Proper sub grounding of a substation is important for:
- It gives a means of dissipating electric current into the earth without exceeding the operating limits of the equipment.
- It provide safe environment to protect persons in the vicinity of grounded facilities from the danger of electric shocks under fault conditions in substation
Substation Grounding is necessary precaution activity.
Electric Shock – Substation Grounding
When an adequately large current passes through the human body, the heart muscle absorbs high energy causing ventricular fibrillation which can cause death of the person. There are four zone can be identified in below graph
No sensation – Zone 1
Perceptibility that current, but no harmful effect to the human – Zone 2
Muscular contraction and also difficulty in breathing for human – Zone 3
Probability that ventricular fibrillation increases of human – Zone 4
a: up to 5%
b: up to 50%
c: greater than 50%
Safe grounding – Substation Grounding
Requires the interaction of two grounding systems:
- Intentional ground,
consisting grounding system buried at the some depth below the earth’s surface in Substation.
- Accidental ground,
temporarily established by person exposed to potential gradient in the vicinity of the grounded facility
Basic shock situations – Substation Grounding
There are various types of shock situations which we can identified,
Step voltage – Substation Grounding
The difference in surface potential experienced by a person bridging 1 m with the feet without contacting any other grounded object.
Touch voltage – Substation Grounding
The potential difference between the Ground Potential Resistance (GPR) and the surface potential at the point where a person is standing while at the same time having a hand in contact with a grounded structure.
Transferred voltage – Substation
Special case of touch voltage where the voltage is transferred into or out of the substation and from/to a remote point external to the substation site.
Mesh voltage – Substation Grounding
Maximum touch voltage within the mesh of ground grid.
Metal-to-metal touch voltage – Substation
The difference of potential between metallic object or structure within the substation sites that can bridged by direct hand to hand or hand to feet contact.
Touch voltage circuit
VTH = Touch Voltage
Zth = Rf/2 Rf = ρ/4b Empirical Euation
Assume b = 0.08 and Rf = 3ρ then
Zth = Rf/2 = 1.5ρ
Accidental Ground Circuit Step Voltage
Estep = IB (RB +ZTH)
Step voltage circuit
VTH = Step Voltage
Zth = 2Rf Rf = 3ρ
Zth = 2Rf = 6ρ