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ABU DHABI GAS INDUSTRIES LTD (GASCO)

EARLY NITROGEN REJECTION UNITS (ENRU)FEED SERVICES 5276

DOCUMENT TITLE:

EARTHING DESIGN CALCULATIONS FOR NEW 220/33 KVSUBSTATION, HABSHAN

COMPANY DOCUMENT NO: 5 2 7 6 - F K - 8 0 0 0 - H - 7 0 1 R e v - 0

GASCO Agreement No: 13527603

Location: Habshan Gas Complex, Abu Dhabi

ENGINEER:

SUBCONTRACTOR:

Technip E&C Ltd (Stone & Webster International,Inc.)

Mott MacDonald Ltd.

Technip Project No: 147491

Approval Class: Class 3

MML DTS ENRU 138MML PROJECT No.: 313780

0 03-OCT-13 ISSUED FOR TENDER SS SSK SSK

B2 18-SEP-13 ISSUED FOR APPROVAL VL JT SSK

B1 15-AUG-13 ISSUED FOR REVIEW & COMMENT VL JT SSK

Rev DateDD-MMM-YY

STATUS WRITTEN BY(name)

CHECKED BY(name)

MML CLIENT

APPROVED BY(name)

DOCUMENT REVISIONS

Sections changed in last revision are identified by a vertical line in the margin

This document has been prepared for the titled project or named part thereof and should not be relied upon or used for any other project without anindependent check being carried out as to its suitability and prior written authority of Mott MacDonald being obtained. Mott MacDonald accepts noresponsibility or liability for the consequences of this document being used for a purpose other than the purposes for which it was commissioned. Anyperson using or relying on the document for such other purpose agrees, and will by such use or reliance be taken to confirm his agreement toindemnify Mott MacDonald for all loss or damage resulting there from. Mott MacDonald accepts no responsibility or liability for this document to anyparty other than the person by whom it was commissioned.To the extent that this report is based on information supplied by other parties, Mott MacDonald accepts no liability for any loss or damage suffered bythe client, whether contractual or tortuous, stemming from any conclusions based on data supplied by parties other than Mott MacDonald and used byMott MacDonald in preparing this report.

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EARLY NITROGEN REJECTION UNITS (ENRU)FEED SERVICES 220/33KV SUBSTATION


COMPANY Document No.: 5 2 7 6 - F K - 8 0 0 0 - H - 7 0 1 R e v - 0

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I N D E X

1.0 INTRODUCTION .............................................................................................................. 3

2.0 OBJECTIVE ..................................................................................................................... 3

3.0 REFERENCE DOCUMENTS ............................................................................................ 4

4.0

BASIS OF EARTHING SYSTEM DESIGN ....................................................................... 4

5.0 INPUT DATA .................................................................................................................... 4

5.1 Soil Resistivity Test Results ......................................................................................... 4

6.0 MODELING METHODOLOGY ......................................................................................... 5

6.1 Soil Resistivity Model ................................................................................................... 5

6.2

Conductor Sizing .......................................................................................................... 7

6.3 Distribution of Earth Fault Current ................................................................................ 8

6.4

Analysis Software......................................................................................................... 9

7.0 RESULTS ........................................................................................................................10

7.1 Case 1 ........................................................................................................................10

7.2

Case 2 ........................................................................................................................10

8.0 CONCLUSION .................................................................................................................11

9.0 ATTACHMENTS .............................................................................................................11

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1.0 INTRODUCTION

GASCO, on behalf of ADNOC, will implement Early Nitrogen Rejection (NRUs) facilities atHabshan gas complex as a mitigation measure for premature breakthrough of nitrogen inwellstream fluids, which would result in sales gas heating value specification being violated.The Early NRUs will treat residue gas streams from Habshan-1 Train-3 and Habshan-3 GasProcessing Facilities to be routed as sales gas to the distribution network.

