Network

NETWORK / 41 / DECEMBER 2016/JANUARY 2017 26% 15% C a s e s t u d y Gas system stock levels and system pressures On 5 September 2016 the amount of gas in the National Transmission System was at its lowest level since 2012, resulting in customers experiencing lower pressures than normal. This was largely driven by market be- haviour; there was a 50 million cubic metre (mcm) shortfall on the NTS on a national demand of approximately 200mcm at the start of the day. The problems began on 4 September when the market was approximately 6mcm light, and this was compounded by late supply losses at St Fergus and Bacton terminals. The Gas National Control Centre (GNCC) took a trade action but there was an insufficient response from the market, which meant the amount of gas in the NTS fell to 326mcm by the close of the gas day. The market opened 47mcm light, meaning there was more gas going out of the NTS than was coming in, leading to a reduction in the overall amount of gas in the NTS. Demand was increasing by approxi- mately 3mcm per hour. With the amount of gas in the system continuing to fall and limited market response, GNCC traded in order to stimulate the market to respond by either reducing demand or increasing supply. A market responded by reducing storage site injection, how- ever there was still a signifi- cant shortfall. A second trade action was taken by GNCC at 1:00pm and Dragon, one of the Milford Haven sub-terminals, started flowing. A further GNCC trade was taken at approxi- mately 2:00pm and the market responded with storage sites withdrawing. This response started to address the national balance position. National Grid received calls from power sta- tions, distribution networks and IUK during the day as they were querying the lower than usual pressures. It did not breach any of its contractual obligations with its customers, however the low pressure was a concern so it outlined why it was happening and what it was doing about it. The increase in supply flow at Dragon in South Wales resulted in a locational pressure chal- lenge for the NTS. There was a planned compression outage at Churchover compressor station for that day. The South Hook sub-terminal at Milford Haven was already flowing 50mcm. The increase in the Dragon supply rate meant the capability of the NTS to move gas away from Milford Haven was going to be exceeded, leading to a localised pressure constraint situation in South Wales. To mitigate this, GNCC requested the return of Churchover compressor station. The initial estimate of return to service was 3:00am on 6 September, but the site team managed to bring it back online by 6:00pm on 5 September. Both Churchover and Wormington compressor stations were then used to allow the movement of more gas out of South Wales. Even with compression being available, the pressure at Milford Haven would reach the maxi- mum operating pressure limit at approximately 11:30pm. A request for locational energy bids was then sent out to the market. A response was received so the GNCC sold gas locationally. This meant the ad- ditional local demand effectively absorbed more gas in area, which reduced the amount that needed to be transported away. Supply and demand profiles on 4 and 5 September Source: National Grid mcm Demand 5 7 9 11 13 15 17 19 21 23 1 3 280 260 240 220 200 180 160 140 120 5 7 9 11 13 15 17 19 21 23 1 3 Supply 4/9/2016 (hour) 5/9/2016 (hour) NTS gas levels on 5 September with key commercial actions taken during the day Source: National Grid GNCC locational action GNCC buy action Actual level of gas Range of gas Opening level of gas Predicted closing level of gas Level of gas in NTS (mcm) 350 325 300 275 250 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 0 1 2 3 4 Hour

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