Grounding Calculation

ψ DANIELIAUTOMATION

ELECTRICAL GENERAL S YSTEMS – GROUNDING

CALCULATIONS

Table of contents Table of contents - .............................................................................................................................1 1 AIM OF THE STUDY..........................................................................................................................2 2 DEFINITIONS AND THERMINOLOGY..............................................................................................2 3 SOFTWARE USED AND INPUT DATA.............................................................................................2 3.1 ELECTRICAL DATA.........................................................................................................................2 3.2 PHYSICAL DATA.............................................................................................................................3 3.3 GEOMETRICAL DATA.....................................................................................................................3 3.4 REFERENCE STANDARD...............................................................................................................4 4 DESIGN AND VERIFICATION CALCULATIONS..............................................................................5 4.1 DESIGN OF THE GRID CONDUCTORS.........................................................................................5 4.2 VERIFICATION OF THE EARTHING GRID LAYOUT.....................................................................6 5 CONCLUSIONS................................................................................................................................17 6 ANNEXES.........................................................................................................................................17

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CALCULATIONS

AIM OF THE STUDY

This report focuses on the calculations made to design the primary earthing network of the UNICOIL Galvanization Plant. The place is located in Jubail Industrial City (Al Jubail , Kingdom of Saudi Arabia), at about 10 km from the Arabian Gulf. The overall area on the plant has a nearly rectangular shape covering a 750x250 m surface. A primary earthing network is already realised on the Painting Line area so that the Galvanization area will be served by an extension of this existing network. The calculations carried out for the design of the earthing network will then account for the overall grid, considering that the former and the new one will be interconnected.

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DEFI DEFINI NITI TION ONS S AND AND THER THERMI MINO NOLO LOGY GY

According According to Europea European n Harmonis Harmonisatio ation n Documen Documents ts CENELEC CENELEC HD 637 S1 will be used used followin following g symbols (brackets the corresponding corresponding definitions IEEE Std 80-2000): 80-2000): Rb (R (Rb) = resistance of of th the hu human bo body Re (Rg) = earthing resistance Ue (G (GPR) = ground potential ri rise Ues = earth surface potential Ut (Et) = touch voltage Ust Ust = tou touch ch volt voltag age, e, as meas measur ured ed with withou outt pre prese senc nce e of of hum human an body body Utp Utp (Rb* (Rb*Ib Ib)) = permis rmissi sibl ble e touc touch h vol voltage tage Us (Es) = step voltage Uss Uss = step step volt voltag age, e, as meas measur ured ed with withou outt pre prese senc nce e of of hum human an body body Usp Usp (Rb (Rb*Ib) *Ib) = permis rmiss sible ible step step volta oltage ge If (If) = fault current Id (Ie) = drawn current Ie (Ig) = earthing current ts (ts) (ts) = in interv terve entio tion tim time of prote rotect ctio ions ns The following terminology will be preferred in this report: “earth” instead of “ground” “ear “earth thin ing g gri grid” d” to to ref refer er to to the the prim primar ary y eart earthi hing ng netw networ ork k “dow “down n lead lead condu conduct ctor ors” s” to refer refer to the the condu conduct ctor ors s whic which h conn connect ect the meta metall llic ic masses to the primary earthing network. Other terms will be in accordance with the used standards.

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SOFT SOFTWA WARE RE USED USED AND AND INPU INPUT T DATA DATA

The software used is GSA (Grounding (Grounding System and Soil Soil Structure Analysis), a tool whose flow chart chart is explained in Annex01 and which has been validated for several situations. Inputs for the program are: elec electr tric ical al dat data: a: sing single le pha phase se to to eart earth h shor shortt circu circuit it curr curren ents ts (If), (If), curr curren ents ts draw drawn n by earthwire(s) and cable shields shields (Id), intervention time of protections (ts) physical da data: so soil ch characteristics geom geomet etri rica call dat data: a: geom geomet etry ry of the the ear earth thin ing g net netwo work rk unde underr stu study dy refe refere renc nce e stan standa dard rd lim limit its: s: max maxim imum um tou touch ch and and ste step p volt voltag ages es (Ut (Utp, p, Usp Usp))

