Every year, the ESO assesses and manages outages on the National Electricity Transmission System across the Scotland, England and Wales.

The Transmission Owners build detailed year ahead plans, including all expected maintenance requirements, and then ESO Network Access Planning work with TOs and other affected stakeholders, like generators and DNOs, to optimise plans and create running arrangements.

Take a look at how a request for an outage on the National Electricity Transmission System is actioned by the Network Access Planning Team of engineers.

What happens when there’s an outage infographic

Further information

Outage Request Submission by a Transmission Owner (TO) via the ESO Outage Planning Database eNAMS.

The outage planning process and how it works for all Users is detailed with the Grid Code’s Operating Code No.2.

Outage requests can be submitted by Users to NGESO for assessment and sanction at any time from six years ahead (and beyond) down to day-ahead and, in certain circumstances such as equipment faults, on the day in real time.

The outage planning process between onshore TOs (Scottish Hydro Electric Transmission, SP Transmission, National Grid Electricity Transmission), offshore TOs and NGESO is further detailed in the System Operator Transmission Owner Code Procedures (STCPs). Those referring to outage planning are:

Under STCP 11-2, NGESO is obliged to maintain an Outage Planning Database to manage all outage requests and assessment outcomes together with release and return details. eNAMS, the electricity Network Access Management System, is the tool that is currently used for this purpose.

ESO challenges:

The STCPs are written such that the ESO can issue a final, year ahead outage plan to all affected users by 49 of year ahead. Where possible, it is preferable to include as many of the planned outages in the year ahead plan as high volumes of outage change in current year, after the final, year ahead plan is issued, produces additional plan re-work for ESO and affected users.

Any outage changes received following the initial TO submission means the ESO detailed outage assessment needs to begin again.

A team of 65 engineers based near Wokingham is responsible for building the final, UK outage plan at the year ahead stage and then assessing and sanctioning the changes to that plan within current year. Typically, around 2500 outages will be placed by NGESO into the final, year ahead plan issues to all affected users in week 49. However, in a typical year, NGESO will go on to process over 17 000 TO outage changes to the final, year ahead outage plan; around 7 500 in Optimisation time scales (Year ahead to 3 week ahead) and 10 000 in Delivery time scales (3 week ahead to day ahead).

ESO challenges:

  • The time required to assess an outage change or a new outage request can vary from a couple of seconds to several weeks, depending on the complexity of the change and the number of users affected. This will result in the TOs and Users experiencing varying outage change & new request processing times.
  • Under certain circumstances due to the urgent nature of a minority of outages, the outage change or new request does not pass through the whole outage planning process. A TO can take an outage on an unplanned basis in the control phase. Under these scenarios, the HV transmission system and connected users’ systems may be placed at additional risk.
  • Due to the rapid changes in both the type and location of new generation sources and due to an increase in interconnections to GB, there are fewer opportunities to easily accommodate changes to the outage plan. Analysis can be protracted, the viability of outages can be marginal and dependent upon several variables outside the direct control of the ESO such as embedded wind and PV.

Analyse the impact of the outage on local demand and transmission connected generation

Transmission outages can inevitably have an impact on users of the system such as generators, distribution network owners and other directly connected customers. If an outage is going to impede the ability of a user to gain access to the network then their agreement must be sought, or acknowledgement of their restriction received depending on their connection conditions. This must be done prior to commencement of a transmission outage and in accordance with the terms of STCP 11-1. Where a change has been proposed to the dates, duration or Emergency Return to Service (ERTS) times of an outage, the ESO must communicate these as succinctly and quickly as possible.

If an outage is going to increase the risk to a demand ‘group’ then robust checks must be undertaken to ensure the integrity of the remaining available transmission infrastructure and that there are contingency strategies in place to restore demand in the shortest possible time in the unlikely event of an incident which results in a loss of supply. There are additional obligations on the ESO where this demand may be considered ‘sensitive’ such as COMAH sites and major transport hubs.