In order to utilize full capacity of existing double circuit 220kV OHL for OGD-III plant and tomeet the power demand of 50 MVA for the new ENRU plant and future expected loads in

Habshan; a new 220/33kV substation shall be built at Habshan in the vacant plot beside theexisting substation SS300, as per local utility company TRANSCO specification. The existing220kV double circuit BAB to Habshan 3 OHL will be diverted to BAB-2 400/220kV GridStation from the old BAB substation before EPC award. Hence, the planned OHL from BAB-2 to OGD-III shall be used to feed the new substation by adopting OHL Loop In Loop Out(LILO) configuration.

The work includes associated modifications at remote end substations (i.e. Habshan 3 andBAB2) and 220KV OHL as well as interface with GASCO new ENRU 33kV substation.

220kV SS SUB-CONTRACTOR (Mott MacDonald Ltd.- MML) Scope of Work is to undertakethe Concept Definition and FEED Engineering Services to define the PROJECT scoperequirements and preparing the Technical part of EPC Enquiry Package in order to supportthe FEED ENGINEER M/s TECHNIP E&C Ltd. in preparation of ENRU project EPC EnquiryDocument.

EPC Enquiry Package shall enable COMPANY to select an EPC Contractor on acompetitive bidding basis and execute the EPC Works without further optimization duringEPC phase for satisfactory completion of the PROJECT.

2.0 OBJECTIVE

The objective of this design calculation is to design the underground earthing system for thenew 220/33 kV substation, Habshan and to ensure that the attainable mesh and steppotentials are within the limits of the tolerable mesh and step potentials respectively underany fault conditions.

The performed calculations are indicative and the EPC contractor shall repeat thecalculations and verify the design and ensure that it is compliant with the latest TRANSCOStandard specifications.

Main parameters that are determined in this document are: Material and size of earth electrodes and earthing conductors Substation grid resistance

Earth fault current Distribution of the earth fault current Grid potential rise Actual and allowable values of step and touch voltage

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3.0 REFERENCE DOCUMENTS

The calculations are based on the following reference documents.

5276-FA-7000-G-401 Project design basisDGS 1630 003 Electrical Design GuidelinesIEEE-80-2000 IEEE Guide for Safety in AC Substation GroundingIEEE-80-1983 IEEE Guide for Measuring Earth Resistivity, Ground

Impedance, and Earth Surface Potentials of a Ground System

S-TR-EARTH-GROUND TRANSCO Earthing/Lighting Specification5276-SOW-00-000-14-001-0 Geotechnical Investigation for Early Nitrogen Rejection Units

(ENRU) Project FEED Services 52765276-FS-8200-G-781 Rev.1 Load Flow and Short Circuit Studies for New 220/33 kV

Substation, Habshan

4.0 BASIS OF EARTHING SYSTEM DESIGN

Definition ExplanationAllowable LG current Maximum LG current that causes safe touch voltage in the entire selected

area in the contour.Symmetrically

arranged ground rods

A symmetric array of ground rods covers a rectangular area in which rods are

located in rows parallel to the X-axis with all rods in a row equally spaced.Asymmetrically arranged ground rods

An asymmetric array of ground rods is a single row of equally spaced rods.

RMS error of soil report The RMS error is computed to indicate the degree of correspondencebetween the calculated soil model and the measured value.

5.0 INPUT DATA

5.1 Soil Resistivity Test Results

Soil resistivity rest results were obtained from the CLIENT and are shown in Table 1.Measurement results are used for making two layer soil resistivity model which is used in

subsequent calculations that are performed using ETAP software.

Table 1. Soil Electrical Resistivity Test ResultsTest No. ER No. Direction In accordance with ASTM G 57-95a

Distance Interval (m) Resistivity Reading R()Apparent Resistivity

S=2aR (m)

1 ER-3

North-South

0.75 76.9 362.381.5 38.6 363.82.25 16.07 227.18

3 10.73 202.264.5 6.11 172.76

East-West

0.75 114.7 540.511.5 44.4 418.462.25 21.5 303.95

3 11.21 211.3

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Test No. ER No. Direction In accordance with ASTM G 57-95a

Distance Interval (m) Resistivity Reading R()Apparent Resistivity

S=2aR (m)4.5 7.01 198.2

6.0 MODELING METHODOLOGY

6.1 Soil Resistivity Model

Calculation of electrical soil resistivity based on the two-layer soil model based on the input

data obtained during site soil resistivity measurement.

