substation grounding

Substation Grounding- Sahadeo Latchmepersad, T&TEC 1

T&T SubsectionLocal IAS Chapter

GROUNDING-DESIGN AND CONSTRUCTIONSaturday 12th November 2005

Presented by

Eng. Sahadeo Latchmepersad, MIEEE, MAPETT, R. Eng.

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ACKNOWLEDGEMENT

The Trinidad and Tobago Electricity Commission

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Range of Large Industrial Customers

Rate D3- 4-25 MVA at 33, 66 or 132 kV

Rate E > 25 MVA at 66 or 132kV

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Transmission

B.C/Union Estate Schematic

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Objectives of Grounding

To provide means to carry electric currents to earth under normal and fault conductors without exceeding any operating and equipment limits or adversely affecting continuity of service.

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Objectives of Grounding-

To ensure that a person in the vicinity of grounded facilities is not exposed to the danger of critical electric shock.

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Basic Shock Situations

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BRIGHTON 66kV SUBSTATION

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Ventricular Fibrillation

The most dangerous consequence of electric current passing through the vital parts of a human body is a heart condition – ventricular fibrillation –immediate arrest of blood circulation.

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Design Algorithm

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Measuring Soil Resistivity

Procedure depends on the instrument

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Soil Resistivity Calculation

ρ = 2 x Pi x a x RWhere R is Maximum measured Value

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Tolerable Step Voltage

Step Voltage Limit 50 kgE Step 50 = (1000 + 6 Cs ρs) 0.116

√ts

E Step 70 = (1000 + 6 Cs ρs) 0.157√ts

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Tolerable Touch Voltage Limit

E touch50 = (1000 + 1.5 Cs ρs) 0.116√ts

E touch70 = (1000 + 1.5 Cs ρs) 0.157√ts

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Constants

Cs = 1 – [0.09 (1 - ρ/ ρ s)][2 hs + 0.09]

ρ = soil resitivityρ s = surface layer resistivityTs = duration of shock current in seconds including clearance time of circuit breaker plus relay

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Conductor Sizing

I = Amm2 [TCAP.10-4] Ln Ko + Tm

[tc αr ρr] Ko + Ta

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Grid Resistance

burial of depth theish conductor oflength total theis

/20111

2011

T

Tg

LyresistivitsoiltheisWhere

AhALR

ρ

ρ

+

++=

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Calculated Step Voltage

Es = ρKs Ki IGLs

Ls = 0.75Lc + 0.85 LR

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Calculated Mesh Voltage

Em = ρ Km Ki IG WhereLM

Lm = Lc + 1.55 +1.22 Lr LR

LX2 + Ly

2

LR = total length of ground rodsLc = length of conductor

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Earthing Designs

Nitrogen 2000 66kV Substation

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Output file

Nitrogen 2000 substation

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Exothermic Joints

Step 1Position clean, dry cables in mold and close clamp.

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Insert disk, insert pour weld metal and starter material into crucible, and ignite to

form the weld.

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Remove finished connection. Clean mold

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FENCE EARTHING

1. Inclusion of the fence within the ground grid area. Under this philosophy, the perimeter conductor of the grid will parallel the fence at a short distance of 1.0m outside. The crushed rock covering will also be required outside the fence.

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FENCE EARTHING

2. Placement of the fence outside the ground grid area with close electric coupling between fence and adjacent earth along its length, but with no electrical tie between the fence and the main station grid

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ANSI STANDARD 80-2000 Date: November 11, 2005Date:

Line-to-line Voltage V 66000 Volts Touch voltage limit E touch 70 1792 VoltsMaximum fault curent HV side 20000 Amps Step voltage limit E step 70 5678 VoltsMaximum Fault Current LV side 20000 AmpsSoil resistivity ρ 15 Ωm Touch voltage limit E touch 50 1324 VoltsSubstation Length L 34 metre Step voltage limit E step 50 4195 VoltsSubstation Width W 30 metreSpacing D 4 metre GPR = 4944 VoltsNumber of ground rods 28 Ea Em= 606 VoltsLength of ground rod 2.44 metres Es= 619 VoltsDepth of grid burial h 0.5 metresFault Duration tf 0.1 s Length of 4/0 Copper Required 574 metresCurrent Division Factor Sf 1 Size of Copper d = 0.01 metresX/R Ratio 7.20 No. of Copper Rods 28 EaCrushed Rock Resistivity ρs 2500 ΩmAmbient temp Ta 40 oC Grid Resistance 0.23 ΩThickness of crushed rock hs 0.102 metresGrid shape (Square=S, Rect.=R, L- shaped=L) rSubstation L-shape Length l 0 metreSubstation L-shape width w 0 metre

50kg Design O.K70kg Design O.K

INPUT DATA RESULTST&TEC Grounding Program

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Development Program

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AutoGrid Pro

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Connection to Customer’s Substation

Fence eathingNumber of TailsIncreased area-lower RgIncreased amount of copper

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The End

Questions?

Thank You