Assess the impact the outage may have on network transfer capability (constraints)

A constraint exists on a transmission system where a prescribed amount of power (MW) cannot be transferred from one area of the network to another. We often refer to these as transmission boundaries and the major ones can be found in the ETYS document. The constraints can be caused by a number of factors and the main ones are listed below

  1. The thermal capability of the network – will the circuits be overloaded if actions are not taken?
  2. The pre and post fault voltages on the network – will we be able to maintain voltage levels to SQSS (Security and Quality of Supply Standards)
  3. Transient stability – will we be able to maintain stability of rotating generating plant on the network

There are other less common types of constraint beyond the above three, such as ROCOF, Vector Shift, Fault Levels and Response and Reserve. Each time an outage is requested, the ESO must perform a very detailed analysis of the impact that the outage might have on any constraint or combination of constraints. The constraints are very dynamic and each time a change is introduced the system a fresh challenge is presented to the ESO to understand the impact on these constraints.

ESO Challenges

  • The transmission system is very dynamic in nature and has become increasingly complex over recent years as we decarbonise our transmission system on the path to achieve net zero. Understanding the impact on security and cost have therefore also become more challenging resulting in not only an increased workload but also an increased interaction between outages. This means that more focus than ever before must be given to any changes in outage pattern.
  • The addition of complex dynamic equipment such as hybrid synchronous compensators, convertor stations & HVDC links to continental Europe have meant that market conditions can be less predictable in the longer term and firm conclusions over cost and security can sometimes only be made in timescales closer to real time.
  • Any change however seemingly insignificant or incremental must now necessarily go through a much more robust sequence of checks and analysis – this in turn can alter the impact on customers and consumers.

Calculate the cost of managing constraints

The constraints detailed in the previous section can have a cost associated with managing them. The ESO is responsible for balancing not only the total system frequency but also the active constraints on the system. The challenge when planning outages is to understand what the cost of managing the constraint will be. In the case of an export constraint it will be necessary to reduce the generation within the boundary and increase the generation outside the boundary (for replacement energy). The reverse is true for an import constraint.

When calculating the cost of managing a constraint the ESO must consider the prevailing generation conditions, the likely market conditions, the duration of the outage and it’s interaction with other transmission outages. The ESO will always endeavour to make the most economic decision and therefore will be working in the interests of the end consumer.

ESO Challenges

  • Volatility in the cost of energy such as that experienced in the 2021-2022 year can result in longer term outage costing being less accurate as we progress to real time management of an outage.
  • Uncertainly over the availability of generation, or changes to the REMIT data received can have an impact of the reliability of a cost forecast. Generators are at liberty to change outage dates for valid technological and commercial reasons and these changes can and do result in changes to the cost of a given outage or group of outages on the transmission network.
  • Constraint costings cannot be produced until detailed study analysis is complete, adding time to the process.
  • Late notice changes limits the number of commercial options available to the ESO.

Study reconfiguration of the network to reduce constraints and to protect equipment from faults

There are a multitude of variables which impact the transmission network and result in a system which is very dynamic in nature. These include, but are not limited to, the distribution and intensity of generation connected to the network, the distribution of demand (load) on the network and the impact of any other transmission and distribution outages. The ESO must ensure that the network is configured appropriately in order to maximise the transfer across boundaries.

Network reconfiguration is typically via adopting alternative switching arrangements at substations and through the use of quad boosters (phase shifting transformers) to alter the direction of travel of current. Each time an alternative substation reconfiguration is proposed to maximise boundary transfers, it is also necessary to ensure that this new configuration has not exposed any other areas of network to a potential excess of current under unexpected fault conditions. This process can become more complex if and when transmission outage requirements change, particularly at short notice.

ESO Challenges

  • Changes which may be required in network configuration for high fault level mitigation can have an impact on constraint costs. Changes to the outage requirements and changes to generation availability both impact fault infeed calculations. The ESO will not operate any substation such that it is at risk of unduly high fault infeed following a securable fault and will take the appropriate action to eliminate the risk. The upshot of this can sometimes be increased constraint costs if a substation can no longer be configured for optimum power transfer.
  • The most economical and secure reconfiguration is sometimes not available at all substations due to technical limitations on equipment (circuit breakers and isolators) at some substations.
  • Late notice of an outage change limits the choice of optimisation options available on the network.

