International Journal of Science, Engineering and Technology Research (IJSETR) s1

International Journal of Science, Engineering and Technology Research (IJSETR)

Volume 1, Issue 1, July 2012

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Design and Performance of Primary Substation Earthing System (230/33 kV)

Hnin Nu Wai , Kyaw San Lwin

Abstract—Earthing practices and designs are provided to reduce grid potential rise as well as earthing resistance and to save people in and around the substation under normal and fault conditions. A good substation earthing is considered three basic conditions: personal safety, protective device operation and noise control. The main functions of an earthing system for a substation are safety of human and animal life by limiting touch and step voltages to safe values (protective earthing, earthing of work), correct operation of the electricity supply network and to ensure good power quality (power system earthing), achievement of a given electromagnetic compatibility (EMC) level, i.e. limitation of electromagnetic disturbances and protection of buildings and installations against lightning [1].This paper is calculated substation earthing resistance, grid potential rise, mesh voltage, step voltage and touch voltage depending on the different depths of burial grid conductor and the earth rod spacing. The calculated results show the amount of decreasing values. Substation earthing is divided into four separated grids such as primary earthing, two transformer earthing and secondary earthing. The advantage of the separated earthing grids is to reduce the installing equipments and to protect human and equipments. The results are showed by using MATLAB figures.

Keywords—Earthing designs, substation earthing resistance, grid potential rise, mesh voltage, step voltage and MATLAB figures.

I. Introduction

Earthing is essential for the outdoor AC substation designs and the installed equipments in the substation. It is also provided and saved for personnel and the animals. In the substation, good earthing is also necessary and important both to provide the protection of people working in the vicinity of earthed facilities and equipments against danger of electric shock and to maintain proper function of electrical system [2]. Earth grid designs depend on the soil resistivity, the number of earth rods, the earth rod spacing and the depth of burial grid conductor in order to reduce the substation earthing resistance, Grid Potential Rise, mesh voltage and step voltage. This paper considers and calculates the safe design values on the base of the earth rod spacing and the depth of burial grid conductor. The main objectives of design of the substation grounding grid under normal as well as fault conditions are-

(1) To ensure that person in the vicinity of grounded object is not exposed to electric shock.

(2) To provide a low impedance path to carry the fault current into ground without exceeding any operating & equipment limits.

(3) During fault conditions, continuity of the service should not be affected [3-4].

II. Important Facts For Earthing Design Theory

Earthing means an electrical connection to the general mass of earth to provide safe passage to fault current to enable to operate protective devices and provide safety to personnel and equipments. All equipments and structures are required to be earthed by two separate and distinct connections with earth. All these earthing points should be interconnected with the substation earth mat. An effective substation earthing system typically consists of earth rods, connecting cables from the buried earthing grid to metallic parts of structures and equipment, connections to earthed system neutrals, and the earth surface insulating covering material. Current flowing into the earthing grid from lightning arrester operation, impulse or switching surge flashover of insulators, and line to ground fault current from the bus or connected transmission lines all cause potential differences between earthed points in the substation. The earthing is broadly divided into

(1)Neutral earthing

The neutral earthing is essential requiring for transformer, generator, star point loads, circuits, star points of CT and PT(secondary).The purpose of the neutral earthing is to hold neutral at grounding potential, prevent arcing ground on OH lines, discharge of voltage surges, path for out of balance current and simple earth fault protection.

(2)Equipment earthing (safety earthing)

Equipment earthing needs for electronic equipments used in the substation such as metallic non-current carrying parts. The aim of the equipment earthing (safety earthing) is to hold metallic parts at earth potential even on earth fault safety.

(3)Discharge earthing

The discharge earthing aims for earth terminals of earth switches, current transformer, surge arrestor, capacitor bank or filter bank. The functions of the discharge earthing do to discharge the surge voltage and capacitors charge current to earth.

A. Basic Considerations for Earth Grid Designs

To do the good earth grid designs for the outdoor AC substations, the following data are necessary and important to install and save the equipments in the substations such as

(1) Materials used for earth electrodes and conductors must be chosen carefully taking into account physical, chemical and economical constraints. Ground conductor must be adequate for fault current (considering corrosion).

(2) Conductor sizing depends on fault current and conductivity as well as mechanical strength of material used.

(3) Resistivity of soil and surface layer determines the step and touch potentials, which determine safe values of operation as described in reference [1, 4-6]. Also the multilayer resistivity has been a subject of continuous attention by the researchers [7-8].

(4) A good grounding system provides a low resistance in order to minimize GPR (ground potential rise)[9].

(5) Grid geometry is a major factor in determining the step, touch and mesh potential contours and current distribution in grid. The limitations on the physical parameters of a ground grid are based on economics and the physical limitations of the installation of the grid.

The substation contains many voltages such as

-  Transient enclosure voltage (TEV)

-  Fast transient (VFT)

-  Metal-to-metal touch voltage

-  Step voltage

-  Touch voltage

-  Mesh voltage

-  Transferred voltage [3]

Figure1. Illustration of the grounding system

B. Advantages for Earth Grid

In the substation earth grids, the basic ideas and concepts for the following points serve such as

(1) A continuous conductor loop not only surrounds the perimeters to enclose as much area as practical but also helps to avoid very high current concentration and very high gradient both in the grid area and near the projecting cable ends. Enclosing more area also reduces the resistance of the earthing grid.

