Select Committee on Environment, Food and Rural Affairs Minutes of Evidence

Memorandum submitted by National Grid (FL 80)


  National Grid electricity and gas engineers worked to maintain security of energy supply throughout the adverse weather and heavy flooding affecting South Yorkshire and Gloucester during late June and July 2007.

  As outlined in the report below, the effects of the weather conditions in South Yorkshire during the week of 25 June impacted on National Grid's operations and assets in a number of ways:

    —  Firstly, risks posed by the potential breach of the Ulley Dam in South Yorkshire impacted on National Grid's gas distribution operations at the Above Ground Installation (AGI) at Guilthwaite and in addition Ulley Dam posed a risk to National Grid's electricity substation at Brinsworth.

    —  Secondly, as a result of severe weather, National Grid's Neepsend electricity substation was affected by flood waters, which led to the local electricity distribution network, CE Electric losing electricity supplies from this supply point.

    —  Thirdly, National Grid's gas distribution network was affected in the Sheffield and Toll Bar region by a number of weather related incidents affecting gas supply pipelines at Kilham farm and Toll Bar detailed in the report below.

  National Grid's operations and assets were also affected in July 2007, as a result of flooding in the Gloucester area.

    —  National Grid's Walham substation was affected by the adverse weather conditions on 21 July which led to parts of the substation becoming inundated with water. National Grid engineers reconfigured electricity systems to ensure continued electricity supply to Gloucester and South Wales.

  National Grid has a number of measures in place to assess flood risks to its electricity and gas assets and has carried out a number of assessments on infrastructure in recent years. Whilst flooding has not been a cause of significant unreliability or unsupplied energy, with concerns over climate change and rising sea levels, National Grid is working to understand the impact of any consequent increase in flood risk, and the requirement for further risk mitigation at our sites to ensure a high level of reliability is maintained.

  National Grid is participating in an Energy Networks Association (ENA) review group, addressing electricity resilience issues, which has been established at the request of Energy Minister, Malcolm Wicks. This group will report to the Energy Emergencies Executive Committee.


  1.  National Grid plc owns and operates the high voltage electricity transmission system in England and Wales, and operates the Scottish high voltage transmission system. National Grid also owns and operates the gas transmission system in Britain and distributes gas in the heart of England, to approximately 11 million offices, schools and homes. National Grid also manages electricity and gas assets in the US, where it operates in the states of New England and New York.

  2.  In addition, National Grid owns and operates other energy infrastructure such as liquefied natural gas importation facilities at Grain and the electricity interconnector with France. National Grid owns around 20 million gas meters in Britain and is at the forefront of gas and electricity smart metering competition.

  3.  National Grid is pleased to have the opportunity to contribute to this inquiry and can report the following account of the effects of the flooding that occurred during summer 2007 on its electricity and gas infrastructure.


  4.  The majority of the electricity transmission system was designed and constructed in the 1960's. Subsequent development has been incremental and generally in close proximity to original installations. Flood risk was considered during the original planning process in accordance with our procedures at the time, using the best information available at that point.

  5.  The majority of the gas National Transmission System (NTS) and Local Transmission System including fixed above ground installations (AGI's) was constructed between 1965 and 1982. At construction, installations were frequently sited to utilise brownfield sites from pre-existing gas distribution installations. Flood risk was evaluated as part of their original design. As with electricity transmission, in recent years the majority of investment on the gas National Transmission System (NTS) has been associated with the development of existing sites.

  6.  In developing existing sites or establishing new sites National Grid is obliged to put forward designs which take into account a range of technical, environmental and commercial drivers with the aim of achieving:

    —  The requirements of the connected party (customer).

    —  Compliance with the relevant planning standards.

    —  Minimal impact on the public, particularly in terms of visual amenity or noise.

    —  The most economic and efficient solution.

    —  Compliance with the relevant National Grid design standards.

  7.  Within the last 10 years only a relatively small number of greenfield sites have been developed for either gas or electricity transmission. As part of the design process Environment Agency flood contour information is considered along with other available information to inform site selection.

  8.  The introduction of Planning Policy Statement 25 (PPS25) in 2006 placed additional responsibilities on National Grid as electricity substations and gas compressor stations are defined as "Essential Infrastructure" within the statement. Any proposed development in a Medium Probability Flood Zone or above (greater than 1 in 1000 risk of flooding) is subject to greater scrutiny with respect to site selection. Where development is to go ahead in a high probability flood zone the design must ensure that the site remains safe and operational during a flood.

