ELECTRICAL SYSTEM OF A THERMAL PLANT CONCLUDING PART- ABC OF THERMAL POWER PLANT.

     Earth pit checking

Each earth pit is checked as mentioned in the standard check list of 

Switch yard earthing system

Check that the depth of the earth pit is 3.5 Mtr in case of pipe 

electrode 3 Mtr incase of rod electrode.

Incase of pipe electrode, earth pit must be having the layers of sand, gravel, salt, and coal with the thickness  as mentioned in the separate detail.

Thickness of the gravel must be 150mm  and the gravel must be spread over the switch yard area.

Main earthing and subsidiary conductors are to be ensured that 40/50 mm dia rod is used for the same.

GENERAL CHECKS :

Minimum spacing between electrodes shall be 6 Mtrs.

Earthing conductor shall be laid at least 300 mm below cable trenches and under ground service ducts, pipes, tunnels, railway tracks etc., if crossed.

Risers of 40/50 mm  dia MS rod shall be brought nearby equipment foundations :  1.  2 Nos. for each equipment earthing.  2.  1 No. for each structural earthing.

Risers end near the foundation shall be brought 300 mm above the ground level near foundation.

Earthing conductors along their run on cable trench ladder columns, beams, walls, etc. shall be supported by suitable welding/cleat fixation at intervals of 750mm .

Galvanized iron sleeves shall be provided for the passage of the conductors where ever it passes through the walls. Both ends of the sleeves shall be sealed to prevent the passage of water through the sleeve.

Earthing conductor embedded in concrete shall have approximately 50 mm concrete cover.

Auxiliary mesh of 1500 x 1500 mm size shall be laid at spacing of 300 mm below operating boxes of circuit breakers, isolators-main switches and earth switches.

All joints  shall be welded as per the drawing and IS code.

LAs, CVTs, and Earth wire down conductor shall be connected to the main earth mat through rod electrodes, rod electrodes shall be made out of 40 dia MS rods at site.

Lightning Masts down conductor shall be connected to earth mat through 1 NO. pipe electrode.

Transformer neutral shall be connected to earth mat through 2  nos. pipe electrodes.

Earth mat shall extend 2 mtrs outside the switch yard fence through rod electrode.

All equipment shall be earthed to at least two points on opposite side which shall be connected to different conductors of the earth mat.

Testing of earth pits  :  Earth pit resistance is measured with earth megger and it should be in the range of 1 to 1.5 ohms. The continuity between two earth pits connected together is checked using a multimeter and it should show exact continuity.

Earth resistance of earth mat is measured and it should be in the range of  0 to 0.05 ohms.

Check overlapping of joints are welded with no gap between two joints.

The connection of earthing network is checked according to the layout drawing. 

We will discuss 220V DC System  as this system is required to start AC system breaker. Therefore  a little knowledge on DC system will help to understand the electrical system.

 continued

ELECTRICAL SYSTEM OF A THERMAL PLANT - ABC OF THERMAL POWER PLANT.

In my earlier post erection of heaters ,de aerators  were discussed . beside that in Turbine package there are other auxiliaries both static and rotary such as different coolers , CEP ( condensate extraction pump ) , vacuum pumps , CPU ( condensate polishing unit) etc. Except CEP erection of other auxiliaries need not require any special strategy. For CEP erection foundation checking and canister erection is slightly crtical. CEP is a vertical pump therefore canister is to be erected before erection of CEP. It is sometimes noticed that CEP foundation is full of water at bottom most area and prevent  canister erection . It is recommended to repair the foundation so that leakage can be stopped or draining the pit before erection.

It was decided that next topics will be steam blowing as we have covered all the major equipment . However i received mails from young engineers to cover electrical system erection and commissioning . I am an electrical engineer but did not work with the system. Therefore what i have seen at site is written down. Value addition is to be done by individual. 
Earthing system is the mother of electrical system. You can not run an electrical machine without earthing.

The  earthing system consists of  following. Earthing is the basic requirement of any  Good electrical system. This is essential to protect human lives and also help us to detect fault in the system . Good earthing  provides smooth electrical operation  and avoid spurious tripping.

i.              Earth Pit

ii.             Subsidiary conductors

iii.            GI Flats

1.            EARTH PIT :   Each earth pit must be made as per their design normally that will be either pipe or rod electrode type earth pits.  In pipe electrode earth pit the depth will be 3.5 Mtr . Where as in rod electrode the depth will be 3 Mtr only.  In pipe electrode,  the earth pit must be made with different layers of  sand, coal, salt  and gravel  whereas in rod electrode, the rod will be inserted into the earth soil. BTG area earth mat was laid after  doing the PCC of the area. Risers were raised along the column foundation from two sides. Rods are welded at least 250 mm overlap so that firm contact is made.  Special earth pits are made for electronic panels and skids which are separated from electrical earthing. 

2.            SUBSIDIARY CONDUCTORS :  The conductors connects between earth pit and various equipments are subsidiary conductors.  They are connected with main earth mat  which is  600 mm depth by means of   raiser  and will be connected to each earth pit through test links.  The other end of the earth pit will be connected to the equipment.

3.            GI FLATS :  All the test links and equipment connections of the switch yard are  done with GI flats of 75 x 12 mm .  Inside the control building, the earthing connection is provided by 50x6 mm GI flats. 

Testing

-             To test the individual earth pits of 400 KV switch yard

-              To test the continuity of the earth mat

-             To measure the resistance of  total earth mat.

Testing Equipment

-          Earth  Megger calibrated.

 -     Calibrated digital multimeter.

State  of the plant

Erection of 400 KV Switch yard earthing network completed.

Test links and connections to various equipments disconnected for test.

Clearance from erection department for carrying out various tests obtained.

continued part -ii