Prepare strategies to return equipment under high cost or emergency situations

When planning outages the ESO compile documents for the Electricity Control Room, including Operational Notes and National Planning Advice (Picasso). These contain an extensive list of all of the factors which must be considered in order to secure the system against any possible unexpected conditions (contingencies). These documents will include switching actions to be taken, commercial actions to be taken to alter the output of generators and quite importantly it will include details of ERTS (Emergency Return To Service) times. These are the timescales in which transmission equipment can be returned to service under emergency situations or in unexpectedly high cost situations. These ERTS strategies and times form and important part of the planning process and must be agreed with relevant parties in the planning stages.

ESO Challenges

  • The efficacy of ERTS strategies, which may have been agreed many months in advance, can be nullified by short notice changes to the transmission outage plan and alternative post fault actions can be more challenging to assess in short notice timescales.
  • If an ERTS changes it can affect the viability of the outage proceeding. For example, an increase could mean that there is not sufficient time to take actions to secure the system. This is particularly pertinent in Winter when considering margins.

Look at alternative dates for the outage if required

The ESO will always be looking to balance the requirements of the Transmission Owners to maintain/reinforce their networks against the balancing costs associated with releasing transmission outages. We will always attempt to optimise the timings of outages to strike a balance between these two factors. The most opportune time to take outages is typically, but not always, during periods of low renewable generation output and not in combination with other transmission outages which may compound the cost or risk associated with them. If an outage is requested, and studies/calculations have shown that the outage will introduce an excessive cost or an increased risk we will therefore not consider it a viable proposal. We will then work with all relevant parties to look for alternative dates to undertake the work.

ESO Challenges

  • Alternative dates and times, chosen against the criteria detailed here are not always preferable to transmission system users. The dilemma between TO requirements and economics becomes a trilemma when the requirements of the users are considered. In these instances we use NAP Papers to gather the information and make the most economic decision for the end consumer.

In order to agree outages with 3rd parties (users) further assessment of the outage is needed. The network configuration at interface points will be discussed and agreed by all connected parties with a focus on the security of the local network.

Where the 3rd party is a Distribution Network Operator (DNO) a demand assessment is required. Our engineers will analyse historic electricity demand figures to understand how much energy is normally supplied at that point in the network. All relevant faults are considered and the most onerous is identified which enables us to calculate a value of firm capacity at the supply point. Further assessment is needed if the firm capacity is lower than the amount of predicted demand. In collaboration with our 3rd parties we will explore ways of reducing the impact of the outage and implement these measures before the outage is released.

We collaborate with all relevant parties to optimize the outage plan by aligning adjacent works. Often this can result in the outage moving by a number of days or even weeks and will restart the outage assessment process. Where priorities differ we employ the use of Network Access Policy (NAP) papers to document the decision making process. The final decision is always made to benefit the end consumer.

ESO Challenges

  • Where it is concluded that the firm capacity of a site is less than the maximum expected demand further action needs to be taken to ensure security of supply. Often there is the capability to transfer demand from one grid supply point to another therefore reducing effective demand at the concerned site. This will then require a re-assessment of firm capacity at the adjacent site meaning consideration must be given to third parties at both sites.
  • Network faults and outages impacting the ability to transfer demand will require a re-assessment of the outage, often the solution will be to re-plan work for when the interconnection is re-established. Delaying the works may then have knock-on implications for all connected parties.

When an outage is agreed the ESO has completed all analysis such that the Security and Quality of Supply Standards will be met all times and that the outage has been planned with minimal impact to the user and overall benefit to the consumer.

The ESO ensure that outages are released as planned and this is continually monitored as part of the Fail to Fly metric reported to Ofgem. Another reportable metric shared is Value Created, which documents the financial result of engineering decisions made every day by the ESO to optimise the network.

System conditions are continually changing and the ESO must ensure the network is resilient against faults, extreme weather conditions and evolving generation profiles. As these factors may change by season, by day or even by hour the ESO continually review our plan to ensure the security of the system.

ESO Challenges

  • Agreeing outages usually means the outage is agreed against a number of conditions. Typically, for one outage to start another outage must return beforehand. It is also agreed on the assumption that the immediate network remains in-tact. Faults or delays in returning work automatically trigger re-assessment, although an outage plan will be developed with a reasonable degree of flexibility such that contingency options are available.