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09789,09789-WR-0104,-,1.B,5 09789

EARTHING PHILOSOPHY      

Dept./Section

: - Electrical Engineering & Design

Project No. Client name Plant name Plant Location

: 09789 : BHP Petroleum Pty Ltd. : Ohanet Development : Ohanet, Algeria

1

2001-05-28

APPROVED FOR DESIGN

AJWA

PTB

PTB

0

2001-05-01

FOR APPROVAL

AJWA

PTB

PTB

REV.

DATE

DESCRIPTION

PREPARED

CHECKED

APPROVED

THIS DOCUMENT/DRAWING IS THE PROPERTY OF ABB LUMMUS GLOBAL B.V. INCLUDING ALL PATENTED AND PATENTABLE FEATURES AND/OR CONFIDENTIAL INFORMATION AND ITS USE IS CONDITIONED UPON THE USER’S AGREEMENT NOT TO REPRODUCE THE DOCUMENT/DRAWING, IN WHOLE OR IN PART, NOR THE MATERIAL DESCRIBED THEREON, NOR TO USE THE DOCUMENT/DRAWING FOR ANY PURPOSE OTHER THAN AS SPECIFICALLY PERMITTED IN WRITING BY ABB LUMMUS GLOBAL B.V.

LGV SFOR 02-7000-02.523 (1999-11-19) 09789,

09789,09789-WR-0104,-,1.B,5

Page 1 of 5

ABB Lummus Global B.V./Petrofac International Ltd.

ABB Lummus Global B.V./Petrofac International Ltd. The Hague, The Netherlands

LGV SFOR 02-7000-02.523 (1999-11-19) 09789,

09789,09789-WR-0104,-,1.B,5

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ABB Lummus Global B.V./Petrofac International Ltd. EARTHING PHILOSOPHY

1.

SCOPE This earthing philosophy describes:    

2.

power system earthing worst case conditions for the electrical safety earthing system versus the related risk of an electric shock for operating personnel. lightning protection of non-steel buildings. the basic set-up of the plant safety earthing system

POWER SYSTEM EARTHING Power system earthing is as follows: 66 kV incoming overhead power line :solid earthing at Alrar power station 5.5 kV system : low resistance earthing (1000A current limitation) 400 V system : solid earthing at the transformers and generator neutrals 230 V UPS : solid earthing at system neutral 110 VDC : unearthed system System neutrals shall have a resistance to earth as follows: 1000 V or less : 10 Ohms max. over 1000 V : 1 Ohms max. Earth returns shall be either: -earthgrid conductors -cable armour -separate earthing conductor in the cable

3.

WORST CASE: EARTH GRID POTENTIAL RISE FROM 66 KV SYSTEM EARTHFAULT Worst case for safety earthing is an earthfault in the main 66 kV supply from Alrar. In areas of high soil resistivities as in Ohanet, it may lift the plant earth grid to a dangerous voltage. For example: assuming an overall earthing grid resistance of 1 Ohm and a calculated 3000 A earthfault current, the earth grid voltage may rise temporarily to 3000 x 1 = 3000 V. This voltage is called the earth grid potential rise. This 66kV earthfault is the worst case since the earth fault current return path is through the body of earth. This situation is different from locally derived systems (with local system neutral earthing) for which earth fault return paths are available through the copper wires of the plant earth grid as well as the cable armours which are used as earth returns.

4.

STEP AND TOUCH POTENTIALS The earthgrid potential itself does not necessarily lead to electric shocks. When all neighboring steel and soil has the same potential, no voltage differences are present and there will not be an electric shock hazard. However, this ideal situation is theory and will not be reached in practice. Electric shock potentials are defined as step and touch voltages. These voltages are calculated using the standard calculation method of IEEE std-80-2000 (6.1). The step and touch voltages are required to stay within the permissible values, which are to be calculated by the same standard.

4.1

CALCULATIONS The attached calculations follow the pattern of the procedure block diagram (sh1 of calc’s) which includes an iteration method.

LGV SFOR 02-7000-02.523 (1999-11-19) 09789,

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ABB Lummus Global B.V./Petrofac International Ltd.

Unsatisfactory outcomes of calculations require a narrow mesh earthgrid. The aim of calculating this mesh grid is to let the soil have approximately the same potential as the earth grid itself and as such reduce the voltage differences to an acceptable level. In view of the wide areas and the rocky soil involved, the installation of a narrow-mesh grid is not seen as a practical approach for the Ohanet project. The approach will be focussed on designing an earthing system with an overall resistance value that is low enough to meet the safety requirements, without the need for a narrow meshed grid. 4.2

FINAL RESULT OF CALCULATIONS The final result of the calculations is that the criteria for touch voltages are met when two deepwell earthing electrodes are included reaching the water table. The resulting mesh width of the grid shall be 300 meter, which is acceptable.

5.

EARTHING ELECTRODES The LGN approach will be to reduce the overall earthing resistance by installation of at least two deepwell earthing electrodes. The resistance of such deepwell earthing electrodes is approaching zero. Final measured resistance values need to prove this. Earthing electrodes are divided into main earthing electrodes and auxiliary earthing electrodes.

