EARTHING SUB-STATION

Equipment Earthing for Safety-Earhing System.

The object of earthing system is to provide as nearly as possible a surface, under and around a station, which shall be at uniform potential and as nearly zero or absolute earth potential as possible with a view to ensure that:

1.    All parts of the equipment (other than live parts) connected to the earthing system (through earthing conductors) shall be at ground potential.

2.    Operators and attendants shall be at ground potential at all times.

Also by providing such a ground surface of uniform potential under and surrounding the station, there can exist no difference of potential in a short distance great enough to shock or injure at attendant when short circuits or abnormal occurrences take place.

Under fault conditions the non-current carrying metal parts of an electrical installation such as frames, enclosures, supports, fencing etc. may attain high potentials with respect to he ground so that any person or animal touching these or approaching these will be subjected to potential difference which results in the flow of a current through the body of the person or animal of such a value as may prove fatal.

For a human body, it has been found experimentally that the safe value of current in amperes (rms) which human body can tolerate is:

            I = 0.165/√t to 0.157 for t <3 sec.

And      I = 9 mA for t>3 sec.

Where, t is the time duration in seconds of the flow of current.

The objective of earthing is to ensure safety of the operating personnel and other human or animal bodies by discharging the electrical energy to the earth. This is done by connecting the non current carrying metallic parts to the general mass of earth by means of an earthing system which consists of:

·         Electrical conductors to conduct fault currents and

·         Connecting leads

The conductors are in the form of:

·         Single electrode in the form of rod, pipe or plate.

·         Multiple electrodes – rods, pipes or plates.

·         Grid or Mat.

Consider a transformer whose tank is earthed by a pipe electrode driven into earth. Let there be a damage to the terminal bushing which has resulted into faulting of one phase conductor to the tank. The earth fault current in this case will flow from the phase conductor into the transformer tank and through the earthing pipe to earth. If the soil is homogeneous in resistivity, the current will flow uniformly outwards in all directions.

The flow of ground fault current results in voltage gradient on the surface of the earth in the vicinity of earthing system.

As the current flows through a constantly increasing volume of earth, its density drops as the distance from the pipe increases. As the current density is highest at the surface of the pipe, the potential at the  surface of the earth will be higher round the pipe. As the distance from the pipe increases, the less is the difference in earth surface potential between two points per unit length.

                                        

If we take a series of measurements with a voltmeter to determine the earth surface potential at different places from the pipe, we will obtain, potential curves as shown in figure. The curves show that the highest potential is possessed by the pipe indicated by V_e (the potential of the electrode). Neglecting the voltage drop in the connecting lead the tank of the transformer can be considered atd the same potential.

Near the pipe, the rate of decrease in potential is very marked, but as the distance further increases it decreases more slowly. As a distance of 20m from the earthing pipe, the potential is so small that it can be neglected, that is considered equal to zero.

Now if an operator touches the tank of the transformer (as shown in figure), the potential difference between his hand end feet will be:

            E_touch = V_e –V

Where E_touch is termed as touch potential. It is the voltage that exists between the hand and both feet of the operator.

On the left hand side of the figure, the operator is shown walking towards the transformer. At any time, the earth surface potential difference between his feet will amount to,

            E_step = V₁ – V₂

Where E_step is termed as step potential and is the voltage that exists between the two feet of a person standing on a ground through which fault current is flowing.

IfR_k = the resistance of the body in ohms.

        = 1000 Ω and

R_t is the grounding resistance of one foot in ohms = taken to be equal to 3P_s where P_s is the resistivity of the soil near the ground in ohm-metre.

Then    E_step =m(R_k + 2R_f)I_k volts

And      E_touch = (R_k + R_f/2)I_k Volts

Where, IK is the current in amps rms flowing through the body.

Substituting the tolerable limits of current:

                                    (1000 + 6P_s) (0.157)

                        E_step = ---------------------------  [for t<3 sec.]

                                                              √t

And                            = (1000 + 6P_s) 0.009 [for t>3sec.]

Similarly                     (1000 + 1.5P_s) (0.15t)

                        E_touch = ---------------------------  [for t<3 sec.]

                                                              √t

and                            = (1000 + 1.5P_s) 0.009 [for t>3sec.]

Simplifying                     (157 + 0.942P_s) 

                        E_step   = -----------------------------  [for t<3 sec.]

                                                              √t

and                            = (9 + 0.054P_s) 0.009 [for t>3sec.sustained fault]

and                                    (157 + 0.2355P_s) 

                        E_touch   = -----------------------------  [for t<3 sec.]

                                                              √t

and                             = (9 + 0.0135P_s) 0.009 [for t>3sec.]

When the earthing system is designed and installed, the objective is to obtain as low values of E_step and E_touch as possible in order to ensure full safety for the operators in the installation. These values should never exceed the values given by the above expressions.

