BISHA CPS EXTENSION PROJECT

PAGE

TCE CONSULTING ENGINEERS LIMITED

i30421007-3CL-00022-P0AGROUNDING SYSTEM CALCULATION FOR NEW POWER PLANT AT ARARSHEET OF iOF 4

SAUDI ELECTRICITY COMPANY

EASTERN OPERATING AREA

Damman, Saudi Arabia

GENERATION EXPANSION OF ARAR POWER PLANT

(CONTRACT NO : 30421007/00)

GROUNDING SYSTEM CALCULATION FOR

NEW POWER PLANT AT ARARDOCUMENT NO. : 30421007-3CL-00022-P0A

AUGUST, 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

1.0 INTRODUCTION:

1.1 Al-Toukhi company for Industry & Trading has been awarded turnkey contract for Open Cycle Gas Turbine based CPS expansion project at Arar 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. 60.16MW @ 50(C, PG7121 (EA) model Gas Turbine Generator unit of GE make along with associated Generator Transformer and other balance of plant electro-mechanical equipment.

2.0 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-1022.

2.4 General site Layout. Dwg No:KA 667182, Rev-1

2.5 AHSL report Ref No.G/3481 dated 31-07-2004: Field measurement of Soil Resistivity

3.0DESIGN BASIS

3.1The average soil resistivity for the purpose of earthing system design is taken as 42.552 ohm metre from the soil resistivity test data by taking the average of the values measured at various locations in the plant as per ASHL Report ref:G/3481 of Test#2 and Test#3.

3.2 The design earth fault currents for the various system voltage levels are considered as follows:

a)132kV System

:40kA for 1 second.

b)33kV System

:31.5kA for 1 second.

c)13.8kV System

:40kA for 3 seconds.

d)4.16KV System

:25KA for 3 seconds.

e)480V System

:50KA for 1 second.

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 1022, Page 244 of 336.)

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 1022, Page 244 of 336)

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

:Bare Copper Conductor

b)Conductors above ground level: Bare Copper Conductor

in trenches and concrete

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.

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. 50kA.

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) = 50 kA (maximum fault current

observed on 0.48kV as per short circuit study Doc no:-.30421007-3CL-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-

1022, page 70 of 336)

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 = 179.47 Sq.mm

Minimum Area of cross section of conductor required is 179.47Sqmm

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

Cl.no 6.18.2-c of PTS 1022, Page 244 of 336.

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 CurrentIfkA50

Fault Current Division factorSf-0.7

Maximum design grid current that flows between ground grid and surrounding earth.IGkA35Refer cl.6.4

Ambient temperatureTa0C50

Diameter of grid conductordm0.015Refer cl.4.2

Average soil resistivity(Ohm m42.552From soil resistivity report

Surface layer resistivity (washed granite)(sOhm m10000From soil resistivity report

Surface layer thicknesshsm0.10

Surface layer resistivity de rating factorCs-0.69097Refer cl.6.2

Duration of shock for determining allowable body currenttssecs0.5

Depth of burial of ground gridhm0.75cl.8.03 of SEC Spec (SES- P-119.10). section 8.2.3

Spacing for mesh voltageKm-0.5322Refer cl.6.5

Spacing for step voltageKs-0.3313Refer cl.6.6

Corrected factor for grid geometryKi-2.568Refer 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-1.32Refer cl.6.5

Resistance of ground gridRgOhm0.3429Refer cl.6.3

Reference depth of the gridh0m1

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

Total length of ground rodsLRm25Refer cl.6.3

Total area enclosed by the gridASq. m3456Refer cl.6.1

Spacing between parallel conductorsDm5Refer cl.6.1

Number of conductors parallel on the longer sidenx-14Refer cl.6.1

Number of conductors parallel on the shorter sideny-12Refer cl.6.1

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 = 64 m x 54 m = 2604 sq.m (GTG Area Ref Dwg no KA667182, Rev 1)

N

64 M

Assuming conductor spacing D = 5m

No. of conductors parallel to the longer side, nx =13+1 i.e = 14

No. of conductors parallel to the shorter side, ny=11+1 i.e. = 12

Therefore length of buried conductor=((13 +1) x 54 )+(( 12 +1) x 64)

= 1524 m

Total buried length, Lc = 1524 m6.2 Calculation of tolerable Etouch and Estep voltage

a) Tolerable Etouch :

For average 70kg body weight(as per email received dated 08.08.2004),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 as10000 as per table 7

of IEEE80(ref: Email received dated 08.08.2004)

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 = 0.690976

Tolerable Etouch = 1864.37 V

b) Tolerable Estep

For average 70kg (as per Email received dated 08.08.2004) 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 = 6965.34 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 buried length of conductors in m

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

Rg = 0.3429

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 = 50 x 0.7 = 35kA.

Therefore,

GPR = 12001.5 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 = 54m

Ly is the Maximum length of grid conductor in y direction = 64m

LR is the total length of ground rods = 2.5x 10 = 25m

LM = 1563.66 m

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 = 5m

'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 = 1.32

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 = 14.11, nb = 0.9185

No. of parallel paths, n = 12.9 = 13

Km =0.5322

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

Ki = 2.568

Substituting the above values,

Attainable Emesh = 1301.71 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 = 1164.25

The spacing factor for step voltage, KS

Ks = 0.3313

By substituting the above values,

Attainable Estep = 1088.324 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 5 m.

Tolerable Estep

=1864.37V

Attainable Etouch =1307.71 V

Tolerable Estep

= 6965.34 V

Attainable Estep

=1088.32 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 body1240 Sq.mm cu

Transformer neutral2240 Sq.mm cu

Generator Switchgear (GCB)1240 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

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

54 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

P0A 28.08.2004MPSR

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