000m. In some cases, delivery cuts can be longer (2000m and above).


Such a circuit of 10km route length with drums of 1000m would have 10x3 i.e. 30 drums in total. It would require 27 joints to join the cables together and there would be 3 terminations or sealing ends at each end (Substation, Transition Compound or Equipment installed on towers).


The cables are typically installed in one of the following arrangements:

  • directly in the ground (trench)
  • in ducts installed in the ground
  • in concrete troughs
  • in a tunnel
  • in a pipe or pipes drilled into the ground to pass under some obstacle or encumbrance
  • on a cable tray attached to a bridge


The design of an underground cable circuit depends on many factors including:


  • conductor size: the size of the conductor is dependent on the current to be carried and the increase of temperature (due to the current flowing through the cable) of the surroundings as allowed by regulations. Of course, the size of the conductor also has an impact on the weight and size of the cable drum being delivered – currently the maximum standard conductor size used is 2500mm2.
  • soil thermal conductivity: the conductor size has an impact on the ability of the cable to dissipate the heat, which is created by the current flowing through the cable when it is delivering power. In the case of a cable installed in the ground this heat must travel through the soil surrounding the cable. Therefore the ground thermal conductivity and temperature also have an impact on the cable sizing.
  • presence and possible impact of other services in the soil which may conflict with the cable route either now or in the future (e.g. other cables, heating or cooling pipes, water supply and waste water).
  • urbanisation: if the cable is to be routed through an urban area future building or road developments may impact on the circuit
  • possibility of flooding: flooding may undermine the installed cable circuit.
  • cable pulling: the route and drum lengths and route topography must be such that the cables can be pulled into the selected installation arrangement i.e. trench, duct, tunnel, etc.
  • electrostatic effects: underground cables have no electrostatic effects initiated by the cable as the electric field is contained inside the cable and shielded by the screen. Electrostatic effect may come from equipment installed above ground (terminations)
  • magnetic effects: the current sets up a magnetic field which must be considered during the design of the underground circuit. As for OHL, the ICNIRP recommendation must be considered taking into account of the costs and benefits and where the time of exposure is significant. It should be noted that underground cables have higher magnetic fields than overhead lines at close distance, but the fields fall off more rapidly with distance.



GIL: Gas Insulated Lines


GIL are generally composed of three parallel aluminium tubes for one three phase circuit. The aluminium tubes are in sections (typically 12-18 m long and 500 mm enclosure diameter). They are bolted together with flanges (sealed with O- rings) or welded together on site to be gas tight (automated welding process including 100% weld quality control by ultra-sonic test). Inside each enclosure pipe a smaller cylindrical aluminium conductor pipe is supported by cast resin post insulators. The GIL enclosure pipe is filled with a gas mixture of 20% sulphur hexafluoride (SF6) and 80% nitrogen at 0.8 MPa pressure to reduce the greenhouse impact from SF6.



GIL may have approximately the same transmission capacity as an overhead line and about double the capacity of a XLPE cable system, depending on actual situations. GIL systems are mostly used to EHV voltages (>245 kV) up to 1000 kV. GIL installation is adapted to pipe line laying technologies and is carried out at local assembly and installation on site. All parts are delivered to the construction site and the laying follows a continuing process. The cost efficiency for this on-site laying process increases with the length of the transmission line to be above 1 km. For shorter length the factory orientated laying process may be more cost effective. This on-site laying process has been verified in many projects world-wide and offers a reliable and safe installation of the GIL. When the outer diameter of the enclosure is enlarged to about 750 mm also a clean air solution of GIL can be offered using Nitrogen and Oxygen only with a GWP (Global Warming Potential) of zero.


GIL are typically installed above ground, in tunnels (phases in vertical or horizontal arrangement) or in underground galleries. Direct buried installations are uncommon today, as it requires additional coatings for passive corrosion protection and cathode corrosion. Experiences with GIL worldwide is constantly increasing with ever larger project sizes (10-20 km route length), higher rated voltages (mainly 400, 500 and 1000 kV) and current ratings (3000, 4000 and 5000 A). The longest installation is the Tokai Line of Chubu Electric in Japan with two three phase systems of 275 kV and 5000 A of 3.3 km transmission route length in a tunnel.


