- 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
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.
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
Voltage phasers V3 can be resolved in the direction of VN and in direction perpendicular to VN as V3Cosθ and V3Sinθ.
Similarly voltage phaser V2 can be resolved as V2Cosθand – V2Sinθ
V2 + V3 = V3Cosθ + V3Sinθ + V2Cosθ – V2Sinθ // Equation-05
V3 = V2
V3Cosθ + V2Cosθ = VN
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)
Ic = j3CwV1
For ungrounded system:
If = IC2 + IC3 = IC = j3CwV1 // Equation-07
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.
There are some advantages of ungrounded system:
- Ungrounded system has negligible earth fault current
- Some continuous process or system and essential auxiliaries where single phase to ground fault should not trip the system.
However below listed disadvantages of ungrounded system are more adverse than advantages:
- Unearthed system experience repeated arcing grounds.
- Insulation failure occurs during single phase to ground faults.
- Earth fault protection for unearthed system is difficult.
- 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:
- 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%
- 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%
Ce = Highest phase to ground voltage of healthy phase / Phase to phase voltage
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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