  9.  National Grid's overhead line, cable and buried pipeline assets are generally secure against flood risk, with the main risks posed by land erosion, and movement of structures caused by floods near transmission assets. However, electricity substations and above ground installations (AGI's) associated with gas transmission are not immune and inundation can result in the need to disconnect critical plant and equipment.


  10.  Following the flooding that took place in the South of England during 2001, National Grid undertook a review of sites within "flood warning areas" as defined by the Environment Agency at the time. Whilst events drew attention to flood risk, National Grid's response was focussed on mitigating the effects of our operations on the environment. To this end we have an on-going environmental investment programme, spending £108 million over the last 6 years.

  11.  Following the floods affecting Carlisle in 2005, National Grid reviewed flood risk at its transmission infrastructure sites using flood risk contour data available from the Environment Agency. In total, 23 sites were identified and considered to be at risk. This work has been revisited recently, using the latest Environment Agency flood contour data. It is now the case that 28 electricity substations and 7 gas installations have been identified as at significant risk of flooding (greater than 1 in 75 risk of flooding in any year).

  12.  In addition, during 2005, National Grid commissioned engineering consultants to consider methods of assessing actual flood risk at electricity substation sites. In 2006 National Grid initiated more detailed site survey work with our engineering consultants to establish indicative costs of mitigation against flooding at high risk sites.

  13.   National Grid reviewed gas transmission sites in 2006 to address installations at risk to flooding and also commissioned an engineering review in July 2007 to consider the risk posed by different levels of flood water.

  14.   Further work is on-going, for electricity transmission, gas transmission and gas distribution, to determine the level of flood waters necessary to render specific sites non-operational and to determine the specific equipment that will first become affected in the event of flooding.


  15.  Whilst flooding has not been a cause of significant unreliability or unsupplied energy, with general concerns over climate change and rising sea levels and the consequent increase in flood risk, further consideration is being given to risk mitigation at our sites to ensure a high level of reliability is maintained.

  16.  Recent events have provided the opportunity to test emergency response procedures and National Grid's ability to react to unplanned events.

  17.  National Grid has initiated further research and development work to improve the understanding of the linkage between flood risk, weather events and levels of monitored waters so that we can better identify flood risk. Most recently, National Grid has collaborated and co-funded industry-wide research and development with the Meteorological Office Hadleigh Research Centre to understand and prepare for the wider impacts of climate change, including assessment of likely flood scenarios on transmission operations. The work is due to report initially in late 2008.

  18.  National Grid will take forward the procurement of mobile flood defences to provide interim cover catering for low probability events and to provide defence against surface water flooding not covered by Environment Agency risk models. The need for any further investment at our sites will be discussed with our regulator, Ofgem.

  19.  National Grid is participating in the Energy Networks Association review group, to examine the resilience of electricity substations. The review has been established at the request of Malcolm Wicks and will report to the Energy Emergencies Executive Committee. National Grid will work with the Environment Agency and other stakeholders to provide a co-ordinated input to this review.


Floods in South Yorkshire Monday 25 June—Friday 29 June 2007


  20.  The "Sheffield Ring" is a group of circuits and substations which transports electricity to the Sheffield conurbation and allows the transit of power more generally across the country. The Sheffield conurbation is supported by the broader transmission system through connections principally at Brinsworth, Neepsend and Thorpe Marsh 275kV substations. (see appendix i).The flooding affecting these sites on week commencing Monday 25 June resulted in a loss of supply to the local electricity distribution network, CE Electric which is fed from National Grid's Neepsend substation.

  21.  National Grid site staff became aware of the flooding at Neepsend substation at midday on Monday 25 June and raised the issue with their local management team at 12:30. Water levels rose rapidly and as such National Grid's site staff were evacuated for safety reasons at approximately 13:15.

  22.  Due to the risk of loss of supply caused by the flooding, Electricity National Control Centre staff and CE Electric Control coordinated efforts to transfer as much electricity demand as possible away from Neepsend substation in order to ensure security of supply to the area.

  23.  At 15:23, approximately 20MW of demand was successfully transferred at which point circuit-breakers on the transformer circuits between National Grid's substation and CE Electric substation opened without instruction, due to the effects of flood water on control and protection equipment. This resulted in the loss of approximately 38MW of demand, affecting an estimated 36,000 CE Electric domestic and commercial customers.