The design of a GIL circuit depends on many special factors including:

  • presence and possible impact of other services which may conflict with the GIL route either now or in the future (e.g. cables, heating or cooling pipes, water supply and waste water).
  • route considerations the given bending radius can be a limiting factor for a route.
  • urbanisation: if the line is to be routed through an urban area, future building or road development may impact on the circuit, that’s why separate tunnels for electric transmission lines may be the best solution.
  • Electromagnetic effects: GIL circuits have negligible electromagnetic effects as the electric field is earthed through the metallic enclosure. The magnetic field is mostly superposed by the induced current into the solid grounded enclosure pipe.


Advantages and disadvantages of various technologies


It is very difficult to compare the three technologies as each circuit installation is different with respect to location, importance of the circuit, reputational and financial impact if there is an outage, method of installation, operational and maintenance aspects, environmental impact, planning/licensing, etc. In view of this no general conclusions can be drawn, and each installation must be treated on a case by case basis. For the comparison of GIL and UGC see CIGRE TB 639.


In Table 1 we endeavor to compare the three technologies under the listed heading:


Table 1 - Comparison of Technologies



The operational and environmental aspects are considered in sections 4 and 5 below.

Each of the above factors needs to be considered specifically for the project being investigated taking the potential installation methodologies into account, which is for UGC and GIL e.g. direct burial, ducting, horizontal directional drilling, tunnelling. Lifetime-costs may give other factors than investment costs. They also depend strongly on the project and must be calculated case by case.

In order to rank the different possibilities, a scoring system could be developed for each of the above factors. Notwithstanding any scoring system experience and mature technology will always be important in any project as the Line owner will not wish to use unproven technology, as that would constitute a high and unacceptable risk.


Operational Aspects


In the table below the various technologies are compared from an operational point of view.


Table 2 - Comparison of Operational Issues


Each of the above factors needs to be considered specifically for the project being investigated taking the potential installation methodologies into account e.g. direct burial, ducting, horizontal directional drilling, tunnelling, etc.


Due to the very different electrical parameters of the UGC, the application of UGC introduces a series of technical challenges that must be addressed during planning, design and operation stages of the UGC system. In AC networks there normally is an offset between the current and voltage. This is due to the different components and loads in the network. The current will fill up the conductor to a certain degree, but only a part of the current can be used as “real power” because of this offset. The rest is “reactive power”. This reactive power shall be compensated to reduce the losses in the network and to control the voltage.


The exchange of reactive power between the UGC and the power system is significantly higher compared to an equivalent OHL. This reactive power must be compensated and therefore a number of additional components are introduced. This adds complexity to the system both in term of operation and maintenance.


Another complication that must be addressed is the shift in system resonance frequencies introduced by the application of UGC. Experience from several countries shows that amplification of background harmonics (electrical noise) occurs due to interaction between the UGC and the power system. Furthermore, the risk of temporary overvoltages is also increased for the same reasons. Both diminished power quality (due to electrical noise) and temporary overvoltages are serious challenges for which mitigation methods are expensive. A further complication is that study and design for the mentioned issues are still comparable immature and little practical experience exist worldwide. Hence, it can be difficult to quantify the risk to the system when a longer UGC is added.


The capacitive load of GIL is much lower (factor 4-5) than for solid insulated cables. Therefore, a phase angle compensation is only needed with GIL transmission length of 100-200 km length. This is depending on the network conditions and needs to be calculated. In principle the GIL can be operated like an OHL including the auto-reclosure function for short time interruption without any danger to the surrounding.


Environmental Issues 


In the table below the various technologies are compared from an environmental impact point-of-view.


