Different Types Of Earthing Devices _ Electrical Engineering Center 5a5i17

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Rs. 1499 Different Types of Earthing Devices

1-Solid Earthing In this case, the neutral of a power transformer is earthed solidly with a copper conductor (see Figure 1). Advantages -Neutral held effectively at earth potential. -Phase-to-ground faults of same magnitude as phase-to-phase faults; so no need for special sensitive relays. -Cost of current limiting device is eliminated. -Grading insulation towards neutral point N reduces size and cost of transformers.

Disadvantages -As most system faults are phase-to-ground, severe shocks are more considerable than with resistance earthing.

-Third harmonics tend to circulate between neutrals.

2-Resistance A resistor is connected between the transformer neutral and earth (see Figure 2).

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-Mainly used below 33 kV. -Value is such as to limit an earth fault current to between 1 and 2 times full load rating of the transformer. Alternatively, to twice the normal rating of the largest feeder, whichever is greater. Advantages -Limits electrical and mechanical stress on system when an earth fault occurs,but at the same time, current is sufficient to operate normal protection equipment.

Disadvantages

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-Full line-to-line insulation required between phase and earth.

3-Reactance earthing

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A reactor is connected between the transformer neutral and earth (see Figure 3).

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-Values of reactance are approximately the same as used for resistance earthing.

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-To achieve the same value as the resistor, the design of the reactor is

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smaller and thus cheaper. 4-Arc suppression coil (Petersen coil) A tunable reactor is connected in the transformer neutral to earth (see Figure 4). -Value of reactance is chosen such that reactance current neutralizes capacitance current. The current at the fault point is therefore theoretically nil and unable to maintain the arc, hence its name. -Virtually fully insulated system, hence current available to operate protective equipment is so small as to be negligible. To offset this, the faulty section can be left in service indefinitely without damage to the system as most faults are earth faults of a transient nature, the initial arc at the fault point is extinguished Sensitive watt-metrical relays are used to detect permanent earth faults.

5-Earthing via neutral earthing compensator This provides an earth point for a delta system and combines the virtues of resistance and reactance earthing in limiting earth fault current to safe reliable values (see Figure 5).

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