HT transmission line and tower earthing

HT transmission line Earthing

1. The earth conductor is placed on the top of structure tower to provide a shield angle over the power conductor to protect against lightning stroke.
2. The shield angle is usually maintained at 25 degrees for outer and 45 degrees for the central conductor.

Specifications of tower earthing

1. Each 132KV transmission tower leg to be provided with one single peg earth electrode.
2.  The electrode length shall be 3 meters with 18 holes of 10mm at the bottom 1 m length.
3. The electrode shall be 38 mm and 3.25 mm thick.
4. All the earthing pegs shall be internally connected via 50*6mm GI flat.
5. The connection of GI flat to tower leg should be done at a height of min. 500mm from the ground level.

Advantages of neutral earthing in electrical power system

TYPES OF EARTHING

1. System earthing: Neutral of the transformer is connected to the earth.
   
2. Equipment earthing: All non-current carrying metallic parts of the generator, transformers, etc.are connected to the earth.
   

ADVANTAGES OF NEUTRAL EARTHING

1. Arcing phenomenon is prevalent in the case of the three-phase ungrounded neutral system. By neutral earthing, the arcing ground is reduced. This is explained in detail via the following example:

•         As shown in fog.1, under Normal conditions due to shunt capacitance the charging current flows from line to ground and sum of three current are zero and no current flows through neutral.

Fig.1: The Normal operating condition

·         Let us consider, L-G fault occurs at phase C, and charging current from phase A and B  to the ground remains the same. The charging current will be the summation of Ia and Ib. The fault current line Ic connects directly to the ground and hence feeds a high magnitude of the current. Since the neutral is not connected to ground that path is considered open.

·         This means that if the neutral was connected to the ground, this faulty current would have moved in that path. As this is not the case, faulty current will leads to arcing in the two capacitances between phases A, B, and ground. So it will originate from the ground and terminate to the unground neutral.

                  Fig.2: L-G Fault at phase C 

      2. The life of insulation is large due to the prevention of voltage surge.

      3. Overvoltage due to lightning is discharged to earth.

      4. Prevention of unnecessary tripping of circuit breakers.

Basics of Earthing in Electrical Power System

WHAT IS EARTHING 

• Earthing is nothing but a connecting specific part of installation to earth conductive surface.

  Choice of earthing system can affect the safety and EMC (electromagnetic compatibility) of the system.
• The objective of earthing is to ensure the safety from the shock and fire hazard arising out of leakage currents.
• The purpose of earthing is to protect the electrical system from the rise in voltage due to lightning, breakage between winding, etc.

WHAT IS TO BE EARTHED?

1.       The neutral of one side of the transformer of three-phase

2. The neutral of 3 phase, 4 wire system

3.The middle conductor of 2 phase,3-wire system

4.One of each 2-wire secondary transformer

5.LA earth terminals

6.One side of CT and PT secondary winding

6.All non-current carrying metallic part of generator, transformers etc.