CONCEPTS LINKED EARTHING SYSTEM IN SUB-STATION

02 Jan 2025

 

 

 

1. Introduction:- ( Earth system should be as per Standard IEEE-80 or IS 3043)

“EARTH” in universal sense is regarded as zero potential and considered as the reference potential in the electrical system/circuit. EARTHING is nothing but burying of conductors in the earth to achieve low resistance value to allow the safe passage of fault current during the disturbance. But in practice earth is poor conductor, non-homogenous in nature, for which the realization of earth resistance becomes complex and calculated by the assumed empirical formulae. It is also seen that maximum of the faults are due to single phase to ground fault ( 95 % of faults are SLG, 3% of the system faults are phase to phase fault and 2% are due to 3phase, other faults). So earthing of the system equipment are highly important and to be maintained seriously for the safety operation of the electrical system and working personnel. The earthing system depends upon the pattern of burying of the conductors, its size, distance of separation and their design of laying inside the earth. These conductors are called electrodes.

 

1.1. Objectives and Importance of earthing system

• Safe flow of fault current to earth.

• Eliminates the rise of potential on the metallic parts of the equipments.

• Develops a uniform electric potential in all non-current parts of the system.

• Manages uniform potential gradient through out the grounding system.

• Does not allow system voltage to rise abnormally under faulty condition.

• Improves the reliability and life of equipments

• Provides safety to the working personnel.

 

1.2. Difficulties of Non-earthing or inefficient earthing.

• Prolonged Fault current will damage the equipment.

• Voltage on the healthy phases will be increased and may cause failure of the insulation in the equipment.

• Step and Touch potential limit may be increased and result accident to human being.

• For the case of transformer heating of the core, damage of the insulation and bushing may result.

• For the case of Instrument transformer (CT) earthing of the star connected secondary provides safe limit voltage during accidental opening of the secondary terminals.

• For CT, insulation between the primary and secondary if fails, then provision of secondary earth may help to avoid severe damage of the CT.

• For PT/CVT, secondary star terminal is generally earthed, if not so then for the fault condition, the development of high voltage on this terminal may damage the equipment.

 

1.3. CASE STUDIES ON EARTHING CONCEPTS

1.3.1. SITUATION 1:-

It was reported by few domestic consumers regarding the burning/failure of electrical appliances during normal electrical supply to them. Investigation and Analysis:- The electrical circuits to the premises were checked . (Ratings, supply connections, earthings system). It was found correct and in order. Then the supply transformer was checked and found with earthing connection in order. Finally it was suspected with the dis-connection of NEUTRAL lead inside the transformer and on opening of the same it was found with the opening of lead. The lead was connected to earth bushing and the problem was rectified.

 

Mathematical explanation for nonearthing of neutral point

 

For healthy and neutral being Earth condition.

 

Refer Fig 1.2.1A and B 

V_R = V , angle( 0⁰ ) ,V_Y = V, angle (-120⁰)

V_B = V, angle (120⁰),

So, V_R+ V_Y+ V_B= V_N = 0

V₁ = ( V_R-V_Y) = √3 V, angle (30⁰ ),

V₂ = ( V_Y-V_B) = √3 V, angle ( -90⁰ ),

V₃ = ( V_B-V_R) = √3 V, angle ( 150⁰ )

For Non-earthing of neutral system

And fault in any one of the phase say in Phase B the following issues will arise.

Refer fig. 1.2.1C

Voltage on the phase B = Zero,

Voltage at N =- V_B=V, angle (-60⁰),  

So V_R1 = ( V_R-V_N) = √3 V, angle (30⁰ ),

V_Y1 = ( V_Y- V_N) = √3 V, angle ( -90⁰ ),

V_B1 = 0

V_RY1 = V_R1 - V_Y1 = 3 V, angle (-60⁰).

So it is concluded that voltage on each healthy phase winding (Here R and Y phase) will have √3 times normal phase voltage for which the electrical appliances, connected on these phases may be damaged during fault on the other phases.   

 

1.3.2. Situation 2

One cow boy was electrocuted, but survived while passing by an electrical installation, not its cow.

Investigation and Analysis:- The cow boy was bare footed and while passing by an electrical installation, fault might have resulted in the system with development of potential on the ground and flow of current though the bare foot of the boy has resulted the electrocution of the boy. But for the case of cow, due to insulating hooves, the current flow has been arrested. This concept of ground is considered as the STEP POTENTIAL

Concepts on Step Potential:-This is the potential difference between a step across by human body on the ground assumed as one meter during the fault condition. The step potential depends upon the fault current flown and the resistance offered by the step circuit as shown in the figure. According to the allowable value of fault current, the limit of step potential is calculated.

Step Potential (Estep) = I _A ( R_B + 2 R_F )

Tolerable Step Potential (Estep)

= (0.155 / √ t ) (1000 + 6 C_s ps)

(Safe value of current I _A = 0.155 / √ t,

Safe body resistance R_B (Between two feet) = 1000 Ohm,

R_F = Contact Resistance immediately under each foot=3 C_s ps )

Attainable Step Pot. (E step max) = (K_s K_{I p} I) / L,

Where K_s = 1/π[(1/2h) +1/ (D+ h) + 1/ (2D) +...+ n terms] 

Where , C _s = Co- relation factor (As per IEEE 80 /1985), its value is obtained according to thickness of crushed rock layer (hs) and Reflection factor K = (p – ps) / (p + ps)

p = Earth Resistivity in Ohm- Mtr, 

 

ps = Surface layer resistivity in Ohm – Mtr

t = Duration of Allowable current in Sec., 

I = Maxm. Current to the grounding System.

