BISHA CPS EXTENSION PROJECT

PAGE

TCE CONSULTING ENGINEERS LIMITED

i30421008-3CL-00014-P0CGROUNDING SYSTEM CALCULATION FOR NEW POWER PLANT AT RAFHASHEET OF iOF 4

SAUDI ELECTRICITY COMPANY

EASTERN OPERATING AREA

Dammam, Saudi Arabia

GENERATION EXPANSION OF RAFHA POWER PLANT

(CONTRACT NO : 30421008/00)

GROUNDING SYSTEM CALCULATION FOR

NEW POWER PLANT AT RAFHADOCUMENT NO. : 30421008-3CL-00014-P0C

DECEMBER, 2004

ENGINEERING CONSULTANTTCE CONSULTING ENGINEERS LIMITED

73/1, ST. MARKS ROAD

BANGALORE 560 001

INDIA

CONTRACTOR:

AL-TOUKHI COMPANY FOR INDUSTRY & TRADING

P.O. BOX 497, RIYADH 11411, KINGDOM OF SAUDI ARABIA

TEL. 00966-1-4633660 : FAX. 00966-1-4631150

CONTENTSCLAUSE No.DESCRIPTIONPAGE No.

1.0Introduction1

2.0Reference drawings / Documents2

3.0Design Basis3

4.0Selection of Earthing Conductors4

5.0List of Abbreviations and Design parameters7

6.0Calculation of Step and Touch potential for Main plant Area8

7.0Conclusion14

ANNEXURES

1.0Annexure-1-Summary of sizes of Equipment Earthing leads15

1. INTRODUCTION:

1.1 Al-Toukhi company for Industry & Trading has been awarded turnkey contract for Open Cycle Gas Turbine based CPS expansion project at Rafha of Saudi Electric Company-Eastern Operation Area (SEC-EOA), Saudi Arabia.

1.2 The project consists of design, engineering, procurement, installation, testing and commissioning of two (2) Nos. 29.92MW @ 50(C, Gas Turbine Generator unit of GE make along with associated Generator Transformer and other balance of plant electro-mechanical equipment.

2. REFERENCE DRAWINGS/DOCUMENTS

2.1 IEEE80 Guide for safety in AC substation Grounding

2.2 SEC-ERB Engineering Standards SES-P-119.10-Grounding.

2.3 Scope of Work and Technical Specifications. PTS-1024.

2.4 General site Layout. Dwg No:KA 666782, P0F

2.5 Grounding Grid at GT Area Dwg No. KA-667088, P0C

2.6 Key Grounding Layout. Dwg No: KA-667089, P0B.

3.0DESIGN BASIS

3.1The average soil resistivity for the purpose of earthing system design is taken as 26.588 ohm metre from the soil resistivity test data by taken the average values measured at various locations.

3.2 Maximum fault current of 44.846kA (= 46kA) is observed on 480V LV station Aux. Swithcgear, as per short ckt. Study doc. # 30421008-3CL-00013 (Refer enclosed ETAP Report 2)

3.3 Fault clearing time for sizing the Main earthing conductor is taken as 1 second.

3.4 In case of equipment earthing, maximum ground fault is considered for frame earthing.

3.5 The maximum allowable temperature rise for cadweld joints (homogeneous conductor) is taken as 1083(C (as per Cl.no 7.2 of SES-P-119.10, Page 14 of 43).

3.6 The minimum shock duration for calculation is taken as 0.5 sec (as per Cl.no 4.0 of SES-P-119.10, page 7 of 43)

3.7 Plant grounding system will be designed such that overall grounding resistance is less than 0.5 Ohm( as per Cl.no 6.18.2-b of PTS 1024, Page 240 of 331.)

3.8 Current Division factor for the calculation is taken as 0.7 (as per Cl.no-4.0 of SES-P-119.10, Page 7 of 43)

3.9 Main Grounding grid conductor not less than 240 Sqmm (as per cl.no 6.18.2-c of PTS 1024, Page 240 of 331)

3.10The grounding grid will be designed to achieve safety from dangerous step and touch voltages in complete plant area from generator transformer upto stack covering both units.