3.1

ELECTRICAL DA DATA

The steel plant is fed from a 110/34.5 kV substation via a MV XLPE cables 95 mm 2 line at 34,5 kV. The three-phase short circuit power at the substation (at about 2 km from the plant) is 1500 MVA, earth fault current and fault duration are not available from UNICOIL. With reference to the above and to similar conditions of supply networks, the single phase to earth fault current at the MV coupling point has been conservatively supposed to be: -

If = 15.000 A

A portion of the fault current is drawn by the feeding cables shield.

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CALCULATIONS

According to typical data (see European Harmonisation Documents CENELEC HD 637 S1), this portion amounts to: -

Id/If = 0,55 The current that the earthing grid is called to draw is then:

-

Ie = If – Id = 6.750 A The intervention time of protections (fault duration) has been supposed to be:

-

3.2

ts = 1,00 s

PHYSICAL DATA Measurements carried out on site with Wenner method on 3 areas of the plant have given the following results(see annex 3): a(m)

Average RW ( ) **

Resistivity( xm)

1 0.34 2 2.89 3 1.88 4 6.28 5 11.68 * a = distance between measurement rods ** RW = Wenner resistance

0,054 0,23 0,1 0,25 0,31

Tab. 1 Measured Measured Wenner Wenner resistances resistances for different different inter-rods inter-rods distances distances

Arranging the data as in Table 1 lead to resistivity figures too low to be realistic. In this specific case, the site is characterised by a soil composed by humid silicate sand due to the presence of sea water at low depth. Typical figures for the resistivity of this kind of soil may be found in literature, where an interval between 150 e 250 m is usually suggested. This may be considered as an average between dry sand resistivities (500 ÷ 5000 m - see CENELEC HD 637 S1) and sea water resistivities (0,3 ÷ 0,5 m at 10°C) In the following, calculations have been carried out considering a more reliable value of: -

= 200 xm

3.3

GEO GEOMETRICA ICAL DA DATA

The geometry of the primary earthing network is shown in Fig.1. The extension of the grid to the existing one is better detailed in the annexed dwg 00MCED90E86100-Electrical 00MCED90E86100-Electrical general systems-Primary Earthing System-Key Plan” These guide lines have been followed to design the earthing grid on the galvanization area: the the hori horizo zont ntal al ele eleme ment nts s have have bee been n cons consid ider ered ed at at a 1,0 m dept depth h in ord order er to gra grant nt a suitable behaviour of the grid under all the possible humidity conditions on the surface. If necessary, the burying depth may be increased on the side parts of the network in order to limit the gradient of the t he potential the laying laying of the the hori horizo zonta ntall elem elemen ents ts is done done accord accordin ing g to the posit position ion of the the plin plinths ths and basements of the buildings, so that to obtain a grid with regular squared meshes the the eart earthi hing ng net netwo work rk is inc inclu lude ded d with within in the the fence fence lim limit its. s. The The exte extern rnal al fen fence ce wil willl be connected to the grid About the possibility or the opportunity opportunity of connecting connecting the fences to the earthing grid, a distinction must be done between “internal fences” (inside the perimeter of the grid) and “external fences” (outside). These considerations may then be done: inte intern rnal al fenc fences es shal shalll be conn connec ecte ted d to the the eart earthi hing ng grid grid:: exte extern rna al fence fences s shall shall not not be conn connec ecte ted d to the gri grid d in orde orderr to avoid void dan dange gero rous us touch voltages

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CALCULATIONS

if the the grou ground nd pot poten enti tial al ris rise e is lowe lowerr than than the the limi limitt for touc touch h and and step step volt voltag ages es,, no additional provisions are required