5.1

AS MAIN EARTHING ELECTRODES THE FOLLOWING SYSTEMS COME INTO CONSIDERATION:  

earthgrid conductors (buried bare copper wires) deep earthing wells (reaching the earth water layer, approx. 300 m deep for Ohanet)

Note: rod electrodes are not considered due to the rocky soil and high resistivity. 5.2

AS AUXILIARY EARTHING ELECTRODES THE FOLLOWING SYSTEMS COME INTO CONSIDERATION       

water wells cathodic protection deepwell anodes underground in-plant pipelines underground off-site pipelines oil or gas transfer pipelines with oil/gas wells connected foundation reinforcing steel (note: non-continuous/non-welded) overhead earthwire of the 66kV overhead line

Note: these auxiliary earthing electrodes are not taken into account in the calculation. 5.3

66 KV OVERHEAD LINE EARTHWIRE. The overhead earthwire of the 66kV overhead power line has an estimated reactance of 45 Ohms if calculated back to the source 108km away. If one includes interconnections with the steel mast structures and earthing through reinforcing steel of the mast foundations, the earthing resistance will drop substantially, however, exact values are difficult to obtain and hence not included as helping in the calculations.

5.4

NORMAL VERSUS FAULT CURRENT SITUATIONS. In many cases, the smaller earth fault currents are not recommended to flow through these auxiliary earthing electrodes, since they serve other primary purposes and are as such generally not designed for conducting earth(fault) currents.

LGV SFOR 02-7000-02.523 (1999-11-19) 09789,

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ABB Lummus Global B.V./Petrofac International Ltd.

However, under main 66kV earth fault conditions, the situation changes drastically. If auxiliary systems would be excluded from the earthing system, high voltage differences may occur between two structures under an earth fault condition. For example: If a deep well anode for cathodic protection is excluded from the earthing system, voltage differences as high as the earthgrid voltage rise may occur across the cathodic protection rectifier. 5.5

OVERVOLTAGE DIVERTERS (E.G. POLARIZATION CELLS). Auxiliary earthing electrodes shall therefore be connected to the earthing system, either by a solid bond or else, through an overvoltage device that will protect the auxiliary systems against damage from overvoltages. Solid bonds are not permitted when auxiliary earth electrodes are provided with cathodic protection. Connections through overvoltage diverters are required in such cases. It is required, that such overvoltage devices are rated for the earthfault current levels available at that device.

6.

PLANT EARTHGRID The earth grid will mainly be composed as follows: One earth main conductor is looped around the plant area, buried at a distance of approximately 0.5 to 1.0 meters outside the perimeter fence. The fence itself will be bonded to this main conductor by a bond approx. every 50 meters. A meshed earth grid is installed of underground earth conductors with a mesh at least every 100 meters to satisfy the touch potential requirements. This grid will be mainly met already by the standard earthgrid for equipment bonding.

6.1

EQUIPMENT EARTHING.

Bonding earth conductors branching off from the main conductors will be used for equipment earthing to:  piperacks (one connection approx. every 25 meters.)  tanks and vessels at two opposite sides or corners  pumpskids, compressor skids etc.  pipework is earthed through pumps, vessels etcetera where they are connected to.  lighting poles  control stations unless plastic enclosures are used for the control switches  junction boxes unless plastic junction boxes are used. For instrument earthing reference is made to typical earthing system drawings DN-XXXXX00603C/D/E. Bonding earth conductors can be buried or above ground as convenient in the situation. 6.2

PIPE EARTHING IN CLASSIFIED AREAS. In classified areas, pipes are required to be earthed. All pipes are inherently earthed through the equipment that they interconnect, i.e. pumps, tanks etc. Additional earthing connections to pipework at the point of leaving/entering a classified area is not required. (IP-code part 1:2.5.2).

6.3

BOLTED CONNECTIONS. Bolted connections of heavy gauge steel, including pipeflanges, are considered to be electrically continuous with regard to reliable earth connections. Bolt sizes considered for this are M12 and larger.

LGV SFOR 02-7000-02.523 (1999-11-19) 09789,

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ABB Lummus Global B.V./Petrofac International Ltd.

7.

LIGHTNING PROTECTION

7.1

METALLIC STRUCTURES Metallic outdoor structures and process equipment are inherently protected against lightning discharges by their connections to the plant earthgrid.

7.2

BUILDINGS Brick or concrete buildings shall be provided with lightning protection as follows: -a ring conductor along the edge of the roof -downcomers at two opposite corners supplemented by additonal downcomers every 20 meters along the walls of large buildings. -a ring conductor buried along the foundation of the wall. -earth rods below all downcomers -at least two tie-in conductors to the plant earthgrid.

8.

ATTACHMENTS

8.1

EARTHING GRID CALCULATIONS  

9.

Ohanet water wells location layout DN-09789-00141W Ohanet plotplan central processing facilities EN-09789-00010A REFERENCES



IEEE-std-80-2000 Guide for safety in AC substation grounding



IP part 1 Electrical Safety Code



IEC 61024 Protection of structures against lightning.



DE-09789-N-0601 General Electrical Specification, subsection 7.3 and the attached drawings for typical earthing system DN-xxx-00603C/D/E.



Short circuit calculations N311-09789-C609B

LGV SFOR 02-7000-02.523 (1999-11-19) 09789,

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