Selection of parameters for two layer soil model is based on the Annex B of IEEE-80-1983.

Two layer soil resistivity model is characterized by its:

First layer height, First layer resistivity, r1

Deep layer resistivity, r2 The reflection coefficient

)(

)(K

12

12

+

=

Two layer soil resistivity model is shown in Figure 1.

Figure 1. Two layer soil resistivity model

A resistivity determination using the Wenner method results in an apparent resistivity whichis a function of the electrode separation, a. In terms of the above parameters the apparentresistivity can be expressed as:

+

+

+= =1n

2

n

2

n

1

ah2n4

K

ah2n1

K41(a)

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Using above equation, for the two-layer soil model, resistivity of the upper and lower layersof soil were calculated along with the thickness of the first layer (or upper layer). The secondlayer (or lower layer) is assumed infinitely thick and its resistivity has been calculated.Calculation results are presented in Table 2.

Table 2. Two layer soil model parametersDirection Layer Height (m) Top layer resistivity (m) Deep layer resistivity (m)

N-S 1.214 403.8 144.4E-W 1.047 614.19 165.31

Average 1.1305 508.995 154.86

Chart 1. Soil Resistivity (Two Layer Model) N-S Direction

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Chart 2. Soil Resistivity (Two Layer Model) N-S Direction

6.2 Conductor Sizing

Main earthing conductor (S) is sized according to IEEE Std 80-2000 recommendations.

Table 3. Design input parametersDescription Unit ValueFault current for earthmat sizing I 40 kADuration of fault current for earthmat sizing Tc 1 secConductor type for buried earthmat Soft Drawn Tinned CopperCorrosion factor 10 %Initial temperature of conductor in the ground Ta 50 CFinal temperature of conductor Tm 450 C

Thermal capacity factor Tcap 3.42 J/(cm3 C) As per IEEE-80,page 42, Table 1

Material Constants

Thermal coefficient of resistivity at 20C r 0.00393Resistivity of conductor at 20C r 1.72 -cmReciprocal of thermal coefficient at 0C K0 234

Earthing conductor size can be determined using (Equation No.37 IEEE 80-2000):

( )

( )

+

+

=

+

+

=

50234

450234ln

100001.720.003931

3.42

40000

TK

TKln

10000t

Tcap

IA

a0

m0

rrc

= 189.67 mm2

Therefore required conductor size with 10% allowance = 1.1*189.67=208.6 mm2

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Selected conductor is: 300 mm2 soft drawn tinned copper wire

6.3 Distribution of Earth Fault Current

According to the IEEE Transactions on Power apparatus and systems vol. PAS-103, No.9,September 1984,page 263-635 the total fault current will not flow through the substationground as part of the current will be diverted by ground wires due to induction andconduction. Sample sketch of current distribution is shown in Figure 2.

Maximum line to ground fault current (I f) was calculated in the short circuit current study(5276-FS-8200-G-781 Rev-B1, page 30). That is the maximum short circuit current that willflow through earthing system in the case of the line to ground fault. Maximum short circuitcurrent was increased by 25% to cater for further system expansion. That way earthingsystem is designed with safety margin.

Current distribution factor (Sf) quantifies percentage of the earth fault current that will flowthrough the earthing system. The worst case scenario was considered in this particularexercise when complete earth fault current flows through earthing system. Therefore factorSfwas set to 100%.

Input data can be summarized as:

Actual fault current (If) 17.187 kAUsed fault current (If) 21.47 kACurrent distribution factor (Sf) 100%

IrIr

IG=Sfx If

OPGW

Total LGFaultCurrent - If

Figure 2. Typical Fault current Distribution

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6.3.1 Tolerable Touch Voltage

The equation for tolerable touch voltage calculation in body weight of 50 kg is calculatedusing Eq. 32 from IEEE 80-2000.

s

SStouch50

t

0.116)C1.5(1000E

+=

where

Etouch50(V) - touch voltageCs - surface layer derating factors - surface layer resistivity in mts - duration of shock current in sec

Surface layer derating factor (Cs) can be determined using IEEE 80-2000, Eq.27.

