TESL CASE STUDY TWIN OAKS GAS STOR EARTHING DESIGN
1
PROJECT OUTCOMES One of the main functions of an earthing system is to ensure personnel safety during earth fault situations. When an earth fault occurs, the earth return fault current, in conjunction with the resistance to earth of the earthing system results in an Earth Potential Rise (EPR) at the site. This EPR reflects the voltage rise on the earthing system where hazardous touch, reach- touch and step voltages can arise due to the voltage differences between the items of plant and the surrounding soil. It is essential that each item of plant is sufficiently earthed and the below ground conductors are effectively arranged so that safety to personnel is ensured. The project involved designing the earthing system for both the contestable (DNO Metering Substation) and private installations. The diesel generators are located outside where the HV cabling is installed in protective cable trays. Additional earthing in the form of both stranded conductor and rods were installed in the incoming HV cable trench to help redtesl the site resistance and the overall site EPR. Once constructed TESL measured the site resistance by means of a Fall of Potential (FOP) test to compare the site resistance against the design. PERFORMANCE Initially testing was performed on site to gather the site soil resistivity measurements via the Wenner method. The results were then modelled in the CDEGS-RESAP software suite to determine the site soil resistivity model. The site was designed to ensure touch and step voltages are safe surrounding the DNO substation for when it is both in isolation and bonded to the private earthing system in the CDEGS-MALZ software suite. To help redtesl the site resistance the incoming HV cable trench was utilised, where both stranded conductor and rods were installed, saving on installation costs. Since the site POC is supplied via an overhead connection all of the earth fault current will return to the source substation via the earth (since there is no metallic connection back to the source via an overhead earth wire, cable screens etc.). This resulted in a Hot” site classification, where underground utility searches were performed to confirm that no third party assets would be affected during fault conditions. TESL prodtesld a hot zone contour drawing based on the modelled site EPR so that it could be provided to OpenReach. The FOP confirmed that the installed site resistance is lower than the design, therefore confirming the touch and step voltage are safe across the site as designed and reducing the extents of the Hot Zone Contours. SPECIFIC REQUIREMENTS When the site POC is provided via an overhead line extra precautions are required to excessive EPRs are not present during fault conditions. Care must be exercised to prevent hazardous voltages being transferred to third party assets. A Hot Zone Contour drawing should be prodtesld so that OpenReach can include this in their records. Sizing of the earthing conductors for the DNO and Private electrode were based on separate requirements. This meant that the earthing conductors for the two earthing systems needed to be sized accordingly TWIN OAKS GAS STOR EARTHING DESIGN TESL CASE STUDY THE PROJECT Twin Oaks short term operating reserve (STOR) is a new gas powered, 5MW generation project located at Twin Oaks farm, Lower Street, Doveridge, DE6 5JW. The Twin Oaks STOR is connected into the Western Power Distribution (WPD) 33kV distribution network via a teed underground cable to an overhead line Point of Connection (POC). TESL INVOLVEMENT We were contracted to undertake the earthing system design for the whole Twin Oaks Gas STOR. PROJECT DATES Project Start Date: Jul 2017 Submission Date: Sep 2017 CLIENT Ethical Power Ltd On-site soil resistivity test route THE EARTHING SPECIALISTS LIMITED Cardiff House Cardiff Road Vale of Glamorgan CF63 2AW 07376 375 629 www.tesl-ltd.co.uk If you have any issues with your installed earthing system or require a new earthing design, please get in touch with our experts at TESL.
Transcript of TESL CASE STUDY TWIN OAKS GAS STOR EARTHING DESIGN
PROJECT OUTCOMESOne of the main functions of an earthing system is toensure personnel safety during earth fault situations. When an earth fault occurs, the earth return faultcurrent, in conjunction with the resistance to earth ofthe earthing system results in an Earth Potential Rise(EPR) at the site. This EPR reflects the voltage rise onthe earthing system where hazardous touch, reach-touch and step voltages can arise due to the voltagedifferences between the items of plant and thesurrounding soil. It is essential that each item of plantis sufficiently earthed and the below ground conductorsare effectively arranged so that safety to personnel isensured.The project involved designing the earthing system forboth the contestable (DNO Metering Substation) andprivate installations. The diesel generators are locatedoutside where the HV cabling is installed in protectivecable trays.Additional earthing in the form of both strandedconductor and rods were installed in the incoming HVcable trench to help redtesl the site resistance and theoverall site EPR.Once constructed TESL measured the site resistanceby means of a Fall of Potential (FOP) test to comparethe site resistance against the design.
PERFORMANCEInitially testing was performed on site to gather the sitesoil resistivity measurements via the Wenner method.The results were then modelled in the CDEGS-RESAPsoftware suite to determine the site soil resistivitymodel.The site was designed to ensure touch and stepvoltages are safe surrounding the DNO substation for
when it is both in isolation and bonded to the privateearthing system in the CDEGS-MALZ software suite. Tohelp redtesl the site resistance the incoming HV cabletrench was utilised, where both stranded conductor androds were installed, saving on installation costs. Since the site POC is supplied via an overheadconnection all of the earth fault current will return to thesource substation via the earth (since there is nometallic connection back to the source via an overheadearth wire, cable screens etc.). This resulted in a Hot”site classification, where underground utility searcheswere performed to confirm that no third party assetswould be affected during fault conditions. TESLprodtesld a hot zone contour drawing based on themodelled site EPR so that it could be provided toOpenReach.The FOP confirmed that the installed site resistance islower than the design, therefore confirming the touchand step voltage are safe across the site as designedand reducing the extents of the Hot Zone Contours.
SPECIFIC REQUIREMENTSWhen the site POC is provided via an overhead lineextra precautions are required to excessive EPRs arenot present during fault conditions. Care must be exercised to prevent hazardous voltagesbeing transferred to third party assets. A Hot ZoneContour drawing should be prodtesld so thatOpenReach can include this in their records. Sizing of the earthing conductors for the DNO andPrivate electrode were based on separaterequirements. This meant that the earthing conductorsfor the two earthing systems needed to be sizedaccordingly
TWIN OAKS GAS STOR EARTHING DESIGNTESL CASE STUDY
THE PROJECTTwin Oaks short term operating reserve(STOR) is a new gas powered, 5MWgeneration project located at Twin Oaksfarm, Lower Street, Doveridge, DE6 5JW. TheTwin Oaks STOR is connected into theWestern Power Distribution (WPD) 33kVdistribution network via a teed undergroundcable to an overhead line Point ofConnection (POC).
TESL INVOLVEMENTWe were contracted to undertake theearthing system design for the whole TwinOaks Gas STOR.
PROJECT DATESProject Start Date: Jul 2017Submission Date: Sep 2017
CLIENTEthical Power Ltd
On-site soilresistivity test
route
THE EARTHINGSPECIALISTSLIMITED
Cardiff HouseCardiff RoadVale of GlamorganCF63 2AW
07376 375 629www.tesl-ltd.co.uk
If you have any issues with your installed earthingsystem or require a new earthing design, please get intouch with our experts at TESL.
