Revised Grounding System Calculaiton Rafha -09.12.2004

TCE CONSULTING ENGINEERS LIMITED 30421008-3CL-00014-

GROUNDING SYSTEM CALCULATION FOR NEW POWER PLANT AT RAFHA

SHEET OF i i OF 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 RAFHA DOCUMENT NO. : 30421008-3CL-00014-P0C DECEMBER, 2004

ENGINEERING CONSULTANT TCE CONSULTING ENGINEERS LIMITED 73/1, ST. MARK’S 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

P0C 09.12.2004

MP

SR

P0B 12.10.2004

RMRB

HRC

REV NO . DATE

PREPARED BY

CHECKED BY

CLEARED BY

APPROVED BY TCE FORM 032 R1

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CONTENTS

GROUNDING SYSTEM CALCULATION FOR NEW POWER PLANT AT RAFHA

SHEET 1 of 1 OF 4

CONTENTS

CLAUSE No.

DESCRIPTION

PAGE No.

1.0

Introduction

1

2.0

Reference drawings / Documents

2

3.0

Design Basis

3

4.0

Selection of Earthing Conductors

4

5.0

List of Abbreviations and Design parameters

7

6.0

Calculation of Step and Touch potential for Main plant Area

8

7.0

Conclusion

14

ANNEXURES 1.0

Annexure-1-Summary of sizes of Equipment Earthing leads

15

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1.

GROUNDING SYSTEM CALCULATION FOR NEW POWER PLANT AT RAFHA

SHEET 1 of 15 OF 4

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.

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GROUNDING SYSTEM CALCULATION FOR NEW POWER PLANT AT RAFHA

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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.

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3.0

GROUNDING SYSTEM CALCULATION FOR NEW POWER PLANT AT RAFHA

SHEET 3 of 15 OF 4

DESIGN BASIS

3.1

The 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. # 30421008-3CL00013 (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 SESP-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.no4.0 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)

3.10

The 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.

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4.0

SELECTION OF EARTHING CONDUCTORS

4.1

Conductor Material

SHEET 4 of 15 OF 4

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

4.2

a)

Conductors buried in earth

:

Soft Drawn Bare Copper Conductor

b)

Conductors above ground level in trenches and concrete

:

Soft Drawn Bare 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.

Size of Conductors

4.2.1 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)

tcxrxrx 10 4

I  TCAPx10  4   K  Tm )   x ln 0   tc x r x r   K 0  Ta 

A = 

or

TCAP

A= Ix

ln 1 

Tm  Ta K 0  Ta ISSUE P0C

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GROUNDING SYSTEM CALCULATION FOR NEW POWER PLANT AT RAFHA

SHEET 5 of 15 OF 4

where: I=

rms current in kA (see below) = 46 kA (maximum fault current observed on 0.48kV as per no:-.30421008-3CL-00001-P0A.

short

circuit

study

Doc

A=

conductor cross section, in mm2

Tm =

maximum allowable temperature in °C = 1083°C (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 PTS1024, 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 SESP-119.10 , Page 14 of 43 Table 10-1.)

tc =



r

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  r=

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 mm2 Minimum Area of cross section of conductor required is 166.17 mm 2 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.

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GROUNDING SYSTEM CALCULATION FOR NEW POWER PLANT AT RAFHA

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4.2.2 Rod 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. 4.2.3 Equipment Earthing Leads The 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-P119.10 page 28 of 43. The sizes of earthing conductor selected for each equipment is listed in Annexure-1.

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5.0 LIST OF ABBREVIATIONS AND DESIGN PARAMETERS Description Fault Current Fault Current Division factor Maximum design grid current that flows between ground grid and surrounding earth. Maximum Grid Potential Rise Ambient temperature Diameter of grid conductor Average soil resistivity Surface layer resistivity (washed granite) Surface layer thickness Surface layer resistivity de rating factor Duration of shock for determining allowable body current Depth of burial of ground grid Spacing for mesh voltage Spacing for step voltage Corrected factor for grid geometry Corrected factor that adjusts the effect of inner conductors on the corner mesh Corrected weightage factor for accounting the effect of grid depth Reference depth of the grid Total number of electrodes Length of each ground rod Total length of ground rods Effective Length of Lc+ LR for step voltage

Symbol If

Unit kA

Value 46

Sf

-

0.7

IG

kA

32.2

Refer cl.6.4

V C m

6504.4 50 0.0185



Ohm m

26.588

s

Ohm m

5000

hs

m

0.10

Refer cl.6.4 Refer cl.4.2.1 Refer cl. 6.5 From Soil Resistivity Report SES-P-119.10 page 14 of 43 SES-P-119.10 page 5 of 43

Cs

-

0.691

Refer cl.6.2

ts

Sec.

0.5

Refer cl.6.2

h

M

0.75

cl.8.03 of SEC Spec (SES- P-119.10). section 8.2.3

Km Ks Ki

-

0.634 0.302 2.399

Kii

-

1

Kh

-

1.323

Refer cl.6.5

h0

m

1

nR

-

20

Lr

m

2.5

LR

m

50

Refer cl.6.5 Refer cl. 4.2.2/6.5 SES-P-119.10 Page 18 of 43 Refer cl.6.5

Ls

m

1157

Refer cl.6.6

GPR Ta d

0

Remarks Refer cl.3.2 SES-P-119.10 page 7 of 43

Refer cl.6.5 Refer cl.6.6 Refer cl.6.5 For grids with ground rods as per IEEE-80

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GROUNDING SYSTEM CALCULATION FOR NEW POWER PLANT AT RAFHA

Total buried length of grid conductors Total / Effective length of Lc + LR for mesh voltage (LT) Peripheral length of the grid Total area enclosed by the grid Spacing between parallel conductors Number of conductors parallel on the longer side Number of conductors parallel on the shorter side Tolerable ETOUCH Attainable ETOUCH Tolerable ESTEP Attainable ESTEP Resistance of ground grid