Fig. 1 Earthing network layout(see Annex 03) The existing earthing network on Painting Line is made of copper rope having section = 120 mm 2, buried at about 1, 0 m depth, to which some vertical rods have been connected on the corners. The existing earthing network in the substation area is made of copper rope having section = 185 mm 2 , buried at about 1, 0 m depth. Columns connections are made of copper rope having section = 70 mm 2 The extension may be obtained using: Hori Horizo zont ntal al elem elemen ents ts (cop (coppe perr stra strand nded ed rope rope)) – Sect Sectio ion n 70 70 mm2 Burying depth = 1,0 m Conn Connec ecti tion on bet betwe ween en pri prima mary ry ear earth thin ing g netw networ ork k and and colu column mns s – Secti Section on 70 70 mm2 Conn Connec ecti tion on bet betwe ween en pri prima mary ry eart earthi hing ng net netwo work rk and and meta metall llic ic mass masses es in ele elect ctri rica call 2 cabin – Section 70 mm In this case, use of vertical rods (copper coated steel) is limited and only related to mechanical stability of the network. Their characteristics could be the following:: following:: Diameter 20 mm length: 3,0 m

3.4

REFERE FERENC NCE E STA STANDARD ARD

The calculations and following conclusions of this study have been carried out in accordance with IEEE Std. 80-2000. 80-2000. With relation to the protection intervention time “ts”, the limits for step and touch voltages for a 50 kg human body are reported in the following. The maximum current through the body may be calculated as:

-

I b

0,116 =

t s

= 0,116 A (IEEE Std 80-2000 Equation Equation 8)

The resistance of the human body is:

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-

Rb = 1000

CALCULATIONS

(IEEE Std 80-2000 Equation Equation 10)

With reference to a surface resistivity of the soil equal to 200 Ωm (see 1.2), the maximum touch and step voltages which may be measured by an instrument having an infinite internal impedance, are: -

E touch = Ib * (Rb + 1,5* ) = 150,8 V (IEEE Std 80-2000 Equation 17)

-

E step = Ib * (Rb + 6,0* ) = 255,2 V (IEEE Std 80-2000 Equation Equation 18)

The maximum voltage on the human body in both case is instead: -

Eb = Rb * Ib = 116,0 V

Then: -

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Utp = 116,0 V Usp = 116,0 V

DESI DESIGN GN AND AND VER VERIF IFIC ICAT ATIO ION N CAL CALCU CULA LATI TION ONS S

The calculations for design and check of the primary earthing network have been carried out with calculation code GSA (see Annex01 for details).

4.1 4.1

DESI DESIGN GN OF OF THE THE GRID GRID CON CONDU DUCT CTOR ORS S

The verification of the section of the grid conductor for thermal purposes may be obtained comparing the maximum current density during fault condition by formula:

-

−  10 =  TCAP ⋅ A  t α ρ

4

I

c

-

r

r

 K + T   ln K + T  0

m

0

a

(IEEE Std 80-2000 Equation 37)

where: I/A: I/A: maximum current density (kA/mm2) Tm: Tm: is the maximum allowable temperature temperature (°C) Ta: Ta: is the ambient ore initial temperature (°C) Tr : is the reference temperature for material constant (°C) o: is the thermal coefficient of resistivity at 0°C (1/°C) r : is the thermal coefficient of resistivity at reference temperature temperature Tr (1/°C) r : is the resistivity of the ground conductor conductor at the t he reference temperature (µΩ cm) Ko = 1/αo tc: tc: is the duration of fault current (s) TCAP: TCAP: is the thermal capacity (J/°C cm 3)

The following values have been considered for the above variables (IEEE Std 80-2000 Table 1) : Tm: Tm: 300°C Ta: Ta: 35 °C Tr : 20 °C o: 1/242 1/°C r : 0,00381 1/°C r : 1,78 µΩ cm Ko = 242 °C tc: tc: 1 s TCAP: TCAP: 3,42 J/°C cm3 which yield: I/A = 0,184 kA/mm2 A copper conductor having section S = 70 mm 2 has then a maximum admissible current = 12,880 A. This section is suitable to withstand the whole earth current, although it may be considered that a