792.00.0915.02

5000

508.99510.09

10.09h2

10.09

1C

s

s

S =

+

=

+

=

where

(m) - resistivity of the earth beneath the surface materials - surface layer resistivity in mhs - thickness of the surface material in m

Therefore tolerable touch voltage is calculated as follows:

V8051

116.0)5000792.05.11000(

t

0.116)C1.5(1000E

s

SStouch50 =

+=

+=

6.3.2 Tolerable Step Voltage

The equation for tolerable step voltage calculation in body weight of 50 kg is calculatedusing Eq. 29 from IEEE 80-2000.

V2872.11

0.1165000)0.7926(1000

t

0.116)C6(1000E

s

SStouch50 =

+=

+=

6.4 Analysis Software

Modeling of the 220/33 kV HABSHAN substation earthing system was performed using theElectrical Transient Analysis Program (ETAP). ETAP is a conventional power systemanalysis tool with set of routines that allowed completion of calculations that are required forthis project.

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7.0 RESULTS

7.1 Case 1

Earthing system defined by parameters in Section 5 and Section 6 is assessed in studyCase 1. Calculations were completed using IEEE 80-2000 standard which is built in modulein the commercially available ETAP software.

Earthing mesh was defined by 40 earthing conductors in each direction which is themaximum number allowed by ETAP software. Also 40 earthing rods were placed along grid

perimeter.

Results indicate that under given conditions it is not possible to meet prescribed designparameters particularly touch voltage potential and equivalent earthing resistance. Summaryof results is presented in the Table 4. Detailed results are shown in Attachment 9.1

Table 4. Case 1 Results summaryDesign Parameter Calculated TolerableTouch Potential (V) 7199 1139Step Potential (V) 4016 4065Earthing Resistance () 1.418 1

Optimization of the earthing system indicated that increasing number of earthing rods andtheir length will not significantly affect critical parameters of the earthing system.

7.2 Case 2

Simulation with lower soil resistivity of the top layer was completed in the study Case 2. Thiswas done to assess the effect of soil resistivity on the main earthing design parameters.

IEEE 80-2000 standard provides recommendations for soil treatment to lower resistivity. It isrecommended that use bentonite, a natural clay containing the mineral montmorillionite willkeep soil resistivity of 2.5 m at 300% moisture. However, top layer soil resistivity was set to

50 m considering it as a conservative approach.

Calculations were completed using IEEE 80-2000 standard which is built in module in thecommercially available ETAP software.

Earthing mesh was defined by 14 earthing conductors in X direction and 9 earthingconductors in Y direction. Also 10 earthing rods were placed along grid perimeter.

Results indicate that under given conditions critical design parameters are within permissiblelimits. Summary of results is presented in the Table 5. Detailed results are shown inAttachment 9.2.

Table 5. Case 2 Results summaryDesign Parameter Calculated TolerableTouch Potential (V) 744 1113

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Step Potential (V) 409 3961Earthing Resistance () 0.205 1

8.0 CONCLUSION

Calculations indicate that top soil needs to be treated to lower its resistivity. Usage ofbentonite is recommended. This approach will allow that earthing system design parametersare kept within prescribed limits.

From the study Case 2 results it can be concluded that:

The overall earthing grid resistance is 0.205 which is less than 1.

The attained step potential is less than tolerable step potential The attained touch potential is less than tolerable touch potential

It can be concluded that the earthing design is safe.

9.0 ATTACHMENTS

1. Attachment 1 - Case 1 - ETAP Software Calculation Results

2. Attachment 2 - Case 2 - ETAP Software Calculation Results

3. Attachment 3 ETAP Software - Earthing Layout Top View

4. Attachment 4 ETAP Software - Earthing Layout Projection

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ATTACHMENT 1 - Case 1 - ETAP Software Calculation Results

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ATTACHMENT 2 - Case 2 - ETAP Software Calculation Results

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ATTACHMENT 3 - ETAP Software Earthing Layout Top View

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ATTACHMENT 4 - ETAP Software Earthing Layout Projection

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5276-FK-8000-H-701 Rev-0