EARTHING DESIGN CALCULATION

A)   OBJECTIVES

The objectives of this calculation are as follows :-

i) To design a common earthing system for a short circuit level of 50 kA for 1 second.

ii) To determine the number of earth pits for earthing and lightning protection.

iii) To determine the size of the main earthing grid conductor

iv) To determine the size of the down comers for lightning protection

         v) To design the overall resistance of the earthing system to be less than one ohm.

B)  REFERENCES

          The following reference material has been used for this calculation.

i) Code of Practice for Earthing IS - 3043 - 1987

ii) IEEE Guide for Safety in AC Substation Grounding - IEEE - 80 - 1986

iii) Code of practice for the protection of buildings and allied structures against lightning   IS - 2309 - 1989

iv) Report on soil investigation for the 2 X 55 MW CPP at DOLVI , MAHARASTRA .

v) Over all Earthing layout (99511-EL-0-03-D-092)

C)  ASSUMPTIONS AND BASIS

          The resistivity of the soil (30 ohm-m) is the average value assumed from the soil investigation report.

          The CPP has many voltages/fault levels viz. 220KV/40KA/1sec , 6.6KV/31.5KA/1sec  

          3KV/26.3KA/1 sec ,and 415V/50KA/1sec.The ear thing system for CPP shall be common for all voltages and shall be as per IS 3043 - 1987.

D)   EARTHING PHILOSOPHY

The earthing system shall consist of:

1.       GI pipe electrodes as per recommendation of IS 3043-1987 installed in test pits with disconnecting facilities. The treated pits (test pit) shall be used for earthing specific items like Transformer neutral, generator neutral apart from being used as the connecting test pits between switchyard to CPP area.

2.       Earthing conductor for connection of equipment to the main earth grid.

3.       Separate earthing electrodes for instrumentation system.

4.       Separate earth electrode for lightning protection connected to the main earth grid is considered.

5.       Roof conductor and vertical air termination rod for chimney shall be 20mm dia x 2000mm long copper conductor / rod coated with lead.

6.       Conductor above ground or in trench shall be galvanised steel & conductor buried in ground or embedded in concrete shall be mild steel.

F)  CALCULATION OF RESISTANCE OF ONE PIPE / ROD ELECTRODE

             Resistance of a pipe / rod electrode is given by

           

          R  =  100r   log e   4l   ohms     (IS:3043 - 1987, 9.2.2, page 20)

                      2pl               d

Where             R         -           Resistance of pipe/rod in ohms

 l          -           length of rod or pipe (in cm)

                        d          -           diameter of rod or pipe (in cm)

                        r          -           Resistivity of soil (in W m)

Substituting the following values in the above equation we can obtain R

                        l           =          300 cm

                        d          =          6.5 cm

                        r          =          30 W m    

                        R         =            100 x 30        log e    4 x 300

                                               2 X p X 300                      6.5

            Resistance of one pipe/rod (R)    =    8.305W

            No. of earth pits required (for 1W earth resistance)   =  8.305           ~ 9 Nos.         

            We will provide 12 No of earth pits to cover the entire CPP.

                                                                                                       

            The following earth pits are also needed :

           

1)    Generator transformer 1        -     2 neutral and 2 body earth pits

            2)  Generator transformer 2         -     2 neutral and 2 body earth pits.

            3) Start up transformer                -    2 body earth pits

          

            4) Auxiliary distribution                -    1 neutral,1 common neutral pit for

 both the ADTs and 2 body earth pits

                 transformer 1

            5) Auxiliary distribution                 -    1 neutral and 2 body earth pits

                 transformer 2

6)   5 MW generator1                  -    2 body earth pits

            7)   5 MW generator2                  -    2 body earth pits

8)  50 MW generator1                 -   1 neutral,1 common neutral for both

          the Generators and 2 body earth pits

            9)   50 MW generator2                -   1 neutral and 2 body earth pits

6)    Lightning earth pits         -      Chimney 1         - 2 Nos.

                                                                  Chimney 2         - 2 Nos.

                                                                  T.G Blg              - 2 Nos.

                                                                Total                  - 34 Nos.

            

            Therefore total of 46  pipe earth electrodes are required.

            Neutral earth pits of 50 MW generator, Generator transformer and Lightning Earth pits shall not be inter connected with main earthing system.

            Considering 33 earth electrodes

              1     =    33                 Where RE effective resistance

            RE           R

            RE   =   R        =  8.305           =  0.2516W

                        33             33