4.0SELECTION OF EARTHING CONDUCTORS

4.1Conductor Material

Copper is considered as the Earthing conductor material as per the CL.No.7-0 of SES-P-119.10, Page, 12 of 43.

a)Conductors buried in earth

:Soft Drawn Bare

Copper Conductor

b)Conductors above ground level: Soft Drawn Bare

in trenches and concrete

Copper Conductor

c)Rod Electrodes

:Copper clad steel.

(as per CL.No 7-0 of SES-P-119-10 Page 12 of 43)

d)Equipment earthing leads

:Insulated Copper

Conductor.

4.2Size of Conductors4.2.1 Main Earthing Conductors

The main earthing conductor for the power plant will be designed for the highest earth fault level in the system. i.e., 46kA.

The Earthing conductor sizes have been calculated as per the following formula (as per Eq 10-13 of SES-P-119.10, page 10 of 43)

A = or A = I x

where:I = rms current in kA (see below) = 46 kA (maximum fault current

observed on 0.48kV as per short circuit study Doc no:-.30421008-3CL-00001-P0A.

A =conductor cross section, in mm2

Tm =maximum allowable temperature in C = 1083C (as per Cl.no

7.2 of SES-P-119.10 Page 14 of 43 table 10-1)

Ta =Ambient temperature in C= 50 C (as per Cl.no 4.01.1 of PTS-

1024, page 67 of 331)

r =Thermal coefficient of resistivity at reference

temperature r (r = 0.00393, see IEEE 80, Table 1)

or (as per Cl.no 7.2 of SES-P-119.10 Page 14 of 43

Table 10-1).

K0 =234 (ref. IEEE.80 Table 1) or (as per Cl.no 7.2 of SES-

P-119.10 , Page 14 of 43 Table 10-1.)

tc = time of current flow, in s =1 sec

TCAP = thermal capacity factor = 3.42, (ref. Table 1) or (as per Cl.no 7.2

of SES-P-119.10 Page 14 of 43 table 10-1)

or =

Resistivity of ground conductor at reference temperature in ((-cm

(Ref. IEEE.80 Table 1).or (as per Cl.no 7.2 of SES-P-119.10

Page 14 of 43 table 10-1) = 1.7421

Substituting the above values,

A = 166.17 mm2Minimum Area of cross section of conductor required is 166.17 mm2

However, earthing conductor cross section of 240 sq.mm. is selected as per

Cl.no 6.18.2-c of PTS 1024, Page 240 of 331.

4.2.2Rod Electrodes

15-mm diameter, 2500mm long, copper clad steel rods will be provided for direct driven electrodes. As per CL.NO 8.4.1 of SES -P-119.10 Page 16 of 43.

4.2.3Equipment Earthing LeadsThe size of the earthing leads depends upon the type of equipment and structure to be earthed and are provided as per Table 10-3 of SES-P-119.10 page 28 of 43.

The sizes of earthing conductor selected for each equipment is listed in Annexure-1.

5.0 LIST OF ABBREVIATIONS AND DESIGN PARAMETERSDescriptionSymbolUnitValueRemarks

Fault CurrentIfkA46Refer cl.3.2

Fault Current Division factorSf-0.7SES-P-119.10 page 7 of 43

Maximum design grid current that flows between ground grid and surrounding earth.IGkA32.2Refer cl.6.4

Maximum Grid Potential RiseGPRV6504.4Refer cl.6.4

Ambient temperatureTa0C50Refer cl.4.2.1

Diameter of grid conductordm0.0185Refer cl. 6.5

Average soil resistivity(Ohm m26.588From Soil Resistivity Report

Surface layer resistivity (washed granite)(sOhm m5000SES-P-119.10 page 14 of 43

Surface layer thicknesshsm0.10SES-P-119.10 page 5 of 43

Surface layer resistivity de rating factorCs-0.691Refer cl.6.2

Duration of shock for determining allowable body currenttsSec.0.5Refer cl.6.2

Depth of burial of ground gridhM0.75cl.8.03 of SEC Spec (SES- P-119.10). section 8.2.3

Spacing for mesh voltageKm-0.634Refer cl.6.5

Spacing for step voltageKs-0.302Refer cl.6.6

Corrected factor for grid geometryKi-2.399Refer cl.6.5

Corrected factor that adjusts the effect of inner conductors on the corner meshKii-1For grids with ground rods as per IEEE-80

