GROUNDING

A properly designed and installed grounding system ensures reliable performance of electrical substations. Just how important is substation reliability? Fast clearing of faults, made possible by good grounding, improves the overall safety and reliability of an electrical system. Therefore, substation reliability must be as "built-in" as possible because of the high available fault current levels present and unlikely occurrence of follow-up grounding inspections.

Types and Methods of Grounding

There are different types and methods of grounding which ensures the reliable performance of a substation.

a. Types

Grounding of earth may be classified as (i) Equipment grounding (ii) System grounding and (iii) Neutral grounding.

Equipment grounding deals with earthing the non current carrying metal parts of the electrical equipment. On the other hand, system grounding means earthing some part of the electrical system e.g. earthing of neutral point of star connected system in generating stations and substations.

i . Equipment Grounding

The process of connecting non current carrying metal parts of the electrical equipment to earth in such a way that in case of insulation failure, the enclosure effectively remains at earth potential is called Equipment grounding.

ii. System Grounding

The process of connecting some electrical part of the power system (neutral point of a star connected system, one conductor of the secondary of a transformer) to earth is called System grounding.

iii. Neutral Grounding

The process of connecting neutral point of 3-phase system to earth either directly or through some circuit element (e.g. resistance or reactance etc.) is called Neutral grounding. Neutral grounding provides protection to personal and equipment. It is because during earth fault the current path is completed through the earthed neutral and the protective devices operate to isolate the faulty conductor from the rest of the system.

b. Methods of Grounding

The methods commonly used for grounding the neutral point of a 3-phase system are:

(i) Solid or effective grounding

(ii) Resistance grounding

(iii) Reactance grounding

(iv) Resonant grounding

i. Solid Grounding

When the neutral point of a 3-phase system is directly connected to earth through a wire of negligible resistance and reactance is called Solid or Effective grounding. Under fault conditions, the voltage of any conductor to earth will not exceed the normal phase voltage of the system.

Advantages:

a. The neutral is effectively held at earth potential.

b. No arcing phenomenon or over voltage condition can occur.

c. Permits the easy operation of earth fault relay.

Disadvantages:

a. It causes the system to become unstable.

b. The increased earth fault current results in greater interference in the neighboring

communication lines.

ii. Resistance Grounding

When the neutral point of a 3-phase system is connected to earth through a resistor, it is called Resistance grounding. The value of R should be neither very low nor very high. If the value of earthing resistance is very low, the earth fault will be large and the system becomes similar to the solid grounding system. On the other hand if the earthing resistance is very high, the system becomes similar to the ungrounded neutral system. The value of R is so chosen such that the earth fault current is limited to safe value but still sufficient to permit the operation of earth fault protection system.

Advantages:

a.The earth fault current is small due to the presence of earthing resistance. Therefore, interference with communication circuits is reduced.

b. It improves the stability of the system.

Disadvantages:

a. This system is costlier than the solidly grounded system.

b. Since the system neutral is displaced during earth faults the equipment has to be

insulated for higher voltages.

iii. Reactance Grounding

In this system, a reactance is inserted between the neutral and ground. The purpose of reactance is to limit the earth fault current. By changing the earthing reactance, the earth fault current can be changed to obtain the conditions similar to that of solid grounding. This method is not always used these days because of the following reasons

a. In this system, the fault current required to operate the protective devices is higher than that of the resistance grounding for the same fault conditions.

b. High transient voltages appear under fault conditions.

iv. Resonant Grounding

When the value of L of arc suppression coil is such that the fault current If exactly balance the capacitive current Ic, it is called Resonant grounding. It is also called as Peterson coil grounding as the arc suppression coil used here is the Peterson coil which is an iron cored connected between the neutral and earth. The resultant current in the fault will be zero or can be reduced by adjusting the tappings on the Peterson coil.

Advantages:

The Peterson coil grounding has the following advantages:

a. The Peterson coil is completely effective in preventing any damage by an arcing ground.

b. This coil has the advantage of ungrounded neutral system.

Disadvantages:

The Peterson coil grounding has following disadvantages:

a. Due to varying operational conditions, the capacitance of the network changes from time to time. Therefore, inductance L of Peterson coil requires readjustment.

b. The lines should be transposed.

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