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CALCULATIONS

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earth current will always follow at least two different paths of comparable impedance. With reference to the above it is possible to assume that the max fault current that the earthing copper conductor conductor can bear is 12.8x2=25.6 kA. This assures a correct thermal design for the earthing conductor. The calculation of the down leads to grid conductor may be calculated with the same procedure. Considering the following values: Tm: 200°C Ta: 50 °C Tr: 20 °C αo: 1/242 1/°C αr: 0,00381 1/°C ρr: 1,78 µΩ cm Ko = 242 °C tc : 1 s TCAP: 3,42 J/°C cm3 the current density G my be calculated as: I/A = 0,145 kA/mm2 A copper conductor having section S = 70 mm 2 has then a maximum admissible current = 10,120 A, being suitable to withstand the overall earth current. This assures a correct thermal design for the down lead conductor. The sections for the grid and down lead conductors are correctly chosen also with reference to the short circuit current which is carried by the earthing grid. If an intervention time of the protections is assumed to be 0.1 s, the maximum admissible currents for the earthing grid and down lead conductors are respectively 70*579 = 40.5 k A and 70*455 = 31.85 kA. For each electrical cabin, two lead conductors will be instead installed: assuming that a current injected in the grid will always be divided in at least two branches, the limit short circuit current for thermal design is about 60 kA.

4.2

VERI VERIFIC FICAT ATION ION OF OF THE EAR EARTH THIN ING G GRID GRID LAYO LAYOUT UT

The verification of the layout as chosen with criteria illustrated in 1.3 may be obtained with the following steps: calcul calculati ation on of eart earth h resis resistan tance ce and and groun ground d pote potenti ntial al rise: rise: if the groun ground d pote potenti ntial al rise rise is lower than the maximum step and touch voltages, the earthing grid is to be considered correctly designed. Conversely, the following must be considered; calc calcul ulat atio ion n of ste step p and and touc touch h volt voltag ages es and and com compa pari riso son n with with the the limi limits ts as giv given en by by the standard IEEE 80: if these values are lower than the limits, the earthing grid is to be considered correctly designed. The calculated resistance of the grid calculated by GSA is: -

Re = 0,254

The ground potential rise will then be: Ue = Re * Ie = 1716 V This calculated value is well above the admissible values for step and touch voltages, hence it is necessary to verify that the maximum touch voltages will not be higher than the limits in correspondence of metallic masses, in particular by the fence, and the step voltages anywhere. The distribution of the current density is shown in Fig. 2.



CALCULATIONS

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Fig. 2 Distribution of linear linear current density on the earthing network(see network(see Annex 03) The earth surface potential distribution on the earthing grid area is shown in Fig. 3 and Fig. 4 (see Annex02 for numerical value).

Fig. 3 Earth surface potential distribution (3D representation) – Calculation grid 8 x 8 m(see Annex 03)



CALCULATIONS

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Fig. 4 Earth surface potential distribution (equi level curves) – Calculation grid 8 x 8 m(see Annex 03) The areas where the touch voltages are within the allowed limits in the earthing grid are illustrated in Fig. 5.

Fig. 5 Areas where the touch voltages voltages with masses connected connected to the earthing grid grid stay below the limits limits light green: actual values Ut (as measured in presence of human body) – dark green: undisturbed values Ust (as measured without presence of human body) (see Annex 03) The whole fence, in particular by the corners and close to the earthing grid, may be subject to voltages above the limits for touch voltages. This is not related at all with the extension made on the earthing network and must be checked with proper measurements of touch voltages.