Corrected weightage factor for accounting the effect of grid depthKh-1.323Refer cl.6.5

Reference depth of the gridh0m1Refer cl.6.5

Total number of electrodesnR-20Refer cl. 4.2.2/6.5

Length of each ground rodLrm2.5SES-P-119.10 Page 18 of 43

Total length of ground rodsLRm50Refer cl.6.5

Effective Length of Lc+ LR for step voltage

Lsm1157Refer cl.6.6

Total buried length of grid conductorsLcm1486Refer cl.6.1

Total / Effective length of Lc + LR for mesh voltage (LT)LMm1565.12Refer cl.6.5/6.3

Peripheral length of the gridLpm258Refer cl.6.5

Total area enclosed by the grid ASq.m3920Refer cl.6.1

Spacing between parallel conductorsDm7Refer cl.6.1

Number of conductors parallel on the longer sideNy-10Refer cl.6.1

Number of conductors parallel on the shorter sideNx-14Refer cl.6.1

Tolerable ETOUCHTolerable ETOUCHV1014.61Refer cl.6.2/7.2

Attainable ETOUCHETOUCHV831.98Refer cl.6.5/7.2

Tolerable ESTEPTolerable ESTEPV3564.78Refer cl.6.2/7.2

Attainable ESTEPESTEPV537.16Refer cl.6.6/7.2

Resistance of ground grid RgOhm0.203Refer cl.6.3

6.0 CALCULATION OF STEP AND TOUCH POTENTIAL FOR MAIN PLANT AREA

6.1 Earthing grid area and Grid spacing

The Earthing grid for Power plant area will be arranged in the form of parallel conductors to form a mat in order to limit the step and touch potential to permissible limits.

Earth Grid area, A = 49 m x 80 m = 3920 m2 (GTG Area Ref Dwg no KA667088, P0C)

N

49 M

Assuming conductor spacing D = 7m

No. of conductors parallel to the longer side, ny = 10

No. of conductors parallel to the shorter side, nx= 14

Therefore length of buried conductor=((10 x 80) +(14 x 49)

= 1486 m

Total buried length, Lc = 1486 m6.2 Calculation of tolerable Etouch and Estep voltage

a) Tolerable Etouch :

For average 50kg body weight, the tolerable touch voltage as per Eq. 32 of IEEE-80 or as per Eq.10-4 of SES-P-119.10, page 6 of 43

Cs = Surface layer de rating factor

(s = the Surface resistivity of soil, which is considered as 5000 as per table 7

of IEEE80

ts = the duration of fault current = 0.5 sec.

hs = Thickness of the asphalt surface layer = 0.1m

(as per SES-P-119.10 Page 5 of 43)

as perEq.10-5 of SES-P-119-10 page 6 of 43

Cs = 0.691

Tolerable Etouch = 1014.23 V

b) Tolerable Estep

For average 50kg body weight, the tolerable step voltage as per Eq. 29 of IEEE-80 or as per Eq.10.3 of SES-P-119.10, Page 6 of 43

Tolerable Estep = 3564.78 V

6.3 For Grid resistance, Rg

The value of grounding resistance shall be calculated using the following formula (as per Eq 10-12 of SES-P-119.10,page11 of 43)

Where,

Rg = Grid Resistance in ohms.

= Average ground receptivity in ohm-m.

A = The area occupied by the ground grid in m2LT = The total / effective buried length of conductors in m (LM)

h = Depth of grid in meters excluding asphalt covering if any.

Rg = 0.203

6.4 Calculation of maximum Grid potential rise

Maximum grid potential rise, GPR is

( as per Eq 10-2 of SES-P-119.10, Page 5 of 43).

IG = If x Sf = 46 x 0.7 = 32.2 kA.

Therefore,

GPR = 6,504.4 Volts.

Since GPR > Tolerable Estep, Emesh & attainable Estep potentials have to be calculated.

6.5 Calculation of Attainable Emesh Voltage

as per Eq.10-8 of SES-P-119.10, Page 8 of 43.