N= Number of parallel conductor in the grid, 

h = Depth of burial in Mtrs 

L = Length of buried conductors in mtrs.

D = Spacing of conductors in grid in mtrs.

Allowable Step voltage as per IS 3043 to be less than 1510 Volt

 

 

 

1.3.3. Situation 3

One person was electrocuted and died while gossiping with his friends near the gate of a 220/132 kV grid sub-station.

Investigation and Analysis:- The person was in bare foot and gossiping with friends being in touch of the newly constructed grill gate, not even cemented, was with temporary support of earth filling. This gate was not even connected to the GRID earth mat. Suddenly earth fault might have resulted with the development of certain potential on the metal structure of the grill gate and flow of current through the hand to one side of the foot and electrocution has caused death of the person. This concept of ground is considered as the TOUCH POTENTIAL and TRANSFERRED POTENTIAL

Touch Potential:- This is the potential difference between ground and the equipment/object likely to be referred by human body, when touched to the equipment during the faulty condition

Touch Potential (E touch) = I _A ( R_B + 0.5 R_F )   

Tolerable Touch Potential (E touch)

= (0.155 / √ t ) (1000+ 1.5C_s ps)

Attainable Touch Pot. (E touch max) = (K_m K_{I p} I) / L,

Where Km = ½π [ln (D / 16 HD)] + 1/π × ln 0.25

           K_I = 0.655 + 0.172 n

 

Transferred Potential: - In the earth system, during fault condition certain voltage is transferred due to the availability of adjacent metallic parts and this potential is termed as transferred potential. So this potential can be regarded as the converted form of touch potential or the special case of touch potential. Practically for the fault situation due to flow of fault current, certain potential is developed on the metallic structure adjacent to the fault affected area. So a person standing within the station area touches a conductor that grounded at remote end or a conductor connected to the station ground mat may experience this transferred potential. This value becomes more than the touch potential as described before.

 

1.3.4. Situation 4

It was reported with the followings in a grid substation of 400/220 kV Grid Sub-station

• Failure and sometimes the burning of electronics cards at the telephone exchange.

• Sparking and fire in the marshal box and cable box at the nearby station.

• Burning of telephone Sets, modems and service main at consumer premises,

• Frequent failure of telephone cables and tags.

• Frequent failure of PLCC equipment

 

Investigation and Analysis:-

These effects basically develop due to the problem of the earthing in the sub-station and non-maintenance of metallic parts in the sub-station at equipotential of the grid mat. For this sub-station, the earthing of the mat was checked and found with value of 5Ohm. The Building, Electronic equipment and other metallic equipment were checked and found with no earth connection and some cases with separate high ohmic earth connection. So during fault condition, due to the development of GPR (Ground Potential Rise) was resulting of flow the fault current to the system and failure of the equipment. The metallic and affected structures were properly earth connected to the mat connection of the GRID sub-station and the problem was rectified. This concept of ground is considered as the GROUND POTENTIAL RISE.

Ground potential Rise:-

Practically it is observed that during the fault condition, the abnormality in the earthing of the ground, causes the rise of potential. For minimization of this potential rise on the non-current metallic parts of the equipment, proper earthing has to be provided. The concept of current flow that results this rise is explained below. The current flow in different equipment and ground system can be realized as shown in following figure. GPR can be defined as the product of ground electrode Impedance and the current that flows through the electrode. (As per IEEE std 367). For the case of heavy fault current flow, like tower fault condition, the ground nearer to the fault current is influenced with potential rise and effect is reduced away accordingly, depending upon the magnitude of fault current. Electrical potentials in the earth drop abruptly around the perimeter of a grounding system, but do not drop to zero. In fact, in a perfectly homogeneous soil, soil potentials are inversely proportional to the distance from the center off the grounding system, once one has reached a distance that is a small number of grounding system dimensions away. The formula is as follows:

Earth potential = soil resistivity x current/(2 π distance)

 

The fault current that flows due to fault occurrence at the tower will try to flow to the earth mat and cause potential rise in the system.

 

I_F = I_M = I_GT + I_GW

 

Where  I_F = Fault current at the tower point

            I_GT = Total ground current through tower footing of each tower = I_G1+ I_G2  + …….. + I_GN

              I_GW = Fault current that flows through ground wire.

            I_M = ground Mat current

                  I_N = Neutral current

For this condition the rise of potential at the ground mat may occur due to the ground fault current and the resistance offered by the system. But if the fault occurs at the sub-station premises or in the equipment, then the potential rise would be reduced due to zero because of non-involvement ground current for this condition. The effect of ground potential rise is severe for the case of any other sub-station or metallic structure if present nearby to the main sub-station. Few cases are described below.

2. CONCLUSION: -

The effect of the earth and its connection in the sub-station is regarded as the most important factor to be maintained properly. Basically the connection of metallic parts in the station and important equipment like Transformers, Lightning Arrestors, Current Transformers etc