An Overview Of Grounding System (Ungrounded)

An Overview Of Grounding System (Ungrounded) (On photo: installed ground clamp. The 2" x 0.022" copper strip is treated with a copper based anti oxidation grease and then clamped to the clean copper plated 8' ground rod - by beevo.com )

Topics:

• Ungrounded system
• Derivation of current expression in ungrounded system
• Advantages & disadvantages of ungrounded system
• Concept of effective and non-effective grounding
• Coefficient of earthing and earth fault factor

Underground Neutral Or Undergrounded System

Before 1950 power system were often without neutral grounding. Such system had repeated arcing grounds, insulation failure and difficult earth fault protection.

Every phase has inherent distributed capacitance with respect to earth. If earth fault occurs on phase B, the distributed capacitance discharges through the fault. The capacitance again gets charged and gets discharged. Because of this sever voltage oscillation is reached in healthy phases.

These voltage oscillation causes stress on insulation of connected equipment.

Figure 1 - Ungrounded neutral or ungrounded system

Ic2 = jCwv2
Ic3 = jCwv3
Ic = jCwv2 + jCwv3
Ic = jCw(v2 + v3) // Equation-01

Now by drawing the phaser diagram as shown below wecan write:

VN + V2 = v2 // Equation-02
VN + V3 = v3 // Equation-03

Substituting equation -02 and equation-03 in equation-01:

Ic = jCw(VN + V2 + VN + V3)
Ic = jCw(2VN + V2 + V3) // Equation-04

Figure 2 - Ungrounded neutral or ungrounded system

Voltage phasers V3 can be resolved in the direction of V_N and in direction perpendicular to V_N as V3Cosθ and V3Sinθ.

Similarly voltage phaser V2 can be resolved as V2Cosθand – V2Sinθ

Hence:
V2 + V3 = V3Cosθ + V3Sinθ + V2Cosθ – V2Sinθ // Equation-05
V3 = V2
V3Cosθ + V2Cosθ = V_N

Substituting in equation-05 we get:

V2 + V3 = VN = V1 (Since V1 is shorted to ground soVN = V1) // Equation-06

Substituting equation-06 in equation- 04 we get:

Ic = jCw(2VN + VN)

Since there is no return path available for fault current in ungrounded system so detection of earth fault current is difficult. This is other disadvantage of ungrounded system.

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Advantages of Ungrounded System

There are some advantages of ungrounded system:

1. Ungrounded system has negligible earth fault current
2. Some continuous process or system and essential auxiliaries where single phase to ground fault should not trip the system.

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Disadvantages of Ungrounded System

However below listed disadvantages of ungrounded system are more adverse than advantages:

1. Unearthed system experience repeated arcing grounds.
2. Insulation failure occurs during single phase to ground faults.
3. Earth fault protection for unearthed system is difficult.
4. Voltage due to lightning surges do not find path to earth.

In order to overcome the above mentioned technical and operation issues the concept of system grounding was introduced. System grounding is connecting the neutral of system to earth.

System grounding is of two types:

1. Effective grounding: Effective grounding is also called solid grounding that is without resistance or reactance. In this case co-efficient of earthing ismore than 80%
2. Non effective grounding: When neutral to earth connection is made through resistance or reactance than the system is said to be non-effectively grounded. In this case coefficient of earthing is greater than 80%

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Coefficient of earthing and earth fault factor

In a system without neutral earth (refer Figure 1), phase to earth voltage phase-1 and phase-2 rises to 3times phase to phase voltage Vrms during single phase to earth fault on phase 3. In a neutral earthed system the voltage ofhealthy phase rises to Ce times Vrms.

Therefore value of Ce:

• For non-effectively earthed system Ce = 1
• For effectively earthed system Ce < 0.8. Hence surge arrester rated voltage is > 0.8 V rms

Surge voltage kV instantaneous is taken as 2.5 times of critical flashover voltage (CFOV) of line insulation. Thus discharge current is given as:

I = (2.5(CFOV) –Residual voltage of arrester) / Surge impedance of line

• V1 = Highest RMS phase to phase voltage of healthy phases (phase 2 and 3 refer to Figure 1) during earth faulton pahse-1
• V2 = RMS value of phase to earth voltage at same location with fault on faulty phases removed

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References:

1. Industrial electrical network design guide By Schneider electric
2. Switchgear protection & power system By Sunil S Rao, Khanna publications
3. EARTHING: Your questions answered By Geoff Cronshaw
4. IEEE Recommended Practice for Electric Power Distribution for Industrial Plants