EARTHING

EARTHING

          INTRODUCTION

Earthing means a connection to the general mass of earth.  The use of earthing is so widespread in an electric system that at practically every point in the system, from the generating system to the consumers’ equipment, earth connections are made.

Earthing is divided into two main categories:

• Neutral Earthing
• General Earthing

          OBJECTIVE OF EARTHING

          Neutral Earthing

This is the earthing of the star or neutral point of power system lines and apparatus.

The objective of neutral earthing are:

a)    To reduce the voltage stress due to switching and lightning surges and to discharge safely into the ground over voltages occurring in the system.

b)   To permit the use of graded insulation in H.V. and E.H.V systems with consequent reduction in weight, size and cost.

c)    To control the fault currents to satisfactory values.

d)   To ensure the operation of ground or earth fault relays.

                  General Earthing

This is a term applied to all earthing of metal parts of lines and apparatus used in electrical systems and equipment used in the utilisation of electrical energy other than neutral earthing.

The objects of general earthing are:

a)    To provide protection to plant and personnel due to accidental grounding of equipment.

b)   To cordon off the zone of dead line working to make it safe during working to prevent electrostatic and electromagnetic induction and also accidental contact from other energized lines and apparatus.

Examples of general earthing are the earthing of the frames of generators, rotors, motors, tanks of transformers, circuit breakers, body of domestic apparatus, lines, electric stoves, electric irons etc.

3.0     NEUTRAL EARTHING

The various methods of neutral earthing are:

a)    Solid Earthing or Effectively Grounded Earthing

b)   Resistance Earthing

c)    Reactance Earthing

d)   Arc suppression coil earthing.

However before discussing the effects, the merits and demerits of the above methods, an isolated Neutral system is considered.

3.1     ISOLATED NEUTRAL SYSTEM

Each line conductor has a capacitance to the earth and the magnitude of this capacitance is the same in a perfectly transposed three phase line.  With balanced voltages applied to such a line, the capacitance currents will be equal in magnitude as shown above.  Assume an earth fault in conductor B.  Hence no capacity current flows between the phase B and earth.

 But the voltage across the other two phases rises to phase to phase voltage, as shown.

The fault phase B supplied the currents ICGR and ICGY.  These being capacitive

Currents, no current flows when the line capacitance is charged.  Hence, an arcing takes place at the faulted point.  During this period, the line capacitance discharges and capacitive current once again flows.  This repetitive cycle of charging and discharging causes intermittent arcing at the point of fault and also gives rise to abnormal voltages across the healthy phases due to the capacitance effect.  In practice, voltages of 3 to 4 times the system phase voltage may occur thereby causing damage to the system insulation.  Hence isolated neutral system is not being practiced.

                                             Solid Earthing

In solid earthing a direct metallic connection is made between the system neutral and the ground.  The ground electrode resistance will be very small usually less than one ohm.

          The Main Advantages are:

a)    There is no abnormal voltage rise on the other healthy phases.

b)   Permits the use of discriminative protective gear.

c)    No voltage stress on the system insulation.

d)   Efficient and correct operation of Earth fault Relays is ensured.

e)    Additional savings are possible in power transformers of 132KV and above with the use of graded insulation.

f)     No arcing grounds.

3.22   Disadvantages are:

a)    On overhead transmission lines, a majority of the faults are to the ground.  Thus, the number of severe shocks to the system is relatively much greater than with resistance or reactance grounding.

b)   The ground fault current is generally lower than the three-phase current.  But near generating stations, it may be relatively higher and may exceed the three phase short circuit currents.  In such cases circuit breakers with higher rupturing capacity are required.

c)    The increased ground fault currents affect neighboring telecommunication circuits.

Most of the adverse effects have been overcome nowadays by the use of high rupturing capacity, high speed circuit breaker and fast acting protective relays.  Hence in the world over, it is the practice to adopt solid earthing for the neutrals of power systems.

3.3     Resistance Earthing

This is one form of impedance earthing and introduced when it becomes necessary to limit the earth fault current.  The resistance used may be a solid metallic resistor or a liquid resistor or a metallic resistor immersed in a liquid like transformer oil.

         The main advantages are:

1)   Permits the use of discriminative gear.

2)   Effects of arcing grounds are avoided with suitable low ohmic resistance.

3)   Ground fault currents are reduced, thus obviating the harmful effects of the large currents associated with solid earthing.

4)   Interference with adjoining communication circuits is avoided.

The disadvantages are:

1)   System neutral will almost invariably be fully displaced in the case of a ground fault, thereby necessitating the use of 100% lightning Arresters at an increase in cost.

2)   Cost of transformers will increase because graded insulation cannot be used.

Resistance earthing, if at all used, is limited to system voltages of 33KV and below and when the total system capacity does not exceed 5000 KVA.

3.4     Reactance Earthing

This is another form of impedance earthing also called `Peferson Coil Earthing' after the name of the inventor.

This is a logical development of reactance earthing and is based on a value of reactance in the system neutral such that the reactance current due to the coil exactly neutralises the network capacitance current at the fault.  The resultant capacity current is theoretically nil and in any case inadequate to maintain the arc.  Hence the name `arc suppression coil'.