SHEET 8 of 15 OF 4

Lc

m

1486

Refer cl.6.1

LM

m

1565.12

Refer cl.6.5/6.3

Lp A

m Sq.m

258 3920

Refer cl.6.5 Refer cl.6.1

D

m

7

Refer cl.6.1

Ny

-

10

Refer cl.6.1

Nx

-

14

Refer cl.6.1

V

1014.61

Refer cl.6.2/7.2

V

831.98

Refer cl.6.5/7.2

V

3564.78

Refer cl.6.2/7.2

V Ohm

537.16 0.203

Refer cl.6.6/7.2 Refer cl.6.3

Tolerable ETOUCH ETOUCH Tolerable ESTEP ESTEP Rg

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 m 2 (GTG Area Ref Dwg no KA667088, P0C) N

MAIN PLANT AREA

49 M

80 M

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GROUNDING SYSTEM CALCULATION FOR NEW POWER PLANT AT RAFHA

SHEET 9 of 15 OF 4

Assuming conductor spacing D = 7m No. of conductors parallel to the longer side, n y

= 10

No. of conductors parallel to the shorter side, n x

= 14

Therefore length of buried conductor

=((10 x 80) +(14 x 49) = 1486 m

Total buried length, Lc = 1486 m 6.2

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 Etouch

 







1000  1.5. Cs . ρ

0.116 s 







. ts

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)    0.09. 1   s   Cs  1  2hs  0.09

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

Cs = 0.691 Tolerable Etouch = 1014.23 V b) Tolerable Estep

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GROUNDING SYSTEM CALCULATION FOR NEW POWER PLANT AT RAFHA

SHEET 10 of 15 OF 4

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

Estep  (1000  6Cs.s ).

0.116 ts

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)

1 1 1 Rg    1  LT 20 A 1  h 20 / A Where, Rg = Grid Resistance in ohms. = Average ground receptivity in ohm-m. A = The area occupied by the ground grid in m2 LT = The total / effective buried length of conductors in m (L M) 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 GPR  IG.Rg

( 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,

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SHEET 11 of 15 OF 4

GPR = 6,504.4 Volts. Since GPR > Tolerable Estep, Emesh & attainable Estep potentials have to be calculated. 6.5 Calculation of Attainable Emesh Voltage  .Km.Ki.IG

Emesh 

LM

  LM  Lc   1.55  1.22    

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

 

Lr

  LR Lx  Ly   2

2

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, Km 

1 2.

2    8  ln D   D  2.h   h   Kii ln      8 Dd 4d  Kh   .( 2n  1)    16.h.d





2

  

Kii = 1 for grids with ground rods. Where, Kii = Corrective weighing factor (Ref as per Cl.no.4.2.2 of SES-P119.10 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)

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Kh  1 

SHEET 12 of 15 OF 4

h h0

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, n  na.nb.nc.nd na 

nb 

2.Lc Lp Lp 4. A

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  0.644  0.148.n Ki = 2.399 Substituting the above values, Attainable Emesh = 831.98 V ISSUE P0C

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GROUNDING SYSTEM CALCULATION FOR NEW POWER PLANT AT RAFHA

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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 Attainable Estep 

 .Ks.Ki.IG Ls

The effective buried length, Ls is Ls  0.75.Lc  0.85.LR

Ls = 1157 m The spacing factor for step voltage, KS 1 1 1 1  Ks     1  0.5n 2    2.h D  h D 





Ks = 0.302 By substituting the above values, Attainable Estep = 537.16 V

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GROUNDING SYSTEM CALCULATION FOR NEW POWER PLANT AT RAFHA

7.0

CONCLUSION

7.1

The Earthing conductor sizes selected are as below:

a)

Conductors buried in earth

b)

Conductors above ground trenches Rod Electrodes Equipment earthing leads

c) d) 7.2

SHEET 14 of 15 OF 4

240 Sq.mm stranded bare copper.conductor. in 240 sq.mm copper : conductor. : 15 mm dia copper clad steel : As listed in Annexure-1. :

level

and

The 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.

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SHEET 15 of 15 OF 4

ANNEXURE-1 SIZES OF MINIMUM CONDUCTORS AND LEADS The electrical equipment shall be connected to the secondary earthing system as described below for equipment safety Grounding as per SES-P119.10, Table 10.3. EQUIPMENT Transformers body Transformer neutral Generator Switchgear (GCB) Generator 13.8kV Busducts MV switchboards LV switchboards & DBs 125V DCDBs. UPS & UPSDB MV motors LV motors Battery charger Main Lighting Distribution Board Lighting Panel Power Receptacles Control and Relay Panels Street Lighting Poles Cable Trays Fuel Tanks Big metal objects and not-electric machinery Main metal structures of buildings Metal stack Fence and Gate

NUMBER OF CONNECTION 2 2 2

CONDUCTOR SIZE

2 2 2 2 2 2 2 2 2 2 1 2 1 1 every 10 m 4

240 Sq.mm cu 240 Sq.mm cu 240 Sq.mm cu 240 Sq.mm cu 240 Sq.mm cu 240 Sq.mm cu 240 Sq.mm cu 240 Sq.mm cu 240 Sq.mm cu 95 Sq.mm.cu Half the power cable size. 240 Sq.mm cu 240 Sq.mm cu 50 Sq.mm cu 50 Sq.mm cu 95 Sq.mm cu 120 Sq.mm cu 50 Sq.mm cu 185 Sq.mm cu

1

95 Sq.mm cu

2 -

120 Sq.mm cu 95 Sq.mm cu 50 Sq.mm cu

As per GE

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