  24.  Further reports from National Grid's site staff indicated that the floodwaters inside Neepsend substation had reached a depth of 1.2-1.5 m. The 275kV circuits connecting Neepsend substation to the rest of the transmission system were opened in order to de-energise the substation for safety reasons.

  25.  Following the flooding at Neepsend substation, a flood risk assessment at other National Grid substations in the Sheffield area was undertaken. The outcome of this assessment showed no risk of breach of flood defences at Brinsworth substation and whilst there were access problems at Thorpe Marsh, the sites operations remained secure (the 275kV and 400kV substations are raised relative to the local ground level and site access roads). National Grid staff remained at Thorpe Marsh substation in order to manage any potential further difficulties.

  26.  At 01:15 on the morning of Tuesday 26 June, National Grid managers joined the Gold Command and were alerted to the possible risk to Brinsworth substation from the Ulley dam. Staff were evacuated from Brinsworth substation and the local area. The substation remained fully operational under the control of National Grid's Electricity National Control Centre (ENCC).

  27.  The impact of dam breach on Brinsworth substation was considered and contingency plans were prepared for the eventuality of having to make Brinsworth substation dead (in the event of deluge from the dam waters).

  28.  Concurrently, the flood risk at National Grid's Thorpe Marsh substation was monitored through communication with field staff and engagement with the Environment Agency. At this time the advice was that National Grid's substation at Thorpe Marsh was secure. However, water levels at Thorpe Marsh continued to rise and by the evening of Tuesday 26 June water levels were approached control and protection systems in the 400kV substation. Sandbags were ordered to protect most critical parts of the site and the army were engaged via Gold Command to provide boats to facilitate movement around the site.

  29.  By early morning on the Wednesday 27 June, flood water started to affect vital site controls and protection at National Grid's Thorpe Marsh 400kV substation. At 07:30 National Grid engineers began to de-energise the 400kV substation circuits in a controlled manner. The 275kV substation at Thorpe Marsh remained fully operational as it is on higher ground than the 400kV substation, and de-energisation of the 400kV substation did not result in loss of supplies to any customers. A huge effort then ensued with National Grid site staff, the military and fire brigade focussed on constructing flood defences around critical plant and equipment. At the same time, National Grid engineers monitored the situation at National Grid's Brinsworth substation and the overhead line circuit which was at risk from the potential breach of the Ulley Dam.

  30.  During Wednesday 27 June the protection and control systems at Thorpe Marsh 400kV substation were secured from flooding by National Grid field staff and the emergency services. The substation was partially restored to operational service by 15:00 and fully restored by 01:50 on the morning of Thursday 28 June.

  31.  By the evening of the Wednesday 27 June water levels in the Ulley Dam had dropped significantly and the risk of flooding at Brinsworth substation was significantly reduced.

  32.  At Neepsend substation, on the morning of the Wednesday 27 June, co-ordinated efforts between CE Electric and National Grid allowed restoration of electricity supplies to all customers by Thursday 28 June. By Friday 29 June at 23:19, National Grid restored sufficient electricity capacity to meet all the local distribution company's energy demand.

  33.  National Grid engineers continued to service the remainder of National Grid's Neepsend substation in order to provide security of supply at Neepsend and restore resilience on the electricity network.


  34.  National Grid operates the gas distribution business serving the heart of England, where it transports gas to over 11 million homes, offices and businesses. Events in Sheffield during the week of 25 June-29 June impacted on gas operations in a number of ways.

  35.  Following heavy rain on Monday 25 June, fears that the Ulley Dam near Rotherham would breach posed a severe risk to National Grid's AGI (Above Gas Installation) at Guilthwaite, which is one of three primary supplies into the conurbation of Sheffield. A number of measures were put in place by National Grid managers and staff to ensure continuity of gas supply to the Sheffield area.

  36.  An engineering solution was put in place to protect gas supplies to Sheffield. Due to adverse weather conditions and significant risk of the dam failing National Grid personnel were not allowed on site at Guilthwaite, as such work took place on Tuesday 26 June at locations either side of the AGI to reduce pressure in both pipelines.

  37.  National Grid staff manned sites either side of the Guilthwaite AGI right through until Friday 29 June when the threat from the Ulley Dam subsided. At this point National Grid engineers were able to re-establish pressure at the AGI. Working with the Gold Command Unit, the Environment Agency and other utilities, National Grid was able to ensure there were no unplanned interruptions to gas supply as a result of the events that took place at the Ulley Dam.