CALCULATIONS

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If these measurements will confirm this, it will be advisable to cover these areas with an insulating layer (around 1.5 m beyond the fence and 1.5 m inside the fence) must be considered (bitumen or crushed rock) to limit the touch voltages. The same for some areas of plant, in particular near the corner. The areas where the step voltages are above the allowed limits in the earthing grid are illustrated in Fig. 6. No areas will result in step voltages higher higher than the limits.

Fig. 6 Areas where the step voltages are higher than the limits(see Annex 03) Other informatio informations ns which which may be obtained obtained from the calculation calculation code GSA output output are reported reported in the following figures



CALCULATIONS

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Fig. 7 Detail of earth surface potential distribution (3D representation) representation) – Calculation grid 3 x 3 m(see Annex 03)

Fig. 8 Detail of earth surface potential distribution (equi level curves) – Calculation grid 3 x 3 m(see Annex 03)

Fig. 9 Detail of areas where the touch touch voltages with masses connected connected to the earthing grid grid stay below the limits limits - ligh lightt green green:: actua actuall value values s Ut (as measure measured d in prese presence nce of human human body) body) – dark dark green: green: undisturbed values Ust (as measured without presence of human body) (see Annex 03)



CALCULATIONS

Fig. 10 Detail of areas where the step voltages are higher than the limits(see Annex 03)

Fig. Fig. 11

Deta Detail il of of eart earth h surf surfac ace e pote potent ntia iall distr distrib ibut utio ion n (3D (3D repr repres esen enta tati tion on)) – Calculation grid 3 x 3 m(see Annex 03)

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Fig. Fig. 12 12

CALCULATIONS

12

Deta Detail il of of eart earth h surf surfac ace e pote potent ntia iall distr distrib ibut utio ion n (equ (equii leve levell curv curves es)) – Calculation grid 3 x 3 m(see Annex 03)

Fig. 13 Detail of areas where the touch voltages with masses connected to the earthing grid stay below the limits limits - ligh lightt green green:: actua actuall value values s Ut (as measure measured d in prese presence nce of human human body) body) – dark dark green: green: undisturbed values Ust (as measured without presence of human body) (see Annex 03)



CALCULATIONS

Fig. 14 Detail of areas where the step voltages are higher than the limits(see Annex 03)

Fig. 15 Calculation direction(see Annex 03)

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CALCULATIONS

Fig. 16 Touch and step voltages along the calculation direction(see Annex 03) Curves (from top of the figure): Ues, Ust, Ut, Uss, Us References (from top of the figure): Ue, Usp = Utp


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Fig. 22 Touch and step voltages along the calculation direction (see Annex 03) Curves (from top of the figure): Ues, Ust, Ut, Uss, Us References (from top of the figure): Ue, Usp = Utp

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CONCLUSIONS

The calculations carried out on the earthing grid under study have not shown any hazardous step and touch voltages on the studied area. The touch voltages could pass the limits only close to the fence and near the corner of plant where additional provisions may be taken as indicated. In any case it is advisable to: take some some measur measuremen ements ts of touch touch voltages voltages to confirm confirm if they they overcom overcome e the limits limits in in the actual actual conditio condition. n. It must be borne in mind that the design of the grid here carried out under pessimistic hypotheses (on current values and resistivity of the soil) is in not substitutive of the measurements of step and touch voltages that must be taken according to IEEE Std. 80 when the plant will be built; if the probl problem em is confirm confirmed ed by measu measureme rements, nts, cover cover the area area with with bitumen bitumen ifif the presen presence ce of peopl people e is to be foreseen on the critical points.

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ANNEXES

Annex01 (GSA Flow Chart).doc: Flow chart of calculation code GSA Annex02 (Ues Distribution):doc: Earth surface potential distribution Annex03 (Colour (Colour prints of Fig . from 01 to 22).pcx: Teach Report Figures Annex 4 – Soil Resistivity measures Drawings 00MCED90E86101/102/103/109: Electrical general systems-On field grounding system lay outPrimary grounding system Drawing A.330.003.dwg: A.330.003.dwg: Installation sketches - Primary Grounding System

Grounding Calculation

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