Where,

Lr is the length of each ground rod = 2.5m

Lx is the Maximum length of grid conductor in x direction = 42m

Ly is the Maximum length of grid conductor in y direction = 80m

LR is the total length of ground rods = 2.5x 20 = 50m

LM (LT)= 1565.12 m The total/effective buried length of conductors

As per Eq.81 of IEEE-80, or as per Eq 10.9 of SES-P-119.10, page 9 of 43

the spacing factor for mesh voltage,

Kii = 1 for grids with ground rods.

Where, Kii = Corrective weighing factor (Ref as per Cl.no.4.2.2 of SES-P-119.10 of page 9 of 43)

D is the grid spacing = 7m

'd' is the diameter of the grid conductor = 18.5 mm (As per Saudi Cable Company catalogues, 240Sqmm conductor dia is 18.5mm)

h is the depth of the grid conductor = 0.75m

h0 is the reference depth = 1m.

Kh = 1.323

As per Eq,84 of IEEE-80 or as per Cl.No 4.2.2 of SES-P-119.10, page 9 of 43.

number of parallel paths,

nc = nd =1 for rectangular and square grid.

na = 11.52, nb = 1.03

No. of parallel paths, n = 11.86

LP = 258 m, peripheral length of the grid

Km =0.634

Corrective factor, Ki (as Cl.No 4.2 of SES-P-119.10 of page 8 of 43)

Ki = 2.399

Substituting the above values,

Attainable Emesh = 831.98 V

6.6 Calculation of Attainable step voltage

From Eq.92 of IEEE-80,or as per Eq.10-10 of SES-P-119.10 , Page 10 of 43.Attainable Estep is

The effective buried length, Ls is

Ls = 1157 m

The spacing factor for step voltage, KS

Ks = 0.302

By substituting the above values,

Attainable Estep = 537.16 V

7.0CONCLUSION

7.1 The Earthing conductor sizes selected are as below:

a) Conductors buried in earth:240 Sq.mm stranded bare copper.conductor.

Conductors above ground level and in trenches:240 sq.mm copper conductor.

b) Rod Electrodes:15 mm dia copper clad steel

c) Equipment earthing leads:As listed in Annexure-1.

7.2The Grid spacing selected for Main plant is 7 m.

Tolerable Etouch

=1014.23 V

Attainable Etouch =831.98 V

Tolerable Estep

= 3564.78 V

Attainable Estep

=537.16 V

It can be seen that with the selected spacing, the attainable step and mesh potentials are within permissible step and touch potentials respectively.

ANNEXURE-1

SIZES OF MINIMUM CONDUCTORS AND LEADSThe electrical equipment shall be connected to the secondary earthing system as described below for equipment safety Grounding as per SES-P-119.10, Table 10.3.

EQUIPMENTNUMBER OF CONNECTIONCONDUCTOR SIZE

Transformers body2240 Sq.mm cu

Transformer neutral2240 Sq.mm cu

Generator Switchgear (GCB)2240 Sq.mm cu

GeneratorAs per GE240 Sq.mm cu

13.8kV Busducts2240 Sq.mm cu

MV switchboards2240 Sq.mm cu

LV switchboards & DBs2240 Sq.mm cu

125V DCDBs.2240 Sq.mm cu

UPS & UPSDB2240 Sq.mm cu

MV motors295 Sq.mm.cu

LV motors2Half the power cable size.

Battery charger2240 Sq.mm cu

Main Lighting Distribution Board2240 Sq.mm cu

Lighting Panel250 Sq.mm cu

Power Receptacles150 Sq.mm cu

Control and Relay Panels295 Sq.mm cu

Street Lighting Poles1120 Sq.mm cu

Cable Trays1 every 10 m50 Sq.mm cu

Fuel Tanks4185 Sq.mm cu

Big metal objects and not-electric machinery195 Sq.mm cu

Main metal structures of buildings-120 Sq.mm cu

Metal stack295 Sq.mm cu

Fence and Gate-50 Sq.mm cu

EMBED Word.Picture.8

EMBED Equation.3

EMBED Equation.3

EMBED Equation.3

EMBED Equation.3

MAIN PLANT AREA

80 M

EMBED Equation.3

EMBED Equation.3

EMBED Equation.3

EMBED Equation.3

EMBED Equation.3

EMBED Equation.3

EMBED Equation.3

EMBED Equation.3

EMBED Equation.3

P0C 09.12.2004MPSR

P0B 12.10.2004RMRBHRC

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_1141145261.doc