Table 3 - Comparison of Environmental Issues



Behaviour under large disturbances


The following table compares the technologies when subject to large disturbances:


Table 4 - Comparison of behaviour under large disturbances


New Technologies


OHL, UGC and GIL continue to evolve with improvements in the manufacturing and in the installation equipment and technologies.

As far as the technology aspect is concerned for OHL, the adoption of composite insulators has been widely adopted. In addition, the use of high temperature conductors and real-time rating applications has become standard use.

For UGC there have been some developments in polymeric materials for insulation other than the currently used XLPE, but it is not yet clear when they will become commercially used. In addition, sensors are often embedded in UGC thus improving the use of real time monitoring and management systems.

There is little further development in the GIL technology, except maybe the routes can become longer. Insulating gases others than N2 / SF6 gas mixtures are under development for clean air (N2 / O2).


Better use of existing lines


One of the areas of interest is about the “better use of existing lines” i.e. the possibility to get more power through the existing lines as this might replace or postpone the need for a new development. In the case of existing OHL it is possible to replace existing conductors with high temperature conductors or to use real time rating applications or to increase the voltage. Existing AC OHL can be converted to DC, if the design of the line allows this. A remarkable increase of transport capacity can be achieved.

In the case of both UGC and GIL the preferred method to increase the power capacity on existing lines consists in the possible use of real time rating applications and the mitigation of hot spots. For buried systems (typically for UGC) this is associated with the longer thermal transients that may allow for cables overloads.




The fundamentals of UGC, OHL and GIL technologies have been outlined in Section 2. It is very difficult to compare the three technologies as each installation is different with respect to location, importance of the circuit, costs, reputational and financial impact if there is an outage, method of installation, operational and maintenance aspects, environmental impact, planning/licensing, lifetime, etc. In view of this no general conclusions can be drawn and each installation must be treated on a case by case basis using the headings outlined in Sections 3, 4 and 5.


In order to rank the different development possibilities a scoring system could be used for each of the headings. Notwithstanding any scoring system, some headings will always be important in any project as the Project Engineer will not wish to propose an installation that would be unacceptable to the planners or to proceed with a development that may appear too costly or where the technology is not suitable for the proposed end-use.


Literature and Glossary


CIGRE published lot of documents and Technical Brochures (TB) that can help to further understand the technologies available for power transmission. Most relevant are:

  • TB 194: Construction, laying and installation techniques for extruded and SCFF cable systems.
  • TB 250: Technical and Environmental issues regarding the integration of a new HV underground cable system in the network.
  • TB 498: Guide for application of direct Real-Time monitoring systems
  • TB 583: Guide to the conversion of existing AC lines to DC operation
  • TB 601: Guide for thermal rating calculations of overhead lines
  • TB 606: Upgrading and uprating of existing cable systems.
  • TB 639: Factors for investment decision GIL vs Cables for AC transmission.
  • TB 680: Implementation of Long AC HV and EHV cable systems.
  • TB 695: Experience with the mechanical performance of non-conventional conductors
  • TB 748: Environmental issues of high voltage transmission lines in urban and rural areas.
  • TB 756: Thermal monitoring of cable circuits and grid operator’s use of dynamic rating systems.
  • CIGRE Green Book “Overhead Lines”
  • CIGRE Green Book “Substations”



The following acronyms are used in the text:


  • AC Alternating Current
  • DC Direct Current
  • EHV Extra High Voltage
  • GIL Gas Insulated Line
  • GWP Global Warming Potential
  • ICNIRP International Committee for Non-Ionizing Radiation Protection
  • OHL Overhead Line
  • PE Polyethylene, insulating material
  • PVC Polyvinyl Chloride, insulating material
  • UGC Underground Cable
  • XLPE Cross-linked Polyethylene, insulating material


Download this Reference Paper : Reference RP_307_1


Your AccountYour Account


may download free of charge all publications including most recent ones


Non Members

may download free of charge publications over three years old and purchase all publications

For any questions in connection with the on-line library, contact:


Publications and Edition Manager