(2) Within the loop, conductors are in parallel lines and along the structures and rows of equipments to provide for short earth connection.

(3) Earth rods may be at the grid corners and at each second junction points along the perimeter. Earth rods must also be installed at major equipments.

(4) This grid system extents over the entire substation switchyard and often beyond the fence line.

Nowadays, the earthing grid is used to reduce ground potential rise and maintain the safe value of the substation earthing resistance [10].

C. Earth Grid Design Formulae

The conductor size for substation earth grid is

The grid potential rise is GPR=IgRg [5] (2)

K is = 0.656 + 0.172 ns (5)

Kim = 0.656 + 0.172nm (8)

The step voltage limits are

The touch voltage limits are

FFၤ္

For Substation Earthing Resistance, the formulae is

Where,

Ac =Conductor size in sq-mm

If =Maximum fault current in kA

tc =Duration of fault in sec

α r =Thermal coefficient of resistivity at reference temperature

ρr =Resistivity of earth electrode at reference temperature µΩ/cm3

TCAP=Thermal capacity factor

Tm =Maximum allowable temperature of conductor in ˚C

Ta =Ambient temperature in ˚C

K0 =Reversed value of thermal coefficient of resistivity at 0 ˚C

Es = Calculating step voltage

Ρ = Soil resistivity in Ωm

Ks = Spacing factor for step voltage

Kis = Corrected factor for grid geometry

Ig = Maximum grid current

L = Length of grid conductor in m

Lr = Length of earth rod in m

N = Quantity of electrode (earth rod)

H = Depth of burial grid conductor in m

D = Earth rod spacing in m

ns = Number of conductor for axis

Em = Calculation mash voltage

Km = Spacing factor for mesh voltage

Kim = Corrected factor for grid geometry

Ig = Maximum grid current

Lg = Length of grid conductor in m

Lr = Length of earth rod in m

Nr = Quantity of electrode (earth rod)

Ρ = Soil resistivity in Ωm

h = depth of burial grid conductor in m

d = Diameter of earth rod

h0 = Reference depth of grid

n, m = number of conductor for X axis and Y axis

Cs = Surface layer resistivity derating factor and1 for no protective surface layer

ρs = The resistivity of the surface material in Ωm

ts = Duration of shock current in sec

Lt = Total length of grid conductor in m

A = Total area enclosed by earth grid in m2

Rg = Grounding resistance in Ω

III. Different Types of Earth Grid

There are different types of earthing grid design such as

- Square grid (without earth rods)

- Square grid (with earth rods)

- Rectangular grid (with earth rods)

- L-shaped grid (with earth rods)

- T-shaped grid (with earth rods)

Among these grids, T-shaped grid is sometime used as the main grid. Therefore, the remaining four grids are commonly used to construct the safe grid for substations.

Figure2. Square Grid (without earth rods)

n = na .nb .nc .nd (14)

n = (L/D) +1 (15)

na = (16)

nb =1 for square grid

nc =1 for square grid

nd =1 for square grid

LT = 2.n.L (17)

A = L.L (18)

Figure3. Square Grid (with earth rods)

LT = 2.n.L +LrNr (19)

Where,

LT = Total length of the grid in m

Lp = Perimeter of the sides of the grid in m

L = Length of one side of the grid in m

D = Earth rod spacing in m

n = Number of x-axis or y-axis

A = Area of earth grid in m2

Lr = Length of earth rod in m

Nr =Number of earth rods

Figure4. Rectangular grid (with earth rods)

nx = (Lx/D)+1 (20)

ny = (Ly/D)+1 (21)

Lg = (Lx.ny)+(Ly.nx) (22)

LT = Lg+Lr.Nr (23)

Where nx=Number of x-axis

ny=Number of y-axis

Lx=Length of x-axis in m

Ly=Length of y-axis in m

Lg=Length of grid conductor in m

Figure5. L-shaped grid (with earth rods)

n = na.nb.nc.nd (24)

na = (25)

nb = (26)

nc = (27)

nd = 1 for L-shaped grid

LT = Lg+ Lr Nr (28)

Where Lg =Total length of the earth grid

Figure6. Layout diagram of Myauk Pyin Substation

IV. Calculation For Actual Result Tables

TABLE.I

Result For Primary Side (230 kV)

TABLE.II

Transformer Result

TABLE.III

Result For Secondary Side (33 kV)

V. Calculation Results For Different Spacing

Figure7. Results for Mesh voltage by using the different depths

of burial grid conductor (Primary side)

This figure describes that the more the depths of burial grid conductor are plenty, the less the mesh voltage decreases. However, the values of the mesh voltage increase because of the wide spacing for the earth rods.

Figure8. Comparison with the values of Substation Earthing Resistance

and Grid Potential Rise due to the depth of burial

grid conductor (Primary side)

The above figure8 shows the results of the substation earthing resistance and the grid potential rise varying with the different depths of burial grid conductor and the earth rod spacing. The more the values of the depths of burial grid conductor and the earth rod spacing increase, the less the substation earthing resistance and the grid potential rise decrease gradually.

Figure9. Various results for Step voltage according to the depths of

burial grid conductor (Primary side)

The values of the step voltage are steadily decreasing because the depths of burial grid conductor and the earth rod spacing increase.