  38.  National Grid's gas distribution teams also worked on a number of river crossings around the Sheffield area affected by adverse weather and rising water levels in the river Don during the week of 25 June and the following week. Maintaining the safety of gas pipelines was the key issue with floodwaters potentially affecting critical infrastructure.

  39.  A National Grid 24 inch diameter seven bar pipeline at Kilham farm became exposed as a result of floodwaters rising, and was found by National Grid gas engineers floating in a field as a result of the breach of river Torne (appendix ii). National Grid engineers worked in difficult conditions in conjunction with control room and planning staff to prepare appropriate contingency arrangements to ensure continuity of gas supply was maintained along this route. In addition National Grid engineers worked in close co-operation with the Environment Agency throughout the operation at Kilham farm.

  40.  In spite of the significant flooding at Toll Bar during the first week in July, National Grid, through configuration of the gas network was able to prevent significant water ingress (ie. flooding of gas main) into the gas network thereby protecting supplies to customers in the Toll Bar area.


Thursday 21 July-Tuesday 24 July Walham


  41.  Walham substation provides approximately 470MW of electricity supplies via four transformers to 450,000 domestic and commercial customers of Central Networks Power distribution company in the Gloucester area. Transmission circuits emanating from Walham form a part of the interconnected transmission network to South Wales supplying approximately 3,000 MW of demand (appendix iii).

  42.  National Grid engineers and staff working at Walham on Saturday 21 and Sunday 22 July carrying out planned repair work were alerted by Electricity National Control Centre (ENCC) staff that there was a risk of flooding in the area caused by rising river levels.

  43.  During Sunday 22 July, ENCC staff developed a contingency strategy in the event that National Grid's Walham protection equipment was inundated by rising water. Through re-configuring transmission circuits at Walham substation, National Grid ensured that in the event of inundation of Walham substation, only one of the two circuits feeding into South Wales from Walham would be lost. This route supplemented the three other available transmission circuits into South Wales ensuring that all demand in South Wales would be secure even in the highly unlikely event of the worst case scenario.

  44.  On Sunday 22 July water levels started to rise. National Grid staff placed sandbags around the critical circuit control cubicles and entry points to the protection (relay) rooms at around 14:00. The fire brigade were also in attendance to provide pumps working closely with National Grid staff to protect critical infrastructure.

  45.  Gold Command (Resilience) was established by the authorities in Gloucester at on the Sunday 22 July. The first Resilience meeting was held with National Grid in attendance. Following this meeting the Environment Agency provided emergency flood defence systems to be used at Walham later on the evening of Sunday 22 July. The military attended Walham substation to assist with the installation of the systems.

  46.  At 21:00, on the Sunday 22 July, one of the four Walham transformers was de-energised and isolated due to rising water levels.

  47.  The temporary flood defences withstood the peak water level which occurred at around 05:30 on the morning of Monday 23 July.However, it was expected that the river level would peak again during the night of Monday 23/Tuesday 24 July at the same or a higher level than had been seen the previous day. National Grid engineers began switching operations to implement the contingency strategy to protect electricity supplies to the area.

  48.  On Tuesday 24 July National Grid took the decision to implement a more resilient flood defence system at Walham substation, thus freeing up temporary resources which were in place. This entailed the construction of the Hesco Bastion barrier with the help of the armed forces (see appendix iv). This structure was completed at 21:15 on the Saturday 28 July and remains in place as a more permanent barrier. No further flooding was experienced at National Grid's Walham substation after water subsided on Tuesday 24 July.

  49.  The circuits at Walham were returned to their normal operational configuration by 18:15 on the Tuesday 24 July. Through the implementation of contingency strategies, and as a result of a significant effort by the armed forces, fire brigade and National Grid engineers, electricity supplies were maintained to the areas of Gloucester and Cheltenham.

  (i)  Sheffield

    (a)  Geographic map of Sheffield and neighbouring cities—National Grid electricity substations marked in yellow.

    (b)  National Grid diagram of its electricity system configuration in Sheffield—Monday 25 June.

  (ii)  Photograph of National Grid gas pipeline floating at Kilham Farm.

  (iii)  Gloucester

    (a)  Geographic map of Gloucester and neighbouring cities—National Grid Walham substation marked in yellow.

    (b)  National Grid diagram of its system configuration including Walham substation.

  (iv)  Photograph of Hesco Bastion wall erected at National Grid's Walham substation.

  (v)  National Grid Network magazine for employees "Flood response special" July 2007

National Grid

August 2007

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