                                         

 Spreading of gravel or crushed rock layer of 0.08 – 0.15m (3 to 6 inches) thickness on the earth surface is useful in retarding the evaporation of moisture and thus in limiting the drying of top soil layers during dry weather periods. It also increases the contact resistance between the soil and the feet. This resistance, however, may be considerably less than that of the crushed rock.

Transferred Potential:

If, a person touches a conductor grounded at a distance, much greater than the dimensions of the earthing system, the shock voltage will be equal to the full voltage rise of the earthing system under fault conditions. Such a touch contact is called Transferred Potential contact as shown in figure.

Grounding of Sub-Stations

                                                 

For any particular value of the fault current, the potential Ve and the potential gradient over the surface out from the electrode in the case considered above will depend upon the ground resistance. In EHV systems maximum ground fault current is very high, and it is not possible to obtain a ground resistance so low as to ensure that the total rise of the grounding system corrected only by control of local potentials.

 A grid (or a mat) is the most practical way of limiting the potentials at sub-station yards. A grid controls the voltage gradient under ground fault conditions so as to keep the potential difference between nearby points within safe limits and avoid danger to the persons working in the area.

A grid (or a mat) consists of a number of square or rectangular meshes of earthing conductor buried horizontally and connected to several electrodes drive at intervals. The typical size of mesh being 4 m x 5 m of earthing conductor or buried horizontally at a depth of about 0.5 m varying to 1.5 m depending upon the soil resistivity and connected to several earth electrodes of size 1.9cm x 3 m driven at intervals. All metal structures and frames including fencing posts are securely connected to the  earthing grid by running multiple connections as far as possible.

DESIGN OF EARTHING GRID.

In designing the earthing grid for a sub-station, it is necessary to obtain the following information about the system:

a)    Maximum earth fault current.

b)    Resistivity of the  soil at the sub-station site.

c)    Fault clearing time.

d)    Area covered by the sub-station.

1.            Earth Fault Current: The maximum earth fault current constitutes a major factor in the earthing system design, this determines the cross section of the grounding conductor and total rise of the earthing system potential. The potential gradients are also a function of this current .

It is common to consider the symmetrical value of the maximum single line to ground fault.

When the fault duration is less than 0.5 sec, the rms value of the earth fault current is increased by a factor as given below to allow for DC offset and Ac and DC decrement.

Fault Duration	Decrement Factor
0.08	1.65
0.10	1.25
0.25	1.10
0.50 or more	1.0

To allow for the future growth of the system and the consequent increase in fault current a multiplication factor of 1.2 to 1.5 depending on the site of growth of the system is also adopted.

The current diverted away from the earthing system by the earthing wires connected to the station earthing is neglected.

2.            Soil Resistivity: The resistivity of earth varies within extremely wide limits; between 1 and 10,000 ohm-meters.

To design the most economical earthing system for a sub-station it is necessary to obtain accurate data on the soil resistivity and its variation at the station site. Even at the same site the resistivity of the soil may vary considerably at different times of the year. It is, therefore, necessary that the measurements are made a number of times during different parts of the year. Where time does not permit measurements should be made in dry season to set the maximum value of the soil resistivity. A number of test locations should be chosen at each site to cover a wide area of the soil. When 4 to 5 test sites are chosen and measurements spread over a period of full one year, it may be quite realistic to adopt for earth resistivity – the arithmetic average of various reading.

3.            Fault Clearing Time: There are many considerations which influence the choice of fault clearing time such ass the system stability, the type of switchgear and relaying used. The Russian practice is to adopt 0.2 sec. as the time of adopting 4 sec. i.e. the same duration as is used for short time rating of switchgear.

The short time rating of switchgear as per Indian practice is based on 3 seconds. Therefore, 3 sec. time may be adopted as the fault clearing time for Earthing Calculations.

However the system design engineer has to use his judgment to adopt any lower value consistent with the operating times of circuit breakers and relays used on the system (0.5 to 1.0 sec.).

4.            Area Covered by Sub-Station: The area covered by the sub-station is governed by the number and type of equipment.

After the layout has been decided and sub-station plan finalized it is a simple matter to calculate the area covered by the sub-station.

The next steps in the design procedure is the

a)    Selection of earthing material.

b)    Determination of size of earthing conductor.

c)    Preliminary arrangement of earthing conductors.

d)    Determination of conductor length required for gradient control.

e)    Calculation of resistance of earthing system.

f)     Checking of design for various voltages.

EXERCISE

1.    What is the general earthing and neutral grounding?

2.    What are problems can arise in case of earth fault, if neutral is not provided with grounding?

3.    State the advantages of neutral grounding.

4.    State the different methods of neutral grounding.

5.    Draw the schematic diagram of the following and explain with drawing vector diagram how the fault is cleared or reduced.

a)    Solid grounding,

b)    Resistance grounding,

c)    Arc suppression coil grounding,

d)    Voltage Transformer grounding.

6.    Explain sub-station grounding.

7.    Compare different neutral grounding systems.

8.    What is earth electrode, earth lead and earth current?